Finite element programs are functional to analyse the behaviour of concrete slabs most notably two way slab systems. However, they lack the current functionality to accurately model long term deflections due to the complexity of factors involved.

This study has explored the factors that contribute to the time-delayed increase in concrete slab deflections known as long-term deflections. The long-term deflections are the results of shrinkage, creep and the decrease in the tension stiffening over time. Previously tested one-way (Gilbert, 2008b)* and two-way (Gilbert & Guo, 2002)** concrete slabs that had been subjected to sustained loads have been used to compare the results. RAPT has been used to explore the effects of age at first loading and the tension stiffening effect. In addition, RAM Concept is a finite element program where the load factors, cracked element analysis and effective curvature ratio (ECR) methods have been used to estimate the concrete slabs long-term deflections.

Conclusions:
RAPT provided the best results within 15% of the measured long-term deflection of the concrete slabs.

The ECR method underestimated the long-term deflection in all cases in the order of 40%-48%.

The cracked element analysis method used in RAM Concept was within 10% of the long-term deflection measured on the one-way slab. The long-term deflection for the two-way slab was underestimated in the order of 72%-80%

The load factors method provided the results within 24% of the measured long-term deflection using Branson’s formula and overestimated the long-term deflection using Bischoff’s formula in the order of 71%-89%.