CIP 36 - Structural Lightweight Concrete

1 CIP 36 - Structural Lightweight ConcreteWHATis Structural Lightweight Concrete ?WHYUse Structural Lightweight Concrete ?HOWis Structural Lightweight Concrete Used? Structural Lightweight Concrete has an in-place density (unitweight) on the order of 90 to 115 lb/ ft3 (1440 to 1840 kg/m3)compared to normalweight Concrete with a density in therange of 140 to 150 lb/ ft3 (2240 to 2400 kg/m3). For structuralapplications the Concrete strength should be greater than2500 psi ( MPa). The Concrete mixture is made with alightweight coarse aggregate. In some cases a portion orthe entire fine aggregate may be a Lightweight aggregates used in Structural lightweightconcrete are typically expanded shale, clay or slate materialsthat have been fired in a rotary kiln to develop a porousstructure. Other products such as air-cooled blast furnaceslag are also used. There are other classes of non-structurallightweight concretes with lower density made with otheraggregate materials and higher air voids in the cement pastematrix, such as in cellular Concrete .

2 These are typically usedfor their insulation properties. This CIP focuses on structurallightweight primary use of Structural Lightweight Concrete is toreduce the dead load of a Concrete structure, which thenallows the Structural designer to reduce the size of columns,footings and other load bearing elements. Structurallightweight Concrete mixtures can be designed to achievesimilar strengths as normalweight Concrete . The same is truefor other mechanical and durability performancerequirements. Structural Lightweight Concrete provides amore efficient strength-to-weight ratio in Structural most cases, the marginally higher cost of the lightweightconcrete is offset by size reduction of Structural elements,less reinforcing steel and reduced volume of Concrete ,resulting in lower overall buildings, Structural Lightweight Concrete provides ahigher fire-rated Concrete structure. Structural lightweightconcrete also benefits from energy conservationconsiderations as it provides higher R-values of wallelements for improved insulation properties.

3 The porosityof Lightweight aggregate provides a source of water forinternal curing of the Concrete that provides continuedenhancement of Concrete strength and durability. This doesnot preclude the need for external Lightweight Concrete has been used for bridgedecks, piers and beams, slabs and wall elements in steel andconcrete frame buildings, parking structures, tilt-up walls,topping slabs and composite slabs on metal Concrete can be manufactured with a combina-tion of fine and coarse Lightweight aggregate or coarse light-weight aggregate and normalweight fine aggregate. Com-plete replacement of normalweight fine aggregate with a light-weight aggregate will decrease the Concrete density by ap-proximately 10 lb/ft3 (160 kg/m3).Designers recognize that Structural Lightweight Concrete willnot typically serve in an oven-dry environment. Therefore, Structural design generally relies on an equilibrium density(sometimes referred to as air-dry density); the condition inwhich some moisture is retained within the lightweightconcrete.

4 Equilibrium density is a standardized valueintended to represent the approximate density of the in-place Concrete when it is in service. Project specificationsshould indicate the required equilibrium density of thelightweight Concrete . Equilibrium density is defined in ASTMC 567, and can be calculated from the Concrete mixtureproportions. Field acceptance is based on measured densityof fresh Concrete in accordance with ASTM C density will be approximately 3 to 8 lb/ ft3 (50 to130 kg/m3) less than the fresh density and a correlationshould be agreed upon prior to delivery of Concrete . Thetolerance for acceptance on fresh density is typically 3 lb/ft3 ( 50 kg/m3) from the target aggregates must comply with the requirementsof ASTM Specification C 330. Due to the cellular nature oflightweight aggregate particles absorption typically is inSprinkling Aggregate in a Stockpile2003the range of 5% to 20% by weight of dry aggregates generally require wetting prior touse to achieve a high degree of saturation.

5 Some concreteproducers may not have the capability of prewettinglightweight aggregates in cold weather if temperaturecontrolled storage is not available. Some lightweightaggregate suppliers furnish vacuum saturated the exception of bridges and marine structures,specifications for Structural Lightweight Concrete do nottypically have a requirement for maximum water-to-cementitious materials (w/cm) ratio. The w/cm ratio ofstructural Lightweight Concrete cannot be preciselydetermined because of the difficulty in determining theabsorption of Lightweight content of Structural Lightweight Concrete must be closelymonitored and controlled to ensure that the densityrequirements are being achieved. Testing for air contentmust be according to the volumetric method, ASTM C 173,or calculated using the gravimetric method described inASTM C 138. Virtually all Lightweight Concrete is Lightweight Concrete requires proper attention todetail.

6 Excessive amounts of water or excessive slump willcause the Lightweight aggregate to segregate from the will generally provide an adequate finish. If thesurface for an interior floor is to receive a hard troweledfinish, use precautions to minimize the formation of blistersor delaminations. See CIPs 13 and 20 for discussions onblisters and delaminations, to the inherent higher total moisture content oflightweight Concrete it typically takes a longer time thannormalweight Concrete to dry to levels that might beconsidered adequate for application of floor splitting tensile strength of Lightweight Concrete is usedin Structural design criteria. The design engineer may requestthe information for a particular source of lightweightaggregate prior to the design. The splitting tensile strengthcorresponding to the specified compressive strength isdetermined in laboratory evaluations. Splitting tensilestrength testing is not used as a basis for field acceptanceof that the requirements of the designer relative to fireresistance or insulation properties of Lightweight concretebuilding elements are in conformance with applicableindustry standards.

7 For a successful project, information isavailable from the supplier of Lightweight aggregate and theready mixed Concrete producer. With proper planning, Structural Lightweight Concrete can provide an economicalsolution to many engineering For PumpingLightweight Concrete placements frequently employ pumps and this can be done successfully when a few precautionsare considered prior to the actual Aggregate should be adequately pre-soaked as pressure during pumping will drive water into the aggregate poresand cause slump loss that may result in plugging of the pump line and difficulties in placement and Pump lines should be as large as possible, preferably 5-inch (125-mm) diameter, with a minimum number ofelbows, reducers or rubber hose The lowest practical pressure should be Pump location should be such that vertical fall of the Concrete is Adjustments to mixture characteristics, such as slump, aggregate content and air content may be necessary toensure adequate pumpability for the job Decide on where Concrete samples for acceptance tests will be taken and what implications this would have on theconcrete mixture proportions and properties as delivered to the for Structural Lightweight Aggregate Concrete , ACI 213R,American Concrete Institute, Farmington Hills, MI, for Determining the Fire Endurance of Concrete Ele-ments, ACI 216R, American Concrete Institute, FarmingtonHills, MI3.

8 ASTM C 94, C 138, C 173, C 330 and C 567, Annual Book ofASTM Standards, Volume , ASTM International, WestConshohocken, PA, Concrete and Aggregates, Tom Holm, ASTM 169C,Chapter 48, ASTM International, West Conshohocken, Structural Lightweight Concrete , Info Sheet # ,Expanded Shale Clay and Slate Institute, Salt Lake City, Utah.