irp chapter a-concrete

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Chapter A: Concrete

Preliminary Remarks 1

1 Technical Regulations 1

2 Raw Materials 1

2.1 General Information 1

2.2 Cement 1 2.3 Aggregate 2 2.4 Concrete Additives 2

2.5 Concrete Admixtures 3

3 Concrete Properties 4

3.1 Water Cement Ratio 4

3.2 Fresh Concrete 43.2.1 Consistency 4

3.2.2 Wet Density of Fresh Concrete 53.2.3 Temperature 5

3.2.4 Segregation 53.3 Solid Concrete 6

3.3.1 Exposure Categories 63.3.2 Compressive Strength 63.3.3 Porosity 73.3.4 Secondary Treatment (Curing)

73.3.5 Special Properties 83.3.6 Load-Deformation-Behaviour 93.3.7 Shrinkage and Swelling 9

3.3.8 Creep 103.3.9 Temperature Strain 103.3.10 Carbonation 103.3.11 Concrete Cover 11

3.4 Quality Assurance 11



Literature 11

Preliminary RemarksBasic knowledge about concrete engineering is required for this course (cp. SIVV-examination regulations). Please refer to the relevant literature (see pp. 11) for thesebasics.

1 Technical Regulations

This chapter is based on the presently valid General Licences of the ConstructionSupervisory Board and on the standards for concrete raw materials, as well as on theEuropean standards DIN EN 206-1 and DIN 1045 (parts 1-4 of which are new).

2 Raw Materials

2.1 General Information

Concrete is an artificial stone consisting of a mixture of cement, sand, aggregate andwater, and, if necessary, concrete additives and concrete admixtures, and is formedwhen the cement paste (water-cement-mixture) hardens.

2.2 Cement

According to EN 197 and DIN 1164 we can distinguish between the following kinds ofcement: CEM I, CEM II, CEM IV and CEM V.CEM 1 (Portland cement) consists of finely ground Portland-cement-clinker.

The group of CEM II-cements (Portland composite-cements) are mixtures consistingof at least 65 % Portland-cement-clinker and granulated blast-furnace slag,pozzolanic or inert materials.

CEM III-cements (blast-furnace cements) consist of Portland-cement-clinker and 36-65 % granulated blast-furnace slag (CEM III/A), or of Portland-cement-clinker and 66-80 % granulated blast-furnace slag (CEM III/B).

All cements are divided into strength categories (see Table A 1).

Table A 1: Compressive strength categories for cementsCompressive Strength (N/mm²)Initial strength

StrengthCategory

2 days 7 daysStandard strength28 days

32.5

32.5 R

-

≥ 10

≥ 16

-≥ 32.5 ≤ 52.5

42.5

42.5 R

≥ 10

≥ 20

-

-

≥ 42.5 ≤ 62.5



When cement reacts with water (so-called hydration) heat is released. NW-cementshave relatively little heat-development. They are used in mass concrete.

Exposing concrete to sulphates can lead to sulphate attack and a substantialdeterioration of the structure. In the case of heavy sulphate attacks HS-cements

should be used. These are, for example, cements with a blast-furnace slag-content ofmore than 65 %.

Certain concrete aggregates, mostly from Northern Germany, are alkali-reactive. Areaction of these aggregates with the calcium hydroxide from the pore solution of thecement-stone may lead to alkali silica reaction (ASR). In these cases NA-cementsshould be used to prevent such a reaction.

A CEM III/B-cement always has the properties NW, HS and NA.

2.3 Aggregate

The aggregate constitutes approximately 70% of the concrete’s volume. Dependingon the size of the aggregate the designations in Table A 2 are used:

Table A 2: Designations of Aggregates

Aggregate DesignationAggregate withSmallest grainsizemm

Largest grain sizemm

Natural Crushed

0 0.125 Powder powder0.125 0.25 micro sand micro-crushed

sand0.25 1 fine sand fine-crushed sand1 4 coarse sand coarse-crushed

sand4 32 Coarse aggregate Coarse aggregate32 - Coarse aggregate Coarse aggregate

When concrete hardens the aggregates and cement paste should form a mostly void-free structure, and also absorb as little water as possible. To ensure this the gradingcurve of the concrete aggregates should not fall within areas 1 and 5 (see Diagram A1). The grading curves of aggregates fall within area 3, that is between standard

grading curves A and B.

