The study compared the performance of lightweight concrete using lightweight aggregate made from three sources: a shale, a clay, and a slate. Three mixtures were made using each lightweight aggregate type:
- An internally cured mixture for which a fraction of the conventional fine grade aggregate (sand) was replaced with prewetted lightweight aggregate.
- A sand lightweight concrete mixture, for which lightweight coarse aggregate and conventional fine aggregate (sand) were used (this is the most commonly used type of lightweight concrete).
- An all-lightweight concrete mixture, for which all of the aggregate was lightweight and, therefore, gave the lowest possible density.
All three types of lightweight aggregate met the requirements of American Association of State Highway and Transportation Officials (AASHTO) M 195 or ASTM C330, Standard Specification for Lightweight Aggregates for Structural Concrete. However, to simplify the discussion, only the sand lightweight concrete data are presented in the table. The performance qualities of a normal-weight concrete made from river gravel acts as a control.
In looking at the results, the shale and clay lightweight aggregate evidently have higher absorption percentages than the lightweight aggregate made from slate yet display similar attributes. The answer to this question is no. The absorption capacity of a raw material does not alter the performance qualities of lightweight concrete as this water is used for internal curing and does not contribute to structural loads post curing. In fact, only minor differences exist between the 28-day compressive and splitting tensile strength data for the shale and clay aggregates and the slate aggregate. Further, the slate aggregate’s splitting tensile strength is the lowest of the three mixtures, despite its low absorption percentage.
Considering the minor differences between lightweight and normal weight concretes, it is interesting to note the absorption percentages do not seem to indicate a linear relationship to performance qualities. Given normal-weight concrete has an absorption capacity of one to two percent, it would indicate how it is the most similar to slate and least similar to shale, all things considered; yet this is not the case. The differences between the materials do not seem related to absorption capacity in a predictable way. Therefore, it can be reasonably concluded the absorption rate of the aggregate does not primarily drive structural lightweight concrete’s strength and durability.
Does it take longer for lightweight concrete to dry?
Lightweight aggregate has a higher absorption capacity than conventional aggregates. Its internal pore network absorbs and stores water before gradually releasing it over time. It is also prewetted prior to placement to maintain slump and allow pumping. For all concrete, water will evaporate until the slab achieves an equilibrium with ambient conditions. Therefore, lightweight concrete would take longer to dry because it absorbs more water.
