semester project paper of maintenance

[ASSESSMENT OF REINFORCED CONCRETE STRUCTURES IN MARINE ENVIRONMENT]

Construction operation and maintenance

By Fikadu G/medhin GSR/024/01

Assessments of reinforced concrete structures in marine environment

1. Introduction

In construction world the 20st century will be known as the century of construction in oceans. There are a number of reasons for this prediction.

Human population is expected to grow to more than six billion by the end of the 20th century. In implanting in living conditions around the world has not kept pace with this increase in population.

Coastal and offshore sea structures are exposed to simultaneous attack by a number of physical and chemical deterioration processes, which provide an excellent opportunity to understand the complexity of concrete durability problems in the field practice.

Oceans make up 80 percent of the surface of the earth; therefore, a large number of structures are exposed to seawater either directly or indirectly (e.g., winds can carry seawater spray for a few miles inland from the coast). Concrete piers, decks, breakwater, and retaining walls are widely used in the construction of harbors and docks.

To relieve land from pressures of urban congestion and pollution, floating offshore platforms made of concrete are being considered for location of new airports, power plants, and waste disposal facilities. Many offshore concrete drilling platforms and oil storage tanks have been installed during the last 3 decades.

Many industrial materials, commonly used for structural purposes, do not show long-term durability in marine environment. Portland cement concrete has proved to be an exception and is, therefore, increasingly used for the construction of concrete structures. Concrete exposed to marine environment may deteriorate as a result of combined effects of chemical action of seawater constituents on the cement hydration products, alkali-aggregate expansion (when reactive aggregates are present), crystallization pressure of salts within concrete if one face of the structure is subject to wetting and others to drying conditions, frost action in cold climates, corrosion of the embedded steel in reinforced or prestressed members, and physical erosion due to wave action and floating objects. Attack on concrete due to any one of these causes tends to increase the permeability; not only would this make the material progressively more susceptible to further action by the same destructive agent but also by other types of attack. Thus a maze of interwoven chemical and physical causes of deterioration is at work




when a concrete structure exposed to seawater is an advanced stage of degradation. Theoretical and practical aspects of Reinforced concrete deterioration by seawater assessments, and recommendations for construction of durable concrete structures in the marine environment are discussed by this semester project. 1.1. Marine environment

The impact of marine environment on reinforced concrete structures was not well understood.

Marine environment throughout the world are characterized by both similarities and differences, which must be clearly understood before constructing concrete structures designed to last hundreds of years.

1.1.1Chemical composition of marine environment (seawater)

Most sea waters are fairly uniform in chemical composition, which is characterized by the presence of about 3.5 percent soluble salts by mass. The ionic concentrations of Na+ and Cl− are the highest, typically 11,000 and 20,000 mg/l, respectively. However, from standpoint of aggressive action to cement hydration products, sufficient amounts of Mg2+ and SO4 2− are present, typically 1400 and 2700 mg/l, respectively. The pH of seawater varies between 7.5 and 8.4; the average value in equilibrium with the atmospheric CO2 is 8.2. Under certain conditions, such as sheltered bays and estuaries, pH values lower than 7.5 may be encountered due to high concentration of dissolved CO2, which would make the seawater more aggressive to portland-cement concrete. Ions Concentration(g/liter) Na+ 11.00 K+ 0.40 Ca2+ 1.33 Cl- 0.43 SO42- 19.80 2.76 Table.1.Average composition of seawater 1.1.2 Marine organisms




Marine growth involving barnacles and mollusks is frequently found on the surface of porous concrete whose alkalinity has been greatly reduced by leaching. Since marine growth is influenced by temperature, oxygen, PH, current, and light conditions it is generally limited to about 20m from the surface of the sea water and is less of a problem in cold climate. Barnacles, sea urchins, and mollusks are known to secrete acid which can cause boreholes in concrete and pitting corrosion on the surface of embedded steel.

The presence of aerobic or sulfur-oxiding bacteria causes the conversion of H2S to sulfuric acid, which is highly corrosive to both concrete and reinforcing steel.

Marine growth may also be problem because it can produce increased leg diameter and displaced volume which would result in increased hydrodynamic loading. Marine growth also prevents adequate visual examination of concrete surfaces for further defects.

1.1.3. Temperature

The surface temperature of seawater varies widely from a low of -20c(freezing point of sea water) in cold regions to a high of about 300c in tropical areas. When the surface temperature is high, it shows a rapid drop with depth until the temperature reaches a steady- state value of about 2-50c, at water depths of 100-1000m.

In addition to its effect on the growth of marine organisms, the temperature of sea water determines the rate of chemical and electrochemical reactions in concrete.

1.1.4. Hydrostatic Pressure

The hydrostatic pressure of sea water on the submerged portion of a structure follows the simple relationship: P=ρh, where P is unit pressure, ρ is density of fluid, and h is depth of water. Although the average density of seawater is 1026kg/m3, for practical purpose the hydrostatic pressure can be assumed as one tone per square meter of depth. The hydrostatic pressure acts as a driving force to push sea water through a permeable material.

1.1.5. Tidal action

A tide comprises the gradual rise and fall of ocean water on a definite schedule twice a day. The gravity of the moon pulls the water nearest the moon slightly away from the solid part of the earth. At same time, the moon pulls the solid




earth slightly away from the water on the opposite side of the globe. In this way, the moon’s gravity is constantly producing two bulges on the ocean waters during its daily journey around the earth. These bulges mark the position of high tide (highest water level).

At any coastal location the time interval between one high tide and the next is 12hours and 25minutes.As the earth turns, the tides rise and fall at each place on the ocean:

A high tide is always followed by a low tide (low water level) after about 6hours and 13 minutes. Tidal action takes place with remarkable regularity, i.e. from its low position seawater rises gradually for about six hours until it reaches the high tide mark and then it begins to fall for the next six hours until it reaches the low tide mark again. Thus, as a result of tidal action, a marine structure is exposed in the tidal range ( between low and high tide levels) to twice-a-day cycles of wetting and drying, heating and cooling(due to differences between air and sea water temperatures),and possibly freezing and thawing( in cold climates).

The tidal range varies considerably from about 0.5m in some locations to as much as 15m in others.

1.1.6. Storm Waves

The forces exerted by ocean waves are enormous and are usually the primary design considerations affecting fixed structures. Waves are caused mainly by the action of wind on water; through friction the wind energy is transformed into wave energy.

1.1.7. Frog and spray

In summer, coastal fog is formed when warm air from land passes over a colder ocean. In winter, the colder air from land passes over the warmer and more humid environment of seawater. In both cases moisture condensation results in fog or low stratus clouds. Coastal frogs are frequently the carrier of fine droplets from seawater arising from the spray action.

Like wave action, spray is created by the action of wind on waves.

In general marine environment is highly in hospitable for commonly used materials of construction, including reinforced concrete.




Seawater contains corrosive ions and gases, and is a home to numerous marine organisms that are harmful to construction materials. Hydrostatic pressure and temperature extremes, capable of accelerating the process of deterioration in materials, are frequently encountered with coastal and offshore structures. Storm waves have destroyed even strong structures. Thus, the hostile and highly complex ocean environment presents both a formidable challenge and a great opportunity to materials engineers.

1.2. Marine structures

The term ‘marine structures’, is normally applied to coastal berthing and mooring facilities, breakwaters and tidal barriers, dry docks, and jetties, container terminals, and offshore floating docks and drilling platforms. Such a description of marine structures is based on their function, and is not useful for design purposes. a classification of marine structures, based on their most prominent design feature, is grouped the wide variety of marine structures, in to five general categories:

Figure1.different types of marine environment

a.piled plat forms

b. flexible bulk heads

c. gravity structures




d. rubble mounds and

e. Floating structures.

2. Quality assurance for new reinforced concrete structures in marine environment

Reinforced concrete is a versatile, economical and widely used construction material. It can be mounded to a variety of shapes and finishes. Usually it is durable and strong, performing well throughout its service life. However, it does not perform adequately as a result of poor design, poor construction, in adequate materials selection, exposed to a more severe environment than anticipated or a combination of these factors. In most countries, concrete structures make up a very large and important part of the national infrastructure, and both the condition and performance of these structures are important for the productivity of the society. Since there is a growing amount of deteriorating concrete structures on marine environment, however, not only the productivity of the society is affected, but it also has a great impact on resources, environment and human safety. The operation, maintenance and repair of concrete structures are consuming much energy and resources and are producing a heavy environmental burden and large quantities of waste. Thus, the poor durability and premature service life of many marine concrete structures do not only represent technical and economical problems. This is poor utilization of natural resources, and hence, also an environmental and ecological problem. For these reasons the reinforced concrete structure should assured its quality trough out the planning, design and construction phase.




2.1. Pre-construction

2.1.1 Planning considerations

Climatic factors such as solar radiation, temperature, water, air contaminations, and wind are particularly important in the degradation of materials used in the exterior surface of structures.

