HARDENED CONCRETE

1. HARDENED CONCRETE DURABILITY: Ability to resist weathering action, chemical attack, abration, or any other process of deterioration. Durable concrete will retain its original form, quality, & serviceability when exposed to environment.

2. Significance of durability • When designing a concrete structure, exposure condition at which concrete is supposed to withstand is to be assessed in the beginning with good judgment. • The soil characteristics with respect to sulphate & chloride content in soil & ground water are also required to be investigated. • Environmental pollution is increasing day by day in urban & industrial atmospheres. • In industrially developed countries over 40% of total resources of the industries are spent on repairs & maintenance. • Every govt departments & municipal bodies have their own “repair boards” to deal with repair of buildings. • But , still we do not give enough attention to durability aspects even when we carry out repairs.

3. PERMEABILITY • It is the ease with which liquids can travel through concrete. • “K” property is of interest in relation to the water tightness of liquid retaining structures & to chemical attack. • “K” of concrete can be measured by means of a laboratory test. • The sides of a concrete specimen are sealed & water under pressure is applied to the top surface. • When steady state condition have been reached (10days) the quantity of water flowing through a given thickness of concrete in a given time is measured.

4. Conti.. • The water permeability is expressed as a coefficient of permeability, k, given by Darcy's equation. • 1/A *dq/dt=k(Δh/L). Where, dq/dt=rate of flow of water. A= C/S area of sample. Δh= drop of hydraulic head through the sample. L = Thickness of sample. k= m/sec.

5. Sulphate attack • Concrete attacked by sulphate has a characteristic whitish appearance, damage usually starting at the edges & corners & followed by cracking of the concrete. • Reason is, the essence of sulphate attack is the formation of calcium sulphate & calcium sulphoaluminate , both products occupying a greater volume so that expansion & disruption of hardened concrete take place. • Sulphate attack depends on its concentration & on the “k” of concrete.

6. Conti.. • If concrete is very permeable, water can percolate right through its thickness, Ca(OH)2 will be leached out . • Evaporation at the ‘far’ surface of the concrete leaves behind deposits of calcium carbonate, formed by the reaction of Ca(OH)2 with carbon dioxide. • This deposit of whitish appearance , is known as efflorescence. • Efflorescence is generally not harmful, however , extensive leaching of Ca(OH)2 will increase porosity so that concrete becomes progressively weaker & more prone to chemical attack. • Crystallization of other salts also causes efflorescence.

7. conti.. • Salts attack concrete only when present in solution, & not in solid form. • The strength of the solution is expressed as concentration (ppm).

8. Chloride attack • It is one the most imp aspects w r t. durability of concrete. • It is particularly imp because it causes corrosion of reinforcement. • Around 40% of failure of structures is due to corrosion of reinforcement.

9. Conti.. • Due to the high alkalinity of concrete a protective oxide film is present on the surface of steel reinforcement. • This protective layer can be lost due to carbonation and also due to the presence of chloride in the vicinity of water and oxygen. • One may recall that sulphates attack concrete, whereas the chloride attacks reinforcing steel. • Chloride enters the concrete from cement,water,aggregets and sometimes from admixtures.

10. Conti.. • The amount of chloride required for initiating corrosion is partly dependent on the ph value of the pore water in concrete. • As per is-456-2000 for RCC and PCC embedded metal maximum total acid soluble chloride content expressed as kg/cu.mt. of concrete is 0.6. • Thus chloride attacks the reinforcement rather than concrete. nothing happens to concrete in the beginning except the reduction in ph. • But the reduction in the ph will cause the corrosion of steel which will create durability problem. • chloride attack comes under type B of deterioration mechanism.

12. CARBONATION • WHAT IS CARBONATION??? “ It’s a reaction between the lime in concrete and the carbon dioxide from air, yielding Calcium Carbonate.”

15. Carbonation of concrete • It is a process by which co2 from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates in presence of water. CH + co2----------------------------------- caco3 + water • This chemical reaction removes CH which was giving high alkalinity to concrete. • This brings down the ph of the concrete which will accelerate the corrosion of reinforcing steel in concrete

16. Conti.. • The extent of carbonation depends on the permeability of the concrete and on the concentration of carbon dioxide in the air. • The penetration of carbon dioxide beyond the exposed surface of concrete is extremely slow.

