Hardened concrete

Concrete Technology

1. Government Engineering College, Bhavnagar . Civil Engineering Department

2. Concrete Technology Topic:- Hardened Concrete

3. Contents Overview of Concrete Hardened Concrete Strength of Concrete Standard Test methods Stress Strain Behaviour of concrete Creep and Shrinkage

4. Overview of Concrete Concrete is one of the most commonly used building materials. Concrete is a composite material made from several readily available constituents (aggregates, sand, cement, water). Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape. Constituents:- Cement Water Fine Aggregates. Coarse Aggregates. Admixtures

5. Concrete Advantages Disadvantages Ability to be cast Economical Durable Fire resistant Energy efficient On-site fabrication Low tensile strength Low ductility Volume instability Low strength to weight ratio

6. PROPERTIES OF HARDENED CONCRETE The principal properties of hardened concrete which are of practical importance can be listed as: Strength Shrinkage & creep deformations Permeability & durability Response to temperature variations Of these compressive strength is the most important property of concrete.

7. PROPERTIES OF HARDENED CONCRETE Of the above mentioned hardened properties compressive strength is one of the most important property that is often required, simply because; Concrete is used for compressive loads Compressive strength is easily obtained It is a good measure of all the other properties.

8. What Affects Concrete Strength ?

9. STRENGTH OF CONCRETE The strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on: w/c ratio Degree of compaction

10. COMPRESSIVE STRENGTH Compressive Strength is determined by loading properly prepared and cured cubic, cylindrical or prismatic specimens under compression.

11. COMPRESSIVE STRENGTH Cylinder : ASTM C470 Cubes : British standard 150x150x150 mm 3 Other sizes: Cylinder: 100 × 200 or 150 × 300 mm Cubes: 100 × 100 × 100 mm 3 or

12. For 150 mm cubes fill in 3 layers compact each layer 35 times. For 100 mm cubes fill in 3 layers compact each layer 25 times. No need for capping.

13. For 150 x 300 mm cylinder, fill in 3 layers compact each layer 25 times. Capping to obtain a plane and smooth surface (thin layer ≈ 3mm), using: Stiff Portland cement paste on freshly cast concrete, or mixture of sulphur and granular material, or high-strength gypsum plaster on hardened concrete.

14. TENSILE STRENGTH Tensile Strength can be obtained either by direct methods or indirect methods. Direct methods suffer from a number of difficulties related to holding the specimen properly in the testing machine without introducing stress concentration and to the application of load without eccentricity.

15. SPLIT TENSILE STRENGTH Due to applied compression load a fairly uniform tensile stress is induced over nearly 2/3 of the diameter of the cylinder perpendicular to the direction of load application.

16. The advantage of the splitting test over the direct tensile test is the same molds are used for compressive & tensile strength determination. The test is simple to perform and gives uniform results than other tension tests. σ st = 2P π Dl P: applied compressive load D: diameter of specimen l: length of specimen Splitting Tensile Strength

17. Factors Affecting the Strength of Concrete Factors depended on the test type : Factors independent of test type: Size of specimen Size of specimen in relation with size of aggregates Support condition o f specimen Moisture condition of specimen Type of loading adopted Rate of loading Type of test machine Type of cement Type of aggregates Degree of compaction Mix proportions Type of curing Type of stress situation W/C ratio Admixtures

18. STRESS-STRAIN RELATIONS IN CONCRETE σ - ε relationship for concrete is nonlinear. However, specially for cylindrical specimens with h/D=2, it can be assumed as linear upto 40-50% of σ ult σ ult (40-50%) σ ult ε ult

19. MODULUS OF ELASTICITY OF CONCRETE Due to the nonlinearity of the σ-ε diagram, E is the defined by: Initial Tangent Method Tangent Method Secant Method

20. Creep in Concrete

21. Consequences of creep Loss in pre-stress possibility of excessive deflection stressing of non load bearing members

22. Shrinkage Concrete shrinkage is the contracting of the concrete due to the water evaporating  from the mixture. This evaporation will cause the concrete to weaken. This can lead to cracks, internal warping and external deflection.

23. Types of Concrete Shrinkage There are numerous  types of concrete shrinkage  including plastic shrinkage, drying shrinkage , Autogenous shrinkage, and carbonation shrinkage . Plastic shrinkage happens soon after the concrete is poured in the forms. The water evaporates and results in a reduction of volume, this causes the concrete on the surface to collapse. It can be reduced by covering the surface with polyethylene sheeting immediately after it is poured. Plastic Shrinkage in Concrete

24. Types of Concrete Shrinkage Drying shrinkage  is the ever lasting process for concrete within drying conditions. The loss of water within the gel pores of the concrete is what causes the concrete to shrink. The  finer the gel  within the pores, the more shrinkage there is. Autogenous shrinkage  is most prevalent within the concrete in the interior of a dam. When the temperature is constant shrinkage may occur, especially when there is no moisture movement. Carbonation shrinkage  is where carbon dioxide penetrates beyond the surface of the concrete. This also depends on the moisture content and the humidity levels. Carbonation shrinkage is caused by the disbanding of calcium hydroxide crystals and the evidence of calcium carbonate.

25. Where Concrete Shrinkage Occurs Concrete shrinkage can occur in any poured concrete. It is most common in slabs, beams, bearing walls, foundations and columns. It can also be found in pre-stressed members as well as tanks. Shrinkage is a problem for any poured concrete, but when it happens in bearing walls and foundations the entire stability and integrity of the structure is in jeopardy .

26. Thank You For Bearing. By, Bhavik Shah – 130210106049, Kartik Hingol – 130210106030, Digvijay Solanki – 130210106055, Nitin Charel – 130210106011.