Effect of time gap and bottom concrete strength on two-lift concrete interface characteristics

Abstract: Compared to conventional cement concrete pavements, two-lift concrete pavements typically have a thin upper strong concrete layer placed over a relatively thicker and weaker grade of concrete layer and are generally preferred over conventional concrete pavements due to the feasibility of using relatively inferior aggregates in the bottom layer. The performance of the two-lift concrete pavements depends on the properties of the two concretes and the characteristics of the interface between the two layers. The degree of bonding between the two layers is mainly influenced by the time-delay (time gap) between the pouring of the two concrete layers and the strengths of individual layers. The influence of the time gap and the strength of the bottom concrete layer on the interface strength and failure patterns was experimentally evaluated in this study. Three concrete mixes (M1, M2, M3) with water/cement ratio ranging between 0.58 to 0.45, were prepared for the bottom layer concrete whereas only one concrete mix, with a water/cement ratio of 0.40, was considered for the top concrete layer. The compressive strength of the top layer concrete was found to be 47.4 MPa whereas the strength of the bottom layer concrete was found to be 23.3 MPa, 29.4 MPa and 41.8 MPa for M1, M2 and M3 mixes respectively. 150 mm two-lift cube specimens were prepared with the thicknesses of both layers being equal. One hour time gap was used for the experiment conducted to investigate the effect of bottom concrete strength on the interface. Similarly, for estimating the effect of time gap on interface condition, specimens were prepared using M1 concrete for bottom layer and with varying time gaps (0.5, 1.0, 3.0 and 24.0 hours). The time gap between the placement of the bottom and upper layers of concrete was varied. The specimens were subjected to direct shear test and axial splitting test for evaluating the shear strength and tensile strength respectively of the interface between the two layers. A reduction of 6%, 22% and 49% in the interface shear strength was observed with increase in the time gap from 0.5 hours to 1.0, 3.0 and 24 hours respectively for the specimens having M1 as bottom layer mix. The interface tensile strength reduced by 0.7%, 18% and 34% for a time gap of 1.0, 3.0 and 24 hours respectively. The smaller time gaps (0.5 and 1.0 hours) led to shear-crack dilation (recorded by a camera) and a rough-failure surface whereas smooth interface failure without any shear-crack dilation was observed for a time gap of 24 hours under direct-shear loading. Aggregate-mortar interface failure was the predominant mode of failure for time gaps of 0.5 and 1.0 hours for direct shear as well as split tensile mode of loading. For the experiment conducted with one hour time gap and three different bottom concrete mixes (M1 to M3), compared to specimens with M1 bottom mix, interface shear strength increased by 4% and 23% respectively for M2 and M3 concretes whereas the corresponding increments in tensile strength were 10% and 37%.