This is the first in a continuing series of articles in ESCSI Lightweight Design eNews. The article in each issue will address some common myth or misconception about lightweight aggregate or lightweight concrete.
But first, let us define what we are talking about when discussing lightweight aggregate and lightweight concrete. When discussing lightweight aggregate in the context of an ESCSI document, we are referring to expanded shale, clay, or slate (ESCS) that has been manufactured using a rotary kiln process at temperatures around 2000 deg. F. When the particles reach these high temperatures, the material softens and bubbles are formed which remain as pores when the aggregate cools. The pores within the vitrified ceramic aggregate particles give the lightweight aggregate its reduced density.
Myth #1 – The absorption of a lightweight aggregate, or the type of raw material from which it is made, is a primary factor that defines the physical properties of the aggregate and the strength, durability, and soundness of structural concrete in which the lightweight aggregate is used.
To address this myth, I will begin by presenting data from a study performed by Byard and Schindler at Auburn University (2010). The study compared the performance of lightweight concrete using from lightweight aggregate made from three sources: a shale, a clay, and a slate. These mixtures were compared to the performance of a normal weight concrete made with river gravel, a common aggregate source for conventional concrete mixtures for bridge decks in Alabama. Three types of lightweight aggregate mixtures were made using each lightweight aggregate type: an internally cured mixture for which a fraction of the conventional sand was replaced with prewetted lightweight aggregate; a sand lightweight concrete mixture, for which lightweight coarse aggregate and conventional fine aggregate (sand) were used, and which is the most commonly used type of lightweight concrete; and an all lightweight concrete mixture, for which all of the aggregate was lightweight and therefore gave the lowest possible density. To simplify the discussion of information in this article, only the sand lightweight concrete data are presented. It should be noted that all three types of lightweight aggregate met the requirements of AASHTO M 195 or ASTM C330 standard materials specifications.
The following table contains data for the control mixture and each of the sand lightweight concrete mixtures tested. The first two rows of the table (see shaded cells) include the factors identified as being defining characteristics for the performance of a lightweight aggregate in Myth #1: the type of the raw material from which the aggregate is manufactured and the absorption of the lightweight aggregate.
Table 1 – Properties of Lightweight Aggregate and Sand Lightweight Concrete Compared to Conventional Aggregate and Concrete (Byard and Schindler 2010)
From the data in the table, it is clear that the shale and clay lightweight aggregate have absorptions that are markedly higher than the lightweight aggregate made from slate. Those who believe Myth #1 would think that this would indicate that the materials with higher absorption would have reduced properties compared to the slate. However, data in the table do not support this assertion. The 28-day compressive and splitting tensile strength data for the shale and clay aggregates are not significantly different from the strengths of the slate aggregate; in fact, the splitting tensile strength for the slate aggregate is the lowest of the three mixtures. The thermal characteristics of the lightweight concrete from the three sources are also very similar. This data, therefore, supports the notion that for these mixtures, the lightweight concrete has fairly similar performance characteristics between the different types of raw materials and absorptions so the use of the absorption or raw material type alone would not be an effective criteria for selection of the aggregate for a project.
Lightweight concrete mixtures using lightweight aggregate from three sources (2 shales and a slate) were also tested by Greene and Graybeal (2013) to study the performance of lightweight concrete for prestressed concrete bridge girders. Test results on material properties for these mixtures indicate that the absorptions of the shales were 14.5% and 10.3% compared to 9.0% for the slate. Therefore, the difference in absorption for these lightweight aggregate sources was not as significant as in the Byard and Schindler study. While a statistical analysis of the data was not performed, test results for the mixtures using the shale lightweight aggregates did not appear to have significantly different compressive and tensile strengths and modulus of elasticity values compared to the mixture using slate lightweight aggregate.
CONCLUSION: Based on the results reported for these two test programs, Myth #1 is not true. The test results discussed above reveal that there is no systematic difference in material properties between mixtures made with different types of lightweight aggregate or aggregates with different absorptions.
AASHTO. 2015. Standard Specification for Lightweight Aggregates for Structural Concrete, M 195. AASHTO.
ASTM International. (2017). Standard Specification for Lightweight Aggregates for Structural Concrete. C330 / C330M-17a. ASTM International, West Conshohocken, PA. doi: https://doi.org/10.1520/C0330_C0330M-17A [AASHTO M 195]
Byard, B.E. and Schindler, A.K. (2010) Cracking tendency of lightweight concrete. Highway Research Center, Auburn, Ala.
Greene, G.G. and Graybeal, B.A. (2013), Lightweight concrete: Mechanical properties, FHWA-HRT-13-062, Federal Highway Administration, U.S. Department of Transportation, Washington, DC.