But the range and importance of these agents vary widely with type of climate, geographic location, time of the year, and even within a relatively small area (microenvironment). In actual service, degradation factors may interact to increase the rate of degradation, or less often, to decrease the rate by one factor cancelling the effect of others. Degradation of material properties mostly sets in under combined action of internal and external factors. It is a complex process determined largely by the physiochemical properties of the material (internal) and the manner in which it is used (external). The processes or reactions lead to a chance in the ability of a material or component to perform as intended. Factors internal to a material are those which determine its quality (i.e., the way it is made, placed, and cured), and others such as shrinkage, creep, and thermal effects which are inherent in its nature. External causes of deterioration are broadly grouped as physical, chemical, or mechanical. Main physical factors are the fluctuations of moisture content, temperature, freezing and thawing (that occurs in natural weathering), and fire. The main chemical factors are aggressive gases and liquids, and the main mechanical factors are load, friction and vibration. To control the source of deteriorations of reinforced concrete structures it is necessary to include the environmental assessment with regard to the effect of reinforced concrete structures in the planning stage ,so as to design the structure sustainably. 2.1.2 Design considerations

The design of concrete structures, while in full accordance with the requirement of the design specification, do not necessarily work well in practice. A survey of structures usually indicates that deterioration occurs repeatedly in connection with certain details, or that certain effects occur which were not anticipated in design. For example, inadequate design, which fails to allow creep of structural elements of a building (e.g. deflection of floors), may result in the load being




transferred to non-structural elements, such as partition walls or cladding panels, and cracking and damage often result (Slater 1980). In designing, there is a need to trace the anticipated water flow over the whole surface of both vertical and horizontal members. Design should include the provision of desired concrete quality to resist the adverse effects produced by in-situ exposure conditions. Thus, the designer needs to be better informed on the particular characteristics of the given environment so that a proper choice in matching the concrete selected with the characteristics of the environment can be made. The quality of concrete with respect to durability can be measured in terms of various parameters. These properties of hardened concrete are governed by the micro and macro structure of concrete which is essential in its ability to resist chemical attack by: External sources(e.g., acids, carbon dioxide, and sulphates from within the concrete (e.g., alkali-aggregate reaction (AAR) and unsound cement), and from other environmentally induced distress related to moisture ingress (e.g., freeze-thaw cycling, leaching). These parameters regarding concrete quality are affected by the quality of cement and aggregate, the w/c ratio and degree of hydration, the effectiveness of compaction, the extent of curing, and the presence of cracks. Exposure conditions vary considerably from location to location. Whatever the in-situ conditions, the designer must acquaint himself with the factors, results of a given environment, and only then design a concrete to meet those demands or take measures to protect the concrete from the aggressive conditions. Having designed the concrete, the specifier should then institute proper quality control procedures to ensure that the placed concrete will meet the specified features. Material selection It is clear that the permeability of concrete is the most important factor determining the long term durability. Therefore, with any new construction not only is it important to select materials and proportions for the concrete mixture that are most likely to produce a low permeability product on curing but also necessary to maintain the water tightness of the structure as long as possible through the intended service life. In short, careful attention should be paid to all of the following aspects of concrete construction.




Selection of concrete making materials and mix proportions Good concreting practice Measures to prevent the widening of pre existing micro cracks in

concrete during service.

It should be noted that many of the recent concrete sea structures, built during the last 15-20 years, are required to with stand unprecedented stress conditions.

For example, coastal and offshore structures in North Sea and arctic are exposed to enormous hydrostatic pressures, impact loading, frost action, and abrasion /erosion loss from floating ice. coastal structures in the middle east are exposed to numerous cycles of temperature extremes (i.e. hot days and cold nights).consequently, these structures made with high-strength concrete are very heavily reinforced as well as pre stressed.

The protection of the embedded steel with a low permeability concrete is of paramount importance from the stand point of durability.

The requirements for the concrete to be free-flowing, cohesive, self-compacting and not prone to bleeding are generally achieved by aiming for a high workability (100–150 mm slump) and high fines content (sand, cement and possibly fine filler). These allow the free water/cement ratio to be kept moderate so that the concrete maintains its cohesiveness and is not prone to segregation. Free w/c ratios in the range 0.40 to 0.45 are common. Cementitious materials The cementitious material content is likely to be 375 -+ 50 kg/m3 in order to increase cohesion; not because some cement will be washed out. This will ensure a good strength but not the full potential as there is likely to be some residual voids as a result of trapped air and the higher than normal water content. The flow of the concrete has been found to be enhanced by the inclusion of about 15 per cent of pozzolanic material. In large pours, internal temperatures can reach 70–95°C and cracking can develop when the concrete cools, particularly in unreinforced concrete. To reduce this risk, the use of a cement blend consisting of about 16 per cent Portland cement, 78 per cent the inert materials and 6 per cent silica fume has been suggested, with the concrete being pre cooled to 4°C prior to placing.




Aggregates Aggregates should be rounded in shape, such as Thames Valley Gravel, as these will have a low water demand and give better flow and self-compaction characteristics. Crushed rock or other angular material should be avoided if possible. Grading should be continuous to reduce the susceptibility to segregation. The maximum aggregate size is usually 20 mm, although this is often dictated by the placing method. The sand content is normally high at 45 per cent-plus of the total aggregate; lower amounts would make the mix prone to washout. Admixtures There is normally sufficient cement and fine sand particles in underwater concrete to obviate the need for an admixture but should the concrete not be sufficiently cohesive a plasticizer could be included. For massive pours there is often a need to keep the concrete workable for extended periods and a retarder would be needed. Super plasticizers are available for high-performance concrete Workability The workability of the mix determines the degree of compaction underwater. As can be seen from Figure below with a slump of 150 mm, concrete which is not vibrated will achieve almost as much strength (95 per cent) as the fully compacted concrete. There is not much benefit in having a slump greater than this as high-slump concrete will be more prone to washout. Even at 100 mm slump, the strength difference is relatively small, at about 88 per cent.

2.2. Construction considerations

To make durable reinforced concrete structures in marine environments the selection of proper materials and mixing proportions is only the first step. Sufficient attention must also be given to the concrete production and construction practice. Reflecting the growing awareness in the concrete construction early age of concrete plays an important part in determining the service life. Usually the early age period is limited to the first 24 hours after production.

Improper procedures or carelessness during any phase of the construction operation results in concrete of inferior quality. Poor transportation, placing, finishing techniques, and inadequate curing conditions are a few examples.




So during the entire process of concrete work it is true to necessary to assure the methods are towards durable reinforced concrete structure for marine environment.

2.2.1. Batching, mixing and transporting

Mixing proportioning is a process by which one arrives at the right combination of cement, aggregates, and admixtures for making a concrete mixture that would meet given requirements. With ordinary concrete mixtures the usual requirements are consistency or slump of fresh concrete, the 28 day un axial compressive strength Which is presumed to be an index of general concrete quality including the long term durability. Of course, for freezing and thawing cycles, concrete has to be protected by air entertainment.

Similarly for corrosive environments, such as sea water, a maximum limit on the permeability of concrete may be specified.

Since test methods for direct measurement of permeability are rather complex, this property may be controlled indirectly by specifying a maximum limit on the water/cement ratio.

2.2.2. Placement, consolidation, and finishing

For those used to concreting on dry land, concreting under water presents various challenges. Transporting, compacting, quality control, finishing and accuracy must all be carried out successfully in this different, and often difficult, environment. There are, however, many common aspects, chief of which is that air is not required for the setting and hardening of concrete – it sets and hardens just as well, and often even better, under water – but it must be fluid enough to flow into position and be self compacting as conventional vibration is not practicable under water. During transporting and placing, conventional concrete and water must be kept apart and, when they inevitably do come into contact, rapid interface flow must be minimized or cement may be washed out to form a weak layer. Washout can be obviated by the use of an admixture to make the concrete non-dispersible but this comes at a cost and contractors unwilling to pay the additional expense involved often adhere to the more traditional methods of placing under water.




1. Tremie The principle of this method is that concrete is poured down a pipe or tube from above the surface and is forced into the mass of concrete already in place by the weight of concrete in the tube. The tube is surmounted by a hopper (‘tremie’ in French) and the whole is suspended from a staging or frame, mounted so that it can be moved vertically when held by a crane. As the pour rises, sections of the tube can be removed to facilitate working. A convenient diameter for the tube is 8 to 16 times the maximum aggregate size and 250 mm is a common diameter. Figure below shows a diagrammatical representation of a tremie.

Figure.2. Schematic of a tremie. Before starting the pour, a plug is inserted into the tube to stop the concrete and water Inter mixing. This plug can be purpose-made (similar to a bath plug), a sponge rubber ball or exfoliated vermiculite, which is the most common method in the world. At start-up the bottom of the tube should be on or very close to the sea or river bed, sufficient to allow the water in the tube to escape and to force the first load of concrete to spread out horizontally into a mound shape. The concrete




pouring should be continuous with the bottom of the tube always inside previously placed concrete. If this immersion depth, normally at least 0.5 m, is not sufficient, a breakthrough will occur and the pour will have to be abandoned for the day. Any air that is in the concrete being placed will pass through the previously placed concrete and bubble to the surface, disrupting the settled concrete as it goes. The flow of concrete in the tube is governed by gravity and friction with the tube wall, so the tremie has to be moved up and down to regulate the flow. A crane driver with a good ‘feel’ for this is useful. The tube should be restrained from lateral movement whilst placing concrete. The placed concrete spreads out horizontally on the bed in a circle, with the top of the pour domed upwards. Tremies are best used for thick pours of any area. For large area pours, multiple tremies are used, spaced at about 4–6 m apart, depending on the flatness required for the top level. The slope of the concrete surface from a tremie is likely to be in the range 1 in 9 for tremies close together to 1 in 6 for those spaced far apart as the slope increases with distance from the pipe. 2. Hydro valve This Dutch innovation is a refinement of the tremie and is shown in Figure below. Instead of a solid pipe, it uses a collapsible fabric tube which is kept closed by the water pressure until the weight of concrete in the system overcomes the hydrostatic pressure and the skin friction. A plug of concrete then descends slowly and the tube is sealed behind it by the water pressure. The bottom section of the flexible tube is encased by a rigid tubular section the bottom of which, except at start-up, is at the desired surface level of the concrete.




Figure.3. Schematic of a hydro valve. The Hydro valve is used for casting relatively thin sections of concrete in a to-and-fro pattern, with each run of concrete placed on the sloping exposed face of the previous run. As it is generally carried out in still water and each subsequent row of placements scours off any disturbed material on the sloping face, the end product is sound. The ‘valve’ sits on top of the pour so there is no pipe immersion, as there is with a tremied placement, and the valve levels off the top of the pour. Placements up to 750 mm thick through reinforcement are possible with this method. Another Dutch refinement of the tremie is the ‘hop dobber’, where the tremie pipe is in two sections. The upper section is slightly smaller in diameter and fits into the top of the lower section, which has a surrounding flotation chamber to keep the bottom of the pipe at a constant level. Its method of operation is very similar to that of the hydro valve. 3. Skips Skips are more suitable for thin pours, although it is possible to bury the mouth of the skip in previously poured concrete to produce deeper pours. The skip should be fully charged in the dry and covered with a pair of flexible and overlapping covers; these prevent washout during lowering and also during discharge because they stay in contact with the top surface of the concrete as it flows out of the bottom.




The best type of skip has vertical sides to allow the concrete to fall freely and vertically downwards. Discharge is the critical part of the operation and to minimize intermixing with water, the skip must have a skirt. Some skip doors are hinged at the sides and open from the centre, which produces a skirt. A better type of skip, shown in Figure shown below has a skirt which encases the lower part of the skip and this descends as the central roller doors open.

