The determination of air content of freshly made concrete is detailed in EN 123450 and ASTM C236, where the importance of two main applications is highlighted. The primary purpose of entraining air in concrete is to give the required resistance to weathering. The use of chemical additives to increase the workability of concrete often requires an air content check to be made.
When conducting a compressive strength test on a concrete cylinder it is important that the ends of the specimen are flat and parallel to each other. The trowelled face of a prepared concrete cylinder, or both ends of a concrete core, will require treatment to obtain these conditions.
The strength, durability and finish of concrete rely in part on the adequate compaction of the mix. An increasing number of contract specifications call for various forms of vibro-compacted concrete as a means to achieve a better and more consistent mixture. It should however be remembered that fluid mixes may segregate when vibrated in which case it may be more appropriate to compact using a tamping bar or rod during laboratory mix design.
The correct environment for curing concrete test specimens is important to achieve consistent and reproducible test results. Two primary factors must be taken into consideration to satisfy the requirements, namely to maintain a stable temperature and to prevent loss of moisture from the specimen. A standard curing temperature of 20ºC is usually specified and should be maintained at the required degree of accuracy. The use of water to prevent loss of moisture is the method most commonly used. In tropical climates a curing temperature of 25ºC is often acceptable.
The density of both fresh and hardened concrete is of interest to the engineer for numerous reasons including its effect on durability, strength and resistance to permeability. Hardened concrete density is determined either by simple dimensional checks, followed by weighing and calculation, or by weight in air/water buoyancy methods.
The apparatus has been designed and manufactured to the recommendations laid down in BS, EN and ASTM standards where tests are required on laboratory specimens, or on specimens taken from existing structures. The test procedure specifies a method for determining the change in length of a concrete or mortar sample brought about by a change in moisture content.
1. Initial drying shrinkage: the difference between the length of the moulded and cured specimen (under specified conditions), and its final (constant) length when dried
2. Drying shrinkage: the difference between the length of a matured specimen cut from concrete and saturated, and its final (constant) length when dried
3. Moisture movement: the difference between the constant length of a speciman whend dried, and its length when subsequently saturated with water.
The efficient mixing of concrete prior to moulding specimens in the laboratory for subsequent testing is essential if quality specimens are to be manufactured. The object of mixing is to coat the surface of all aggregate particles with cement paste, and bring the mix to a uniform condition. Pan or rotating drum mixers are suitable for the mixing of small quantities of concrete, which are generally used in a laboratory.
Test procedures require that specimens are cast in a number of standard sizes convenient for compressive and flexural strength determination. The engineering tolerances specified for moulds are very stringent and the internal finish of the surface must be of a high order to comply with the recommendations laid down in many International Standards. Moulds must not deform during manufacture of concrete specimens if the specimen dimensions are to be maintained.
The correct sampling and mixing of fresh concrete is important if test results are to be reliable. Most of the equipment necessary for efficient sampling and mixing is standard laboratory equipment detailed in the Laboratory Equipment section.
To ensure that concrete achieves its maximum possible strength and yet retains its ease of placing on site, it is essential that the design of the concrete mix, in relation to the water-cement ratio and workability, is closely controlled.