MAINTANANCE AND REHABILITATION OF STRUCTURES

UNIT – 1

GENERAL

2 MARKS:

1. Suggest the factors that are influenced in strength of concrete.

Strength of concrete is its resistance to rupture. It may be measured in a number of ways such as strength in compression, in tension, in shear or flavor. The strength that may be developed by workable, properly placed mixture of cement, aggregate and water influenced by,

a) Ratio of cement to mixing water.

b) Ratio of cement to aggregate.

c) Grading, surface texture, shape and strength of aggregate particles.

d) Maximum size of aggregate.

2. What are the methods used for making high strength concrete?

The methods used for making high strength concrete are,

a) Seeding.

b) Reverberation.

c) High speed glarry mixing.

d) Use of admixtures.

e) Inhibition of cracks.

f) Sulphur impregnation.

g) Use of high cementitious aggregates.

3. What are the techniques used for producing high strength concrete?

The techniques used for producing high strength concrete:

a) Compaction by pressure.

b) Helical binding.

c) Polymerization in concrete.

d) Reactive powder concrete.

4. Briefly explain the permeability of concrete?

The introduction of aggregate of low permeability into cement paste, it is

expected to reduce the permeability of the system because the aggregate particles

intercept the channels of flows and make it take a circuitous route. Compared to neat

cement paste, concrete with the same w/c ratio and degree of maturity, should give a

lower co-efficient of permeability. The introduction of aggregate, particularly large

size of aggregates increases the permeability considerably.

5. Give the reason for the higher permeability of concrete in actual structure?

The higher permeability of concrete in actual structure is due to the following

reason:

a) Formation of micro cracks developed due to long term drying shrinkage

and thermal stresses.

b) The large micro cracks generated with time in the transition zones.

c) Cracks generated through higher structural stresses.

d) Due to volume change and cracks produced on account of various minor

reasons.

e) Distance of entrapped air to insufficient comparison.

6. What are the important aspects that will be stacked under thermal properties

of concrete?

The important aspects that will be stacked under thermal properties of concrete:

Thermal conductivity.

Thermal diffusivity.

Specific heat.

Co-efficient of thermal expansion.

7. Briefly explain about thermal conductivity of concrete?

Thermal conductivity:

This measure the ability of material to conduct heat. Thermal conductivity

is measured in joules per second per square meter of area of body when the

temperature deference is 10c per meter thickness of the body. The conductivity

depends on type of aggregate, moisture content, density and temperature of concrete.

8. Write short notes on fire resistance of concrete?

Fire resistance of concrete structure is determined by three main factor,

1) The capacity of the concrete itself to with stand heat and the subsequent

action of water without losing strength, without cracking or spelling.

2) The conductivity of the concrete to heat and

3) The co-efficient of thermal expansion.

In case of reinforced concrete, the fire resistance not only depend upon the

type of concrete but also on the thickness of cover to reinforcement. The

fire introduces high temperature gradient and as a result of cooler interior.

9. Briefly explain about cracks in concrete?

Cracks in concrete->before hardening:

a) Drying:

1. Plastic shrinkage.

2. Settlement shrinkage.

3. Bleeding.

4. Delayed curing.

b) Constructional:

1. Formwork movement.

2. Excess vibration.

3. Sub grade settlement.

4. Finishing.

c) Early frost damage.

Cracks in concrete ->after hardening:

1. Unsound material ->cement, aggregate, excess silt, mad and dust.

2. Long term drying shrinkage.

3. Thermal.

a) Heat of hydration.

b) External temperature.

c) Joints in concrete.

d) Elevated temperature.

e) Freezing and thawing.

4. Moisture movement.

5. Transition zone.

6. Biological.

7. Structural design deficiency.

8. Chemical.

a) Sulphate attack.

b) Alkali aggregate reaction.

c) Acid attack.

d) Sea water.

e) Carbonation.

f) Chloride attack.

9. Corrosion of reinforcement.

10. In nut shell, what are the conditions, which facilitate crack prevention?

Internally induced stresses in building components head to dimensional

changes and whenever there is restrain to movement cracking occurs. Thus vertical

cracks occur in well more frequently due to horizontal movement. Volume changes

due to chemical action within a component result in either expansion or contraction

leading to occurrence of cracks in components.

