Concrete, so commonly accepted in our buildings, bridges, highways and an infinite variety of other structures, is taken for granted as massive and weighty. Not necessarily so! A broad spectrum of lightweight concretes is being used today. Initially there were natural lightweight aggregates that, though durable enough, had little relative strength. Much later, aggregates processed by man were developed that were not only light in weight but also provided the strength missing from those early mixes. Because of these developments we have excellent lightweight aggregate concrete today that provides both durability and structural integrity.

EARLY ROMAN LIGHTWEIGHT CONCRETE

Volcanic aggregates of basalt, scoria, pumice and tuff were used in concrete by Roman engineers and designers. Their awareness of the differences between heavy basaltic aggregates and extremely light pumice and tuff made possible the construction of structures which were quite sophisticated. A famous example of this expertise is the Pantheon in Rome.

PORTLAND CEMENT: A TEMPORARY SETBACK FOR LIGHTWEIGHT AGGREGATES

In 1824, the English bricklayer Joseph Aspdin concocted and patented a new cement that contained pulverized raw limestone and pulverized impure siliceous materials in varying percentages. The cement was called portland cement. When the new, stronger portland cement was used, concretes made with natural lightweight aggregate were no longer as strong as those made with heavy rock and sand aggregates.

MANUFACTURED LIGHTWEIGHT AGGREGATES BRING NEW STRENGTH

For centuries, brickmakers had problems with some brick bloating abnormally during the burning process because the shale expanded under high heat. Based on his considerable construction experience, Stephen J. Hayde thought this unusable, bloated lightweight material might make a good lightweight aggregate for concrete. Hayde was granted a patent for his method of producing lightweight aggregate in 1918.

Processed aggregates today include: rotary kiln expanded shales, clays and slates; sintered shales, clays and slates; cinders; pelletized or extruded fly ash; and expanded slag.