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Concrete slabs on grade and basement slabs are weak and vulnerable to the detrimental effect of moisture which may lead to various problems. For instance, lifting tile off the floor, rotting hardwood, damping carpet, and coatings delamination.

Added to that, beneath continuous flooring, moisture can also cause floor darkening or discoloration, efflorescence deposits, and other deterioration. It can even damage the concrete itself and corrode steel reinforcements within the concrete. It, therefore, becomes very important to study and examine the intrusion and the precautionary measures to avoid the moisture intrusion.

Usually, free water in concrete and rising moisture from beneath the concrete are the main sources of moisture. Factors that lead to moisture accumulation in concrete and subsequent related problems include fast track construction schedule which does not allow free water to evaporate naturally, inadequate moisture protection, and wet construction site.

Regarding fixing moisture problem, it can be divided into two groups namely, protective measures to avoid the moisture-related problems and suitable procedure to tackle the issue after it has occurred.

Sources of Moisture in Concrete

1. Free Water in Concrete

It is from within the Concrete itself which with an excess of water is required to hydrate the cement particles and brings a concrete mixture to a workable consistency for placement.

Free Water in Concrete
Fig. 1: Free Water in Concrete

2. Moisture Rising from Below the Slab

Concrete slab is exposed to a perpetual source of moisture rising from below provided that suitable moisture barrier or any other suitable technique is not installed to prevent moisture migration form below the slab.

Moisture Movement from below the Slab
Fig. 2: Moisture Movement from below the Slab

Causes of Moisture Accumulation in Concrete

  1. Wet Building Sites
  2. Fast-Track construction schedules
  3. Inaccurate, insufficient or misinterpreted moisture Tests
  4. Inadequate Sub-slab moisture protection
  5. Changing materials
  6. Inadequate water drains off from the building.
Wet Construction Site
Fig. 3: Wet Construction Site

Preventive Measures

  1. A Low-Permeance Vapor Barrier / Retarder can effectively keep below-slab moisture from reaching the flooring system.
  2. Sometimes, providing adequate time for natural drying of concrete slab would solve the problem. In this case the concrete slab shall be tested prior to the installation of tiles, coatings, or paints.
  3. Sometimes, accelerated slab drying techniques can be used or a number of topical moisture and pH suppression systems can be applied as a preventive measure.
  4. Use a low water/cement ratio.
  5. Keep below grade excavations free of water to prevent potential reservoirs of moisture that can migrate upward through slabs. Excessive moisture below concrete slabs can cause structural degradation of soil bearing strengths, and swelling and shrinkage of soil. These can adversely affect a concrete slab on grade.
Vapor Barrier
Fig. 4: Vapor Barrier

Rehabilitation of Deteriorated Concrete Slabs

If the concrete is experienced and has moisture-related problems, then certain measures shall be taken to halt further deterioration. Reparation should begin only after the breach has been identified and cured.

  1. Installing sub-drains at the building perimeter to carry water away from a building: This technique incurs some cost but is the most effective solution to tackle any further detrimental effect of moisture on the concrete slab.
  2. Remove, clean, and dry the damaged area of the floor and place vapor retarders to restrict moisture movement
  3. Place a vapor retardant underneath the new floor finish.
  4. Make the floor finish vapor permeable. 
Wet Concrete Floor
Fig. 5: Wet Concrete Floor
Concrete slab suffered from moisture problems
Fig. 6: Concrete slab suffered from moisture problems

About Madeh Izat HamakareemVerified

Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at theconstructor.org.