Inspection of the existing pavement will indicate where problem areas are likely to occur. Existing signs of slipping should be isolated and repaired. If the surface of the existing pavement has the presence of oil and grease from traffic or a prior seal coat, the tack is not likely to adhere correctly and a slip is often the result.
If the existing pavement in a high-stress area is polished and has a fine textured finish, it may be necessary to scratch the surface with a milling machine to allow for a mechanical connection between the layers, in addition to the adhesion bond. This technique is becoming more prevalent because it ensures a textured surface for the overlay to bite to and simplifies the complications of the tacking process. Thermoplastic "stop bars" should be milled off prior to overlaying.
To repair a slipped pavement it is necessary to saw cut and remove the old pavement. The limits of the sawed area must be outside the distorted area, so that a good seam to sound pavement is possible. The asphalt must be removed to the full depth of the slip, which is usually just the top layer. Inspection of the surface below the removed layer will often indicate why the pavement was able to slip.
If the underlying layer is oiled, seal coated, polished or dirty, it must be corrected before a new layer can be applied. It's often easier to scratch the surface with a small milling machine to ensure a tight connection between the layers. After this is accomplished, the pavement is patched normally. The alternative is to mill the slipped area into the next sound layer to complete both steps at once.
When the layer underlying a thin pavement is an aggregate base, it's important that the aggregate surface has a texture to allow the pavement to interlock with the rock particles. In the finishing process, if the rock is very wet and bladed and rolled to a slurry and smoothed to a very slick surface, the prime will penetrate into dust sized rock particles. This type of construction will build a weak joint between the two layers.
If the base-pavement joint is where the slipping occurs the pavement may not be thick enough to withstand the traffic stress. This means some of the base must be removed and a deeper layer of pavement laid to key the asphalt into the base. The other repair option is to replace the slipped area and overlay the whole area to thicken the pavement and provide additional strength.
When you analyze your next overlay, be aware of high stress areas so that you can include the appropriate cost for the prevention of slippage problems. If you believe the owner's proposed design is not suffient to avoid slipping, agree "up front" on the responsibility for the cost of repair. A slipping pavement can be a construction defect, an existing condition or a design flaw. Failure to consider the potential problems makes the lawyers happy and everyone else suffer.