18. Factors contributing for cracks in concrete • Plastic Shrinkage Cracks. • Settlement cracks. • Bleeding. • Delayed Curing. • Constructional effects. • Early Frost Damage. • Unsound Materials. • Shrinkage. • Drying Shrinkage. • Thermal Shrinkage.

19. Plastic Settlement Cracks • plastic concrete when vibrated or otherwise settles. if the concrete is free to settle uniformly, then there is no cracks. • If there is any obstruction to uniform settlement by way of reinforcement or large piece of aggregates, then it creates some voids or cracks. • This is called settlement cracks. this generally happens in a deep beam.

20. conti.. • concrete should be poured in layers and each layer should be properly compacted, building up of large quantity of concrete over a beam should be avoided. • The settlement cracks and voids are so severe it needs grouting operators to seal them off. Re-vibration, if possible is an effective step. • Otherwise, they effect the structural integrity of the beam or any other member and badly affects the durability.

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22. Plastic Shrinkage Cracks • Water from fresh concrete can be lost by evaporation, absorption by sub grade, formwork and in hydration process. • When the loss of water from surface of concrete is faster than the migration of water from interior to the surface, the surface dries up. • This creates moisture gradient which results in surface cracking while concrete is still in plastic condition.

23. CONTI.. • The magnitude of plastic shrinkage and plastic shrinkage cracks are depending upon ambient temperature, relative humidity and wind velocity. • In other words, it depends upon the rate of evaporation of water from the surface of concrete.

25. MEASURES TO REDUCE OR ELIMINATE PLASTIC SHRINKAGE CRACKS • Moisten the sub grade and formwork, • Erect temporary wind breakers to reduce the wind velocity over concrete. • Erect temporary roof to protect green concrete from hot sun. • Reduce the time between placing and finishing. if there is delay cover the concrete with polythene sheets.

26. CONTI… • Minimize evaporation by covering concrete with burlap, fog spray and curing compound. • Plastic shrinkage cracks are very common in hot weather conditions in pavements, floor and roof slabs.

27. JOINTS Types of joints: • Construction joints. • Expansion joints. • Contraction joints. • Isolation joints.

28. Construction joints • Construction joints are temporary joints left behind subsequent concreting operations. • Position of Construction joints should be pre-planned before concreting is started. • The joints must be made at places that the concrete is less vulnerable to maximum bending moment & maximum shear force. • In walls & columns joints should be horizontal & arranged at a level to coincide with the general architectural features.

29. Conti.. • In columns, the concrete should be filled to the level, preferably, a few inches below the junction of beams. • Joints in beams & slab should be formed at the point of minimum shear. & also not desirable at the point of maximum bending moment. • So joints may be made at the extreme position of the middle third.

32. TESTS ON HARDENED CONCRETE • Compressive strength. • Split tensile strength. • Flexural strength. • Non-destructive testing of concrete.

33. Non destructive testing of concrete(NDT) • Power full method of evaluating existing concrete structure with regard to their strength & durability. • The investigation of crack depth, micro cracks, & progressive deterioration of concrete. • Though NDT testing methods are relatively simple to perform , the analysis & interpretation of the test results are not so easy. • Special knowledge is required to analyses the hardened properties of concrete in NDT method.

34. Various NDT testing methods • Schmidt’s rebound method. • Rebound number & strength of concrete. • Penetration techniques. • Pullout test. • Dynamic or vibration methods. • Resonant frequency method. • Usefulness of resonant frequency method. • Pulse velocity method. • Combined methods. • Radio active methods. • Nuclear methods. Etc….

35. Schmidt’s rebound method.

37. limitations - Rebound hammer provides a quick inexpensive means of checking uniformity of concrete. • It has serious limitations. • The results are affected by: • Smoothness of surface under test. • Size, shape & internal moisture condition of the concrete. • Age of specimen. • Surface & internal moisture condition of the concrete. • Type of coarse aggregate.

38. Conti.. -Type of cement. -Type of mould. -Carbonation of concrete surface.