Figure.4. Bottom-opening skip with skirt.

Skips normally trip open automatically on touching the base or previously placed concrete but any projections from the base will also trigger the mechanism, causing premature discharge. 4. Pumps The constitution of concrete for underwater work is similar to that required for pumping, so pumps provide an ideal placing method. Both static and mobile pumps can be used. Water pressure helps to counteract gravity, which can be a problem when placing in the dry. Underwater pours are seldom at depths greater than 30 m, the depth limit when pumping in the dry. As with tremie work, broken seals and air pockets can be a nuisance and the operator of a pump does not have the same ‘feel’ for the pour as a tremie operator. A pump pipeline is normally at an angle when used under water and is therefore not as controllable as a tremie tube; preventing it moving sideways is essential in preventing washout. 5. Toggle bags




Where small amounts of concrete are required, such as in repair work, the toggle bag is ideal. The waterproof bag is filled in the dry with wet concrete and the mouth is closed with a tie rope and toggle. At the placing location the concrete is squeezed out by a diver and rammed into place. The use of a diver adds to the cost of the operation. 6. Bag work The type of bags used here; are normally made from an open-weave material such as hessian. They should be half-filled with plastic concrete, sealed and then taken under water and placed by a diver. Partial filling allows them to be moulded into shape and gives them good contact areas with adjacent bags. Grout from the mix seeps through the open textured material allowing bond to be established with adjacent bags. For additional stability the bags can be spiked together with small-diameter reinforcing bars. Divers prefer to handle bags of dry-mixed concrete and to grout up between bags. However, this system places too great a responsibility on the diver. The dry mix concrete is never fully wetted-out by water seeping in, the concrete cannot be fully compacted and contact surfaces are minimal. Diver-handled bags are usually of 10 to 20 litres capacity but 1 m3 bags can be placed using a crane. 7. Concrete packaged under water Quilted revetment bags or mattresses are used as protection against erosion, scour and water seepage. They consist of a double skin of woven permeable fabric connected together by threads. Sections are zippered together for continuity, laid on banks below and above water level and then pumped full of sandy grout. Cable or rope reinforcement is threaded through them to ensure integrity after settlement and shrinkage cracking have taken place. Filling gaps between structural units can be done by inserting collapsed plastic or nylon bags into which grout is then pumped until the gaps are filled. 8. Grouted aggregates In order to obtain good results with this system, also known as pre-placed aggregate concrete, it requires considerable skill and experience in the application of the process. It must be undertaken by a skilled, specialist contractor. The space to be concreted, often determined by formwork, is filled with single-sized aggregate, into which are inserted grout pipes, as shown in Figure shown below. The aggregate represents about 65–70 per cent of the overall volume to be concreted. Grout is then pressure-pumped in to fill the voids left by the




aggregate particles. One patented process uses a colloidal suspension, produced by mixing the cement, and possibly pfa, with water in a high-speed mixer, followed by the addition of sand and further mixing. Colloidal grout is stable and has good flow properties.

Figure.5. Pre-placed aggregate. The grouted aggregate technique is especially useful where the water is flowing and where undercuts which are inaccessible to tremies and skips need to be filled. Stones placed in a formwork box reduce the flow of water through them, which allows the grout to penetrate rather than be washed out but, prior to grouting, it can cause sediment to be deposited on the aggregate and form a barrier between grout and stones if the water is silty. To minimize this the whole operation should be carried out on the same tide so that silt does not settle when the water is relatively still – at the top and bottom of a tide. The grout pipes are rigid and spaced about 2 m apart, with their ends close to the bottom of the pour. As with tremie work, they are raised as grouting proceeds. A typical grout consists of a blend of Portland cement and pozzolana (at a ratio of between 2.5:1 and 3.5:1 by mass) mixed with sand (at a ratio between 1:1 and 1:1.5 by mass) and with a w/c ratio of 0.42 to 0.5. An intrusion aid is added to improve the fluidity, suspending and cohesive qualities of the grout. The intrusion aid also delays stiffening of the grout and contains a small amount of aluminum powder, which causes a slight expansion before setting takes place. The drying shrinkage of grouted aggregate concrete is lower than that of ordinary concrete, due to the contact between the large aggregate particles. This contact restrains the amount of shrinkage that can take place but shrinkage cracking can occasionally develop.




Strengths of about 40 MPa are usual but higher strengths are possible. Provided a strong aggregate such as flint is used, the resulting concrete will have excellent abrasion resistance.

3. Assessment of the extent of Reinforced concrete deterioration in marine environment

Before diagnosing the causes of deterioration or failure of a concrete structure, a sound understanding of the physical, chemical and mechanical actions that lead to defects is necessary. An extensive amount of research work has been carried out in order to better understand and control several of the most important deteriorating mechanisms such as alkali aggregate reactions, freezing and thawing and corrosion of embedded steel. In particular, much work has been carried out on corrosion of embedded steel, which represents the greatest threat both to the safety and economy of the structures. No single parameter controls the durability of concrete. Instead, there are a number of contributing factors which affect its durability. In practice, several degradation mechanisms can act simultaneously with possible synergistic effects. The schematic diagram below illustrates how different degradation mechanisms can act on concrete exposed to sea water.




Figure. 6. Possible degradation mechanisms acting on concrete exposed to sea water The main degradation process for durability of reinforced concrete structures in marine environment is, without a doubt, the corrosion of the concrete reinforcement. 3.1. Modes of concrete deterioration Deterioration is any adverse change of normal, mechanical, physical, and chemical properties either in the surface or in the body of concrete, generally due to the disintegration of its components The phenomenon which induces such distress may be associated with one of the phases (e.g. design, construction, or service). The effects of deterioration may or may not be manifested visually. There are three basic visual symptoms of distress in a concrete structure: • cracking




• Spalling

• Disintegration.

Figure.7. the three possible concrete deteriorations Although each of the basic symptoms is readily differentiated from the others, each occurs in several forms, each having a different significance. Furthermore, in a given structure, the three basic indicators of distress may occur not only in combination, but with several forms of each symptom being manifested simultaneously. In addition to the deterioration of concrete, the breakdown of other auxiliary materials, such as sealants, coatings, membranes, which form the complex assembly of a structure, should also be considered. Polymeric products often interact with other materials with which they are in contact to form compounds which are devoid of the characteristics of the original. Degradation agents can he defined as any group of factors that can affect the performance of a construction material, component, or system. The different forms of deterioration of a reinforced concrete structure can be presented with chart as shown below.




chart.1. different forms of deterioration of reinforced concrete structure

Based on the above chart the source of deterioration for reinforced concrete structures are many such as; Alkali attack, ASR gel expansion, sulfate attack, Corrosion of reinforcement, frost attack and salt attack. But if you see the diagram most serious problem economically and safety cases is reinforcement corrosion, with this semester project I try to only focus on the assessment of the rebar corrosion of reinforced concrete structures .

3.2. Corrosion of reinforcing steel The corrosion of reinforcing steel in concrete is a major problem facing professionals today as they maintain an ever increasing ageing infrastructure. The economic loss and damage caused by the corrosion of steel in concrete makes it arguably the largest single infrastructure problem facing industrialized countries. Bridges, public utilities and buildings are ageing. Some can be replaced, others would cause great cost and inconvenience if they were taken out of commission. With major political arguments about how many more bridges and other structures we can build, it becomes crucial that the existing structures perform to their design lives and limits and are maintained effectively. In general, there are two major factors, which cause corrosion of reinforcement in concrete to proceed to an unacceptable degree. They are carbonation and the presence of chloride ions, which may either have been present in the concrete constituent’s right from the beginning or are




introduced into the concrete through ingress from the surrounding environment during the service life. Although carbonation-induced corrosion still represents a durability problem for many concrete structures, it is primarily uncontrolled penetration of chlorides which represents the most technically difficult and serious problem to the durability and safety of concrete structures. Concrete provides a high degree of protection to the reinforcing steel against corrosion, due to the high alkalinity (pH ≈ 13) of the pore solution. Under high alkalinity steel remains passivated. In addition, well-consolidated and properly cured concrete with a low w/c ratio has a low permeability, which minimizes penetration of corrosion inducing agents, such as chloride, carbon dioxide, moisture, etc. to the steel surface. Furthermore, the high electrical resistivity of concrete restricts the rate of corrosion by reducing the flow of electrical current from the anodic to the cathodic sites. In resume, if the concrete is properly designed, applied and maintained, there should be little problem of steel corrosion during the design life of the structures. Unfortunately, the durability requirements are not always achieved in practice due to which the corrosion of reinforcement in concrete has become a commonly encountered cause of deterioration. Once reinforcement corrosion is initiated, it progresses almost at a certain rate (depending on availability of oxygen and humidity, etc.) and shortens the service life of the structure, by causing surface cracking and subsequently spalling of the cover concrete due to expansion of the corroding steel. The rate of corrosion directly affects the extent of the remaining service life of a corroding reinforced concrete structure. 3.2.1. Mechanism of corrosion of steel in concrete The strongly alkaline nature of concrete, due to Ca(OH)2 with a pH of about 13, prevents the corrosion of the steel reinforcement by the formation of a thin protective film of iron oxide on the metal surface. This protection is known as passivity. However, if the concrete is permeable to the extent that carbonation reaches the concrete in contact with the steel or soluble chlorides can penetrate right up to the reinforcement, and water and oxygen are present, then corrosion of reinforcement will take place. The passive iron oxide layer is destroyed when the pH falls below about 11.0. Carbonation lowers the pH to about 9. The formation of rust results in an increase in volume compared with the original




steel so that swelling pressures will cause cracking and spalling of the concrete. Corrosion of steel occurs because of the electro-chemical action which is usually encountered when two dissimilar metals are in electrical contact in the presence of moisture and oxygen. However, the same process takes place in steel alone because of differences in the electrochemical potential on the surface, which forms anodic and cathodic regions, connected by the electrolyte in the form of the salt solution in the hydrated cement. The positively charged ferrous ions Fe2+ at the anode pass into solution while the negatively charged free electrons e- pass along the steel into the cathode, where they are absorbed by the constituents of the electrolyte and combine with water and oxygen to form hydroxyl ions (OH)-. These then combine with the ferrous ions to form ferric hydroxide and this is converted by further oxidation to rust. Thus, it can be written: Fe → Fe 2+ + 2e------------------------ (anodic reaction) 4e- + O2 + 2H2O → 4(OH) ------------ (cathodic reaction) Fe2+ + 2(OH)- → Fe(OH)2 ------------(ferrous hydroxide) 4Fe (OH)2 + 2H2O + O2 → 4Fe(OH)3 ----(ferric hydroxide)

