The Importance of Cement Content for Structural Strength of Concrete
Cement content is important to allow engineers to gauge the structural stability of any building or concrete structure where it is used. High cement content on the other hand can cause concrete to suffer from excessive shrinkage and thermal cracking, which can cause the concrete to spall and thus be dangerous for a structure.
Cement is a material that is manufactured as per strict quality controls and thus has strengths that are known. Structural engineers use this known factor to design concrete mixes that have a specified cement content that allows the concrete to gain the strength that is desired for the structure where it is to be used. While the quality of the other aggregates also play a part in the composition, and therefore strength of concrete, this has more to do with their capacity to form compact concrete that will give the necessary strengths. The other aspects of concrete that determine the strength of the concrete are the water content, the placing of the concrete, compaction, presence of honeycombs, and air content. Designers lay down standards for all these parameters when they design a concrete structure.
Image source: Wikimedia: compression test
Is Cement Content Testing Relevant?
Testing of concrete for cement content is done by crushing a sample of the concrete obtained from the concrete through core sampling. The concrete is then crushed and the resulting fine powder tested chemically. This determines the insoluble residue, lime content, and soluble silica. This is then compared with control samples of the cement or from assumptions made after ascertaining the source of the cement used in the concrete. Such test will give an accuracy that may be plus or minus fifteen percent of the actual cement content.
Determining cement content of concrete that is already part of a structure becomes relevant when failures have occurred and it is necessary to find the causes of a failure. Otherwise other methods for testing of concrete are more important.
Other Tests for Concrete
Testing of cement content is more of an academic exercise, as other tests to determine the strength of the concrete are of more relevance to engineers. These tests are done while concreting operations are in progress and can include slump tests and compression tests for samples taken during the placing of concrete. These tests are conducted on cubes made from the concrete that is being placed and the tests are conducted at intervals of 3, 7 and 28 days for which laboratory standards have been set. It is also possible to hasten these tests by steam curing of the samples so that earlier results are obtained.
Tests are also possible to determine concrete strengths, which are linked to cement content, and are conducted subsequent to the completion of the structure. Core samples are taken and tested for compressive strength. There are also nondestructive tests available to test concrete strength with the use of ultrasound. Engineers also resort to actual load tests of a structure, if indicative compressive strengths of samples taken during concreting show values lower than desired.
Other tests for concrete can include air content tests to determine how the concrete would fare in frozen conditions. Testing concrete for moisture assumes importance where additional membranes or flooring is to be installed above a concrete slab. Excess moisture in a slab can cause blisters, bubbles, or delamination, and such moisture content is tested using a number of chemicals or the use of moisture meters.
Carbonation caused by carbon in the air reduces the otherwise high value of pH in concrete. This causes it to fall from a pH of 13 to about 8.5. These values assume importance before using any adhesives that may be required for installed flooring, as pH above 9 affects adhesives and their effectiveness. There are various methods for concrete pH tests mainly based on chemicals. A concrete surface pH test can be performed by spraying of phenolphthalein on a cut surface of concrete. The color changes if pH values are in excess of 9.
Image source: Wikimedia: Slump test