Dealing with cracks in concreteFacts About Concrete Voids and Concrete Damage

September 19, 2019by Jerald Sargent0

Though concrete is a durable and inexpensive material used both inside and outside the home, concrete does not last forever. Cracked, uneven, or sunken concrete can occur for many reasons. In rare cases, damaged concrete results from ice, earthquakes, impacts, or overloading. More commonly, however, voids under the concrete are responsible for damaged concrete. When a void is responsible for concrete damage, concrete crack repair may be ineffective without first filling the void. Here are some facts about concrete damage and the process for repairing it effectively:


Voids are underground areas where soil is less dense and, consequently, able to support less weight than compacted soil. This fact is confusing to some people because the general view is that the ground is stable and should always remain so. However, voids under a concrete slab can be caused by a number of conditions:

  • Water flowing under the concrete slab causing soil erosion
  • Underground water flows dissolving limestone shelves and causing soil to collapse
  • Insufficient soil compaction before construction, exacerbated by the weight of the structure built on top of the uncompacted soil
  • Loss of lateral support due to nearby excavation, mining, or construction activity

The risk of voids can be reduced by properly draining soil around the structure and completely compacting soil before construction. However, some voids are, no pun intended, unavoidable. Voids due to dissolved limestone or subsidence probably cannot be prevented in any event.

Concrete Damage

Concrete is strong under compression forces, but weak under tension forces. Prestressed concrete, which has steel rebar embedded in a concrete slab, was designed to compensate for concrete’s weakness in tension. In tension, the steel bars bear the load so that the concrete does not need to. However, there are limits to the tension forces any concrete slab can bear. When those limits are reached, the concrete will crack necessitating concrete crack repair.

Imagine, for example, a concrete slab that is supported in the middle by compacted soil but has voids under each end. The concrete slab would be balanced like a see-saw, with both ends sagging. At this point, gravity takes over and the concrete slab will eventually reach a more stable position. The slab may pivot and slide laterally into a void, resulting in a raised or sunken concrete slab. Additionally or alternatively, the sagging ends will cause the concrete slab to crack in the middle.

Concrete Repair

Concrete crack repair is not as simple as filling the crack. Anyone who has used DIY concrete crack filler knows that filling the crack will be an ineffective form of crack repair absent filling the void that created the crack in the first place. Merely filling the crack will often fall short as a form of concrete crack repair because the crack will reopen as the concrete continues to sag into the underground voids.

The process used to fill concrete voids has existed for nearly 100 years. The process was formerly referred to as mudjacking or slabjacking because the process is exactly as the name implies – grout (referred to as mud in those days) is injected into the void, where it expands and hardens. Filling the void under a concrete slab with grout accomplishes a few important tasks:

  1. Injecting grout into the void densifies the soil, supporting the concrete slab and filling the void
  2. Grout expands, lifting the concrete slab and straightening it
  3. Grout hardens, preventing further soil collapse

There are a few different forms of grout used to fill voids. The “mud” version of grout is a mixture of water, cement, and a fine aggregate, such as fly ash or sand. The grout is usually looser than concrete, since the goal is to fill all the cracks and crevices in the void. As an aside, concrete and cement are not synonymous; cement holds concrete together but cement also holds grout together.

Another form of grout is polyurethane, a polymer. Polyurethane is delivered as a foam which expands and hardens inside the void. The primary benefit of polyurethane foam over cement grout is that polyurethane foam cures quickly and can be injected even when the soil is moist.

In sum, the most common cause of concrete damage voids under the concrete slab and, consequently, repair of cracked, uneven, or sunken concrete often requires that the voids be filled first.

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