By MOIRA A. HARDING

W ith a typical unit weight of 90 to 120pounds per cubic foot (pcf) and a com-pressive strength from 2500 psi to morethan 8000 psi (Ref. 1), structurallight-

weight concrete is a versatile building material. Sinceit is generally 20% to 40% lighter than normalweightconcrete (Ref. 2) , a structure's dead load can be re-duced, its foundation costs lowered, and its concreteand rebar needs lessened. Structural lightweight con-crete also resists fire better than normalweight con-crete because of its lower thermal conductivity andits lower coefficient of thermal expansion (Ref. 3) .In

many structures, these benefits justify the use oflight-weight aggregate concrete, which generally costsmore than normalweight concrete. Contractors whosuccessfully use structural lightweight concrete sharethe following characteristics:.An understanding of the product, particularly the

unique qualities of lightweight aggregates.An ability to establish good communication, espe-

cially with the lightweight aggregate supplier.A knowledge of the field tests and adjustments

needed to evaluate and maintain the quality of theconcrete

CONCRETE CONSTRUCTION / JULY 1995

Using structural lightweight concrete should posefew problems for knowledgeable contractors

.An appreciation for the proper handling of light -

weight concrete

and properties of the aggregate. Most aggregates,however, can be saturated in fewer than three days.The particles can also be treated by the aggregatesupplier; before shipping and storing, the lightweightaggregate is sprayed with water as the particles travelon a belt.

Thermal quenching. This process occurs at thelightweight aggregate supplier's facility. Thermal sat-uration requires immersing heated aggregates in wa-ter. Particles subjected to thermal quenching absorbsignificantly more moisture than particles sprinkledfor only 24 hours (Ref. 4).

Vacuum saturation. Another process that occurs atthe aggregate supplier's facility, vacuum treatmentconsists of placing the aggregates in a tank and satu-rating them under a vacuum. In 30 to 45 minutes,vacuum -treated aggregates may achieve almost 100%saturation (Ref. 4).

It is critical for either the aggregate supplier or theready mix producer to perform the prewettingprocess correctly. If the aggregate particles in light-weight concrete are properly prewetted, then theconcrete should behave much like normalweightconcrete.

Establish CommunicationMaking good use of structural lightweight concrete

calls for excellent communication throughout theproject. Especially critical is the relationship betweenthe contractor, ready mix producer, and lightweightaggregate supplier. To design the best mix, the aggre-gate supplier and ready mix producer should know,for example, the methods the contractor will use toconvey, place, consolidate, finish, and cure the con-crete. The ready mix producer and lightweight aggre-gate supplier typically work together to provide agood mix, with the aggregate supplier recommendingthe best ways to handle its product.

One established method of communication is theprejob conference between the concrete contractor,ready mix supplier, lightweight aggregate field servicerepresentative, testing agency representative, andowner. Participants need to thoroughly discuss theproject's specifications, coordinate the ordering ofthe concrete, and establish procedures for sampling,testing, and handling the concrete. To facilitate theprocess, the contractor may want to review the mixand work with the ready mix and aggregate suppliersto ensure desirable fresh concrete properties, such asworkability and finishability .

Field Tests and Adjustments

The American Concrete Institute recommendssampling the lightweight concrete to verify its in-place strength as well as testing its unit weight, aircontent, and slump to monitor the quality of the mix(Ref. 5). The results of the last three tests can help the

Lightweight Aggregate Characteristics

Although lightweight aggregates are discussed gen-erally here, it is very important for concrete contrac-tors to contact the lightweight aggregate suppliers todiscuss the specific characteristics of their aggregate.Since the availability and properties of aggregatesvary, lightweight aggregate suppliers provide recom-mendations on handling procedures. Problems thatoccur on a lightweight concrete project are probablydue to lack of communication between the contrac-tor and the lightweight aggregate supplier.

By definition, structural lightweight concretecontains aggregates that are either all-lightweightor a combination of lightweight and normalweightaggregate (Ref. 3). Generally, an all-lightweight con-crete mix weighs between 90 and 100 pcf; a three-way mix of coarse lightweight aggregate, fine light-weight aggregate, and natural sand weighs between100 and 110 pcf; and a concrete mix of coarse light-weight aggregate and natural sand weighs from 110to 120 pcf (Ref. 2).