Prevention of cracks:

Proxy adhesives are the most common adhesives used to crack repair. They

are usually introduced into cracks by injection. High molecular weight metha

Crystalates are used to some plat surface application by floading the surface with

adhesive and they have been used occasionally to insect into fine cracks because of

their of their low viscosity.

PART – B

12 Marks:

1. a) Briefly explain the maturity concept of concrete.

While dealing with curing and strength development, we have considered

for only time aspect. It has been pointed out earlier that it is not only the time but

also the temperature during the early period of hydration that influence the rate of

gain of strength of concrete. Since the strength development of concrete on both

time and temperature, it can be said that strength is a function of summation of

product of time and temperature. This summation is called as maturity of

concrete.

Maturity = Σ (time*temperature)

The temperature is reckoned from an origin lying between -120c and -100c.

it was experimentally found that the hydration of concrete continues to take place

up to about -110c. Therefore -110c is taken as a datum line for computing

maturity.

Maturity is measured in degree centigrade hours (0c hrs) or degree

centigrade days (0c days).

A sample of concrete at 180c for 28 days is taken as fully matured concrete

its maturity would be equal to 28*24(18-(-11)) = 194880ch.

However, in standard calculation the maturity of fully cured concrete is

taken as 19,8000ch. (The discrepancy is because of the origin or the datum is not

exactly being calculated). Corresponding temperature is recorded for each interval

of time the summation of the product of time and temperature give an accurate

picture of the maturity of concrete. In the absence of such detailed temperature

history with respect to the time interval, the maturity fig can be arrived by

multiplying duration in hours by the average temperature at which the concrete is

cured. Of course, the maturity calculated as above will be less accurate.

Maturity concept is useful for estimating the strength of concrete as a

percentage of strength of concrete of known maturity. In other words, if we know

the strength of concrete at full maturity (19,8000ch), we can calculate the

percentage strength of identical concrete at any other maturity by using the

following equation given by plowman.

Strength at any maturity as a percentage of strength at maturity

19,8000ch = A+B log10 maturity/1000

The values of co-efficient, A and B depend on the strength level of

concrete. The values are given in table & below.

Strength after 28 days at 180c

(maturity of 19,8000ch) MPa

Co-efficient

Less than 17.5

17.5 – 35.0

35.0 – 52.5

52.5 – 70.0

A B

10 68

21 61

32 54

42 46.50

Cube strength at the maturity of 19,8000ch. It is to be noted that the maturity

equation holds good for the initial temperature of concrete less than about 380c

gives the value of constant A and B when strength and temperature are expressed

in lbs/sq inch and 0f respectively.

1. b) The strength of a sample at fully matured concrete is found to be 40.00 mpa.

Find the strength of identical concrete at the age of 7 days when cured at an

average temperature during day time at 200c, and night time at 100c.

Maturity of concrete at the age of 7 days.

= Σ (time*temperature)

= 7*12(20-(-11)) + 7*12(10-(-11))

= 43680ch.

The strength range of this concrete falls in zone III for which the constant

A= 32 and B = 54.

The percentage strength of concrete at maturity of

43680ch = A+B log10 maturity/1000.

= 32+54 log10 (4368/1000)

= 66.575.

The strength at 7 days = 40*66.5/100

= 26.5MPa.

2. a) Briefly explain the water/cement ratio, gel/space ratio and gain of strength

with age of concrete.

Water/cement ratio:

Strength of concrete primarily depends upon the Strength as cement paste.

The Strength of cement paste increase with cement content and decrease with air and

water content.

A general rule defining the strength of the concrete paste and concrete in

terms of volume fraction of the constituents by the equation

S = K (c/c + e + a) 2

Where, S = strength of concrete.

c, e, and a = volume of cement, water and air respectively.

K = a constant.

In this expression the volume of air is also included because it is not only

the water/cement ratio but also the degree of compaction, which indirectly means the

volume air filled voids in concrete taken into account in the estimation of strength.

For lower water/cement ratio could be used when the comparatively higher

water/cement ratio is below the introduction of voids the compressive strength of

concrete is directly proportion to the cement/water ratio.

Gel/space ratio:

This is the ratio of the volume of the hydrated cement paste to the sum of

volume of hydrated cement and of the capillary pores.

