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Quality control of lightweight concrete
To get good normal weight concrete, an engineer writes a good specification and sees that concrete quality is assured by proper control procedures at the job. With lightweight concrete, the engineer specifies a C330* aggregate and the 28- day strength and air-dry weight necessary to meet design requirements. Slump and air content should also be specified.
The combination of strength and unit weight will, in most cases, eliminate undesirable or unsatisfactory materials. For example, suppose a lightweight aggregate has difficulty in achieving good strength. It will require an excess of cement to meet specifications, and this will boost both the unit weight and the cost. It will lose out on two counts. An engineer today can obtain reliable test data from aggregate producers on their material showing shrinkage values, modulus of elasticity, strength vs. cement content, and other properties.
More and more companies that provide aggregate for structural concrete have pushout test results on their material and will be able to provide an engineer this additional information. With such data, the specification can be closed to one type of aggregate or even to a given brand, taking into account all of the local conditions and the job requirements.
Quality control of lightweight concrete is achieved by:
(1) Periodic slump measurements will control the amount of water being mixed with concrete and, since lightweight concrete is proportioned with a given cement content and mixed to a given slump, this will in effect control the net effective water-cement ratio and all subsequent concrete properties.
(2) Fresh unit weight of the concrete, another simple check, is measured in half or quarter cubic foot containers. This weight should conform to the fresh unit weight determined from trial mixes and it is related to the 28-day air-dry weight, which is used as the basis for design. When the weight and slump are satisfactory, the mix and the yield are reasonably correct.
(3) If the weight changes, the usual cause is a change in air content (entrained air is generally used in lightweight concrete to improve its workability and handling characteristics). Then the third control test is run, namely, an air content test using the volumetric method. If the percent of air is incorrect, an adjustment is made at the plant to get the air content back into line.
(4) If the air content is satisfactory, further checks must then be made on gradation and specific gravity of the aggregate and possibly on the batching and handling procedures.
Generally, with attention to the basic principles of concrete mix design, good quality lightweight structural concrete is furnished to the field without difficulty. With increasing frequency, compressive strength evaluations of lightweight concrete have shown coefficients of variation under ten percent, rated excellent for job-furnished concrete.
*ASTM designation C330 defines lightweight aggregates for structural concrete in a number of ways: it names most available materials; it lists maximum permissible unit weights of coarse and fine fractions; it describes limiting aggregate tests; and it specifies tests for concrete-making ability.