Aggregate types. lightweight aggregates suitable forstructural concrete may be natural materials such aspumice or scoria, or they may be processed aggregatessuch as expanded shales, clays, slates, and slags. Ex-panded slag is blast-furnace slag that is treated withwater while in a molten state, and expanded shales,clays, and slates have been heated-either in a rotarykiln or on a sintering grate-until they reach a plasticstate. In this plastic state, gases are released within theparticle, expanding it and forming an internal cellularstructure that remains even after the particle is cooledand hardened.

Absorption. More porous than normalweight parti-cles, lightweight aggregates that are not presaturatedwill absorb the water in the concrete mix. To controlslump, the lightweight aggregates should be prewet-ted before being used in a mix. This absorbed water isnot part of the mix water in the lightweight mix, and itdoes not directly affect slump, water-cement ratio, orquality of the paste. Since the aggregates' water ab-sorption varies, to maintain a high-quality mix, thelocal supplier should be contacted.

Prewetting Processes

There are three ways to prewet lightweight aggre-gate particles: thorough sprinkling, thermal quench-ing, and vacuum saturation (Ref. 3). What's impor-tant is both the amount of water that is absorbed andthe uniformity with which it is absorbed.

Sprinkling. This process occurs either at the readymix plant or the aggregate supplier's facility. If theparticles are treated at the ready mix plant, they aresprinkled with water for a period of time that canrange from hours to weeks, depending on the type

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had cause to avoid using structural lightweight con-crete. But knowledgeable contractors are able to uselightweight concrete to produce strong, durable struc-tures. For the contractor who still hesitates, manylightweight aggregate suppliers provide service engi-neers to make the contractor's first experience withlightweight concrete as productive as possible. £

tar, which is needed at the surface for finishing, sinks.Finishing. Metal tools are recommended through-

out the finishing process because they are less likelyto tear the surface of the concrete. After strikeoff op-erations, ACI allows finishers to use a grid tamper, orjitterbug, to keep the lightweight aggregate particlesfrom floating to the surface (Ref. 3) .To help finishnormalweight concrete, a jitterbug can only be usedon concrete with a slump that is less than 3 inches.

During floating operations, it is still important notto overwork the concrete. Too much darbying or bullfloating can bring the lightweight aggregate to thesurface and drive down the mortar needed for finish-ing. Lightweight concrete takes the same amount oftime to set as normalweight concrete with a similarcementitious content.

Curing. Begin curing the concrete as soon as possi-ble after the final finishing. The curing of lightweightconcrete tends to differ slightly from that of normal-weight concrete, since lightweight aggregates pro-duce an internal curing effect as they slowly releasetheir stored moisture. The presence of this internalmoisture, however, does not replace recommendedcuring practices. As in any curing process, it is impor-tant to keep the concrete continuously moist, so thatit can develop the requisite tensile strength to keepthe concrete from cracking.

For many years, ready mix producers have been ableto supply lightweight concrete that behaves in muchthe same way as normalweight concrete. Before thisknowledge was widely known, perhaps contractors

References

1. Thomas A. Holm, "Lightweight Concrete and Aggregates,"in Significance of Tests and Properties of Concrete and Con-crete-Making Materials, STP 169C, ASTM, Philadelphia, 1994,pp. 522-532.

2. Lightweight Concrete, Expanded Shale Clay and Slate Insti-tute, Salt Lake City, 1971, p. 33 and p.16.

3. ACI Committee 213, "Guide for Structural Lightweight Ag-gregate Concrete, ACI 213R-87 ," in Part 1 of ACI Manual ofConcrete Practice, American Concrete Institute, Detroit, 1987 ,pp. 213R-1 to 213R-27.

4. ACI Committee 304, "Placing Concrete by Pumping Meth-ods, ACI 304.2R-91 ," in Part 2 of ACI Manual of ConcretePractice, American Concrete Institute, 1991 , pp. 304.2R- 7 to304.2R-11.

5. ACI Committee 304, "Batching, Mixing, and Job Control ofLightweight Concrete, ACI 304.5R-91 ," in Part 2 of ACI Manualof Concrete Practice, American Concrete Institute, 1991 , pp.304.5R- 7 to 304.5R-8.

6. Robert E. Tobin, "Handling Lightweight Concrete on theJob," in Lightweight Concrete, SP-29, American Concrete In-stitute, 1971, pp. 63- 70.

7. "Pumping Structural Lightweight Concrete, Info. Sheet#477.1 ," Expanded Shale Clay and Slate Institute, Salt LakeCity, 1992, 2 pp.