These differences in potential are due to the inherent variation in structure and composition (e.g., porosity and the presence of a void under the rebar or difference in alkalinity due to carbonation) of the concrete cover, and differences in exposure conditions between adjacent parts of steel (e.g., concrete that is partly submerged in sea water and partly exposed in a tidal zone). The reactions involved in the process can be represented by the following schematic equations: 2Fe (metal) —> 2Fe2+ + 4e-----------------------(anodic reaction) 2H2O + O2 + 4e- → 4OH------------------------ (cathodic reaction) 2Fe2+ + 4OH- → 2Fe (OH)2 --------------------(ferrous hydroxide ) 2Fe3+ + 6OH- → 2Fe (OH)3 --------------------(ferric hydroxide ) 2Fe(OH)3 → Fe2O3 + 3H2O --------------------(ferric oxide)




a. b. Figure.8. Schematic representation of electro-chemical corrosion: (a) electrochemical process, and (b) electrochemical corrosion in the presence of chlorides It is shown that oxygen is consumed, but water is regenerated and is needed only for the process to continue. Thus there is no corrosion in a completely dry atmosphere, probably below a relative humidity of 40 per cent. Nor is there much corrosion in concrete fully immersed in water, except when water can entrain air. It has been suggested that the optimum relative humidity for corrosion is 70 to 80 per cent. At higher relative humidities, the diffusion of oxygen is considerably reduced and also the environmental conditions are more uniform along the steel. Chloride ions present in the cement paste surrounding the reinforcement react at anodic sites to form hydrochloric acid which destroys the passive protective film on the steel. The surface of the steel then becomes activated locally to form the anode, with the passive surface forming the cathode: the ensuing corrosion is in the form of localized pitting. In the presence of chlorides, the schematic reactions are (see figure above (b)): Fe2+ + 2C1- → FeCl2 - FeCl2 + 2H2O → Fe (OH)2 + 2HCl

Thus, Cl- is regenerated. The other reactions, and especially the cathodic reaction, are as in the absence of chlorides. It should be noted that the rust contains no chloride, although ferric chloride is formed at an intermediate stage. Because of the acidic environment in the pit, once it has formed, the pit remains active and increases in depth. Pitting corrosion takes place at a certain potential, called the pitting potential. This potential is higher in dry concrete than at high humidities. As soon as a pit has started to form, the potential of




the steel in the neighbourhood drops, so that no new pit is formed for some time. Eventually, there may be a large-scale spread of corrosion, and it is possible that overall and general corrosion takes place in the presence of large amounts of chloride. 3.2.2. Factors affecting corrosion of steel in concrete The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in that they do not attack the integrity of the concrete. Instead, aggressive chemical species pass through the pores in the concrete and attack the steel. This is unlike normal deterioration processes due to chemical attack on concrete. Other acids and aggressive ions such as sulphate destroy the integrity of the concrete before the steel is affected. Most forms of chemical attack are therefore concrete problems before they are corrosion problems. Carbon dioxide and the chloride ion are unusual as they penetrate the concrete without significantly damaging it. The factors affecting corrosion of steel in concrete may be classified into two major categories: External factors and internal factors. 3.2.2.1.External factors affecting corrosion of steel in concrete They include mostly environmental parameters, such as: • Availability of oxygen and moisture at reinforcement level: presence of moisture and oxygen supports the corrosion. Moisture fulfills the electrolytic requirement of the corrosion cell, and moisture and oxygen together help in the formation of more OH- thereby producing more rust component, i.e., Fe (OH)2. Oxygen also affects the progress of cathodic reactions. In the absence of oxygen, even in a situation of depassivation, corrosion will not progress due to cathodic polarization. In structures which are submerged or exposed to long-term or cyclic water application that causes water saturation of the concrete for periods of several weeks, the availability of oxygen is the only limiting factor for the corrosion rate of the reinforcement when the concrete surrounding the steel is water saturated and most of the oxygen within the concrete near the reinforcement surface has been consumed by the cathodic reaction of the corrosion process. Therefore, in the case of common outdoor structures being exposed to rain and not submerged or constantly water saturated due to other reasons, no




reduction of the corrosion rate induced by limited oxygen diffusion is to be expected. • Relative humidity: The moisture condition is one of the parameters that governs the hardening of concrete, especially close to the surface, and consequently governs the permeability to gases, water and ions. Moisture variations cause shrinkage and shrinkage cracking. Moisture plays a significant role in chemical reactions in concrete and in physical and chemical processes in various deterioration phenomena. The initiation time for reinforcement corrosion is highly influenced by the moisture content since high moisture delays the intrusion of carbon dioxide (diffusion of carbon dioxide is far slower than in the gas phase), however a certain moisture content is necessary for carbon dioxide to react with the portlandite (Ca(OH)2). Chlorides need moisture to penetrate the concrete. In the splash zone of a marine structure moisture plays a more active role when salts penetrate due to convection, moving with the water and depositing where and when the moisture evaporates. When the corrosion starts the rate of corrosion is influenced by moisture to a great extent. In dry conditions (electrolytic resistance of the concrete) or very wet conditions (O2 diffusion becomes the controlling factor) the rate is slow but intermediate moisture conditions give an electrolyte and permits the intrusion of oxygen to the corrosion process. Therefore, knowledge of the amount of pores described by the porosity, the pore size distribution and the water saturation degree of the pores are vital parameters to understand the electrochemical processes in concrete. • Temperature: Temperature has a large influence on the corrosion process of steel in concrete, especially, on the corrosion potential, the corrosion rate, concrete resistivity and transport processes in concrete. A rise in temperature may result in a twofold effect: the electrode reaction rates are generally increased, and the oxygen solubility is increased resulting in increase in the rate of corrosion. If the situation is conducive for corrosion to take place, the corrosion rate is increased by high temperature and high humidity. • Carbonation and penetration of acidic gaseous pollutants to the reinforcement level: Carbonation is the result of the interaction of carbon dioxide gas in the atmosphere with the alkaline hydroxides in the concrete. Like many other gases carbon dioxide dissolves in water to form an acid. Unlike most other acids the carbonic acid does not attack the cement paste, but just neutralizes




the alkalis in the pore water, mainly forming calcium carbonate that lines the pores: CO2 + H2O → H2CO3 (2.12) H2CO3 + Ca(OH)2 → CaCO3 + 2H2O (2.13) There is a lot more calcium hydroxide in the concrete pores than can be dissolved in the pore water. This helps to maintain the pH at its usual level of around 13 as the carbonation reaction occurs. However, eventually as the locally available calcium hydroxide reacts, precipitating the calcium carbonate and allowing the pH to fall to a level which may cause initiation of reinforcement corrosion, loss of passivity of concrete against reinforcement corrosion, and reinforcement corrosion (Berkeley 1990), as indicated in Table below. PH of concrete State of reinforcement corrosion Below 9.5 Commencement of steel corrosion

A t 8.0 Passive film on the steel surface

disappears

Below7 Catastrophic corrosion occurs

Table.2. – State of reinforcement corrosion at various pH levels Carbonation damage occurs most rapidly when there is little concrete cover over the reinforcing steel. Carbonation can occur even when the concrete cover depth to the reinforcing steel is high. This may be due to a very open pore structure where pores are well connected together and allow rapid CO2 ingress. It may also happen when alkaline reserves in the pores are low. These problems occur when there is a low cement content, high water cement ratio and poor curing of the concrete. The carbonation rate reaches a maximum at a RH value range of 50 to 70%, while above and below this range, the rate is significantly slowed down. Corrosion may commence once a critical RH of about 50% has been reached and gradually increase up to a RH around 100%, at which point, it decreases due to lack of oxygen. • Aggressive anions reaching the reinforcement level: The corrosion of the reinforcement due to chloride ions from deicing salts or seawater is the main cause of damage and early failure of reinforced concrete structures. A high chloride concentration in the concrete cover results in




depassivation. The depassivation mechanism for chloride attack is somewhat different. The chloride ion attacks the passive layer but, unlike carbonation, there is no overall drop in pH. Chlorides act as catalysts to corrosion when there is sufficient concentration at the reinforcement surface to break down the passive layer. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. Obviously a few chloride ions in the pore water will not break down the passive layer, especially if it is effectively re-establishing itself when damaged. Severe corrosion attack usually occurs when alternating drying/wetting cycles take place, typically in the splash zone of a structure. It is generally recognized that only the ‘‘free chloride’’ ions influence the corrosion process. The resistivity decreases and corrosion rate increases with an increase in the chloride content. However, the change in pH is found to be insignificant due to a change in the chloride content of concrete. Chloride penetration into concrete determines the time to depassivation initiation of localized corrosion and is thus one of the most decisive processes for durability and service life of reinforced concrete structures. Water and chlorides are transported rapidly into concrete by capillary suction. Besides the dominating influence of concrete porosity and pore size distribution the rate and amount of chloride ingress are related to the humidity gradients present in the concrete matrix as well as to the chemical properties of the hardened cement. Usage of cements with high C3A (3CaO·Al2O3) content is considered to be conducive to good resistance to corrosion because of its ability to bind chlorides chemically by forming calcium chloro-aluminate, 3CaO·Al2O3·CaCl2·10H2O, sometimes referred to as Friedel's salt. However, sulphate attack from seawater results in a decomposition of calcium chloro-aluminate, thus setting chlorides free by formation of calcium sulpho-aluminate. Chlorides become therefore again available for the corrosion process. Carbonation of hardened cement paste in which bound chlorides are present has a similar effect on setting free the bound chlorides and thus increasing the risk of corrosion. It has been found that the presence of even a small amount of chloride ions in carbonated concrete enhances the rate of corrosion induced by low alkalinity of carbonated concrete. Whereas active corrosion of the reinforcing steel bars can




