Pavement Concepts > Pavement Selection

cncPAVE was developed to assist the user in selecting the most appropriate concrete pavement for roads and streets carrying cars and trucks.

Jointed (J) pavements are normally best used in confined or smaller applications such as aprons, hard standings, parking areas, access roads, streets, etc. Continuously reinforced concrete (CR) is normally used in highways, freeways and particularly in overlays and inlays of existing pavements. Doweled pavement (D) can be used anywhere but requires careful control of dowel alignment and the compaction of concrete around dowels. Ultra-thin continuously reinforced (UT) concrete can also be used anywhere, particularly as overlays when the top structure is relatively stiff and erosion resistant. The UT concrete without steel fibres should only be used on lightly trafficked roads and streets.

Concrete has a tendency to shrink as it dries out which results in cracks in the slab. The crack width depends on many factors, but primarily on temperature changes, drying shrinkage of the mix and the mix properties. Water entering the cracks results in creating weak spots in the slab support, and in a reduction of the bearing capacity of the slab. Design methods are aimed at reducing the negative effects of cracks in the slab. This is done by either forcing the slab to crack at closer intervals (4.5 m) through crack initiation (partial-depth sawing of the concrete at an early stage to initiate cracks), or by introducing a relatively high percentage of longitudinal reinforcement. The first method results in a jointed pavement and the second method results in a continuously reinforced concrete pavement (CR) with crack spacing of about 1.5 m where the crack width is narrow enough to prevent the ingress of water and incompressible material. In order to enhance load transfer at joints in a jointed concrete pavement (J), smooth steel bars can be introduced and this type of pavement is called a doweled pavement (D).

Sometimes steel mesh or fibres are introduced into the jointed concrete slab, to increase tensile strength, decrease the slab thickness, and/or increase panel size.

Introducing steel fibres, increasing the flexural strength and increasing the steel reinforcement in CR leads to a stiff concrete layer protecting the layers below. This results in a drastic reduction in slab thickness - to between 50 mm and 70 mm.

The configuration called ultra-thin continuously reinforced concrete (UT) pavement has typically:

•some 80 kg/m3 of fibres,

•about 12 MPa flexural strength, and

•5.6 mm diameter of steel spaced at approximately 50 mm.

The ultra-thin reinforced concrete (UT) without steel fibres is also between 50 mm and 70 mm thick but has no fibre reinforcement and a steel mesh consisting of 5.6 mm bars spaced at 100 - 200 mm. However, large spacing of bars makes this pavement particularly susceptible to ingress of water, poor or non-uniform supporting layers, and heavy traffic loading.

Slab support stiffness is critical to the successful structural performance of a concrete pavement To ensure uniform and adequate support, a subbase is often introduced. The subbase may be constructed using natural gravel materials but in order to reduce the risk of erosion and thus voids under the influence of water and moving traffic, the subbase is normally stabilized by using cementitious or bituminous materials, especially when high traffic loading is to be accommodated.

Apart from traffic loading, the biggest contributor to concrete pavement failure is the ingress of water into the pavement. Not only does water weaken the subgrade, but penetrating through joints and cracks, it accumulates in the space between subbase and concrete slab. Under the dynamic loading of traffic, hydrostatic pressure develops. This then results in movement of materials in the case of unstabilised materials, or stripping in the case of bituminous treated sub bases, thereby creating voids under the slab. These voids result in a loss of support which means over stressing and ultimately failure.

Pavement design is aimed at providing an adequate thickness of concrete slab, possibly on a subbase, to reduce stresses on the subgrade, which, if too high, will result in deformation and loss of slab support. Slab thickness depends on slab support stiffness, traffic loading and the stiffness properties of the concrete itself. The design process involves calculating the stress within the concrete slab and comparing it with the strength of the concrete, to ensure that the ratio 'stress over strength' is lower than 1. The lower this ratio, the more load applications can be accommodated.

The properties of materials used in constructing the pavement, as well as traffic loading and environmental influences, vary within themselves and with time. It is important to consider these variations in the design process and to calculate the risk of failure, rather than to assume fixed values and thus arrive at a single number of load-applications to failure, which may be incorrect.