2.4 Concrete Additives

Concrete additives are micro-granular admixtures, which influence certain propertiesof the concrete.

Inert additives (e.g. quartz- or limestone-powder) do not react with cement and water.They can improve the composition and consistency of aggregate combinations.

Pozzolanic materials (e.g. pulverised fly(or fuel) ash, trass and silica-powder)improve the composition of the aggregate combinations and contribute to thehydration process by reacting with the calcium hydroxide produced during cementhydrates.



Ilustration A 1: Grading curve areas for aggregates with maximum grain size 32mmsieving stagemesh sizehole size

mesh sieves (DIN 4188 part 1)square-holed sieves (DIN 4187 part 2)

Pigments are used to dye the concrete.

Organic additives (synthetic resin dispersions) do not react with the hydration-products. They have innate adhesive properties.

2.5 Concrete Admixtures

Concrete admixtures are liquid or powder materials, which influence certainproperties of fresh concrete and / or hardened concrete. They can be divided intogroups depending on the function or performance (see table A 3):

Table A 3: Property groups for concrete admixtures

Property Group Abbreviation for German codes or standardsonly

Plasticizer or water reducer BVSuper plasticizer or high range water reducer FMAir-entraining agents LP

Retarder VZSealing agent DMAccelerator BEGrouting aid EHStabilizer STChromate-reducing agent CRRecycling aids for washing water RHFoaming agent SB

Following concrete admixtures are most important: Plasticizers improve theworkability of fresh concrete at the same water content or allow a water-reduction of5-10 l/m³ concrete, thus improving the quality of the concrete. Super plasticizers

have a similar, albeit much stronger, effect as plasticizers. Depending on their periodof effectiveness they may be added to the fresh concrete at the concrete plant orlater at the construction site.

Super plasticizers must be used in the production of concrete with a consistencyclass > F4.

Air-entraining agents create small discrete air-voids (diameter up to 0.3 mm) in freshand solid concrete. These voids remain filled with air even in water-saturatedconcrete and reduce the pressure of water freezing and thawing in capillary pores.Thus they improve the concrete’s resistance against frost- and de-icing salts.

Retarders slow the hydration of the cement paste, thus extending the concrete’sworkability.



3 Concrete Properties

3.1 Water Cement Ratio

The water cement ratio indicates the mass ratio of water to cement. It is one of themost important parameters for concrete and influences all properties of concrete,especially strength, porosity and durability. Therefore observing the water cementratio is highly important and adding water to ready-mixed concrete at the constructionsite is strictly forbidden. If adding water is necessary in special cases it must besupervised and documented by a concrete engineer.

Diagram A 2 shows the relationship between the water cement ratio and the

compressive strength of concrete subject to the cement strength category.Diagram A 2: Relationship between concrete cube strength f c of concrete and thewater cement ratio and on the strength category of the cement (according to Walz)

strength categories of cementwater cement ratiocompressive strength of concrete f c (N/mm²)

3.2 Fresh Concrete

3.2.1 Consistency

The consistency is a measure for the workability of the concrete.

According to DIN EN 206-1 the consistency may be measured by the slump test, thesettling time, the compaction test or the spread flow test. In Germany the spread flowtest is most commonly used. DIN EN 206-1 distinguishes the slump measures shownin Table A 4:

Table A 4: Slump Measure CategoriesCategory Spread Measure

(diameter) in mmF1* ≤ 340F2 350 – 410F3 420-480F4 490-550F5 560-620F6 ≥ 630

*due to non-existing sensitivity this test procedure should not be used

Slump measure category F3 is used mainly in building construction.



