3 Structural Lightweight Concrete Myths Examined

Brief 

Lightweight concrete can be a useful solution for weight and durability challenges, but despite a long history several misconceptions about the material endure. Ken Harmon examines three prevailing myths, beginning with the subject of water absorption and how it affects durability.

Insight

Lightweight concrete has been a staple of the built environment for centuries, predating the Roman Empire. While some early lightweight concrete structures still stand, their materials bear little resemblance to today’s offerings.

Industry professionals now recognize structural lightweight concrete as a strong, low-density mixture of Portland cement, water, and various combinations of normal weight aggregates and lightweight aggregates.

In such concrete, lightweight aggregate produced from shale, clay, or slate is heated to approximately 2000 degrees F in a rotary kiln process. During this process, it softens and bubbles form that remain as unconnected pores when it cools.

Since the early 1920s, industry professionals have effectively employed structural lightweight concrete to solve weight and durability problems.

Yet misconceptions about the material’s light weight and porous form and how they impact end-use performance in composite metal decks for floors and roof slabs still exist. This article reviews three relevant misconceptions to help set the record straight.

Myth #1

The water absorption of lightweight aggregate reduces the strength and durability of structural concrete.
A pervasive industry belief is that greater water absorption by aggregates reduces the performance properties of structural lightweight concrete.

However, data from a study by Byard and Schindler at Auburn University (2010) indicate that the aggregate’s raw material has little bearing on performance.The study compared the performance of structural lightweight concrete using three common mixture ratios:

• An internally cured mixture for which a fraction of the conventional fine aggregate (sand) was replaced with prewetted lightweight fine aggregate;
• A commonly used sand lightweight concrete mixture, for which lightweight coarse aggregate and conventional fine aggregate (sand) were used; and
• An all-lightweight concrete mixture that uses lightweight coarse aggregate and lightweight fine aggregate (no normal weight aggregate).