be detected in the laboratory and onsite by non-destructive electrochemical techniques as potential measurements, no such possibility exists for the detection and quantification of chloride ions in concrete. Only qualitative information on chloride distribution can be obtained from potential mapping. • Stray currents: Stray currents from the various sources, e.g., building power supply systems, cathodic protection systems, locomotive power supply systems, etc. cause electrolytic corrosion. • Bacterial action: Bacterial action is found to be effective in three ways: 1. The bacteria decrease the amount of cover by disintegration of the cementitious materials. 2. The anaerobic bacteria produce iron sulfides in the oxygen deficit condition, such as concrete sewers, which enables the corrosion reaction to proceed even in the absence of oxygen. 3. Aerobic bacteria may also aid in the formation of differential aeration cells, which can lead to corrosion. 3.2.2.2 Internal factors affecting reinforcement corrosion: they include concrete and steel quality parameters, as discussed below: • Cement composition: The cement in the concrete provides protection to the reinforcing steel against corrosion by maintaining a high pH in the order of 12.5-13 owing to the presence of Ca (OH)2 and other alkaline materials in the hydration product of cement, and by binding a significant amount of total chlorides as a result of chemical reaction between C3A and C4AF content of cement in concrete. Thus the threshold chloride value shifts to higher side with an increase in the C3A content of cement. The use of blended cement, such as micro silica-blended high-C3A cement, is found to be concomitantly resistant to sulfate attack and chloride corrosion of reinforcement. • Impurities in aggregates: Aggregates containing chloride salts cause serious corrosion problems, particularly those associated with sea-water and those whose natural sites are in ground water containing high concentration of chloride ions. • Impurities in mixing and curing water: Mixing and curing water, either contaminated with sufficient quantity of chloride or being highly acidified due to any undesirable substance present in water, may prove to be detrimental as far as corrosion of reinforcement is concerned.




• Admixtures: Addition of calcium chloride in concrete, as a common admixture for accelerating the hydration of cement is perhaps the most significant reason for the presence of chloride in reinforced concrete structures exposed to normal weather conditions. Some water reducing admixtures also contain chlorides. • w/c ratio: Low w/c ratio decreases the concrete permeability, which in turn reduces the chloride penetration, carbonation penetration, and oxygen diffusion in concrete. However, a low w/c ratio does not by itself assure concrete of low permeability. For example, 'no fines' concrete can have a low w/c ratio and yet be highly permeable. Thus, in addition to the low w/c ratio, the concrete must be properly proportioned and well consolidated to produce a concrete of low permeability. When reinforced concrete structures are immersed in some aggressive solution, it is the permeability of concrete, which is a function of w/c ratio, affects the corrosion of rebar. The depth of penetration of a particular chloride threshold value increases with an increase in the w/c ratio. Carbonation depth has been found to be linearly increasing with an increase in the w/c ratio. The oxygen diffusion coefficient is also found to be increasing with an increase in the w/c ratio. • Cement content: The cement content in concrete does not only affects the strength but it also has a significant effect on durability. Due to reduced amount of cement in mix the concrete is not consolidated properly leading to the formation of honeycombs and other surface defects. These honeycombs and surface defects help in the penetration and diffusion of corrosion causing agents, such as Cl-, H2O, O2, CO2, etc., in concrete. This results in the initiation of reinforcement corrosion due to the formation of differential cells. Furthermore, concrete with low cement content has a lack of plastic consistency due to which it does not form a uniform passive layer on the surface of the steel bars. Therefore, it is important to maintain minimum cement content from the durability point of view. • Aggregate size and grading: Since the size of aggregates has a bearing upon the consistency of concrete, it may have an effect upon reinforcement corrosion. Aggregate grading is another factor, which should be considered for high quality impermeable concrete. It has been observed that for a given w/c ratio, the coefficient of permeability of concrete increases considerably with increasing size of aggregates. Keeping this in view, t is recommended maximum size of aggregate as: 25-50 mm for 35 MPa concrete, 18.5 mm for 40 MPa




concrete and 9 to 12.5 mm for concrete with a compressive strength of more than 40 MPa. The proportioning of coarse and fine aggregates is important for the production of a workable and durable concrete. The aggregate proportioning for this purpose consists of fixing the optimum volume fraction of sand in the total aggregate content. • Construction practices: Serious corrosion problems may occur if enough care, such as listed below, is not taken at the construction stage

(i) aggregate washing for deleterious materials, if any; (ii) control of chloride in almost all ingredients of concrete, i.e., water,

cement, aggregate, and admixtures; (iii) strict enforcement of designed and recommended levels of w/c ratio,

cement content, cover thickness, etc.; (iv) proper consolidation of freshly placed concrete; and (v) Proper curing of concrete.

• Cover over reinforcing steel: Cover depth has a significant effect in case of corrosion due to penetration of either chloride or carbonation. This effect of cover is limited within the time of casting to the time at which the reinforcement is depassivated and corrosion is initiated. The rate of corrosion, once it has started, is independent, among other things, on the cover thickness as well. Cover thickness is one of the factors, which affects the cracking and spalling of the concrete due to the reinforcement corrosion. • Chemical composition and structure of the reinforcing steel: The differences in the chemical composition and structure of reinforcing steel and presence of stress in the reinforcement, either static or cyclic, create different potentials at different locations on the surface of reinforcement, causing the formation of differential corrosion cells, which leads to its corrosion. • PH of the concrete pore water: In reinforced concrete structures steel is protected against corrosion by the high alkalinity of the concrete pore water resulting in a passive film on steel. Concrete, with its continuous pore system and tendency to form surface cracks, is far away from being a perfect barrier. The real importance of the concrete cover is related mainly to its ability to preserve the conditions of high pH needed to maintain the reinforcement in a passive condition by preventing the rate of ingress of "acidic substances" from the external environment causing a lowering in pH. Generally those are atmospheric carbon dioxide and, in polluted locations, other gases such as sulphur dioxide.CO2,reacts with the




alkaline constituents of the cement paste to form a carbonated zone, which gradually penetrates into exposed concrete, reducing the pH of the affected region to a value where corrosion will occur. This carbonation of concrete is characterized by a pH shift of about 4 units to lower values. Carbonation monitoring is therefore possible by pH measurements. As the carbonation front is generally well defined, requirements on the accuracy of pH sensors are not necessarily high, but are required for long term performance. In combination with chloride measurements the ratio of [Cl-]/[OH-] describing the real corrosiveness of the concrete pore solution could be assessed. The so-called critical chloride concentrations do not usually consider the role of pH. Monitoring may be used to assess existing deterioration and predict future performance.




4. Deterioration Assessment process Several structures situated in a marine environment were investigated with the objective to collect data for an assessment of their durability performance and an evaluation of their service life, using the proposed model. In order to obtain the information necessary to assess the structures performance, several test can be performed on the structures to gather sufficient relevant information. An assessment of existing deterioration is a standard requirement in the repair of corrosion damaged concrete . The current condition of reinforced concrete may be defined by the state of the reinforcement and the deterioration and effective loss of the cover concrete. If the steel is corroding, spalling of the concrete cover can be a safety hazard, while the loss of steel cross-section and bond strength may affect load carrying capacity. Following the outline of the various causes of defects in concrete and mechanisms of deterioration, three stages in an assessment process are considered.

· Investigation of the current condition of the structure · Diagnosis of the causes of defects or deterioration · Selection of an appropriate solution




Chart.2. General Assessment process

4 .1 Preparation (preparatory desk work)

Many investigations are initiated because a routine inspection has revealed a defect, or because deterioration has accelerated or because an obvious fault has developed. Among the common reasons for carrying out investigations are:

· A defect or form of deterioration has been noticed. e.g. Cracking, spalling or erosion of the surface.

· General or local damage has occurred · Some form of defect is suspected · As part of a routine maintenance programme. · The use of the structure is to change · The ownership of the structure is to change

Planning the investigation of a concrete structure has to be seen with in the context of developing an appropriate repair solution.

The european standard products and systems for the protection and repair of concrete structures suggests that the following information should be considered during the assessment:




· Orginal design approach

Design information

– Plans/drawings, as-built drawings, specifications and other related information

– Location of the structure and topology and accessibility

– Structural drawings for reinforcement details

· Environment,including exposure to contamination · History

The history of the structure and its use should be investigated. The actual loads carried by the structure, particularly intense loads, and details of any change of use should be noted. Information should be obtained on any repairs or structural alterations. Changes in processes or use may be important; for example, stored or transported through chemicals or chemicals used in an industrial process may have leaked or been split onto the surfaces of concrete members. Generally it is necessary to look at the history of the reinforced concrete structure with respect to:

– Service history – Condition of use(loading) – Present condition – Future use

A preliminary walk –round survey is essential. During the walk-round there may be an opportunity to obtain samples of loose concrete or lumps from spalled areas that may be easily detached. At this stage, opportunities to view all the structure may be limited, example, high parts of the structure hidden areas behind false ceilings.

4.2. Preliminary investigation (visual inspection and appraisal)

Each structure is unique and it is only possible to suggest broad guidelines for a preliminary inspection, as it is normally limited in its scope and access may be restricted. It is usually conducted in the accessible locations around a structure and might include: Determination of the location, general extent and nature of readly discoverable deterioration or distress




Assessment of the requirements for access for the main survey The objective of this stage of investigation is to develop a preliminary diagnosis and to obtain information for the planning of a more detailed inspection, if required. In parallel with the investigation it will be advisable to find out what records are available from the client or elsewhere. The main part of the preliminary inspection is carried out by systematically working through the structure and noting the occurrence and location of the principal signs of deterioration or distress, as indicated below. Photographs should be taken of signs of deterioration or of features of particular interest. Sampling locations should be identified and any particular access requirements for the full survey noted. Visual inspection may be used to assess a problem in its advanced state, i.e. after cracking of the concrete cover has occurred. A flow diagram illustrating this process is given in chart below. It may be followed by further non-destructive and destructive testing (detail investigation).

· Cracks are one of the most helpful features in making a preliminary diagnosis of a concrete structure. The nature and extent of any cracking on external exposed surfaces is best observed when the surface is drying after rain, as this highlights fine cracks and other defects. Cracks are symptomatic of various types of deterioration, overloading or deficiencies in design or detailing. The location orientation, width and length of typical cracks should be recorded. As the age of the structure when cracks first developed is important, such information should be sought.