3.2.2 Wet Density of Fresh Concrete

The wet density of fresh concrete is the mass in kg per m³ of fresh, compactedconcrete. It indicates the uniformity of the concrete composition, and, if compared to

the standard wet density, it can show up possible mistakes in the manufacture of theconcrete specimens.

3.2.3 Temperature

When using the concrete the minimal concrete temperatures in Table A 5 must beobserved.

Table A 5: Minimal and maximum temperatures for fresh concreteAir Temperature (°C) Temperature of Fresh Concrete (°C)

+5 to -3 ≥ 5for NW cement: ≥ 10

< 3 ≥ +10afterwards, for 3 days: > 10

> 30 ≤ 30

3.2.4 SegregationDue to the segregation of solid materials and the subsequent bleeding of water anexposed horizontal concrete surface usually has a layer of watery, and thereforeweaker, fine mortar. This condition is especially pronounced in more watery fine

mortar and concretes with low water retention properties.

3.3 Solid Concrete

3.3.1 Exposure Categories

The effects of environmental conditions are divided into exposure categoriesaccording to DIN EN 206-1 (see Table A 6).

Table A 6: Exposure CategoriesNo risk of corrosion Corrosion of reinforcement Corrosion of concrete

CarbonisationXC 1XC 2XC 3XC 4

Frost with /without thawingagentsXF 1XF 2XF 3XF 4

ChloridesXD 1XD 2XD 3

Chemical AttackXA 1XA 2XA 3

XO

Seawater ChloridesXS 1XS 2XS 3

WearXM 1XM 2XM 3



Depending on the exposure category, specifications stipulate the maximum watercement ratio, the minimal compressive strength category of the concrete, the minimalcement content, and, in some cases, the minimal void content.

3.3.2 Compressive Strength

According to its compressive strength concrete is divided into categories (see TableA 7). The strength is tested after 28 days, using either 150 mm diameter, 300 mmlength cylinders, or 150 mm cubes.

Table A 7: Compressive Strength Categories for ConcreteCompressive StrengthCategory

Characteristic minimalcompressive strength ofcylinders f ck, cyl

N/mm²

Characteristic minimalcompressive strength ofcubes f ck, cube

N/mm²C 8/10 8 10C 12/15 12 15C 16 /20 16 20C 20/25 20 25C 25/30 25 30C 30/37 30 37C 35/45 35 45C 40/50 40 50C 45/55 45 55C 50/60 50 60C 55/67 55 67C 60/75 60 75C 70/85 70 85

C 80/95 80 95C 90/105 90 105C 100/115 100 115

3.3.3 Porosity

Cement chemically and physically binds around 40% of its mass in water. Tofacilitate workability a certain amount of excess water is usually added to the freshconcrete. This water leaves absorptive capillary voids when it evaporates. Anincreasing water cement value means:

• an increasing porosity of the cement paste, therefore decreasing concretestrength, lower frost resistance, lower corrosion protection for thereinforcement

• the concrete is more permeable to water and less durable, because of the fastabsorption of water through the capillary voids

• increased shrinkage (volume decrease during the evaporation of excesswater) with higher strain and possible cracking

The size of capillary voids depends not only on the water cement ratio, but also onthe degree of hydration. The degree of hydration is low if the water needed for thehydration of the cement is missing due to early evaporation (see Picture A 3).

Picture A 3: Water permeability of cement paste, depending on the capillary porosity,the water cement ratio and on the cement’s degree of hydration



degree of hydration in %water permeability in cm/s 10 -12 ratio of capillary voids in vol.-%spec. surface in cm²/g

Age in dayswater cement value

3.3.4 Secondary Treatment

Freshly laid and green concrete must be protected until sufficiently cured to:• minimize early shrinkage• ensure a sufficiently strong concrete surface• ensure sufficient durability of the concrete• prevent freezing• avoid destructive concussions, impacts or damages.

The exposed surfaces of the concrete (to a depth of about 3 cm) is affected by earlyevaporation, and may, if not treated properly, exhibit low durability.

