The stabilization of concrete slab roads

Areas of Expertise (1)


Old roads with a concrete slab pavement that has been in service for several decades are often subject to step faulting, which may result in vibrations. This distress can originate in the lack of a base layer or in the lack of stability of that layer, which is often associated with poor drainage or a weak subgrade. In old roads, this is an “ageing phenomenon” which can often be ascribed to open joints or to very old age with consequent long-term traffic loading. In younger roads, the cause of step faulting and slab rocking needs to be investigated before making repairs.

Concrete slab instability originates at the joints, where water can penetrate. Water entrapped under the joints is ejected (“pumped up”) as vehicles pass over. The fines entrained with this ejected water leave growing cavities under the slabs. The consequently increasing step faulting and slab rocking cause discomfort to the road user and sometimes lead to premature deterioration of the entire road structure, as cracks are formed where the slabs deflect.

This problem can be addressed in several stages:

  • determining the cause of the distress: high age of the concrete slabs, unsealed or inadequatly sealed joints, poor drainage, low quality of the base layer material, etc.;
  • stabilizing the slabs;
  • possible overlaying with asphalt.

The slabs can be stabilized by injecting the underlying cavities with grout. Step faulting can be remedied to a certain extent with this technique. Any remaining differences in level at joints can be removed by milling.

When opting for overlaying with asphalt, it is recommended to “crack and seat” the slabs first. This will result in smaller slabs with less potential for inducing reflective cracks in the asphalt overlay. An anticracking interlayer should always be applied before overlaying. Cracking and seating is not allowed where buildings stand less than 5 m clear from the road, to avoid damage. The slabs then have to be reduced by splitting.

Where slabs have subsided owing to inadaquate load transfer at the joints, dowels can be inserted. Although this technique is rather laborious, it can certainly be recommended where dowels are missing, or in cracks in B1-B5 category roads (designed for a whole-life traffic greater then or equal at 8.106 standard 100-kN axles). However, it will only be useful if the concrete slabs themselves are still in good condition.

The durability of the repair will depend partly on the cause of the distress. For example if the quality of the base layer material is poor, subsidence will soon reappear, even if the cavities have been well injected.

The Concrete Roads – Pavings Unit and the Asphalt Pavements, other Bituminous Applications and Chemistry Division of the Belgian Road Research Centre have jointly carried out research into the causes of those defects and the effectiveness of the various solutions that can be implemented.

During this research, the effectiveness of stabilization by cracking and seating of the concrete prior to overlaying with asphalt was assessed, as well as that of under-slab injections and slab jacking techniques. The effect of inserting dowels in the joints of an existing road was investigated as well.

More information on the progress of this project can be found in the article Vibration-controlled stabilization of concrete slabs for durable asphalt overlays with anticracking layers.