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Crack and Seat for Concrete Pavement Maintenance

The condition of the load transfer dowel bars at the joints is a major influence on the performance of unreinforced jointed concrete pavements. Corrosion of the dowel bars inhibits thermal movement and this causes spalling at the joints and leads to mid-bay cracking. Water infiltrating the joints can lead to lack of support from the lower layers. One way to improve the serviceability is to overlay the pavement with asphalt after carrying out selected repairs to the concrete. However, thermal movement within the overlaid jointed concrete frequently cause transverse cracks to develop in the new surfacing above the original joints. To minimise the cracking, overlays of at least 180mm are applied.

Flexible composite roads also suffer from transverse reflection cracks at the road surface which develop above shrinkage cracks in the underlying cement-bound layer. Typically these reflection cracks are spaced between 6m and 10m apart.

Since 1991, the technique of crack and seat in which fine cracks are created in unreinforced concrete slabs or exposed lean concrete road bases before overlaying, has been used in trials in the UK. The approach was to induce fine vertical transverse cracks in the unreinforced concrete in order to create more locations where thermal contraction would take place whilst retaining satisfactory load carrying and load transfer characteristics. However, when the concrete or cement-bound road base is cracked and seated, its load spreading ability is reduced depending on the remaining degree of interlock at the crack and on the crack spacing. The effective stiffness modulus of the cemented layer is thus reduced and this must be taken into account when designing an overlay on a cracked and seated pavement.

Structural Design

A structural design procedure has been developed to determine the thickness of asphalt overlay needed to carry the future traffic loading for the next 20 – 40 years on pavements with cracked and seated concrete layers. Based on the trials and on the initial application of the technique elsewhere, a range of typical effective stiffness values for the cracked and seated concrete have been derived from Falling Weight Deflectometer (FWD) measurements. Clearly the back analysed effective stiffness depend on the thickness of slab and on the position of the FWD loading plate and geophones with respect to the induced cracks. The effective stiffness of the cracked concrete is also a function of the crack spacing.

The design procedure uses linear elastic multilayer analysis to calculate the values of the maximum tensile strain developed at the bottom of the asphalt overlay and the maximum vertical strain at the top of the subgrade produced by a standard axle load of 8.2 tonnes. Appropriate stiffness moduli are used for the asphalt layers and foundation. The thickness of the concrete and the sub-base layers for the pavement are assumed to be constant and the critical strains are calculated for a range of effective stiffness values of the cracked concrete strains are calculated for a range of effective stiffness values of the cracked concrete and for a range of overlay thicknesses. Using the criteria given by Powell et all (1984) to ensure that these traffic induced critical strains are maintained within acceptable limits, the overlay thicknesses can be derived for the range of effective concrete stiffness being considered. These are in the form of design curves relating overlay thickness to design life, in terms of the cumulative number of 8.2 tonne axles to be carried over the next 20 – 40 years, for various assumed stiffnesses of the cracked and seated concrete.

From these curves the engineer can select an appropriate overlay thickness for the asphalt that would satisfy the structural requirements for a particular threshold stiffness value for the cracked and seated concrete. At present however an overriding constraint has been set for a minimum overlay thickness of 150mm to ensure that reflection cracking due to thermal stresses does not occur under UK conditions.

Crack and Seat Specifications

As part of the specifications, a small but comprehensive trial is carried out on an area of the works that is representative of the complete scheme. The trial has several objectives. First, the contractor must demonstrate that his plant is capable of consistently producing fine, predominantly vertical transverse cracks in the concrete that comply with the requirements of the specification. Secondly, the contractor must show that the crack spacing can be controlled within the tolerance specified. Thirdly, different crack spacings are tried and FWD measurements are made to determine the effective stiffness of the cracked concrete to ensure that it is above the threshold value required in the structural design.

The crack pattern on the surface of the concrete must be predominantly transverse and longitudinal cracks longer than the transverse crack spacing are not permitted. Cores are taken through several cracks on each slab in the trial to ensure that they are predominantly vertical and most importantly that the concrete is not shattered nor does it have multiple cracking. The cores also confirm the slab thickness used for design. When the contractor has satisfied these requirements, the settings on his machine are not changed during the main works without permission of the overseeing organisation.

The effective stiffness of the materials within the pavement is calculated using a back analysis technique that requires knowledge of the thickness and position of the various pavement layers. The process can be carried out using different computer programme; for this application TRL uses MODULUS V5.1 (Texas Transportation Institute 1995). The back-calculated effective stiffness of the cracked concrete depends on the positioning of the FWD relative to the cracks. Within the trial, FWD measurements are made every 0.5m along each slab to determine the location of the minimum effective stiffness value within each slab to determine the location of the minimum effective stiffness value within each slab for the crack spacings investigated. Four slabs cracked at each spacing are tested.

When the main works are carried out, the contractor is responsible for observing the surface crack pattern and ensuring that it remains the same as that agreed from the trial. If it changes, due, for example, to a change in slab thickness, the overseeing engineer requires the contractor to adjust the drop height of the guillotine to correct the pattern. The main requirement, however, is to ensure that the concrete is not shattered internally by the cracking operation. Cores, 150mm diameter are extracted, at least one per 300 sq.m of cracked concrete, and the cracking operation is not allowed to proceed more than 100m beyond the last acceptable core. If a core shows that the concrete is shattered or has multiple cracking or is not cracked, the cracking is stopped and the reason for the non-compliance is investigated. This ensures that long length of concrete are not under or over cracked.

FWD measurements are made on each cracked slab at the position of minimum effective stiffness determined from the trial or every 5m along a lean concrete roadbase. If the back-calculated effective stiffness falls below the threshold value used for design than the reasons for this are determined and it might be necessary to adjust the crack spacing.

Although the close control might appear to slow the cracking operation down, it ensures that it will not be necessary to replace slabs or lengths of lean concrete due to a failure to meet the requirements of the specifications and this is expensive and time consuming. However, experience has shown that the close control has not reduced the overall speed of the operation.

The cracked concrete is seated with a pneumatic tyre roller with a ballasted weight of at least 12tonees. Tests have shown that a minimum of 6 passes over every point of the cracked and seated concrete is sufficient to seat the pavement into any small voids that might have existed. The specification requires that the number of passes is monitored. FWD measurements have demonstrated that after cracking and seating, the range of effective stiffness value along the scheme has been reduced compared to the range before the treatment.

Cost Effectiveness of Crack and Seat

The crack and seat technique has been cost effective. It has been found that contracts have been completed in one third of the time and at one third of the cost of traditional reconstruction. Also substantial savings in time and cost have been made compared to repairing the concrete (particularly at the joints) and overlaying with at least 180mm of asphalt.

As the existing pavement is left in place, the crack and seat technique supports sustainable construction. The reduction in contract period reduces delays and therefore the cost to the travelling public. Energy consumption during construction is reduced compared to traditional reconstruction or repair. Use of primary aggregate is also reduced together with a reduction in materials taken to landfill tip.

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