Federal Highway Administration - Research, Development, and Technology

CONCRETE MATERIALS AND MIX DESIGN FOR ASSURING DURABLE PAVEMENTS

CONCRETE'S ROLE IN PAVEMENT STRUCTURAL DESIGN/PERFORMANCE

The three most common rigid pavement types are JPCP, JRCP, and CRCP. Several concrete characteristics may influence the structural performance of these pavement types(see Table 2).

Strength

Concrete strength is directly considered as a part of the pavement structural design. Requirements are typically in the range of 5 MPa (700 psi) flexural modulus of rupture or 28 MPa (4000 psi) compressive strength. In order to provide some additional insight on the influence of strength on performance, one of the factors being investigated in the Long-Term Pavement Performance Program (LTPP)is the effect on pavement performance of two levels (3.8 and 6.2 MPa (550 and 900 psi))of flexural strength (3). The performance of adjacent pavement sections built using concrete of these two strengths should provide some empirical evidence on the influence of strength on performance.

For the two reinforced pavement types, JRCP and CRCP, there is the additional subset of strength concerned with the bond strength of the concrete to the reinforcement, and so-called bond development length. These properties influence the performance of the slab at joints and cracks.

Tensile Stresses and Uncontrolled Cracking

Concrete is much weaker in tension than compression, and therefore transverse cracks which occur in pavements are due to development of excessive tensile stresses in the concrete. Pavement design must consider this property of concrete, and incorporate features to either control crack location or maintain structural performance in spite of the occurrence of random cracks.

Since, by definition, JPCP has no reinforcement, uncontrolled cracking must be prevented. Therefore, for JPCP, contraction joint spacing must be kept small, in order to induce cracking at the weakened plane of the joint, and prevent cracking at mid-slab, between joints. If an uncontrolled (mid-panel) crack does occur, the slab will soon fail due to crack-opening, subsequent loss of load transfer and faulting. Prevention of uncontrolled cracking can be assured in JPCP by using short transverse joint spacing (4 - 5 m,(12 - 15 ft)), and timely sawing of those joints during construction.

Transverse cracking in JRCP is as serious a problem as it is in JPCP. Contraction joints are used in JRCP to control the location of some cracking and all horizontal separation. Because of the size of the joint opening which can occur, the use of load transfer devices is required. The pavement design incorporates the allowance for mid-slab cracking, and hence transverse joint spacing can be greater than for JPCP. The purpose of the reinforcing, then, is not to prevent these cracks, but to keep these cracks tight once they occur, so that load transfer across the crack is accomplished through aggregate interlock and no faulting occurs. The steel percentage must be sufficient to keep the cracks tight, since the steel is not designed to serve as a load transfer device, and will soon fail in that situation.

Transverse joints are not a part of the CRCP design, therefore transverse cracks are expected to form soon after placement. The critical performance parameter is the spacing of those cracks: close enough to keep any one crack from opening too wide, but far enough apart so that good bond is maintained with the reinforcing, and the pavement between cracks is long enough to be stable.

Influence of Materials & Design | Influence of the Environment

http://www.tfhrc.gov/pavement/pccp/concr4.htm