Properties of Lightweight Concrete
The most significant property of lightweight concrete is reduced weight at no sacrifice in strength. Structural lightweight concrete available today are rotary kiln expanded shale, clay or slate (roughly 80% of structural use) and sintered expanded shale or clay (20%).
Lightweight Concrete provides the same compressive strength as normal weight aggregates with approximately the same cement content. A typical performance chart of a given aggregate shows the various strengths attainable with different amounts of cement for both 7-day and 28-day tests (Fig. 1).
Fig.1: Effect of cement content on compressive strength
Composite design, except when beams are encased, assumes no bonding action between the concrete and the steel, even though there is a considerable amount of bond under most conditions of load and building usage.
The interaction between the steel and the concrete is obtained through shear connectors, and the loading on the concrete is basically that of bearing, which is directly related to concrete’s compressive strength.
If the lightweight concrete is comparable in compressive strength to normal weight concrete, the shear capacity (or, more correctly, the bearing capacity) of the connectors should be comparable. Pushout tests on shear connectors in lightweight concrete have indicated comparable values.
However, because of some uncertainties of materials and a lack of complete test data to prove this point, many engineers and most connector manufacturers recommend some reduction in permissible load per connector when using lightweight concrete.
Generally, 80% to 90% of normal weight concrete capacity is used. On the other hand, many engineers do not require any reduction in their designs.
The modulus of elasticity of lightweight concrete differs from normal weight concrete. It can range from one-half to three-fourths of the E-value of normal weight concrete at a given strength level, depending on the weight of the concrete. The ACI Building Code uses this formula for estimating the E-value of both types of concrete:
In composite design, the modular ratio, n = Es/Ec, is important. For 3,000 psi, the n-value for normal weight concrete is 9; for lightweight concrete weighing 100 pcf, the n-value is 15; and at 115 pcf, the n-value is 12.
In designing with lightweight concrete in composite design, it is recommended that no differentiation be made in n-values for preliminary design only. By using n=9 for 3,000 psi lightweight concrete, the composite design tables in the AISC Manual and other sources can be used.
However, in checking the actual stresses in the concrete and in computing deflections it is recommended that the applicable n-value be determined from the above formulas.
Higher n-values mean smaller transformed areas; hence, slighter smaller moments of inertia and, theoretically, greater deflections. This effect is offset by the reduced dead load due to lower concrete weight.
Other properties of lightweight concrete that may be of interest in composite design are the creep and shrinkage characteristics. Many engineers feel that lightweight concrete has much higher creep and shrinkage.
Actually, a very extensive study of these properties—NBS Monograph 74, Creep and Drying Shrinkage of Lightweight and Normal- Weight Concretes—shows creep to be comparable to most normal weight concrete and, on an average, shrinkage to be only moderately greater.
In some areas, lightweight structural concrete is being specified because it has less shrinkage cracking potential than normal weight concrete.
Although there are no definitive values available, the feeling exists with some researchers that lightweight concrete under test performs better in composite design, possibly because the slightly higher creep and shrinkage may tend to distribute the Vn-load to more connectors than when normal weight composite beams are tested.
The other property is the better performance of lightweight concrete in fire tests, because of its improved insulation characteristics.