GENERAL PROPERTIES OF RIGID PAVEMENT
Load Spreading Capability to Subgrade
As mentioned earlier, rigid pavements have a much higher stiffness (modulus of elasticity) than asphalt pavements (about 10 times). Any construction that receives a load from above, will transmit or spread the load down. In terms of road pavement construction, one of its functions is to distribute and spread the traffic load received by the layers below it to the subgrade. The load that is transferred to the layer below it produces less pressure, due to the wider area that accommodates the load, so that the subgrade can be carried.
With a greater stiffness or modulus of elasticity of cement concrete, rigid pavement construction has a higher load dispersion capability than flexible pavement. As a result, the deflection becomes smaller and the stress acting on the subgrade is also low, therefore rigid pavement does not require a strong foundation bearing capacity. It is very important to pay attention to the uniformity of the carrying capacity of the subgrade, where there should be no significant changes in the carrying capacity. This is in stark contrast to the principle of flexible pavement design where the base and subbase layers require high strength to distribute the stresses of the wheel loads acting on the asphalt layer. Illustration of load distribution on rigid pavement and flexible pavement, shown in the figure below.
Illustration of load distribution on rigid pavement and flexible pavement
The results of stress tests on subgrade on rigid pavement, from a wheel load of 12,000 lb, or a working stress of 106 psi, are reduced to a stress in the subgrade to 3 to 7 psi with a distribution area of more than 20 ft. Tests conducted by the Portland Cement Association (PCA) also showed that the heavier wheel loads were distributed over the subgrade with a wide area and did not cause higher stresses.
Rigid Pavement Structure
At the beginning of rigid pavement development, the pavement is built directly on the subgrade without regard to the type of subgrade or drainage conditions. In line with the increase in traffic after World War II, the problem of pumping became important even though it was raised as early as 1932. Thickening of the edges was common in the 1930s and 1940s. For example, a rigid pavement is constructed that is 15.2 cm (6 in) thick in the center, and 20.3 cm (8 in) of edge layer thickness along the edge of the slab, is called an 8-6-8 plan. The pavement itself generally only has a width between 5.5 - 6.1 m. When the design is progressing, to prevent pumping the pavement is built over the sub-base layer with granular material. The current development uses a thicker design and is often used for highways with heavy traffic loads.
In rigid pavement construction, the main construction is one layer of high-strength cement concrete, and the sub-base layer only functions as a supporting construction. While the flexible pavement construction generally consists of several layers (3 or more), namely the surface layer, the foundation layer and the sub-base layer, all of which are the main construction. From the explanation above, it can be concluded that rigid pavement is a single layer construction, while flexible pavement is a multi-layered construction. The schematic of the rigid pavement and flexible pavement construction layers is shown in Fig Illustration of load distribution on rigid pavement and flexible pavement.
Foundation layer (Sub Base)
The purpose of using a rigid pavement foundation layer is to increase the bearing capacity of the concrete slab and provide resistance to erosion prevention in the foundation layer due to traffic and environmental loads. For rigid pavement, the foundation layer with binder material, can vary, one of these three types:
- cement-stabilized crushed stone with conditions not less than 5% (weight ratio) to prevent erosion. The cementitious material may contain cement, lime, fly ash and/or granulated blast furnace slag
- tightly graded asphalt mix
- lean concrete which has a compressive strength at the age of 28 days, between 80 and 110 kg/cm2
Breaking the bond between the subbase layer and the concrete slab layer, is to use a bond breaking layer on top of the foundation layer to provide a smooth surface and uniform friction. Lean concrete mixes are made like ordinary concrete without transverse joints and therefore cracks will occur. This is intended to achieve a close crack pattern and a narrow crack width, thereby providing a level of load transfer, which corresponds to the bond breaking layer and will not propagate to the overlying concrete slab.
Rigid Pavement Construction Capacity
Rigid pavement construction generally has a concrete slab thickness of about 25 cm, with a compressive strength of concrete equivalent to a flexural tensile strength of 45 kg/cm2. The rigid pavement has a capacity or serviceability of 8 million repetitions of standard axle load, which is equivalent to a 55 cm thick flexible pavement construction. Thus for the same load and subgrade, rigid pavement construction requires a thinner construction thickness. Illustration of the structural equivalent of rigid pavement and flexible pavement is shown in the figure below.
Illustration of the structural equivalent of rigid pavement and flexible pavement
Effect of the Shoulder of Wood Pavement Construction
Bonded concrete shoulders were first tested in Illinois in 1964, and were found to add significant structural value to rigid pavements, resulting in thinner slab thickness of pavements using concrete shoulders. The concrete shoulder here must be a concrete shoulder that is integrated with the slab integrally or a concrete shoulder that has structural properties, and must have a concrete quality and thickness equal to the thickness of the slab itself with a minimum width of 60 cm. While the width of the shoulders that are integrated with the concrete slab and located in the median path, the width can be reduced by a minimum of 50 cm. Look at the picture below
Typical concrete shoulders on rigid pavement
(Source : Diklat Perkerasan Kaku, 2017)
Bonded concrete shoulders that are structural in nature are concrete shoulders whose joints are formed in notches with a minimum width of 150 cm. Figure 4 shows a typical concrete shoulder on a rigid pavement.