· Deterioration of the concrete (e.g. spalling, pop-outs, discolouration). · Leaks, damp patches or lime-scale on structures should be recorded.

This is particularly necessary for water-retaining structures, tunnels and basements where the function of structure may be affected. It is also important as many deterioration processes depend on the presence of moisture. Damage may be concentrated at damp location.

· Any evidence of reinforcement corrosion, such as staining, cracking, spalling or delamination of surface concrete and exposed corroded reinforcement, should be noted.

· The structure should be checked for signs of previous repairs. Repairs

may be different in colour or surface texture or show up as irregularities in the surface between repaired and original concrete. The properties of the repair material will be different to those of the original concrete and




this need to be taken into account when choosing sampling locations for the main investigation.

Chart 3. Procedural flow of the different options to detail assessment

Structural condition, extent of cracking, damage

Safety of the structure

Need for commissioning a detailed inspection

Accessories needed for detailed inspection

Traffic control requirements

Any unusual problems facing the structure

The result of the preliminary (visual) inspection should be presented in the form of sketches, photographs and written reports. The sketches should show general layout, principal dimensions, location and nature of defects, position and direction from which photographs were taken and any other pertinent information. Where appropriate, a compass bearing should be added to sketch plans. Photographs should illustrate the general lay out and condition of the structure, and detailed photographs should show individual defects or areas which have deteriorated. Various approaches for classifying defects have been proposed, using systems appropriate for visual inspection




4.3. Detailed investigation

Several structures situated in a marine environment were investigated with the objective to collect data for an assessment of their durability performance and an evaluation of their service life, using the proposed model. In order to obtain the information necessary to assess the structures performance, several tests needed to be performed on the structures to gather sufficient relevant information. Inspection procedures involved quality assessment of the built reinforced concrete structures as well as the measurement of the environmental response. The inspection of the built quality of the structures was performed through the evaluation of the concrete cover depth, the chloride diffusivity and by electrical potential mapping of the concrete cover. The environmental response was evaluated by measuring the chloride penetration (profiles), the carbonation depth, and the corrosion activity through visual damage assessment. 4.3.1. Sample collection

The sampling rate, type of testing and test locations will vary from case to case.There are no general rules but there are some basic principles.It is important to consider the aims of the evaluation before measurements begin.No measurements should be taken or tests carried out if it is not known what the results will be used for.

Where possible, the spread of sampling and testing locations should include:

o Members of different types e.g.column,beams,slabs o Typical areas of each type of deterioration o Corresponding areas that are in good condition o Corresponding areas with different exposure conditions o Where appropriate, areas that have been previously repaired.

Selection of the locations for sampling or testing depends on the nature of the sample or test and the underlining purpose.Ease of access is also aconsideration.It should be noted that a property may vary at different points on the same member, for example the strength of concrete in a column. To obtain the mean value of aproperty the locations have to be chosen at random and the number of samples or tests has to be sufficient to give a sufficient to give a satisfactory level of confidence.If a particular defect is being investigated, samples may be extracted or tests carried out at locations close to the areas of




damage, with other tests and samples at undamaged locations for comparison. Sample locations may have to be chosen to avoid reinforcement or cracks(for example when extracting cores and dust samples).

Lump samples: are easily obtained from corners or using a hammer and chisel. They have the advantage that they can be which they can be used in limited. Lump samples can be used for visual and petrographic examination, chemical testing and, if they rarely big enough to permit sub- samples to be taken for strength determination except possibly by point load testing techniques. However, the sampling and so the strengths would probably be un representatives. Cores: cores are extremely useful as they provide a means of determining the strength of the in situ concrete. They can be used for visual and petrographic analysis and can be sectioned or drilled to provide samples for chemical analysis. They can also be used to investigate depth and width of cracks. In such cases the core holes, as well as the cores themselves, should be examined as cores may suffer damage during cutting or extraction. The condition of the surface of the core hole will confirm the state of any crack shown by the core in its original state. Samples from cores can be used in the determination of chloride content. Dust samples: dust samples have the advantage that they can be obtained rapidly and inexpensively with readily-available hand-held equipment. Their use is limited to various types of chemical analysis 3.1.2. Laboratory and in situ test methods Non-destructive and semi-destructive techniques will be used for the assessment of the various structures and their concrete properties, either in situ by direct or indirect measurement, or in the laboratory. Several test methods were used. In situ tests consisted of concrete cover measurements, half-cell potential measurements, core extraction for carbonation depths and diffusion coefficients, and collection of samples for chloride profiling. Laboratory test methods consisted of chloride profile determination (chloride contents calculation and diffusion coefficient determination through curve fitting of profiles) and diffusion coefficient from migration testing. Property under investigation test Interpretation of result




Concrete strength Core Near-surface test Rebound

Dirct Indirect Qualitative

Concrete quality Visual examination of cores and lump samples Ultra sonic pulse velocity Petro graphic examination Expansion of cores Chemical analysis

N/A Indirect Direct/indirect Direct Direct

Corrosion of reinforcement Carbonation depth Cover depth Chloride content Half –cell potential and potential mapping Corrosion rate Resistivity

Direct Indirect Direct Indirect Indirect Indirect Indirect

Table.3.tests and their interpretations Direct indicates that the equipment gives a direct value for the property being determined. Indirect indicates that the required property is determined indirectly, e.g.by calibration or an assumed relationship with the measured property. Quantitative indicates that the test will not yield quantitative results. Common Less common Infrequent -Visual inspection -Crack mapping -Cover -Location of reinforcement -Cores -Chloride profiles -Carbonation depth -Potential mapping -Resistivity -Cement content

Near-surface tests Petrography Ultrasonic pulse velocity Chloride diffusion

Rebound hammer Porosity Water absorption Initial surface absorption Water permeability Gas diffusion Corrosion rate Thermography Expansion cores Radar Gamma radiography Acoustic emission

Table.4..Tests and techniques used on marine structures The most frequent used types of tests listed on the first column shown on the above table, I try to focus on some of the most common types of tests done on reinforced concrete structures in marine environment mainly the unique characteristics of marine environment.




I. Concrete cover measurements The depth of reinforcement below the surface of concrete was measured using a cover meter. Cover meters are electro-magnetic devices consisting of a search head and a control box. This equipment has digital readout and an audible output which increases in loudness or pitch as the search head approaches the position of a reinforcing bar. This method for locating reinforcing steel within a concrete member is truly non-destructive. Under reasonable conditions, a site accuracy of estimated cover of ± 5 mm within the working range of the instrument may be expected. The effect of bar size is important if this is less than 10 mm or greater than 32 mm. Calibrations are sensitive to steel type, bar diameter and deformity, aggregate and cement type, and these factors have to be taken into account. Most commercially available equipments are calibrated for medium-sized mild steel round bars in ordinary Portland cement concrete. The range of the equipment is limited according to type, but care and experience in interpretation of data are required in the following cases: a. Multiple bars, e.g. laps, transverse steel or closely spaced parallel bars; b. Light wire mesh, buried nails or other metals between the reinforcing bars and the surface; c. Metal tie wires; d. Aggregates with magnetic properties; e. Stability of calibration, which may be particularly important in relation to temperature changes within the magnetic field; f. Stray magnetic fields.

Fig. Cover meter




The principal applications are the location of reinforcement and estimation of cover, orientation and, in some cases, diameter of reinforcing bars. II. Cores Test The examination and compression testing of cores cut from hardened concrete is a well-established method, enabling visual inspection of the interior regions of a member to be coupled with strength estimation. Other physical properties which can be measured include density, water absorption, and indirect tensile strength and movement characteristics including expansion due to alkali–aggregate reactions. Cores are also frequently used as samples for chemical analysis following strength testing. In most countries standards are available which recommend procedures for cutting, testing and interpretation of results; BS EN 12504-1(135) in the UK, whilst ASTM C42 (136) and ACI 318 (137) are used in the USA. Core location and size Core location will be governed primarily by the basic purpose of the testing, bearing in mind the likely strength distributions within the member.

i. Where serviceability assessment is the principal aim: tests should normally be taken at points of likely minimum strength.

For example from the top surface at near mid span for simple beams and slabs, or from any face near the top of lifts for columns or walls. However, core cutting may impair future performance, cores should be taken at the nearest non-critical locations. Aesthetic considerations concerning the appearance after coring may also sometimes influence the choice of locations.

ii. If specification compliance determination is the principal aim, the cores should be located to avoid unrepresentative concrete, and for columns, walls or deep beams will normally be taken horizontally at least 300mm below the top of the lift. If it is necessary to drill vertically downwards, as in slabs, the core must be sufficiently long to pass through unrepresentative concrete which may occupy the top 20% of the thickness. In such cases drilling upwards from the soffit, if this is feasible, may considerably reduce the extent of drilling, but the operation may be more difficult and may introduce additional uncertainties relating to the effects of possible tensile cracking.

Reinforcement bars passing through a core will increase the uncertainty of strength testing, and should be avoided wherever possible.