Power’s experiment showed that the strength of concrete bears a specific

relationship with the gel/space ratio. He found the relationship to be 240x3, where x is the

gel/space ratio and 240 represents the intrinsic strength of gel in mpa for the type of

cement and specimen used. The relationship between the strength and water/cement ratio

will hold good for 28 days strength and for fully computed concrete, whereas the

relationship between the strength and get/space ratio is independent of age.

Gain of strength with age:-

The concrete develops strength with confined hydration. The ratio gain of

strength in faster to start with and the rate get reduce with age. It is customary to assume

the 28 days strength as the full strength of concrete. Actually concrete develops strength

beyond 28 days also. The increase in strength beyond 28 days used to get immersed with

the factor of safety. There is normally a gain of strength beyond 28 days. The quantum of

increase depends upon the grade and type of cement, curing and environmental evidence

of justify a higher strength for a particular structure due to age.

b) Calculate the gel/space ratio and the theoretical strength of a sample of concrete

made with 500g of cement with 0.5 water/cement ratio, on full hydration and at

60% hydration.

Gel/space ratio on full hydration = 0.657c/0.319c +Wo

Where,

C = wt of cement in gm.

Vc = specific volume of cement = 0.319ml/m.

Wo = volume of mixing water in ml.

Volume of gel = C*0.309*2.06

= 0.657C

Space available = C*0.309*Wo = 0.319C +Wo

Gel/space ratio on full hydration = 0.657*500/0.319*500 + (500*0.50)

= 0.802 say 0.80.

Strength of concrete = 240(0.80)3 = 123mfa.

Gel/space ratio for 60% hydration

X = 0.657Cα/0.319Cα+Wo

= 0.657*500*0.60/0.319*500*0.6+250

= 0.57.

Theoretical strength of concrete at 60% hydration

= 240*(0.57)3 = 44.4mpa.

3. Explain in detail about the methods used for making high strength for

construction as quality assurance.

There are special methods used for making high strength concrete, they are

given below

a) Seeding.

b) Reverberation.

c) High speed slurry mixing.

d) Use of admixtures.

e) Inhibition of cracks.

f) Sulphur impregnation.

g) Use of high cementitious aggregates.

a. Seeding:

This involves adding a small percentage of finely ground, fully hydrated

Portland cement to the fresh concrete mix. The mechanism by which this

is supported to strength development is difficult to explain. This method

may not hold much promise.

b. Reverberation.

Concrete undergoes plastic shrinkage mixing water creates continuous

capillary channels bleeding and water accumulates at some selected

places. All these reduces the strength of concrete and increase the strength

of concrete.

c. High speed slurry mixing.

This process involves the advance preparation of cement and water mixture which then blended with aggregate to produce concrete. Higher compressive strength obtained is attributed to more efficient blending of cement paste.

d. Use of admixtures.

Use of water reducing agents are known to produce increased compressive strength.

e. Inhibition of cracks.

Concrete fails by the formation and propagation of cracks. If the propagation is inhibited the strength will be larger replacements of 2-3% of fine aggregate by polythene or polystyrene “lenticules” of 0.025mm thick and 3 to 4mm diameter result in high strength. Concrete cubes made in this way have high yielded strength upto 105mpa.

f. Sulphur impregnation.

Satisfactory high strength concrete have produced by low strength process concrete with sulphur. The process consists of 1200c for 24 hours, immersing then molten sulphur for 24 hours under vaccum and releasing the vaccum. The sulphur infiltrated concrete has given strength up to 58 mpa.

g. Use of high cementitious aggregates.

It has been found that use of cementitious aggregate has yielded high strength. Cement found is kind of clinker. This glass clinker is finely ground results in a kind of cement. When coarsely crushed, it makes a kind of aggregate known as ALAG. Using ALAG aggregate, strength up to 125 mpa has been obtained with water/cement ratio 0.32.

4. Explain in detail about the surface treatments of concrete.

There are large varieties of material, which are applied to the surface of concrete either to water proof the surface to render it to attack quick hardening effect so that concrete is made strong within a short period to enable forces. Some of the material used for surfaces treatments is listed below.

a) Aqueous solutions of sodium silicate.

b) Magnesium or zinc selico fluoride.

c) Drying oil such as linseed oil or tung oil.

d) Chlorinated rubber paints.

e) Neoprene paints.

f) Epoxy paints or coal far epoxy paints.

g) Silicon fluoride treatment.

