Long-Term Service Life of Structural Lightweight Concrete

Roman period : The first known use of lightweight concrete occurred more than 2000 years ago. There are several lightweight concrete structures in the Mediterranean region, but the three most notable structures were built during the early Roman Empire and include the Port of Cosa, the Pantheon Dome, and the Coliseum.

The Port of Cosa, built about 273 B.C., used lightweight concrete made from natural volcanic materials. These early builders learned that expanded aggregates were better suited for marine facilities than the locally available beach sand and gravel. They went 25 miles (40 km) to the northeast to quarry volcanic aggregates at the Volcine complex for use in the harbor at Cosa. Broken shards of calcined clay vases were also used in the piers…. the first usage of manufactured aggregate. This harbor is on the west coast of Italy and consists of a series of four piers (~ 13 ft [4 m] cubes) extending out into the sea. For two millennia they have withstood the forces of nature with only surface abrasion. They only became obsolete because of siltation of the harbor.

The Pantheon, built in A.D. 120-126, incorporates concrete of decreasing density from bottom to top of the dome. Roman engineers had sufficient confidence in lightweight concrete to build a dome whose diameter of 142 ft (43.3 m) was not exceeded for more than nineteen hundred years. The structure is in excellent condition and is still being used to this day for spiritual purposes.

The Coliseum, built in 75 to 80 A.D., is a gigantic amphitheater with a seating capacity of 50,000 spectators. The foundations were cast of lightweight concrete using crushed volcanic lava. The walls were made using porous, crushed-brick aggregate. The vaults and spaces between the walls were constructed using porous tufa cut stone. After the fall of the Roman Empire, lightweight concrete use was limited until the twentieth century when expanded shale, clay and slate lightweight aggregate became available for commercial use (ESCSI 1971).

Ships : While it is clearly understood that the terms high strength and high performance are not synonymous, we may consider the first modern use of high performance concrete to be when the American Emergency Fleet Corporation built lightweight concrete ships (1917-1920) in which specified compressive strengths of 5000 psi (35 MPa) were obtained with a unit weight of 110 lb/ft3 (1760 kg/m3) or less, using rotary kiln produced expanded shale and clay aggregate. Commercial normalweight concrete strengths of that time were approximately 2500 psi (17 MPa).

Oil Platforms : In energy-related floating offshore concrete structures, great efficiencies are achieved when a lower density material is used. A 25% reduction of mass in air will result in a 50 % reduction when submerged. Because of this, the oil and gas industry recognized that lightweight concrete could be used to good advantage in its floating structures as well as structures built in a graving dock and then floated to the production site and bottom founded.

Bridges : Several hundred bridges have incorporated lightweight concrete into decks, beams, girders, or piers. Transportation engineers generally specify higher concrete strengths primarily to ensure high-quality mortar fractions (high compressive strength combined with high air content) that will minimize maintenance. Thousands of bridges in the United States are functionally obsolete with unacceptably low load capacity or an insufficient number of traffic lanes. Structural lightweight concrete has played a major roll in bringing these structures up to modern compliance in an environmentally responsible way.

Buildings: Many thousands of residential, commercial and industrial buildings, ranging from one story to multilevel high-rises, have been constructed around the world using lightweight concrete masonry and/or structural lightweight concrete.

The first major building project employing structural lightweight concrete in the United States was in 1928 and 1929, with an addition to the Southwestern Bell Telephone Company office in Kansas City. The building was originally built as a 14-story structure, and the company had found that the foundations and underpinning would support an additional eight floors, taking into account the additional dead load of conventional normalweight concrete. Upon analysis, the designers determined that by using lightweight expanded shale concrete instead of conventional concrete 14 floors could be safely added rather than eight, doubling the height of the building to a total of 28 floors. The concrete was mixed onsite (this was before the day of the ready-mix plant) with the relatively crude mixing equipment of the day. There were naturally some technical problems, primarily in producing a uniform and workable mix and placing the concrete in column and beam forms, but these were overcome by applying technical knowledge developed at the University of Kansas.

When completed, the building addition showed a total dead load reduction of more than nine million pounds through the use of lightweight expanded shale aggregate: six million pounds through the use of lightweight structural concrete, and three million pounds through the use of Haydite lightweight brick in the walls in place of structural clay units. Compressive strength of the lightweight concrete was 3,500 psi at 28 days, an almost unprecedented high concrete strength at the time. The building has stood for more than 80 years as a demonstration of the practicality and economics of structural lightweight concrete.

The first structural lightweight concrete high-rise building was the Chase Park Plaza is St. Louis. Built in 1929, this 28-story building used structural lightweight concrete in both frame and floor systems, as well as for fireproofing.

Another early “sustainable” application that is still used today is the 1923 development of lightweight concrete masonry with a higher insulation value, normal shrinkage, and a uniform compressive strength equal to normal weight concrete masonry. The many green benefits of lighter units are covered in the Masonry section.

Fire Resistance

Lightweight concrete is more fire resistant than ordinary normalweight concrete because of its lower thermal conductivity, lower coefficient of thermal expansion, and the inherent thermal stability of an aggregate already heated to more than 2000º F degrees, as reported in ACI 216 Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, when slab thickness is determined by fire resistance and not by structural criteria (joists, waffle slabs e.g.), the superior performance of lightweight concrete, will reduce the thickness of slabs resulting in significantly lower concrete volumes.