The use of a cover meter to locate reinforcement prior to cutting is therefore recommended. Where the core is to be used for compression testing, British and American Standards require that the diameter is at least three times the nominal maximum aggregate size. In many countries, a minimum diameter of 100mm is used, with 150mm preferred, although in some countries like Australia 75mm is considered to be generally acceptable. In general, the accuracy decreases as the ratio of aggregate size to core diameter increases and 100mm diameter cores should not be used if the maximum aggregate size exceeds 25 mm, and this should preferably be less than 20mm for 75mm cores. In some circumstances smaller diameters are used, especially in small-sized members where large holes would be unacceptable, but the interpretation of results for small cores becomes more complex. The choice of core diameter will also be influenced by the length of specimen which is possible. It is generally accepted that cores for compression testing should have a length/diameter ratio of between 1.0 and 2.0, but opinions vary concerning the optimum value. BS EN 12504-1 (135) recommends a ratio of 2.0 if results are to be related to cylinder strengths or 1.0 for cube strengths. The Concrete Society (36) suggest that cores should be kept as short as possible l/d = 1.0→1.2 for reasons of drilling costs, damage, variability along length, and geometric influences on testing. Drilling A core is usually cut by means of a rotary cutting tool with diamond bits. The equipment is portable, but it is heavy and must be firmly supported and braced against the concrete to prevent relative movement which will result in a distorted or broken core, and a water supply is also necessary to lubricate the cutter. Vacuum-assisted equipment can be used to obtain a firm attachment for the drilling rig without resorting to expansion bolts or cumbersome bracing. Uniformity of pressure is important, so it is essential that drilling is performed by a skilled operator. Hand-held equipment is available for cores up to 75mm diameter. A cylindrical specimen is obtained, which may contain embedded reinforcement, and which will usually be removed by breaking off by insertion of a cold chisel down the side of the core, once a sufficient depth has been




drilled. The core, which will have a rough inner end, may then be removed using the drill or tongs, and the hole made good. This is best achieved either by ramming a dry, low shrinkage concrete into the hole, or by wedging a cast cylinder of suitable size into the hole with cement grout or epoxy resin. It is important that each core is examined at this stage, since if there is insufficient length for testing, or excessive reinforcement or voids, extra cores must be drilled from adjacent locations. Each core must be clearly labeled for identification, with the drilled surface shown, and cross-referenced to a simple sketch of the element drilled. Photographs of cores are valuable for future reference, especially as confirmation of features noted during visual inspection, and these should be taken as soon as possible after cutting. Testing Each core must be trimmed and the ends either ground or capped before visual examination, assessment of void age, and density determinations. 1. Visual examination Aggregate type, size and characteristics should be assessed together with grading. These are usually most easily seen on a wet surface, but for other features to be noted, such as aggregate distribution, honeycombing, cracks, defects and drilling damage, a dry surface is preferable. Precise details of the location and size of reinforcement passing through the core must also be recorded. The voids should be classified in terms of the excess void age by comparison with ‘standard’ photographs of known void age provided by country technical report for instance with the Concrete Society Technical Report 11 (36). These reference photographs are based on the assumption of a fully compacted ‘potential’ void age of 0.5%. If a more detailed description of the voids is required, this should refer to small voids (0.5–3 mm), medium voids (3–6 mm) and large voids (>6mm) with the term ‘honeycombing’ being used if these are interconnected. It is also helpful to describe whether voids are empty, or the nature of their contents, for example white gel from ASR. 2. Trimming Trimming, preferably with a masonry or water-lubricated diamond saw, should give a core of a suitable length with parallel ends which are normal to the axis of the core. If possible, reinforcement and unrepresentative concrete should be removed. 3. Capping




Unless their ends are prepared by grinding, cores should be capped with high alumina cement mortar or sulfur–sand mixture to provide parallel end surfaces normal to the axis of the core. (Other materials should not be used as they have been shown to give unreliable results.) Caps should be kept as thin as possible, but if the core is hand trimmed they may be up to about the maximum aggregate size at the thickest points. 4. Density determination This is recommended in all cases, and is best measured by the following procedure: (i) Measure volume (Vu) of trimmed core by water displacement (ii) Establish density of capping materials (Dc) (iii) Before compressive testing, weigh soaked/surface-dry capped core in air and water to determine gross weight Wt and volume Vt. (iv) If reinforcement is present this should be removed from the concrete after compression testing, and the weight Ws and volume Vs determined (v) Calculate saturated density of concrete in the uncapped core from

If no steel is present, Ws and Vs are both zero. The value thus obtained may be used, if required, to assess the excess Void age of the concrete using the relationship Estimated excess void age =Dp−Da ×100% Dp−500 Where Dp = the potential density based on available values for 28-day-old cubes of the same mix. And Da is the actual density. 5. Compression testing The standard procedure in the United Kingdom is to test cores in a saturated condition, although in the USA dry testing is used if the in-situ concrete is in a dry state. If the core is to be saturated, testing should be not less than two days after capping and immersion in water. The mean diameter must be measured to the nearest 1mm by caliper, with measurements on two axes at quarter- and mid-points along the length of the core, and the core length also measured to the nearest 1 mm. Compression testing will be carried out at a rate within the range 12–24N/(mm2.min) in a suitable testing machine and the mode of failure noted. If there is cracking of the caps, or separation of cap and core, the result should be considered as being of doubtful accuracy.




Ideally cracking should be similar all round the circumference of the core, but a diagonal shear crack is considered satisfactory, except in short cores or where reinforcement or honeycombing is present. 6 Other strength tests on cores Although compression testing as described above is by far the most common method of testing cores for strength, recent research has indicated the potential of other methods which are outlined below. Two of these measure the tensile strength, although neither method is yet fully established. Tests for other properties of the concrete, such as permeability, alkali–aggregate expansion or air content may also be performed on suitably prepared specimens obtained from cores and these applications of core test are the concern of this paper. So even if core test can have multiple purposes with concrete performance tests, its application for testing concrete with the environmental response gives me sense and I gave more focus on the Marine environment response tests for reinforced concrete structure by considering the most driving problem is reinforcement corrosion. The below shown two parts of tests are mainly classified to assess the corrosion potentials on the environment and the degree of the corrosion of the existing structures in more structured manner II. Environmental response

A. Chloride profiles Chloride contents should be measured at increments of distance from the surface of say 10mm to establish chloride profile. If the chloride level at the steel is above the threshold level at which steel corrosion may occur, conditions exist for corrosion to be initiated and further investigations are needed to determine if corrosion has started. The measurement used in determining chloride profile can also be used to predict the likely hood of the corrosion level for the future.

Dust sample collection The chloride profiles can be measured either on cores immediately after extraction by grinding off material in layers parallel to the exposed surface, or directly with dust samples collected from the structures. Generally dust-samples are taken in intervals of 5-10 mm, up to a depth of 40-70 mm into the structure.




The dust samples are analyzed and the total chloride content in each sample is determined. The chloride analyses are made in the laboratory. The equipment used consists of:

· a hammer drill with depth-measurement gauge, · equipment to measure the thickness of the concrete cover and localize

the reinforcement-bars, and · equipment to collect the dust-samples (a short brush or a glazing knife, · a plastic bag and cup to collect the dust, · a tire inflator, to clean the holes from dust and scalable plastic bags or

cups to store the dust.) The diameter of the drill is 12 mm. Finally, repair material to repair the holes is also needed. All the equipment used for drilling, collecting and storing of dust-samples must be properly cleaned and free from contamination, especially thawing-salt and salt from sea water. The drilling-places should be selected in such a way, that the reinforcement-bars are avoided and the dust-samples be the most representative for each depth. A cover-thickness-measuring gauge can be used to localize the reinforcement-bars. The total depth of the boreholes should be adjusted to the specific construction (concrete cover thickness, penetration depth and the required accuracy of measurements), the division in different depth-levels and the purpose of the investigation. The outer layer of each structure, approximately 5 mm, shall be treated as the initial layer. The dust is dependent of the equipment used for the drilling, collected in filter bags, special dust collectors or cups made for collecting dust. The samples should be stored in scalable plastic bags or cups to prevent the samples from being contaminated. When samples are taken from different depth-layers the hole shall be cleaned between each sampling. The number of holes should be adjusted to depth-layer interval, the amount of dust and the method for collecting the dust. As a basic rule each dust sample should contain dust from at least two holes. The amount of concrete dust from each hole is between 25-40 g (with a thickness of each layer equal to 10 mm). Profile grinding is used when cores if extracted from the structure. It is a simple method where concrete dust is cut from different layers of a concrete




sample. The samples used in the method are cores with a diameter of 100 mm. The concrete dust is used to determine the chloride content in different layers of the concrete structure.

Figure.9. Example of location of dust samples extraction from a concrete structure. The equipment used consists mainly of a turning lathe, to which the sample is attached, and a cutter. The area of the cutter should be smaller than the area that is to be cut. A container, to collect the dust samples, is placed under the cutter. The dust from each layer is collected separately and put into a plastic bag. Chloride content determination In a spectrophotometric analysis, a sample solution in a glass or quartz "cell" is inserted into a spectrophotometer. Radiant energy (ultraviolet, visible, or infrared) of a very narrow (monochromatic) wavelength range is selected from a source by means of a diffraction grating, and the resulting beam is directed at the cell containing the sample. Some of the radiant power incident on the cell is absorbed by the chemical substance(s) in the sample, and the remainder is transmitted. The amount of energy absorbed is proportional to the concentration of the chemical substance(s) absorbing it. This proportionality is the quantitative basis of spectrophotometry, or spectrophotometric methods. Spectrophotometry is applied to dust samples prepared either as dust directly taken from the structure or with profile grinding on cores.




The concrete dust will be dissolved in hot nitrous acid (80°C) and then filtered. A certain amount of the filtered material is mixed in a chemical-solution which together with chlorides becomes coloured brown. Depending on the chloride content in the solution the intensity of the orange colour changes, as shown in Figure below. The chloride content is determined with a spectrophotometer and compared with analysis made with a solution with known chloride contents. The acid-soluble chloride content of the samples is determined to three decimals using a spectrophotometric method.

Figure.10. chlorine content effect testing solution

Colour changes according to chloride concentration. The dust samples used in the spectrophotometry become representative for the structure when they average the dust from various extractions. For all the structures evaluated, the dust was collected from five different locations. This is to avoid hitting large aggregates, when dust samples are taken directly from the structure. The test results are influenced by the temperature and the accuracy of the drilling or profile grinding. Diffusion coefficients Migration testing The chloride migration coefficient determined by the method is considered to be a measure of the resistance of the tested material to chloride penetration. This non-steady-state migration coefficient cannot be directly compared with chloride diffusion coefficients obtained from the other test methods, such as the non-steady-state immersion test or the steady-state migration test.




The method requires cylindrical specimens with a diameter of 100 mm and a thickness of 50 mm obtained from drilled cores with a minimum length of 100 mm. An external electrical potential is applied axially across the specimen and forces the chloride ions outside to migrate into the specimen. Figure below shows the set up of the test procedure used in the laboratory. After a certain test duration, the specimens are axially split and a silver nitrate solution is sprayed on to one of the freshly split sections. The chloride penetration depth can then be measured from the visible white silver chloride precipitation, after which the chloride migration coefficient can be calculated from this penetration depth.

