Design of external in-situ concrete paving
Thickness design methods for unreinforced and reinforced concrete pavements have been established for many years. The tabulated values and factorial equation in Concrete Society Technical Report 66, External in-situ concrete paving, are based on the equations given in TRL Research Report 87, Thickness design of concrete roads, which is the most widely-used method of designing concrete pavements that are either unreinforced or which include fabric reinforcement (in the top of the slab). The method has the advantage that, because it is an empirical method based on the performance of sections of pavement in service, it automatically includes the effects of fatigue, temperature and shrinkage. Pavements reinforced with fabric (generally in the bottom of the slab) can also be designed using some of the methods described for fibre-reinforced slabs (see separate entry).
One of the parameters that influences the required pavement thickness is the support given to the slab by its foundation; the TRL equations use the equivalent surface foundation modulus of the subgrade and sub-base acting together, where equivalent foundation modulus is the modulus of a uniform foundation giving the same slab support as the actual foundation.
In the preparation of Technical Report 66, it was considered that CBR (California Bearing Ratio) is the most useful and easily measured starting point. To simplify matters, standard sub-bases of 150mm Type 1 granular material and 150mm HBM (Hydraulically Bound Material) were chosen and the equivalent surface foundation moduli of these sub-bases on subgrades with a range of CBRs from 3 to 30% were determined. Once the foundation moduli of the standard sub-bases on a range of subgrade CBRs had been determined, this made it possible to calculate pavement thickness using the TRL equations with CBR as the starting point.