1.5 Concrete (Part I) This ction covers the following topics. • Constituents of Concrete • Properties of Hardened Concrete (Part I) 1.5.1 Constituents of Concrete
Introduction
Concrete is a composite material compod of gravels or crushed stones (coar aggregate), sand (fine aggregate) and hydrated cement (binder). It is expected that the student of this cour is familiar with the basics of concrete technology. Only the information pertinent to prestresd concrete design is prented here.
The following figure shows a petrographic ction of concrete. Note the scattered coar aggregates and the matrix surrounding them. The matrix consists of sand,
hydrated cement and tiny voids.
Figure 1-5.1 Petrographic ction of hardened concrete
(Reference: Portland Cement Association, Design and Control of Concrete Mixtures )
Aggregate
The coar aggregate are granular materials obtained from rocks and crushed stones. They may be also obtained from synthetic material like slag, shale, fly ash and clay for u in light-weight concrete.
The sand obtained from river beds or quarries is ud as fine aggregate. The fine aggregate along with the hydrated cement paste fill the space between the coar aggregate.
The important properties of aggregate are as follows.
1) Shape and texture
2) Size gradation
3) Moisture content
4) Specific gravity
5) Unit weight
6) Durability and abnce of deleterious materials.
The requirements of aggregate is covered in Section 4.2 of IS:1343 - 1980.
The nominal maximum coar aggregate size is limited by the lowest of the following quantities.
1) 1/4 times the minimum thickness of the member
2) Spacing between the tendons/strands minus 5 mm
3) 40 mm.
The deleterious substances that should be limited in aggregate are clay lumps, wood, coal, chert, silt, rock dust (material finer than 75 microns), organic material, unsound and friable particles.
Cement
In prent day concrete, cement is a mixture of lime stone and clay heated in a kiln to 1400 - 1600ºC. The types of cement permitted by IS:1343 - 1980(Clau 4.1) for prestresd applications are the following. The information is revid as per IS:456 - 2000, Plain and Reinforced – Concrete Code of Practice.
1) Ordinary Portland cement confirming to IS:269 - 1989, Ordinary Portland Cement,
33 Grade – Specification.
2) Portland slag cement confirming to IS:455 - 1989, Portland Slag Cement –
Specification, but with not more than 50% slag content.
3) Rapid-hardening Portland cement confirming to IS:8041 - 1990, Rapid Hardening
Portland Cement – Specification.
Water
The water should satisfy the requirements of Section 5.4 of IS:456 - 2000.
“Water ud for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete and steel”.
Admixtures
IS:1343 - 1980 allows to u admixtures that conform to IS:9103 - 1999, Concrete Admixtures – Specification. The admixtures can be broadly divided into two types: chemical admixtures and mineral admixtures. The common chemical admixtures are as follows.
1) Air-entraining admixtures
2) Water reducing admixtures
3) Set retarding admixtures
4) Set accelerating admixtures
5) Water reducing and t retarding admixtures
6) Water reducing and t accelerating admixtures.
The common mineral admixtures are as follows.
1) Fly ash
2) Ground granulated blast-furnace slag
3) Silica fumes
4) Rice husk ash
5) Metakoline
The are cementitious and pozzolanic materials.
1.5.2 Properties of Hardened Concrete (Part I)
The concrete in prestresd applications has to be of good quality. It requires the following attributes.
1) High strength with low water-to-cement ratio
2) Durability with low permeability, minimum cement content and proper mixing,
compaction and curing
3) Minimum shrinkage and creep by limiting the cement content.
The following topics are discusd.
1) Strength of concrete
2) Stiffness of concrete
3) Durability of concrete
4) High performance concrete
5) Allowable stress in concrete.
Strength of Concrete
The following ctions describe the properties with reference to IS:1343 - 1980. The strength of concrete is required to calculate the strength of the members. For prestresd concrete applications, high strength concrete is required for the following reasons.
1) To sustain the high stress at anchorage regions.
2) To have higher resistance in compression, tension, shear and bond.
3) To have higher stiffness for reduced deflection.
4) To have reduced shrinkage cracks.
Compressive Strength
The compressive strength of concrete is given in terms of the characteristic compressive strength of 150 mm size cubes tested at 28 days (f ck). The characteristic strength is defined as the strength of
the concrete below which not more than 5% of the test results are expected to fall. This concept assumes a normal distribution of the strengths of the samples of concrete.
The following sketch shows an idealid distribution of the values of compressive strength for a sizeable number of test cubes. The horizontal axis reprents the values of compressive strength. The vertical axis reprents the number of test samples for a particular compressive strength. This is also termed as frequency. The average of the values of compressive strength (mean strength) is reprented as f cm. The characteristic strength (f ck) is the value in the x-axis below which 5% of the total area under the curve falls. The value of f ck is lower than f cm by 1.65σ, where σ is the standard deviation of the normal distribution.
Frequency
1.65σ
5% area f ck f cm
28 day cube compressive strength
Figure 1-5.2 Idealid normal distribution of concrete strength (Reference: Pillai, S. U., and Menon,
D., Reinforced Concrete Design)
The sampling and strength test of concrete are as per Section 15 of IS:1343 - 1980. The grades of concrete are explained in Table 1 of the Code.
The minimum grades of concrete for prestresd applications are as follows.
•30 MPa for post-tensioned members
•40 MPa for pre-tensioned members.
The maximum grade of concrete is 60 MPa.
Since at the time of publication of IS:1343 in 1980, the properties of higher strength concrete were not adequately documented, a limit was impod on the maximum strength. It is expected that higher strength concrete may be ud after proper testing.
The increa in strength with age as given in IS:1343 - 1980, is not obrved in prent day concrete that gains substantial strength in 28 days. Hence, the age factor given in Clau 5.2.1 should not be ud. It has been removed from IS:456 - 2000.
Tensile Strength
The tensile strength of concrete can be expresd as follows.
1) Flexural tensile strength: It is measured by testing beams under 2 point loading
(also called 4 point loading including the reactions).
2) Splitting tensile strength: It is measured by testing cylinders under diametral
compression.
