Before a void fill solution can be devised, it is important to determine what has caused the hollow space beneath the concrete surface. These empty spaces can develop for many reasons, including soil consolidation, erosion, and flood and drought cycles. Proper soil composition and compaction are critical to providing a stable foundation for concrete surfaces.

In our experience poor retaining and drainage systems can lead to voids under concrete structures. The steel reinforcement inside the concrete will hold strong while the erosion process is developing underneath causing a void. Immediate action is required to save on future major damage and cost.


Because they are hidden beneath the concrete surface, it is nearly impossible to tell whether or not you have a void under your concrete by visual inspection alone. Because of this, property owners often describe the symptoms of a concrete void to our technicians. These symptoms include:

  • Hollow sounds. Do you notice a hollow sound from beneath the surface of the slab with activities such as walking, driving, bouncing balls, or skateboarding? If so, this may be a clear indication that there is insufficient soil beneath the surface.
  • Cracking slabs. If your concrete surfaces have experienced cracking and/or sinking over time, the soil beneath may have become unstable. This is one of the most common visual clues of potential subsurface voids.
  • Deterioration of joints. Expansion joints and control joints are installed in concrete to prevent slabs from shifting and moving against one another during ground movement. If these are deteriorating or commonly have water draining through them, there may be a void beneath the slab to blame.

If allowed to remain, hollow spaces beneath your concrete surfaces can contribute to a vicious cycle of improper drainage and settling. Unresolved hollows will result in increasing cracks in the slab. These cracks will allow water to penetrate the soil beneath the concrete, exacerbating the washout and settling cycle. To prevent costly damage to your property, it is important to take action to fill the voids as soon as possible.

We have the methods and equipment to fill voids without mess through our injection process using polyurethane foam which reacts when coming into contact with water. The end product formed is harmless to the environment and resistant against biological attack. The premixed resin is pumped by means of single component injection pump that is suitable for high pressure.

Contact BPQ Construction for a quote or further information