Strength of Lightweight Concrete

Since lightweight aggregate is weaker than the cement paste, lightweight concrete generally fails through the crushing of the coarse aggregate. At high stresses the aggregate particles are crushed and cease to contribute to the strength of the concrete, becoming in effect voids. Since large voids weaken a material more than small voids it is the strength of the coarse aggregate that is critical.

In accordance with suggestions given in clause 3.2.7 of the FIP Manual of Lightweight Aggregate Concrete, References 25, Firstmix uses the residual absolute volume/ cement ratio, thus allowing for the effects of both the free-water content and the air content in one operation, to estimate concrete strength, instead of using the free-water/cement ratio with a separate allowance for the influence of the air content as in the normal DOE Method.

The concrete strength is first estimated as in the DOE Method for the given coarse aggregate angularity and cement type, but the residual-absolute-volume/cement ratio is used instead of the free water/cement ratio.

The estimated concrete strength is then reduced to allow for the weakness of the coarse aggregate in accordance with the following factors, which are estimated from data given in the FIP Manual of Lightweight Aggregate Concrete:

Modulus of deformation

of coarse aggregate.

Critical strength of concrete

made with coarse aggregate.

Strength reduction factors for


ratios of

MPa psi Cylinder Cube 0.4 0.5 0.6 0.7 0.8 0.9
    MPa psi MPa psi            
4380 635100 7.2 1044 9 1305 0.25 0.25 0.25 0.25 0.25 0.30
8880 1287600 15.2 2205 19 2756 0.63 0.63 0.66 0.73 0.79 0.88
13155 1907475 23.5 3408 28.5 4134 0.86 0.86 0.89 0.93 1.00 1.00
36330 5267850 70 10153 80 11603 1.00 1.00 1.00 1.00 1.00 1.00

The strength reduction factors represent the ratio of the strength of concrete made with a particular lightweight coarse aggregate to the strength of concrete made with normal weight aggregate.

The strength characteristics of the lightweight coarse aggregate must be entered on the Laboratory Measurements Dialog. The user can specify either

Whichever method is used to specify the strength characteristics of the lightweight coarse aggregate, Firstmix then uses linear interpolation to obtain the strength reduction factor, which applies to the appropriate residual-absolute-volume/cement ratio.