Strength of Concrete

• Strength is the most valuable property in the concrete because it is directly related to the structure of cement paste.

• The most important factor in the strength of concrete is water/cement ratio. The other mix proportions are of secondary importance.

• Concrete strength:- Concrete is strong in compression and weak in tension-Compressive strength of concrete is the maximum

compressive load(stress) it can carry per unit area and it is measured in N/mm²or Mpa. It gives the general view of the concrete quality. When strength improves other properties (density, durability, impermeability. Volume stability etc.) improves as well.

- Tensile strength of concrete is usually considered about one-tenth of its compressive strength.

• Concrete cubes are taken from batches and tested for 7 days and 28 days.

• The seven-day tests are guide to the degree of hardening. The strength at this age for concrete should not be less than two-third of the strength required at twenty eight days.

• With ordinary Portland cement concrete, about 60% of the strength is reached at twenty-eight days, 70% in two months, and 95 % in six months.

• Influencing factors on strength of concrete:1. Water / cement ratio.(Abram’s Law)2. Degree of compaction: If full compaction is not

achieved, porous and honey-combed concrete will be produced resulting in loss of strength.

3. Age. With increase in age, the strength increases because degree of hydration increases.

4. Temperature: High temperature increases early strength of concrete but affect negatively on long term strength.

5. Aggregate/cement ratio: Increase in A/C ratio lead to higher strength( lower volume of voids)

6. Quality of the aggregate( grading, surface texture, shape, strength, size and cleanliness.)

7. Curing: Concrete strength increases with longer period of curing. Concrete will be fully developed only if its cured. Since the hydration of cement proceeds only in the presence of an adequate amount of water, moisture must be maintained in the concrete during the curing period.

8. Cement content: the increase in cement content and fineness of cement particles increases the concrete strength, whereas the strength decreases as the cement content is decreased.

9. Admixture: Admixtures effect concrete strength by modifying certain properties like rate of hydration or setting time and workability.

10.Steps of concrete preparation: Proper mixing, handling, transporting, placing.

- The time interval between mixing and placing the concrete should be reduced to the minimum possible.

11. Microcracking : : it is very fine cracks exist at the interface between coarse aggregate and hydrated cement paste.

• “Abrams’ Law”: Strength of concrete is inversely proportional to the water/cement ratio

• Sources of weakness in strength of concrete: - pores and voids - micro cracking -High water/cement ratio - improper compaction - Poor curing

• Strength depends on the effective water/cement ratio.

• Effective water/cement ratio is the mix water less the water absorbed by the aggregate.

• Total water in concrete mix consists of the water absorbed by the aggregate to bring it to a saturated surface –dry condition and the free water available for the hydration of the cement and for the workability of the fresh concrete.

• Aggregate/Cement ratio: High aggregate/cement ratio lead to higher strength. Why: Since the paste represents a smaller portion of the volume of concrete then the total porosity of the concrete is lower which means lower volume of voids, hence high concrete strength.

• Angular crushed aggregate has better bond and less microcracking than smooth aggregate.

• Transition zone: It is the interface between the aggregate and hydrated cement paste. It has higher porosity and therefore weaker than the hydrated paste further away from the aggregate.

• Durability :Durability of concrete is one which can withstand the conditions for which it has been designed. Durability is affected by external factors like environment, freezing and thawing, wetting and drying, abrasion , chemical attack and by internal factors like volume change, corrosion and alkali aggregate reaction.

• Permeability and porosity: - permeability is one of the main characteristics

influencing the durability and strength of concrete. It is the property of concrete which permits liquids to pass through it.

- Porosity is the property in which liquids can penetrate into it by capillary action. And it depends on the total volume of the spaces occupied by air or water between solid matter in hardened concrete.

- A higher permeability or porosity leads to deterioration of concrete.

• Causes of voids in concrete:1. High Water/cement ratio: excess water leaves

voids and cavities after evaporation.2. Poor compaction results in accumulation of

air voids.3. Porous aggregate.4. Poor curing increase permeability.

- In general the best method to make the concrete less permeable is: good mix design with appropriate water/cement ratio followed by careful placing, compacting, and curing.

- As per ACI recommendation: In order to have low permeability concrete, the structural concrete should have a w/c ratio of not more than 0.50 for exposure to fresh water and not more than 0.40 for exposure to sea water.

• Shrinkage: It is the contraction that occurs in concrete when it dries and hardens. The volume changes due to shrinkage results in cracks.

• Types of shrinkage:1. Plastic shrinkage: it takes place before

concrete has set and caused by water evaporation due to air , temperature, humidity and wind velocity.

2. Autogenous shrinkage: It is self produced shrinkage occurs due to hydration and aging within the concrete.

3. Drying shrinkage: It takes place after the concrete is set and hardened. It is directly proportional to the w/c ratio and inversely proportional to the aggregate /cement ratio.