Carborundum fused alumina and finely divided iron are effective in rendering a concrete surface less slippery and more resistance to abrasion dusty. Treatments with a solution of sodium silicate harden a concrete surface and also render it less dusty. Treatments with a sulphate solutions and silico fluoride are also effective.

Tung oil and linseed oil are applied to concrete surface neither neat, hot or thinner with turpentine or white spirits. The treatments gives a hard surfaces and not heavy. Oil paints with a tung-oil medium or bituminous paints can be used, but paints certaining synthetic resins particularly polyurethanes or epoxy yester or chlorinated rubber have a greater resistance to wear. None of these surface treatments are on a weak or friable concrete surface.

Treatment with the sodium silicate and silicoflouride only affords protection against mild conditions of attack either by aqueous solution or organic liquids. Drying oil can be expert a protective influence for some years ago against dilute aqueous solution of aggressive solids. The surface water with chlorinated rubber paints, neoprene paints, epoxy paints etc are found to solution of salts and dilute acids. Epoxy paints or synthetic lacquers specially prepared for the treatment of concrete surfaces have given good protection.

The above material have been often applied for protecting concrete surfaces from abrasion, erosion and general deteriorating action of concrete piles jetty pires and other hydraulic structure. A treatment by bitumen and coal has been found to give protection against insects and borers. Some plastic materials, rubber, latex glass fiber coating, PVC linings have been given to concrete in solution for increasing durability.

One of the durability of concrete is known as oscrate process. In this concrete member is imprecated with silico fluoride under pressure. This method has been adopted to increase the durability of concrete piles and pipes carrying sewage.

Lastly one of the recent applications to improve the durability of concrete is the treatment of polymer impregnation. Polymer application to the concrete surface improves the durability of concrete manifold. This method has been adopted to increase the durability of precast concrete piles and pipes carrying sewage.

5. Explain in detail about the permeability of concrete for concrete construction as quality assurance.

Theoretically, the construction of aggregate of low permeability in to cement paste, it is expected to reduce the permeability of the system because the aggregate particles intercept the channel of flows and make it take a circuital route. Compared to neat cement paste, concrete with the same w/c ratio and degree of maturity, should give a lower co-efficient of permeability. But in practice it is seen from test date it is not the case. The introduction of aggregate particularly larger of size of aggregate increase the permeability considerably producing calcium sulpho aluminates known as attringite molecules of water may be 32 or 31.

On the other hand magnesium sulphate has more for reaching action than other sulphate because it reacts not only with calcium hydroxide and hydrated calcium alumino like other sulphate but also decomposes the hydrated calcium silicates completely and makes a friable mass.

The rate of sulphate attack increase with the increase in the strength of solution. A standard solution of magnesium sulphate can cause serious damage to concrete with higher water/cement ratio in a short time.

Another factor influencing the rate of attack is the speed in which the sulphate gone into the reaction is replenished. For this it can be seen that when the concrete is subjected to the pressure of sulphate bearing water on one since the rate of attack is higher. Similarly alternate wetting and drying due to tidal variation or spraying leads to rapid attack.

6. Discuss the method of controlling the sulphate attack on concrete.

The method of controlling the sulphate attack:

a) Use of sulphate resisting cement.

b) Quality concrete.

c) Use of air-entertainment.

d) Use of pozzolana.

e) High pressure stream curing.

f) Use of high alumina cement.

a) Use of sulphate resisting cement.

The most effective method of resisting the sulphate attack is to use cement with the low C3A content. In general, it has been found that a tricalcium aluminates (C3A) content of 7% gives a rough division between cements of good and poor performance in sulphate water. High resistance attack was found for Portland cements containing not more than 5.5%of tricalcium aluminates.

b) Quality concrete.

A well designed placed and compacted concrete which as dense impermeable exhibits high resistance to sulphate attack. Similarly, a concrete with low water/cement ratio also demonstrate a higher resistance to sulphate attack.

c) Use of air-entertainment.

