Cement Americas

WIN 2018

Cement Americas provides comprehensive coverage of the North and South American cement markets from raw material extraction to delivery and tranportation to end user.

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20 CEMENT AMERICAS • Winter 2018 • www.cementamericas.com FEATURE C ement performs a vital role in the construction industry, but as a manufactured material created by a complex process involving multiple ingredients, testing is essential to ensure compliance with specification and application specific requirements. Here are the most important tests and an update on relevant standards. Compressive Strength Compressive strength is one of the most important prop- erties of concrete and mortar. The strength of the bind- er (cement) therefore has a significant effect on the per- formance characteristics of the mixture and ensures the overall quality of the finished product. The test for compres- sive strength is gen- erally carried out by crushing cubes of hard- e n e d c e m e n t - s a n d mortar in a compres- sion machine. The com- pressive strength of the cubes is determined by the highest stress applied to a cube speci- men that causes it to fracture. The test equipment required for this purpose includes a compression machine, a mortar mixer, suitable molds, a humidity cabinet, the cement itself and test sand. The applicable standards for Compressive Strength are: EN 196-1, ISO 679, EN 459-2, EN 1015-11, EN 13454-2, ASTM C109/C109M, ISO 7500-1 and ASTM E4. Tensile Strength The tensile strength of cement is relatively low when com- pared to its compressive strength. Concrete structures are vulnerable to tensile cracking due to a variety of effects including dynamic loading and temperature variation. Tensile strength tests can be carried out to gain an under- standing of the cohesion between the cement particles. The tensile strength is proportional to the compressive strength and the tests are simple and inexpensive to per- form, so they are relatively popular. The most common ten- sile strength test on cement is conducted by the application of uniaxial tension. This test is carried out by molding cement-sand mortar bri- quettes, which are placed in a machine that can apply a ten- sion load. The tensile strength is calculated by measuring the load required to split the sample in half over the section of the fracture. When a uniaxial tension machine is not available, tensile tests can also be carried out by other indirect methods. For example, in the split cylinder test, a sample in the shape of a cylinder is loaded laterally until fracture occurs. An alter- native method is the flexure test in which a mortar beam is loaded between two supports to apply a bending stress; which causes the fibers in the lower half section of the beam to develop tension stresses, until failure occurs. It should be noted that these tests give a higher tensile strength value than the uniaxial tension method. Typical laboratory equipment for tensile strength testing of cement would include a 10kN flexural/tensile testing machine, briquette molds, a mortar mixer, a humidity cab- inet, the cement itself and test sand. Applicable standards include ASTM C307 and EN 196-1. Fineness The final stage of cement production involves grinding to form a fine powder containing particles of significantly different sized particles. The particle size distribution has a major influence on the rate at which a cement sets and gains strength, and can affect other factors such as work- ability and drying shrinkage. The smaller the particle, the larger the surface area of the powder in relation to the volume, so as cement reacts with water, the smallest particles contain the largest number of contacts and have a high participation in the process of set- ting and hardening. The fineness of cement is tested by measuring the air-permeabil- ity-specific surface area of the cement powder. The measure- ment of fineness of hydraulic cement is undertaken with the Blaine air-permeability appa- ratus to determine the specific surface expressed as the total surface area in square centime- ters per gram (or square meters per kilogram). The test is carried out on a com- Passing the Test Testing Cement for Quality and Reliability. By Alfonso Rivera Flow table. Mortar mixer.

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