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A cracking idea

New techniques are extending the useful life of all types of concrete road. Sally Ellis reports.

In recent years, research on the in-service performance of concrete roads has led to the development of technologies aimed at reducing future maintenance requirements, and make the treatment of rigid pavements economic, without the need to remove the concrete.

Research has also stimulated the introduction of measures during the construction of flexible composite pavements to reduce 'reflection cracking' which can allow water to enter the pavement structure and cause long-term breakdown.

Implementation of these research findings has effectively extended the design life for these types of pavements and this may encourage the increased use of concrete in future road construction.

Rigid pavements have a long design life due to their inherent strength and minimal maintenance requirements in their early life (15-20 years). After 20 years, long-term maintenance may be required. For this maintenance, the traditional method in the UK is to overlay with a thick asphalt layer - for example, 180mm - to inhibit the development of reflection cracking.

Reflection cracks in the asphalt surfacing, above cracks or joints in the rigid pavement, are caused by the thermal contraction of the concrete. Techniques that will alleviate the occurrence of reflection cracking have been developed for application in the UK through research carried out by TRL on behalf of the Highways Agency.

More specifically, there are two methods that are being used by the Agency in maintenance schemes on trunk roads. These are to 'crack and seat' the concrete pavement prior to overlay or to 'saw-cut and seal' the overlay after overlaying. Also, a combination of the two may be used.

The crack and seat method is to induce fine, vertical transverse cracks in jointed unreinforced concrete road slabs before overlaying with asphalt. This allows the seasonal contraction to take place at locations other than at the joints and reduces reflection cracking in the asphalt overlay.

The fine cracks retain the aggregate interlock necessary to ensure the satisfactory loadspreading ability, while still allowing for thermal movements to take place.

In general, concrete slabs are 'cracked' at 1 to 1.5m centres using a guillotine and then 'seated' using a pneumatic-tyre roller. Initially, this technique was developed for the maintenance of unreinforced pavements but more recently the technology has been applied to reinforced concrete.

For these pavements, the technique involves saw cutting through the concrete slab, until the reinforcement has been severed and then proceeding with the guillotine to crack and seat the pavement.

The reinforcement needs to be severed to allow the concrete to expand and contract in smaller sections.

Although the saw cutting is time consuming, this technique has been applied on a full maintenance contract and with several saw cutting teams in operation, the technique does not take significantly longer than crack and seat on unreinforced pavements.

The saw-cut and seal technique is suitable for both unreinforced and reinforced jointed concrete pavements. A partdepth, 25mm wide slot is cut in the asphalt overlay above each joint in the concrete, with a fine saw-cut also being made in the base of the slot. The slot is then cleaned by water jetting and dried.

Bond breaker tape is applied to the bottom of the slot and an approved sealant installed. The fine saw-cut ensures that any cracks that form occur between the bottom of the saw-cut and the concrete joint. The sealant prevents water ingress that would affect the pavement foundation and reduces surface deterioration that would otherwise be associated with reflection cracks.

Draft specifications for these new maintenance techniques have been produced and are being used by the Highways Agency on trunk roads in England. The minimum thickness of the overlay has been reduced from 180mm to 150mm and validation work is ongoing to confirm if the thickness can be reduced further.

Use of these techniques increases the pavement life and improves the potential for the use of rigid pavements in new construction. The techniques also mean that existing concrete pavements can be effectively maintained with reduced implications for traffic disruption.

In the 1970s, the construction of flexible composite pavements declined when the repeated occurrence of cracks in the asphalt surface was interpreted as a high risk of future maintenance need. In an attempt to overcome this potential problem, the thickness of the asphalt layer was increased to 200mm, making the design uneconomic.

Transverse cracks occur in the surface of flexible composite pavements above thermal stress cracks that occur naturally in the cement bound roadbase. The thermal stress cracks occur at a natural spacing of 10-30m. Precracking the roadbase at a closer spacing reduces the magnitude of the thermal movements at individual cracks, and hence the tensile stresses in the asphalt, and reduces the occurrence of cracking in the surface.

If cracks do occur, they are much finer and less likely to lead to deterioration of the surfacing and hence the whole pavement stability is retained. In this way, the location and size of surface cracks can be controlled.

Minimising the size of the surface crack allows the pavement life to be extended and reduces future maintenance costs, not only in terms of maintenance works but also the cost to the road user of the delays during roadworks.

Full-scale trials of precracked flexible composite pavements were constructed at four locations in the UK during the summer and autumn of 1996 and pre-cracking has been used as part of DBFO schemes. The report describing the trials and discussing the initial results of the monitoring that was carried out during construction has been published as TRL Report 289.

Five years later, the trial sites continue to demonstrate that pre-cracking can reduce the occurrence of cracks in the surface of flexible composite pavements. This, combined with in-service evidence that flexible composite pavements are outliving their design life, means that the Highways Agency design standard is likely to be amended, doubling the current design life and reducing the required thickness of asphalt in these pavements.

As part of normal maintenance, saw-cut and seal, and crack and seat will play a key role in the implementation of that part of the Highways Agency's 10-year plan to cover all concrete surfaces with quieter surfacings. It is also likely that thin asphalt surfacings will be required, directly on the concrete surfaces. This must be achieved without introducing a future maintenance problem due to surface cracking, but the behaviour of the new generation of asphalt thin surfacings in respect of surface cracking has not yet been fully explored.

Further research will therefore be required and its results implemented quickly if the application of quiet road surfacings is to be effective.

Sally Ellis is group manager, TRL

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