A Framework for the Estimation of Effective Flexural Strength of Two-Lift Concrete Pavements

Abstract

The performance of two-lift concrete pavements is significantly influenced by the strength of each lift and the degree of bonding between the two concrete lifts. Research on the influence of the bottom-lift concrete strength on the effective flexural strength of two-lift concrete pavements is scarce. The paper examines, numerically and experimentally, the influence of the strength of the bottom-lift concrete on the effective flexural strength of two-lift concrete pavements for both bottom-up and top-down cracking considerations. The current practice of design of two-lift concrete pavements considers the flexural strength of either the top or the bottom layer for estimating the bottom-up or top-down fatigue damage of the pavement, leading to an under- or over-design of the pavement. Thus, unlike existing design methods, the present approach enables selection of the effective flexural strengths of the two-layer system to produce designs to avoid the under-/over-estimation of fatigue life. Two-lift concrete beams were prepared by maintaining a time gap of 1 h between the placement of the bottom-lift and that of the top-lift to ensure a strong bond between the two layers. The grade of concrete used in the top-lift concrete (with a compressive strength of 36 MPa) was the same for all the combinations of beams investigated. The experimentally measured effective flexural strength (for bottom-up cracking case) of two-lift concrete beams increased by 33% and 63% with an increase in compressive strength of bottom-lift concrete (from 17.1 MPa to 25.4 and 32.7 MPa), respectively. The corresponding increments in the effective flexural strengths for top-down cracking were found to be 2.6% and 14.9%. The present study developed a numerical framework to evaluate the effective flexural strength of two-lift concrete by using the concrete damaged plasticity (CDP) model proposed by Lubliner/Lees/Fenves. The damage variables required in the CDP model were evaluated using the modified Alfarah method. The values of direct tensile strength and secant elastic modulus are taken as 0.9 times the split-tensile strength and static elastic modulus obtained respectively from the experiments instead of the original relations adopted in the Alfarah method. The proposed numerical framework successfully predicted the effective flexural strength of two-lift concrete beams for both bottom-up and top-down cracking cases, using the calibrated CDP parameters of both bottom- and top-lift concretes. In addition, the fatigue lives of two-lift concrete beams for both bottom-up and top-down cracking cases were estimated experimentally and numerically.