Floor flatness/levelness must be measured not just at installation, but also later, to see if there have been dimensional change.
ACI highlights need for good subfloor finish
Moisture-related flooring damage has been on the rise, according to the American Concrete Institute (ACI), which outlines some of the key factors that need to be addressed to mitigate the problem.
01 April 2019
Concrete slabs are a common subfloor material, especially in commercial buildings and high-rises. They can be slab-on-ground construction or – if they are aboveground and attached to the building’s structure – suspended slabs.
Any region experiencing rapid development – as the Gulf states currently are – will have countless examples of finish flooring materials being installed on top of a concrete substrate.
Such installations can lead to problems if the finish floor surface traps moisture within the slab. Water contained in the concrete mix itself, as well as groundwater below a slab-on-ground, are the sources of moisture.
Flooring may experience delamination, blistering, staining, mould growth, adhesive degradation and more, especially with slabs-on-ground, because these slabs don’t have the ability to dry from underneath. Transmission of moisture from the ground can be an ongoing circumstance.
Moisture-related flooring damage has been on the rise in recent years. One reason is that fast-track construction methods allow less time for concrete drying. Reformulation of floor covering adhesives to meet low volatile organic compound (VOC) targets may be another factor.
Moisture damage can be mitigated if appropriate steps are taken during construction. A critical question for design and construction teams to ask is: how does moisture behave in concrete slabs and, in turn, how does it affect the performance of applied flooring materials?
The answer entails more than simply reaching a desired moisture content within the slab. Factors such as floor flatness, surface texture, cracking, curling and more can have an effect. The team should address the following considerations, decide which ones are applicable, formulate a mitigation strategy, and write the strategy into the specification language. The specification should have input from floor covering and adhesive manufacturers.
Floor flatness
Even when a concrete floor is built flat, the top of the slab may lose more moisture than the bottom, causing differential shrinkage that results in curling or warping. It’s also important to remember that floor flatness and levelness are different measures; suspended slabs may become unlevel if they sag between support beams. Floor profile changes can result in inadequate surface area available for bonding with flooring.
While engineering the floor with a corrective factor is possible, this is difficult to do so effectively because of the number of variables involved. Installing reinforcing steel, specifying low-shrinkage concrete or repairing the curl with grinding and patching are better options. It’s also recommended to measure floor flatness/levelness not just at installation, but at a later time, to see if there have been dimensional changes.
Testing
The key to moisture control is measuring vapour movement through the slab. The goal is to understand flux rate, or the rate at which water moves through the slab, as well as the force needed to move the water. Traditionally used is the container method, in which a container of dry calcium hydroxide is placed over the concrete surface and sealed. After 72 hours, the calcium hydroxide is weighed and the results are compared to the starting weight. The amount of weight gain represents the amount of water absorbed during evaporation from the concrete.
More recently, measuring relative humidity (RH) has become common. Testing relative humidity involves drilling a hole in the concrete, lining it, and inserting a sensing probe with an RH meter. Relative humidity measurements can be taken continuously or at selected intervals using the same holes. A drawback of this method is that it’s mildly destructive.
The design team should set numerical limits for RH and vapour pressure. The construction team and/or consultants must measure moisture often enough, and at enough locations across the slab, so that results are statistically accurate. ASTM standards will give requirements for the number of tests to be performed in a given circumstance.
Concrete surfaces generally require pH testing, since adhesives can degrade and lose strength when exposed to highly alkaline solutions. Adhesive manufacturers will determine a maximum value or range for the pH. Rewetting and surface preparation can alter carbonation of a concrete surface, increasing the pH. When these situations occur, additional drying time for the concrete surface may be needed for it to carbonate to an acceptable pH level.
Vapour retarder/barrier
Engineers should approach slab-on-grade design just as they would a roof, considering all moisture sources and detailing the vapour barrier construction. Specifications should identify not only the location of a vapour retarder/barrier but also its composition, thickness, and installation method. Floor slabs should be placed directly on the barrier and should conform to ASTM E 1745. Low-permeance flooring materials or floor coverings with low moisture requirements, as defined by ACI standards, will benefit from the use of a vapour retarder material with a permeance level well below the current ASTM minimum requirement. Installation of the barrier should be in accordance with ASTM E 1643, which addresses such issues as laps and sealing around penetrations as well as puncture repair.
Curing
Concrete drying time and curing are related to the water-cementitious material ratio (w/cm), which is an indicator of its porosity. With proper curing that achieves a low w/cm, hardened concrete will have pores that are comparatively closed off from one another, thereby limiting the paths by which water can travel through the slab.
ASTM F 710 contains non-mandatory information on selecting a w/cm. ACI suggests specifying both w/cm and the corresponding compressive strength, because field measurements of w/cm for fresh concrete aren’t reliable enough for use in assuring that the specified value has been achieved.
Curing with water (for example, ponding or using wet burlap) is effective because it promotes the chemical reaction between cement and water that leads to optimum strength in finished concrete. However, adding water to a slab’s surface delays the start of drying and increases the amount of water that must later exit the concrete. It also constricts the path through which the water must exit. If drying time is a concern, curing slabs by covering with plastic sheets and/or waterproof paper is an option.
Similarly, using a curing compound creates a film that will slow initial drying, resulting in longer drying times. The film will typically have to be removed before the floor covering adhesive can be placed, otherwise poor adhesion may result. It is possible to work with adhesive and/or floor covering manufacturers to identify curing compounds or cure-and-seal materials that are appropriate for use.
Surface preparation
Most buildings have a variety of floor coverings installed. Since concrete surface finish requirements are unique for each product, floor covering installers typically handle surface preparation. The architect or engineer should, however, authorise all preparation methods being used. Power washing or acid etching should generally not be allowed. The effects of repair methods such as grinding and patching may also have an effect on the slab’s moisture and pH, so both the techniques and materials used for repair should be addressed by the design team.
Concrete slabs are usually tested, prepared and then covered, so if tests are required after surface preparation begins, they should be specified.
Protection
Protection is the most difficult design and construction item to incorporate into the project. If drying time is critical and the moisture-sensitive floor covering is an important feature of the facility, the slabs should be constructed after the building is enclosed and the roof is watertight. However, this extends the construction schedule and increases costs. It is far more typical that slabs be exposed to wetting-and-drying cycles.
Some architects incorporate a specification section that outlines possible moisture issues and remedies so that potential floor drying problems are brought to the attention of the owner and contractor, facilitating decision-making when concrete is not drying fast enough.
There’s no one-size-fits-all procedure that will protect against flooring damage. Conditions for each project are unique. However, identifying moisture sources and pathways – and creating mitigation strategies – during the project design phase can improve outcomes.
ACI 302.2R: “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials” provides indepth guidance on this issue.
