Optimized Joint Spacing for Concrete Overlays with and without Structural Fiber Reinforcement

In thin concrete overlays (10 to 15 cm, 4 to 6 in), field observations have sometimes shown that not all contraction joints activate initially and, in some cases, do not activate until many years after construction. Contraction joints that do not activate may be considered an inefficient design that can lead to unnecessary maintenance efforts, unnecessary costs, and negative impacts on concrete overlay performance. Optimum joint spacing design for concrete overlays may need to be determined based on factors different from those that are currently considered. This study included an analysis for recommended joint spacing using pavement design software, as well as a field review of joint activation in existing concrete overlays using non-destructive testing. Test sections were also constructed in conjunction with new concrete overlay projects to analyze a wider range of variables and study early-age joint activation behavior. The data showed that joint spacing was the most significant factor affecting joint activation in thin concrete overlays. The addition of 2.4 kg/m3 (4 lb/cy) structural synthetic macro-fibers did not affect the rate of initial joint activation compared to overlays without fibers. A design parameter, slab length over the radius of relative stiffness (L/l), was identified to correlate with joint activation rate and timing. Designing joint spacing to achieve L/l between 4 and 7 may provide the desired balance between maximum, timely joint activation and good overlay performance.