This Article Covers What Is “Compressive Strength Of Concrete” And Factors Affecting On Strength Of Concrete.
What Is Compressive Strength Of Concrete
The compressive strength of Concrete defined as the resistance to failure under the action of compressive forces. Especially for concrete, compressive strength is an important parameter to determine the performance of the Concrete/Materials during service conditions.
- Strength of concrete is its most valuable property that determines the amount of resistance it can offer against the applied loading.
- Further, it indirectly gives indication about other important characteristics, such as, durability, impermeability and stability against volume changes.
- Strength of concrete is more related to physical structure of the hardened concrete compared with the actual chemical composition.
- It basically depends upon factors that are difficult to study directly including presence of flaws, discontinuities and pores.
- To control the quality of concrete, there is a need to concentrate on aspects that may reduce the flaws, discontinuities and pores.
- For better strength of concrete, a minimum amount of cement required to coat the aggregate particles and bind them together.
- The concept that greater amount of cement increases the strength directly is not always correct.
- Greater cement increases workability and compaction and reduces W/C ratio, which increase the strength.
The strength of concrete mainly depends on two factors:
- Degree of compaction
- W/C ratio
- If more compaction achieved at a lower W/C ratio, using lesser content of cement with addition of plasticizers etc, the strength of concrete may be higher.
- In general, compressive strength of concrete measured by performing tests on concrete cylinders or cubes.
- The strength against other actions, like tensile and shear strengths, determined indirectly.
- However, even the compressive strength also depends upon the tensile strength.
- A compression test specimen fails by crushing that may also considered as the splitting in the perpendicular direction due to Poisson’s effect and slenderness effect.
- The concrete strength indeed controlled by limiting strain rather than a maximum stress level.
- In pure tension, concrete fails at a strain value within the range 0.0001 to 0.0002.
- The compressive strain causing failure is approximately 0.002 for 70 Mpa concrete to 0.004 for 14 Mpa concrete.
Mechanism Of Concrete Failure In Compression
Very fine cracks present in hardened concrete at the surface of coarse aggregate before the application of load called bond cracks.
These cracks appear due to differential volume changes between the aggregate and the cement paste.
The differential volume changes caused by different stress-strain behavior, coefficient of thermal expansion and moisture movement patterns.
The bond cracks do not open up to approximately 30 per cent of the ultimate strength.
At a stress level of approximately 30 to 70 per cent of the ultimate strength, bond cracks grow in number, length and width.
At this stage, the stress-strain curve becomes somewhat nonlinear due to development of more strain.
In this range of applied stress, the cracks not usually interconnected with each other and termed micro-cracks.
At stress levels of 70 to 90 per cent, cracks through the mortar appear that connect the bond cracks to form longer interconnected cracks.
Factors Affecting Strength Of Concrete
- The reduction in concrete strength with the amount of voids given in below table,
- This is the main factor that influences strength, but by itself this mainly controlled by the W/C ratio of the mix.
- Pores produced in the hardened concrete due to two main reasons,
- The first reason is the presence of entrapped air in concrete due to compaction less than 100 per cent (air voids are produced).
- The second reason is the evaporation of water from the hardened concrete (may called water pores).
|Voids Percentage||Strength Reduced|
- The water added to a concrete mix remains as combined water, gel water and capillary water in the hardened moist cured concretes.
- The combined water is the water that chemically combined or permanently held within crystals as water of hydration.
- This water does not evaporate upon drying up to 105° C, and amounts to approximately 23 per cent of the mass of dry cement.
- The gel pores are very small pores of about 2×10-6 mm diameter formed by the crystals of hydrates containing gel water.
- The gel water is approximately 28 per cent of the cement gel volume.
- Capillary pores relatively larger pores, approximately 0.001mm in diameter, formed within the gel due to the fact that the hydrate-crystals occupy less volume than the original unhydrated constituents (water and cement).
- The volume of these pores approximately 18.5 per cent of the dry volume of cement provided that full hydration takes place and there no excess water added.
- Capillary pores may filled with capillary water during moist curing.
- At a water/cement ratio of 0.6, all types of pores for fully compacted concrete may occupy 47 to 60 per cent volume of the cement paste.
- Strength of concrete not only depends upon the total volume of pores but also upon the other features such as whether these pores interconnected or they are separate.
- As already stated, the exact effect of porosity of cement gel indirectly determined based on the W/C ratio.
Degree Of Compaction
- Compaction is related with the removal of air voids from the concrete, whereas, porosity deals with the presence of both air and water pores.
- If full compaction not carried out, air-voids left in concrete. These large voids significantly reduce the strength as discussed in the topic of workability.
- After the degree of compaction, W/C ratio is the single largest factor affecting the strength.
- Larger W/C ratio from a basic value required for full hydration, reduces the strength drastically by increasing the porosity of the cement gel.
- Effective water-cement ratio to used for the comparison, which based on water added for saturated surface dry aggregate.
Age Of Concrete
- The strength of concrete increases with age and this effect caused by more hydration of cement and reduction of capillary pores.
- The effect is more pronounced at early ages and becomes slow at higher ages.
- After an age of one year, the effect is insignificant.
Improper Curing And Temperature Changes
- Improper curing and larger temperature changes produce shrinkage and thermal cracks.
- These cracks further reduce the strength of concrete.
- For a constant W/C ratio and full compaction, a leaner mix with larger aggregate/cement ratio develops a higher strength.
- This effect produced due to lesser porosity of the resulting concrete.
- The factors like grading, surface texture, shape, strength and maximum size of aggregate also influence the concrete strength.
- For example, the bond cracks may easily develop and propagate for smooth gravel than for rough and angular crushed aggregate.
- Hence, crushed aggregate may develop larger strengths. For high strength concretes, failure planes may pass through the larger size aggregate particles and their individual strength also becomes significantly important.
Effect Of Additives And Admixtures
- The presence of various additives and admixtures may influence the strength of concrete.
- Usually strengths at ages lesser than 28 days affected the most.
- However, the final strengths may also vary with the type of cement and presence of any additional chemical in it.
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