Question

what is compression test and why is it important when testing materials for civil engineers?

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Answer 1

The compressive strength of any material is 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 material during service conditions. Concrete mix  can be designed or proportioned to obtain the required engineering and durability properties as required by the design engineer. Some of the other engineering properties of hardened concrete includes elastic modulus, Tensile Strength, Creep coefficients, density, coefficient of thermal expansion etc.

PURPOSE OF THE TEST:
• The compressive strength of cubes gives us the information of the potential strength of the concrete mix from which it is sampled.

• It helps in determining whether correct mix proportions of various mix proportions of various materials were used to get the desired strength.

• It helps in determining the time of removal of formwork or the time of taking the concrete structure into service.

• It helps in determining the rate of gain of strength of concrete samples if cubes from the samples are crushed at different periods of time.

The most significant parameters are briefly discussed below:
3.1. CEMENT:
The chemical composition and fineness of cement can influence the aggregate relation of concrete quite significantly. The early strengths of cement are attributable to the higher content of Tricalcium Silicate than Dicalcium silicate content. Similarly, finer ground cement will give higher early strength than the coarser ground cement with similar chemical composition. Even though the early strengths may differ due to the above factors the ultimate strength at 28 days may not change significantly

• The variations in the results obtained at the site, from time to time for a particular grade of concrete can help in determining the quality control exercised and uniformity of concrete produced.
Grade of cement= (Average compression strength for a month)- (1.65 times the standard deviation
Of the same month.)
WATER:
The water to cement ratio (W/C) by weight plays a very important role in the concretes and durability. Lower the W/C, higher the strength and durability of concrete. Slight variations in W/C can cause considerable reduction in durability. The changes in strengths are more significant in the lower ranges of W/C as well as when higher grades of cement are used. It is therefore very important to accurately batch water such that it does not exceed more than 1%.

3.3. CEMENT STORAGE AND TRANSPORTATION:
Storage of cement 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 moisture or water.
The transportation of cement is equally important. Cement must arrive at site from the production unit in the shortest possible time with minimum numbers of handling. Delays during transportation and increase in numbers of handling can cause considerable drop in strength and other properties as cement may get exposed to moisture or water especially during monsoon.

The cement, which is not stored, transported and handled properly, will lose its strength and in turn, influence the strength of concrete in which it is used.

3.4.CEMENT PACKAGING:
Cement packed in bags using very porous material is more susceptible to loss of strength and hence packaging material of cement bags can greatly influence the strength. Preservation of cement strength 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 strength and durability of concrete.

3.5. AGGREGATES:
The following characteristic properties of aggregates influence concrete compressive strength.

• The size of aggregates- Larger the maximum aggregate size (MAS) lesser is the cement paste required and hence will need lesser cement and water paste for some compression strength and workability as compared to aggregates with smaller MAS.

• The shape of aggregates- Rounded aggregates have lesser surface area than crushed cubical aggregates of the same specific gravity and hence will need lesser cement and water paste for same compressive strength and workability.

• Grading of aggregates- 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 to coarser fine aggregate.

• Porosity- Porous aggregate may crush when compressive loads are applied before the failure in the mortar bond between the aggregates can occur. The compressive strength will lower if less dense or porous aggregates are used.

To conclude, all parameters remaining identical, concrete compression strength will be higher for concrete made using fine aggregates having lesser finer fines, aggregates having maximum aggregate size, aggregates having rounded shape and/or aggregates having less porosity/ having density.

3.6.CONCRETE WORKABILITY:
If all concrete mix parameters are identical and only water is added to increase the workability then the compression strength of mix with increased workability will be less than the compressive strength of the original mix.

3.7.CONCRETE TRANSPORTATION AND PLACEMENT:
Generally concrete cubes are taken at the batching/mixing plant. The concrete mix if not transported and/or placed properly it will segregate and/or lose its workability. Segregated concrete forms honeycombs and is porous having large void content. 5% voids means 30% loss of strength and 10% voids means 60% loss of strength. The concrete cube test result will be satisfactory but the concrete in the structure would have a much lower strength.

3.8.CONCRETE COMPACTION:
Concrete ingredients when mixed have 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 the 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 until such time entrapped air is less than 2%