There are several ways of secondary treatment:• leaving the concrete within the mould• covering with plastic sheets• constantly moist, water retaining coverings• applying liquid curing membranes with proven suitability• underwater-storage or constant spraying with water• a combination of the treatments above• a constant relative humidity of > 85% for the duration of the secondary

treatment

Duration of the secondary treatment according to DIN 1045-3:For indoor-structures: 0.5 daysFor all other structural parts: Apply treatment until the compressive strength of

the concrete, in those areas of the structure whichare close to the surface, is at 50% (for XM 70%) ofthe nominal strength f ck. At temperatures of 15 to25°C this requirement leads to a curing period of 1to 15 days, depending on the strength developmentof the cement (DIN 1045-3).

3.3.5 Special Properties

Concretes, which are exposed to certain stresses, must have a resistance against

these factors. Most important is the density of the concrete, which depends on itscomposition processing and on the secondary treatment (curing).



Concrete can be manufactured to have the following special properties:• concrete with high water proofing• concrete with a high resistance to frost• concrete with a high resistance to frost and de-icing salts• concrete with a high resistance to chemical attack• concrete with a high resistance against wear and tear and abrasion• concrete for high service temperatures of up to 250 °C• concrete for underwater use (underwater concrete).

Waterproofing is usually tested on slabs with the measurements of 200 mm x 200mm x 120 mm. During these tests, according to DIN 1048 part 1 (now replaced), thegreatest permeability depth (average of 3 tests) may not exceed 50 mm forwaterproof concrete.

3.3.6 Load-Deformation-BehaviourEvery construction material warps under stress. The modulus of elasticity (e-modulus) of the construction material indicates this behaviour. The smaller the e-modulus, the smaller the strength necessary for a certain deformation. The e-modulus of concrete C25/30 is approximately 30,500 N/mm², that of steel approx.210,000 N/mm².

Concrete can only withstand very small expansions if exposed to tensile stress. Itbreaks at expansions of approx. 0,1 mm/m.

The e-modulus of most plastics is very dependent on temperature.

3.3.7 Shrinkage and Swelling

Shrinkage is the decrease in volume due to evaporation, while swelling is theincrease in volume due to water absorption. In the case of constricted shrinkage-deformation tensile stress develops, which may lead to cracks.

In the laboratory mortar prisms reach shrinkage coefficients of 1.3 mm/m. Shrinkagecoefficients measured on structural parts are usually much smaller. The shrinkagecoefficient is dependent on environmental conditions, the dimensions of the structural

part and on the water content.

3.3.8 Creep

The elastic deformation is proportional to the applied load. Creep is the time-dependent increase in the permanent deformation of the structural part under acontinuous load. The creep value can amount to a multiple of the elastic deformation.

3.3.9 Temperature Strain

A concrete structure, which is 10 m in length, will expand or shrink 1 mm if exposedto a temperature change of 10 C. If temperature-induced deformations are



constricted, restraint stresses will develop within the structural part. Concrete andsteel have approximately the same coefficient of thermal expansion.

3.3.10 Carbonation

During the hydration of the concrete calcium hydroxide Ca(OH) 2 is produced. Thisproduces within the cement paste a pore water solution with a pH-value of approx.13. This highly alkaline solution creates a passive layer on the reinforcement steel,which prevents corrosion of the reinforcing steel, even if they are exposed to oxygenand moisture (however, not in the case of chloride exposure). The carbon dioxide ofthe air, together with moisture turns the calcium hydroxide into calcium carbonateCaCo 3. This process is called carbonation. In an aqueous solution calcium carbonatehas a pH-value of below 10. At such a low alkalinity the passive layer on thereinforcement steel is no longer durable. Thus reinforcement corrosion becomespossible if there is enough moisture and oxygen.

There is practically no carbonation if the concrete is dry (no moisture to facilitate thechemical reaction) or water-saturated (no diffusion of the carbon dioxide). Thestrongest carbonation of concrete takes place at a relative humidity of 40 to 60 %.

