4CONCRETEConcrete is a construction material composed of cement (commonly Portland cement), coarse aggregate, fine aggregate, water and admixture.The cement and water form a paste that hardens and bonds the aggregates together.Concrete is the most widely used construction material in the world.4
5CONCRETE Concrete has strength, durability, versatility, and economy. It can be placed or molded into virtuallyany shape and reproduce any surface texture.5
8IntroductionCompressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength.When a specimen of material is loaded in such a way that it extends it is said to be in tension. On the other hand if the material compresses and shortens it is said to be in compression.
9IntroductionOn an atomic level, the molecules or atoms are forced apart when in tension whereas in compression they are forced together. Since atoms in solids always try to find an equilibrium position, and distance between other atoms, forces arise throughout the entire material which oppose both tension or compression.the compressive strength of a material is thatvalue of uniaxial compressive stress reached whenthe material fails completelyusually obtained experimentally by means of acompressive test.compression
10IntroductionThe major difference between the two types of loading is the strain which would have opposite signs for tension (positive—it gets longer) and compression (negative—it gets shorter).Another major difference is tension tends to pull small sideways deflections back into alignment, while compression tends to amplify such deflection into buckling.
11What is the Compressive Strength(CS) of Concrete? Concrete mixtures can be designed to provide awide range of mechanical and durability propertiesto meet the design requirements of a structure.The compressive strength of concrete is the most common performance measure used by theengineer in designing buildings and other structures.CS is measured by breaking cylindrical concrete specimens in a compression-testing machine.
12compression tests of cylinders of concrete which are crushed 28 days after they are made.
13What is the Compressive Strength(CS) of Concrete? CS is calculated from the failure load dividedby the cross-sectional area resisting the load(pound-force per square inch (psi) in US) and Customary units or megapascals (MPa) in SI units.Concrete CS requirements can vary from2500 psi (17 MPa) for residential concrete to4000 psi (28 MPa) and higher in commercial structures.Higher strengths up to and exceeding 10,000 psi (70 MPa) are specified for certainapplications.
14What is the Compressive Strength(CS) of Concrete? Compressive strength. The amount of force a material can support in a single impact
15Why is Compressive Strength(CS) Determined? Compressive strength test results are primarily used todetermine that the concrete mixture as delivered meets therequirements of the specified strength in the jobspecification.Strength test results from cast cylinders may be used forquality control, acceptance of concrete, or for estimating the concrete strength in a structure for the purpose ofscheduling construction operations such as form removalor for evaluating the adequacy of curing and protectionafforded to the structure.
16Why is Compressive Strength(CS) Determined? Fractured Test Specimen at Failure
17Why is Compressive Strength(CS) Determined? Concrete structures, except for road pavements,are normally designed on the basis that concreteis capable of resisting only compression, thetension being carried by steel reinforcement.
18What is Compressive Stress? applies to materials resulting in their compaction (decrease of volume).When a material is subjected to compressivestress then this material is under compression.Usually compressive stress applied to bars,columns, etc. leads to shortening.
20Modes of failure of standard concrete cylinders 20
21What is Tensile Strength? Concrete has substantial strength in compression, but is weak in tensile.The Tensile strength of concrete is roughly 10% of its compressive strengthNearly all reinforce concrete structures are design on the assumption that the concrete does not resist any tensile forces.Tension will create cracking of the concrete.
22What is Tensile Strength? Importance in design of concrete roadsand runways.E.g, its flexural strength or modulus of rupture(tensile strength in bending) isutilized for distributing the concentrated loads over a wider area of road pavement.
23What is Tensile Strength? Tensile strength. The amount of stretching force a material can withstand
24What is Flexural Strength? FS is one measure of the tensile strength of concrete.Measured on unreinforced concrete beam or slab to resist failure in bending.Is expressed as Modulus of Rupture(MR) in psi (Mpa)Flexural MR is about 10 to 20 percent of CS depending on size, type and volume of coarse aggregate used.
25What is Shear Strength? Shear strength in engineering is a term used to describe the strength of a material orcomponent against the type of yield or structural failure where the material or component fails in shear.A shear load is a force that tends to producea sliding failure on a material along a planethat is parallel to the direction of the force.
26What is Shear Strength? the shear strength of a component is important for designing the dimensions and materials tobe used for the manufacture/construction ofthe component (e.g. beams, plates, or bolts).In a reinforced concrete beam, the main purpose of stirrups is to increase the shear strength.
27What is Shear Strength? Stirrup and column ties Steel in place in a abeam
28What is Shear Strength?Shear strength is the maximum shear stress that a material can absorb in one impact before failure ness of the material tested.Shear strength. The maximum shear stress a materialcan absorb in one impact
30Spalling Concrete ( concrete cancer) Concrete cancer can affect any building inwhich reinforced concrete is used.This includes floor slabs, stairs, balconies, walls, columns, beams and pathways.Essentially, the steel responsible for reinforcement has begun to rust.
31Spalling Concrete ( concrete cancer) Spalling concrete is concrete which has broken up, flaked, or become pitted.This is usually the result of a combination of poor installation and environmental factors which stress the concrete, causing it to become damaged.On a low level, concrete spalling can be purely cosmetic in nature. However, it can also result in structural damage such as damage to reinforcing bars positioned inside the concrete.
32Spalling Concrete ( concrete cancer) Spalling concrete is largely due to a natural deterioration process called carbonation.Carbon dioxide in the air diffuses into theconcrete and reacts with the alkalis in it.The concrete becomes carbonated and this allows the embedded steel bars to corrode.These corroded steel bars expand and exert a force on the surrounding concrete causing the concrete to bulge and crack.
33Spalling Concrete ( concrete cancer) The early stages of spalling concrete willnot affect the safety of the building. However, the spalling concrete should be repaired as soon as possible before the steel bars corrode further and damage larger areas hence the term 'concrete cancer'.
34Spalling Concrete ( concrete cancer) - Corrosion in Reinforced Concrete
36Concrete ShrinkageShrinkage of concrete is defined as the contraction due to loss of moisture.Due to the shrinkage of concrete, the prestress in the tendon is reduced with time.Prestressed concreteconcrete with stresses induced in it before use so as to counteract stresses that will be produced by load; often contains stretched steel bars or wires called tendons
37Concrete Shrinkage Due to water loss to atmosphere (volume loss). Plastic shrinkage occurs while concrete is still “wet” (hot day, flat work, etc.)Drying shrinkage occurs after concrete has setMost shrinkage occurs in first few months (~80% within one year).Reinforcement restrains the development of shrinkage
38Concrete Shrinkage As concrete harden there is reduction in volume It caused a shrinkage- Absorption of the water by the concrete and the aggregate- Evaporation of the water which rises to concrete surface.This contraction can lead to cracks or breaks in the surface of the concrete, or in tiles and other floor finishes installed over the slab.To minimize cracks associated with concrete shrinkage, builders place control joints at specific intervals along the concrete.
39Concrete Shrinkage A wet mix makes pouring easier but it also tends to encourage shrinkage.The more shrinkage the higher chancesof finding cracks later.
40Restrained shrinkage cracking Parallel cracking perpendicularto the direction of shrinkage4040
41Concrete Creep Creep of concrete is defined as the increase in deformation with time under constant load.Due to the creep of concrete, the prestress in thetendon is reduced with time.Basically, long term pressure or stress on concretecan make it change shape.This deformation usually occurs in the direction the force is being applied.
42Concrete CreepLike a concrete column getting more compressed, or a beam bending.Creep does not necessarily cause concrete to fail or break apart.Creep is factored in when concrete structures are designed.creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep is a "time-dependent" deformation.