Figure11. migration testing apparatus

Chloride profile fitting

To evaluate the chloride profiles the second Fick's law is applied. It is assumed that no significant deviation of Fick's second law occurs under constantly immersed conditions. When evaluating the achieved chloride profiles which were found in concretes exposed to real environmental conditions, it is necessary to consider the possibility, that deviations from the ideal shape can occur (due to carbonation, leaching, temperature variations, different chloride loadings, etc.) Different possibilities to determine the parameters DCPM (apparent diffusion coefficient, determined by chloride profiling) and CSN (surface chloride concentration, determined by chloride profiling) and which the values obtained are applied in practice are based on a best fit least squares analysis.




Figure.12. Curve fitting (Fick's 2nd law of diffusion) by the method of least squares. The determined output derived from the curve fitting carried out will be: • DCPM, an apparent diffusion coefficient, determined by chloride profiling in m/s2 and • CSN, the notional surface chloride level in % by weight of concrete.

B. Carbonation depth

Carbonation depth is assessed using a solution of phenolphthalein indicator in ethyl alcohol that appears pink when it is in contact with un carbonated concrete with PH values above 9 and colourless in contact with concrete which has lower PH, i.e. which has carbonated. The test is most commonly carried out by spraying the indicator on freshly exposed surfaces of concrete brocken from the structure or on split cores.

Alternatively, the powder from drill holes can be sprayed or allowed to fall on indicator- impregnated paper. Un carbonated cement past has a PH of about 13 and reinforcing steel loses its passivation at about PH11.Hence there may be corrosion in narrow zone ahead of front defined by the indicator.

In general the change in PH occurs in this zone which is only a few millimeters a head of the line indicated by the test and the phenolphthalein method provides a good indication of the location of the depassivation front.

The rate of penetration of carbonation is dependent on the quality and moisture content of the concrete and its environment. In a simplified form. Penetration rate is assumed to obey a square root law of the form X=Kc√t




Where, x=carbonation depth T=time Kc=carbonation constant This relationship applies in condition of constant and modrate humidity, but where there is wetting and drying the relationship may be more complex. With only asingle measurement it may be difficult to predictions of carbonation rate and is safe for design purposes. III. Corrosion potential assessment tests

1. Half-cell potential measurements Half-cell potential measurements are often carried out when reinforcement corrosion is suspected or evident. It is a measure of the electrical potential on the surface of the reinforcement and can be interpreted in terms of the likelihood of corrosion activity. The equipment consists of a half-cell and a high-impedance voltmeter. The half-cell is a tube with a porous end, which contains a metal rod in a saturated solution of its own salt e.g. copper in copper sulfate or silver in silver chloride. Silver in silver chloride half-cells, while more expensive, and provide a more stable system, although the copper in copper sulfate system is widely used. The equipment is simple and enables a non-destructive survey to produce isopotential contour maps of the surface of a concrete member. Zones of varying degrees of corrosion risk may be identified from these maps. This method cannot indicate the actual corrosion rate.

Fig13. Half-cell potential measurements It is important to recognize that the use and interpretation of the results obtained from the test require an experienced operator who is aware of its




limitations such as the effect of protective or decorative coatings applied to the concrete. This technique is most likely to be used for assessment or monitoring of the durability of reinforced concrete members where reinforcement corrosion is suspected. Procedure The procedure usually requires a small hole to be drilled to enable electrical contact to be made with the reinforcement in the member under examination and no surface preparation is required. One terminal of the voltmeter is connected to the reinforcement and the other terminal is connected to the half-cell as shown in Figure above. It is usual to take readings on a grid pattern on the concrete surface. A closer grid may be used in areas of special interest. It may be necessary to spray water on the surface locally to obtain good electrolytic contact Results Reported uses include the location of areas of high reinforcement corrosion risk in marine structures, bridge decks and abutments. When used in conjunction with other tests, it has been found helpful for investigating concrete contaminated by chlorides.

2. Electrochemical potential mapping. The electrical potential relative to a half-cell indicates the risk of reinforcement corrosion. It is only a qualitative measurement and may be affected by several factors, such as reinforcement connectivity, concrete cover depth, chloride presence, relative humidity and temperature. Readings are normally taken over the concrete surface, initially on a large grid. The greatest corrosion risk is usually associated with areas in which the potential gradient is steep; where these occur the grid should be reduced. If there is uncertainty, further testing (e.g. drillings or cores to inspect the state of the steel) is required to indicate the corrosion risk. For ease of interpretation, results are usually plotted as contours of equal potential.

Fig14.map equal potential contour (values in millivolts)




3. Resistivity

Resistivity measurements are sometimes carried out in conjunction with half-cell potential surveys to help in predicting the likelihood of corrosion. The most common techniques is the four-probe or wenner array. Four metal electrodes are inserted into shallow holes, drilled at equal spacing (commonly50mm) in a straight line on the concrete surface. A current is passed between the outer two electrodes and the potential drop across the inner electrodes is measured.

Test equipment is available commercially. The test can be carried out rapidly and easily with little damage to the concrete surface. Readings can be affected by moisture or salt in the concrete, reinforcement close to the surface or poor coupling between the probes and the concrete surface.

Skill and experience are required in the interpretation of results which is based on fairly broads of corrosion risk.

Fig15. Typical layout of resistivity test array

Concrete resistivity depends on temperature, cement type, degree of cement hydration, W/c ratio, pore water composition and the water content of the concrete. The resistivity is related principally to moisture content and to a lesser extent to chloride content. Moisture content has a profound effect on corrosion rate of reinforcement.




Resistivity is a measure of the pore continuity of concrete and the degree to which the pores are filled with water. It controls the amount of current flowing once the reinforcing bar becomes active and hence controls the metal loss in a corrosion cell.

A known current `I' is impressed on the outer probes and the resulting potential drop `V' between the inner probes is measured and resistance `R' is given by V/I. Resistivity of concrete (p) = 2ΠσR Where σ is the inner electrode distance in cm R is the measured resistance in ohm. Generally, the electrical resistivity of concrete can be measured with good reproducibility using different techniques for specimens in various shapes and sizes on the assumption that the applied electrodes are well bound to the concrete and the spacing between them is adjusted to the dimensions of test sample. The electrical resistivity of concrete is being increasingly used indirectly to evaluate concrete characteristics such as the chloride ion diffusivity, the degree of concrete saturation and its aggressiveness.

Resistivity(kohm cm) Likely corrosion rate

<3 Very high

5-10 High

10-20 Low

>20 Negligible

Table4.. resistivity measure-corrosion rate

Resistivity measurements are only valid at the time of measurement,and should be repeated at intervals in climates with highly variable seasonal changes.

For example, in the Middle East a value of 100 Kohm cm could be recorded in the dry season (negligible potential for corrosion), but a reading of 5 Kohm cm after a downpour (high potential for corrosion of active reinforcement).




4.3.7 Investigation report

When required by a major rehabilitation project, a concrete materials design memorandum, in the form of a separate report or a part of The Rehabilitation Evaluation Report, will be prepared. The need for repairs can vary from such minor problems as shrinkage cracks and pop outs to major damage resulting from severe cavitation or abrasion or from structural failure. Therefore, the scope of the investigation will also vary, depending on the criticality of the project. For minor (noncritical) repair of concrete structures in moderate climates, the investigation may be limited to determining the availability of satisfactory repair materials. Examples of such repairs include patching small spalls and sealing dormant cracks in southern states. If the environment of the project is known to be deleterious to concrete, such as cycles of freezing and thawing, sulfate attack, or acid attack, the investigation must address the measures to be taken to mitigate deterioration, regardless of the quantity of repair materials involved. A more detailed investigation will normally be required for rehabilitation of structures such as locks, dams, large pumping stations, and power plants. 5. Conclusion and recommendations

The marine environment is highly in hospitable for commonly materials of construction, including reinforced concrete. Sea water contains corrosive ions and gases which make the environment uncomfortable for sustainable construction work.

The knowledge about the service life of a structure is uncertain due to the random variation of the geometry, material characteristics, execution and environment. This random variation can be assessed and in part controlled by testing and quality control at several stages during the life span of a structure.

The influence of the environment on deterioration of reinforced concrete structures could be estimated by observation of causes and quantity of arisen deteriorations. Concrete during its lifetime is affected by influence of various external and internal factors. The results of regular reinforced concrete structures inspections showed that the major influence on service life of reinforced concrete structures in marine environment is a corrosion of reinforcement. Other types of damages of the aggressive environment, did not affect the concretes residual service life so much as this case. Reinforcement corrosion is major and more common type of reinforced concrete deterioration in most




marine environments. The general way how to describe the environmental actions on concrete structure is to use the surface climate that depends of surface conditions. Even if the effect of marine environment is not an issue to Countries like which do not have sea ports( sea water) is burning issue in most developed and developing countries with regard if un friendly relationship with most construction materials. So a lot of researches have been taken on this study area but still there are recommendations on the unsolved problems of marine environment to enhance development of on shore concrete structures construction.

As I mentioned above even it has no direct effect on the countries construction, but it can have indirect effect on knowing the marine environment deteriorating effects of concrete with different construction planning’s such as bridge constructions.

So I recommend for universities or higher institutions who gives construction related field of studies to make awareness on the following:

ÿ They should give more cover on their curriculum to concrete technology because it is the most dominant and developing construction material in the country.

ÿ They should equipped their students with the characteristics of concrete as construction material while using in combination with other materials like steel bars, plastic fibers and other emerging materials to properly use the material.

ÿ They should also give enough coverage of topics to the environmental fitness of the material( reinforced concrete).this should include the tropical weather effect, a marine effect on the concrete performance by having knowledge on causes and methods of deterioration and its corrective measures for reinforced concrete structures.




References 1. concrete in the marine environment, by P.Kumar Mehta, university of California (department of civil engineering) USA 2. PROBABILITY-BASED DURABILITY ANALYSIS OF CONCRETE STRUCTURES INMARINE ENVIRONMENT by Rui Miguel Ferreira 3. Diagnosis of deterioration in concrete structures, by concrete society 200, report of working party 4. Advanced concrete technology process Edited by John Newman, Department of Civil Engineering Imperial College London First published 2003 5. Construction of offshore and marine structures,2nd edition, University of California at Berkeley 6. Testing of concrete structures, 4th edition, By John H. Bungey, Stephen G.millard &Michael G.Grantham

semester project paper of maintenance

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