The denser the concrete and the further the carbonation has advanced, the slowerthe carbonation will progress (see Diagram A 4).Concrete used outdoors, which is exposed to rainfall, will carbonate slower thanoutdoor-concrete, which is protected from rainfall.

3.3.11 Concrete Cover

The reinforcement corrosion is (even if there are narrow cracks) reduced, the denser(influenced also by the concrete composition and secondary treatment) and thickerthe concrete cover is.

Therefore it is especially important to ensure sufficient concrete cover achieved bythe correct positioning and size of spacers, that are placed before concreting.

3.4 Quality AssuranceAccording to DIN 1045-3 there are three supervision categories when placingconcrete on site. Concrete belonging to the supervision category 1 are only testedeach once pour it (internal quality control). Concretes belonging to supervisioncategories 2 and 3 also have to be tested according to a set supervision plan by arecognized supervisory board (also called “external quality control”).

A construction site must be marked as “supervised”, visibly displaying the regulationsunderlying the construction project (DIN 1045-3) and the address of the recognizedsupervisory board. The inspection effort for supervision category 3 is higher than for

supervision category 2.



Literature

Bundesverband der Deutschen Zementindustrie e.V., Bauberatung Zement, Cologne (Ed.)Zement Merkblätter. Betontechnik, Hochbau, Tiefbau, Straßenbau, Landwirtschaftliches BauenCologne: Selbstverlag 1999Content Keywords: concrete technology; concrete processing; building construction; civil engineering;road construction; agricultural construction

Bayer, Edwin; Kampen, Rolf; Bundesverband der Deutschen Zementindustrie e.V., Cologne (Ed.)Beton-Praxis. Ein Leitfaden für die BaustelleSchriftenreihe der Bauberatung Zement7th edition, Düsseldorf; Beton-Verlag 1997Content Keywords: concrete production; reinforced concrete building; concrete building; constructionsite; solid concrete; concrete processing; secondary treatment; quality assurance; moulds; formwork;reinforcement; joint; crack development; fresh concrete; mortar; precast concrete element

Weber, Robert; Tegelaar, RudolfBundesverband der Deutschen Zementindustrie e.V., Bauberatung Zement, Cologne (Ed.)Guter Beton. Ratschläge für die richtige Betonherstellung19th Edition; Düsseldorf: Beton Verlag 1995Content Keywords: concrete production; concrete composition; ready-mixed concrete; Fließbeton;concrete inspection; concrete properties; concrete quality, materials engineering

Deutscher Beton-Verein e.V. –DBV-, Wiesbaden (Ed.)Beton-Handbuch. Leitsätze für Bauüberwachung und Bauausführung3rd updated edition, Wiesbaden: Bauverlag 1995Content Keywords: concrete composition; concrete production; aggregate; binder; admixtures;additives; concrete composition; processing; secondary treatment; inspection; quality supervision;reinforcements; mould; formwork; joint; standard; regulation; guideline; exposed joint; mass concrete;heavy concrete; light-weight concrete; concrete aggregate; additional water; concrete additive;

concrete admixture; quality assurance; concrete (high-strength)Iken, Hans W.; Lackner, Roman R.; Zimmer, Uwe P.Handbuch der Betonprüfung. Anleitung und Beispiele4th edition, Düsseldorf: Beton Verlag 1994Content Keywords: concrete inspection; inspection procedures; source of error; cement; aggregate;grouting mortar; DIN V ENV206; ISO-Norm; additional water; ground (aggressive)

Kupfer, Herbert (Ed.); Weigler, Helmut; Karl, SieghartBeton. Arten – Herstellung – EigenschaftenHandbuch für Beton-, Stahlbeton- und SpannbetonbauBerlin: Ernst und Sohn 1989Content Keywords: concrete; production; concrete composition; processing; secondary treatment;

fresh concrete; green concrete; solid concrete; light-weight concrete; gunite; fibrous concrete; concreteengineering; property; raw material

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