The most common test hardened concrete is to test its compressive strength at a particular period of time, from the time of casting the concrete cubes. The concrete strengths are generally specified by compressive strengths and the structural design is worked out on that basis.
It is very important for the Engineers and Technicians to obtain accurate results of compressive strengths of concrete. Compressive strengths over a period of time also indicate the extent of quality control is exercised at the site.
The parameters which can affect the compression strength of concrete and the purpose of compressive strength of concrete test cubes, equipment required for such tests, the procedure and precautions for this test. It also discusses the different types of cube failures that can occur.
Although the grade of cement and quality as well as the water to cement ratio of the concrete mix play a very important role in the compression strength of concrete, other parameters can also influence the compressive strength to a great extent.
The most significant parameters are briefly discussed below.
Cement
The chemical composition and fineness of cement can influence the age-strength relation of concrete quite significantly. The early strengths of cement are generally attributable to the higher content of Tricalcium Silicate (C3S) than Dicalcium Silicate (C2S) content. In the same way, finer ground cement will give higher early strength than the coarser ground cement with similar chemical composition. Even though the early strengths may differ to the above factors the ultimate strength at 28 days may not change significantly.
Variations in compressive strengths of concrete can also occur to a considerable extent, if the supply of cement is received from different brands or if the supply of cement is received from different production units of the same brand. It is, therefore, very important to obtain cement at the site from one consistent source.
Diversifications in compressive strengths of concrete can also occur if there are considerable variations in the compressive strength of cement produced by the same production unit. It is, therefore, necessary to obtain the monthly standard deviation of the compressive strengths of cement and also ascertain the coefficient of variation from the cement manufacturer to co-relate the chances of concrete cube strength variation, due to variations of cement strength from time to time.
It is, therefore, incorrect to assume the cement grade from the test report or from the average monthly strength or from the grade printed on the cement bag but by using the formula given below.
Fck= Fm-1.65S
Where, Fck is Grade of Cement
Fm is the average 28 days cement strength for the month
S is Standard Deviation for the same month
Water
The water to cement (W/C) ratio by weight plays a very significant role in concrete strengths and durability. Lower the W/C, the higher is the strength and durability of concrete. Slight variations in W/C can cause a considerable reduction in strength and a very significant reduction in durability.
The changes in strengths are more significant in the lower ranges of water-cement ratio (W/C) as well as when higher grades of cement are used. It is, therefore, very vital to accurately batch water such that it does not exceed more than 1%.
Cement Storage and Transportation
Cement storage in a proper manner is of importance, as it is a hygroscopic chemical and can easily lose its strength and other essential properties if it comes in contact with water or moisture.
The transportation of cement is equally important. The cement must arrive at the site from the production unit in the shortest possible time with minimum numbers of handling.
Cement Packaging
Cement packed in bags using very porous material is more susceptible to loss of strength, and hence, the packaging material of cement bags can greatly influence the strength. Preservation of strength of cement for a longer duration under similar conditions of exposure is greatly dependent on the type of packaging material used. This in turn can greatly influence the durability and strength of concrete.
Aggregates
The following characteristic properties of aggregates influence the compressive strength of concrete.
- Size of the Aggregate: Larger the maximum aggregate size (MAS) lesser is the cement paste required and hence will need lesser cement and water paste for the same compression strength and workability as compared to aggregates with smaller MAS.
- The shape of Aggregate: Rounded aggregate has lesser surface area than crushed cubical aggregates of the same specific gravity and hence will need lesser cement and water paste for the same compressive strength and workability.
- Grading of Aggregates: The presence of finer fines in aggregates causes an increase in surface area. Therefore, excess finer fines in aggregates will increase the water demand and will, therefore; require more cement for the same compression strength as compared to coarser fine aggregate mixes.
- Porosity: Porous aggregate may crush when compressive loads are applied before the failure in the mortar bond between the aggregates can occur.
Concrete compressive strength will be higher for concrete made using fine aggregates having lesser finer fines, aggregates having the higher maximum aggregate size, aggregates having rounded shape, and/or aggregates having less porosity/high density.
Concrete Workability
If all concrete mix parameters are identical and only water is added to increase the workability then the compression strength of the mix with increased workability will be less than the compression strength of the original mix.
Concrete Transportation and Placement
Generally, concrete cubes are taken at the batching or mixing plant. The concrete mix if not transported and placed properly it will segregate and/or lose its workability. Segregated concrete forms honeycombs & is porous having large void content. 5% voids mean 30% loss of strength of concrete and 10% voids mean 60% loss of strength of concrete. The concrete cube test result will be satisfactory but the concrete which is cast in the structure would have a much lower strength.
Concrete Compaction
Concrete components when mixed have a considerable amount of entrapped air depending on the workability. High workable mixes have lesser entrapped air than low workable mixes. The entrapped air is driven out by compaction using vibration techniques. If compaction is not done properly, the entrapped air remains within the concrete mass, with the result that for every 1% entrapped air there will be around 5% to 6% drop in strength. It is, therefore, a must to compact concrete in the cube moulds as well as in the structure properly till the time entrapped air is less than 2%.
Curing of Concrete
If the curing of concrete is not proper, the water from the concrete surface will evaporate and adequate water for the hydration process will not be available for achieving the desired strength. Poorly cured concrete will have lower strength than adequately cured concrete of identical concrete mix proportions and materials.
It is, therefore, essential to cure both concrete test cubes and concrete structures equally well to achieve the desired strength. Concrete kept continuously moist for 28 days will develop full strength. Concrete cured for less than 28 days will not develop full strength. Concrete gains strength with age, however, if left in dry condition strength will not increase. It will only gain strength so long as water or moisture is available for the hydration process to continue. Change in ambient temperature or temperature at the time of curing significantly influences concrete strength, especially strength, especially when ambient temperatures are above 10oC. This influence is more and more significant at a higher temperature range. As per standard practice, concrete test cubes are cured at a temperature of 27oC ± 2oC.
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