Fracture-based method to determine the flexural load capacity of concrete slabs

The increasing service demands and limited availability of raw materials for rigid pavements pose a unique challenge in terms of the initial pavement design, concrete material selection, and the slab's cracking performance throughout its service life. New pavement technologies and concrete materials continue to be developed to face these challenges, but existing design methods based on empirical fatigue curves cannot easily relate the fundamental hardened properties of new concrete materials, the pavement layers, and slab geometry to the slab's cracking performance. This research aims to contribute to filling this gap by proposing a new method to discretely account for crack initiation, crack growth, and the flexural load capacity of concrete slabs using mechanical and fracture properties extracted from small-scale concrete specimens. The method predicts the flexural capacity and crack growth of large-scale concrete slabs with different geometries and materials by inserting cohesive surface elements on the expected crack path in the 3-D finite element model that represents the slab. The cohesive elements were defined by the total fracture energy (GF), initial fracture energy (Gf) measured from notched, three point bending beams, and tensile strength (f't) measured from split tensile cylinders. The soil underneath the slab was characterized as a linear elastic spring elements with their stiffness obtained from a beam on soil test. The simulations from the 3-D slab models reasonably matched experimental tests of notched concrete slabs confirming the feasibility of the proposed method. The softening curve type (i.e. bilinear, linear or exponential) embedded into the cohesive elements and the variation in soil stiffness had a similar effect on the flexural capacity of the 150 mm concrete slabs. In contrast, the soil stiffness was a more significant factor affecting the flexural capacity of the 63 mm slabs relative to the softening model. Although the bilinear softening model more readily represented the fracture behavior of concrete beams, the linear softening model applied to slabs was able to reasonably predict the flexural load capacity of the experimental slabs while significantly reducing the computation time. Alternative concrete materials were simulated with the fracture-based methodology. Relative to normal concrete, higher crack resistance and flexural load capacity were attained for concrete containing structural fibers while lower load capacity and more rapid crack propagation occurred in concrete slabs with recycled concrete aggregates. This fracture-based methodology constitutes an important step toward a more fully mechanistic pavement design approach that would integrate the specific concrete material fracture parameters, slab geometric properties, and support conditions into the prediction of the rigid pavement cracking performance.