A punchout is a type of distress that typically occurs between closely spaced transverse cracks in CRCP. It is defined as a block or wedge of CRCP that is delimited by two contiguous transverse cracks, a longitudinal crack, and the pavement edge.
A punchout commonly initiates in conjunction with excessive erosion of the support between two closely spaced transverse cracks. The natural opening and closing of cracks is caused by temperature and moisture changes in the slab. There is also a tendency of the aggregate interlock along the transverse crack to wear out under traffic. This results in a loss of load transfer.
Traffic loads and the curling and warping behavior of the slab segment combined with these circumstances can induce both high transverse shear and flexural stresses leading to the characteristic spalling along the transverse cracks and longitudinal crack formation typically 2 to 5 ft (0.6 to 1.5 m) from the pavement edge between transverse cracks. This combination of behavior describe the prerequisites for a punchout distress. Progression of the punchout distress continues with traffic loads and the formation of severe faulting. Loss of support, pumping of the base material, and the consequent reduction are all factors in the further development of the severity of the punchout distress.(1)Pavement Analysis and Design (ISBN: 978-0131424739)
The most important factor in preventing punchouts is use of a non-erodible pavement base material to minimize loss of support. Other factors that can be considered during the design stage to enhance the control of punchouts include:
- Adequate steel reinforcement to maintain tight crack widths.
- Sufficient concrete strength and slab thickness to reduce tensile stresses and cracking.
- Selection of hard and angular aggregates with a low coefficient of thermal expansion (CTE) that can improve load transfer. While most any aggregate can be successfully used in a CRCP, aggregates with these properties will improve the behavior of the cracks.
- The use of a stress relieving interlayer beneath the slab, sometimes referred to as a “bond breaker”.
- Specification of curing techniques that allow for concrete hydration without excessive temperatures and subsequent drying shrinkage.
- Specification of mix designs that are suited for the environmental conditions.
- Slab thickness that is adequate given the magnitude of the traffic loading.
- Tied shoulders and widened lanes.
It is ideal for a pavement designer to obtain experience on those factors that influence punchout development under local conditions. For example, the Texas Department of Transportation (TxDOT) has performed extensive inves- tigations into the effect of different aggregate materials on the performance of CRCP.(2)Evaluation of the Performance of Texas Pavements Made with Different Coarse Aggregates (TX-01/7-3925-1)
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