Areas of Concern: New Construction & Renovation
There are two areas of concern when identifying and preventing moisture-related flooring issues in construction. Depending upon whether it’s new construction or a renovation project, building professionals need to understand the underlying issues inherent in both scenarios when it comes to the presence of moisture in concrete slabs. We’ll address new construction first, including what to look for and what steps to take when confronted with excess moisture, and then repeat the process for existing slabs and trenches.
1. New Construction Slab
When it comes to new building projects, deadlines and delays mean time is at a premium, and construction schedules rarely contain sufficient time to facilitate thorough drying of concrete. This can become a major issue on the floor surface, as contractors or project managers may push to get product installed before the concrete has had ample time to eliminate excess moisture that contributes to failures.
Under optimal conditions, new construction slabs require one month per inch of drying time for normal weight or hard-rock concrete before installation of flooring products. The ideal water-to-cement ratio should be between .45 and .50 under ambient conditions (a minimum temperature of 70° F with a maximum relative humidity of 50 percent, and constant air movement at 15 m.p.h.) For concrete batched with lightweight aggregate, on the other hand, the drying time is two months. Lightweight aggregate concrete requires twice as much time to dry as normal weight concrete of the same thickness due to the pre-saturation of lightweight aggregates.
One of the reasons design and construction professionals run into moisture issues with concrete is because they often mistake the curing process with drying. “Curing” is the chemical reaction that creates the agglomerate better known as concrete, while “drying” is the loss of water not needed to hydrate the cement. Curing compounds, commonly used today as an alternative to keeping the concrete surface physically wet during the initial curing period, will substantially lengthen the drying period.
According to the Portland Cement Association (PCA), slabs on ground such as pavement, sidewalks, parking logs, driveways, floors, etc.; and structural concrete used for bridge decks, piers, columns, beams, slabs, cast-in-place walls, retaining walls, etc., require a minimum curing period of seven days for ambient temperatures above 40° F. Alternatively, the American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compression strength, which commonly matches the typical seven-day curing time specified by the PCA. Further, the PCA notes that the 70 percent strength level can be achieved more quickly when concrete cures at higher temperatures (and inversely at longer intervals when curing during lower temperatures) or when certain cement/admixture combinations are used.
During the drying process, the largest portion of water that must leave the slab is known as “water of convenience.” If the water-to-cement ratio is higher than the recommended level (.45 to .50), it means more water of convenience will be present in the concrete, which will negatively impact the quality of the slab in a number of ways, including:
- substantially increasing the concrete dry time
- creating a more porous and permeable concrete
- increasing the likelihood that the pores will be interconnected (an interconnected pore system from the bottom of the slab to the top will serve as an avenue of transport for liquid water or water vapor to move through the slab)
- creating higher chance of Moisture Vapor Emission (MVE) in the future with vapor barrier breakdown/penetration or drainage
- contributing to concrete laitance or weak surface
Moisture-related problems are also encountered in suspended slabs, where the concrete used often has a high water-to-cement ratio to make it more fluid and easier to pump. Because suspended slabs typically contain lightweight aggregate, they cannot be tested with calcium chloride tests (see sidebar for details about concrete testing).
In order to achieve specified flatness and levelness criteria, it can be tempting to use ride-on power trowels to get the slab to a hard, level finish quickly—but it comes at a substantial risk. The power troweling process can actually have the opposite effect, and slow down a project and add costs. This is because the smooth, glossy surface that results from power troweling actually retards the drying process by smoothing over the naturally porous characteristics of concrete and sealing in the moisture that needs to be released before the direct installation of a floor covering. Additionally, the slab can be overworked and even “burned” by some high-speed blades, closing up the natural pores or pathways that allow moisture within the concrete slab to evaporate. In other words, specifying a power trowel finish should be done carefully with the end flooring application in mind and never at the expense of the recommended drying schedule.
Another so-called solution to expedite the drying process of new concrete slabs is to apply temporary heat using propane- or oil-based sources. Unfortunately, this process produces the opposite effect desired by introducing “wet” heat, which actually inhibits drying of the slab. As a general rule of thumb, for every gallon of propane burned in the drying process, a gallon of water is produced. Using heat sources creates an abundance of carbon dioxide, which accelerates the formation of calcium carbonate and may inhibit or prohibit adhesive bond or create a “dusty” surface.
What Do You Do?
In order to achieve a properly formed concrete slab in new construction projects that will help prevent moisture problems from arising in the future, there are a number of steps that should be taken at the onset of the project. These include:
- Plan ahead with concrete specs. In your concrete mix specifications, be sure to require a .45. to .50 water-to-cement ratio, and limit fly ash content to no greater than 15 percent or 20 percent of the cementing material.
When curing, go “old school” and wet cure the slab. Curing blankets are now available for this purpose, and there is a negligible cost difference between wet curing and topical material systems. Most membrane forming curing compounds do not degrade over time, if at all, making them a poor choice for new construction slabs. Penetrating “cure and seal” products are intended not to deteriorate and should not be applied on concrete intended to host floor coverings. Also, be sure to avoid cold conditions or plan to use heating blankets. The same goes for windy conditions—avoid them or prepare for wind protection of the slab.
If a curing compound is used, be sure it is removed within 30 days after application. If the compound is prone to degradation, it typically happens late in the construction process, dramatically slowing down the drying time. If they do not degrade and flooring is installed, they will break the adhesive bond and release the “free” water causing flooring failure.
Finally, for the concrete finish, be sure to have the contractor leave a bull-floated finish, which should cost less than that artist finish. Pay the flooring contractor the cost difference to skim the floors—they are typically going to skim one coat anyway.
- Push for closure of building. The sooner the slab is enclosed, the sooner effective drying will start. Prior to HVAC installation, avoid the temptation to run propane or oil heat, which slows the drying process. Instead, run electric heaters and dehumidifiers; since moisture vapor is the great equalizer, the lower the relative humidity of the room, the more moisture tries to equalize that pressure from the ground up.
Once the HVAC is ready for use, push to have it running as quickly as possible. Doing so ensures stabilization of the concrete slab and creates an accelerated drying condition within the interior environment.