National Concrete Pavement Technology Center

Project description

Bonded concrete overlay of asphalt (BCOA), previously known as ultra-thin whitetopping (UTW), has been widely used to repair aged asphalt concrete (AC) pavements with moderate distress in many states in the United States. Due to the increasing costs of roadway maintenance, Louisiana has a great interest in determining if the thin bonded concrete overlay (usually 2-6 in.) is a suitable and cost-effective alternative to the current practice of roadway maintenance. The objective of the study was to evaluate the structural performance and load carrying capacity of BCOA pavements and to characterize the influence of in-situ bond strength on the performance of BCOA pavements. In this study, three full-scale BCOA test sections with 6-in., 4-in., and 2-in. Portland cement concrete (PCC) over an aged asphalt pavement were tested under accelerated pavement test (APT) loading under a typical pavement condition in southern Louisiana. The aged asphalt pavement consists of 3-in. AC over 8.5-in. crushed stone and 10 in. cement treated subgrade. A heavy load simulation device – ATLaS30, equipped with a hydraulically-adjusted dual-tire wheel load, was used. Each section was trafficking-loaded to a failure (i.e., all the slabs in loading path were cracked) under alternated load magnitudes of 9 kips and 16 kips of the ATLaS dual-tire wheel load. It was found that the 6-in. PCC overlay had a superior load carrying capacity compared to the 4-in. and 2-in. concrete overlays. The predicted pavement lives for the 6-in., 4-in, and 2-in. BCOA sections were 8.9-, 3.5-, and 1.2- million ESALs, respectively. As expected, the majority of load-induced cracks were not at a slab corner but along the wheel path (or longitudinal direction), presumably because the accelerated load in this study was applied along the centerline of the slabs. The load-induced tensile strains (measured at bottom of the slabs) also revealed a longitudinal cracking potential. Several Non-Destructive Test (NDT) methods indicated that the crack initiation of a BCOA slab could be coupled with a possible debonding at the slab-asphalt interface. A trench cutting investigation further revealed that a good bond was established between the PCC and AC layer. A performance review and in-situ pull-off test (also known as bond test) of the BCOA slabs suggests that the main distresses, such as longitudinal and corner cracks, develop primarily as a result of debonding of the asphalt layer from the concrete overlay. Debonding, which reduces the contribution from the underlying asphalt layer, increases the stress in the concrete layer, leading to the development of cracks. Therefore, under what level the bond strength should be specified in a BCOA pavement design is still debatable. Based on the results, it is recommended that a 6-in. BCOA pavement may be used in a medium to high volume pavement design where heavy and overloaded trucks are abundant and a 4-in. BCOA may be suitable to be used in a pavement rehabilitation project with a medium volume traffic. A newly-developed Short Joint Portland Concrete Pavement (SJPCP) module in the Pavement ME software was employed to predict the performance of the BCOA sections of this study. The predicted results were discovered to be roughly comparable to the in-situ cracking performance of this study. Finally, a failure criterion in terms of fatigue cracking and bond strength was proposed and the corresponding construction cost savings when implementing BCOA pavement as a design option for a medium to high volume pavement were estimated.