Use of air-entertainment to the extent of above 6% has beneficial on the sulphate resisting qualities of concrete. The beneficial effect is possible due to the

reduction of segregation improvement of workability, reduction in bleeding and in general better imperability of concrete.

d) Use of pozzolana.

In-corporation of or replacing a part of cement by pozzolana material reduces the sulphate attack. The admixtures of pozzolana which converts the leachable calcium hydroxide to insoluble non-leachable cementitious product. This pozzolana action is responsible for the impermeability of concrete. Secondly the removal of calcium hydroxide reduces the susceptibility of concrete by magnesium sulphate.

e) High pressure stream curing.

High pressure stream curing improve the resistance to sulphate attack. This improvement is due to the removal or reduction of calcium hydroxide by the reaction of silicate which is invariably mixed when high stream curing method is adopted.

f) Use of high alumina cement.

The cause of grate resistance shown by alumina cement to the action of sulphate is still not fully understood. However it is attributing in part to the Portland cement. High calcium cement contains approximately 40% alumina, a compound very sulphate attack when in normal Portland cement. But these percentages of alumina present in high alumina cement behave in different way. The primary cause of resistance is attributed to formation of protective films which inhibit the penetration or diffusion of sulphate ions into the interior. It should be remembered that the alumina cement may not show higher resistance to sulphate attack in higher temperature.

7. Explain in detail about the reaction with chloride and moisture effects in concrete.

Reaction with chloride:

Presence of cacl2 even in small percentage lead to rapid corrosion of reinforcement as it reduces the electrical resistivity of concrete and helps of promote galvanic cell action.

Presence of chlorides increase shrinkage cracks in concrete further accentuating corrosion of reinforcement in aggressive environment.

The ingress of chloride ions in excess of the threshold concentration value reduces the alkanity of the concrete and breaks down the protective film to set off the process of corrosion.

It increases the corrosion rate through their conductivity to the ionic activity.

Chloride salt can enter concrete in two ways

i. Chloride may be present in the concrete mix itself.

ii. Chloride can penetrate into the hardened concrete wherever it is permeable and reaches the reinforcement at isolated points.

8. Discuss in detail about the PH value and carbonation of concrete.

PH value:

The PH value of most concrete is normally about 120 which is sufficient to passivate the reinforcement against corrosion. When it reduces below 8.0 the carbonation of concrete takes place and in turn corrosion initiates.

The fig shows the relationship between the PH value corrosion rate and the acidic media, which encourages corrosion by destroying the protective layer around steel.

Carbonation of concrete:

Carbonation is a process by which co2 converts free time into caco3 and water, there reduces the PH further. As the PH reaches about 9.0 the passivation of reinforcement is lost and corrosion starts based on the moisture.

Moisture:

The presence of moisture in a R.C snember is due to two reasons.

i. Water that is due to making the concrete mix remains well distributed and enclosed in the concrete mass.

ii. Water that finds its way into the hardened concrete from outside due to the subsequent absorption of water which may circulate freely inside the mass.

Concrete shrinks when allowed to air at lower relative humidity and it swells when kept at 100% relative humidity or when placed in water. Just as drying shrinkage is an ever continuing process, swelling continuously placed in water is also ever continuing process.

Drying shrinkage is due to loss of absorbed around gel particles swelling is due to the absorption of water by the cement gel. A sample which shrinks due to drying cannot by absorption water to retain its portion after absorption.

The moisture movement in concrete includes alternatively compressive stress and tensile stress which may cause fatigue in concrete which reduces the durability of concrete owing to reversal of stresses availability of oxygen and water. The penetration depth of this PH is generally called the carbonation depth.

The rate of carbonation depends on

Permeability of concrete.

Co2 concentration in the air.

Moisture in the gel and capillary pores.

Relative humidity of atmosphere.

As the carbonation reaches the reinforcement, the passivating influence of concrete is lost and the reinforcement starts corroding in the presence of moisture and oxygen carbonation time decrease with increase in w/c ratio and with decrease in lower.

The influence of relative humidity of the atmosphere on the rate of carbonation is shown below.

Phase Process Relative air humidity Rate of carbonation

1. Diffusion inwards of co2. <30% Low

2. Reaction between co2 and water molecules.

40-75% High

Reaction between resultant carbonic and the alkaline components of concrete.

>75% low