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Moisture, concrete and tiling installations

By Michelle Costigan,
Technical Pro duct Manager,Laticrete UK.

After many years of trouble shooting membrane installations I have learned many lessons when it comes to moisture in concrete substrates and its effects on these applications, including subsequent finishes.

The following information on how to identify and mitigate moisture problems is based on Laticrete’s technical data sheets, and a spectrum of global and national standards.

Materials used in tile and stone installation applications can be affected by moisture during the installation and curing phases. Some materials, such as waterproofing membranes, may not cure properly or may delaminate from a continually wet substrate, or a substrate with a high Moisture Vapour Emission Rate (MVER).

A substrate which is damp, or has a high MVER, may also contribute to the formation of efflorescence.  Concrete, masonry and associated levelling beds with a high moisture vapour emission rate may also have an impact on a tile or stone installation.  The nature, extent and movement of moisture in concrete can be ascertained using a range of tests.

Two tests that are generally used to determine moisture content to internal concrete areas.

1. Relative humidity (RH) in-situ probe test
2. RH surface mounted insulated hood test
Other tests that can be performed to internal concrete areas include:
3. Calcium Chloride (ASTM F1869: Standard Test Method for Measuring Moisture Vapour Emission Rate of Concrete Subfloors Using Anhydrous Calcium Chloride)
4. Plastic Sheet Method (ASTM D4263: Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method).

Concrete contains moisture from the day it is poured and a generally accepted minimum cure time of 28 days (the time required to achieve the designed strength) is required before a finishing material is applied.  

It is important to note that 28 days is not a magic number that relates directly to every concrete installation reaching optimal moisture content. Simply relying on 28 days can be insufficient and can lead to failure of materials which may be affected by high moisture levels.  The national  tiling standard BS 5385-4: 2015 recommends a minimum further six weeks air drying after curing of a concrete swimming pool shell before the use of non-proprietary materials, in optimal conditions.
The cure time of the concrete can vary depending on:-

1. The water to cement ratio at the time of placement.  The higher the amount of water in relation to the cement at the time of pouring can have a profound effect, not only on the amount of time that is required to reach acceptable moisture levels of the concrete, but also on the physical properties of the concrete. Excess water in the mix can have a negative impact on the performance of the concrete throughout its life.

2. Concrete thickness.  Thicker concrete means more water. More water means more time to reach sufficient moisture levels.

3. Method of curing.  Concrete slabs can be wet cured, damp cured or allowed to dry without additional water placed on top of curing concrete to improve the physical properties of the concrete. Wet curing and damp curing can have a profound effect on the time it takes the concrete to reach a level where the moisture content is suitable for installation of waterproofing membranes and/or finish materials.
4. Environmental conditions.  Temperature, humidity, exposure to water, placement of vapour retarder, foundation drainage, when the building is fully enclosed, and when heating or air conditioning is turned on, will all affect the amount of moisture in a concrete slab over extended periods of time.

5. Placement of the concrete pour.  Installations of concrete on grade or below grade can lengthen the amount of time required for a concrete slab to reach suitable moisture levels.

6. Proper placement of a vapour retarder and drainage layer.  On grade or below grade concrete usually requires the proper placement of a suitable vapour retarder and moisture drainage layer.  Properly designed and installed these two components can greatly reduce the movement of moisture and moisture vapour through a slab and allow the concrete to maintain a reasonably consistent moisture level for many, many years.

From a tiling prospective there are essentially two types of moisture that can move through a concrete slab; negative hydrostatic pressure and moisture vapour emission.

Negative Hydrostatic Pressure is actual water in contact with the bottom of the slab. This water freely moves through to the top of the slab by high head pressure through the capillaries of the concrete slab.  Essentially, concrete is a very dense, very hard sponge.  Concrete will use any available moisture to continue the hydration process and get harder and harder over time. In other words, concrete can continue to cure indefinitely when exposed to water.  Negative Hydrostatic Pressure is a relatively rare occurrence and can be alleviated or dissipated using sump pumps, foundation drainage or good landscaping practices.

Moisture Vapour Emission is moisture that passes through a concrete slab caused by slight pressure differentials between the exterior and interior of the slab.  Moisture vapour emission from concrete is a natural and necessary process for any concrete pour.  Since the Portland cement in concrete only requires approximately 25% water content to hydrate properly then any excess water will more than likely pass out of the concrete slab as moisture vapour moving upwards through the slab into the structure. Considering that concrete, without the addition of plasticizers or superplasticizers, can have water content of 50% means that there can be a considerable amount of water that must pass through the slab as moisture vapour.

Part two of this article will appear in TSJ, April.

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