Why do we have problems with
Concrete Moisture
What do we do?

Situation Critical: Slab Moisture Problems in Hospitals

Life, in its many states of transition, changes no more dramatically than in hospitals. From one ward to another and from one state of being to another, life’s fragility is fully experienced. Little wonder, then, that people gripped by such moments do not question the inherent safety of the hospital floor. However, that very floor can be as fragile as life itself when it is poorly installed and badly maintained. In this context, the stakes of quality floor installation form the difference between saving lives under duress and asking them to make way for reinstallation contractors.

In hospitals, flooring failure is the most expensive post-construction nightmare short of an earthquake for multiple reasons. The buildings may be massive, but the human toll is greater. Imagine the reinstallation logistics: shut down the facility; move patients to another location; remove all medical equipment from every room; store the equipment while waiting for the repair; and, when the fix is in, reverse the whole process in order to restore the hospital to “situation normal.” Not only that, but the legal ramifications and human frustrations could swell to historic proportions.

All that for a wet concrete slab? Yes.

Flooring installation includes a dynamic moisture process hidden from the human eye. Concrete slabs emit and absorb moisture after they have been poured into a foundation. Water vapor rises from the bottom to the surface of the slab and evaporates into the surrounding air. When contractors apply floor coverings without leaving sufficient time for concrete to dry, moisture is trapped between the concrete and the floor covering. The excess volume (RH) and chemistry (PH) of the moisture will eventually destroy the flooring’s adhesive bond. The symptoms? Bubbles, seam splits, and curling in the flooring to name a few. The potential for mold represents a breach of hospital sterility, not to mention floor failure.

Most people who design medical buildings understand these industry-specific risks. Moisture can permeate the building through its floor, its roof, its walls, its windows and even through its heating, ventilation, and air-conditioning (HVAC) system. In fact, this is how a substantial number of construction litigation cases begin. Builders may recognize the dangers of moisture permeation in a hospital structure, but it is amazing how few builders really understand flooring moisture migration. It is sadly ironic that a lack of know-how in one area destroys so much know-how in all the others. Such is the dilemma facing hospital construction contractors.

It is useful to provide some of the foundation knowledge that leads to successful concrete construction:

● “Dry” condition concrete still contains a manageable amount of moisture.
● Concrete is made partly from water, so water is always part of the mix.
● Floor coverings interact with the concrete slabs on which they are applied.
● Vapor barriers can be installed to slow down, or prevent, moisture from permeating the slab via the ground below.
● Vapor retarders, including floor coverings, can still absorb moisture from the air.
● Proper moisture measurement, as RH testing, informs builders of when concrete slabs are prepared to bond with floor coverings.

In Theory

Concrete is often poured over materials that keep unwanted moisture at bay. This is achieved by installing a sand-bed on the ground with a vapor barrier laid on top. These form the means of stopping ground moisture from migrating into the concrete. Under-slab vapor retarders do not resolve moisture content (MC) in the concrete, but they assure that ground moisture will not ruin the slab.

Then the concrete curing process begins. Moisture moves from the wettest, deepest parts of the slab towards the surface in order to evaporate. Surface vapors evaporate first. As they do, the drier condition of surface concrete invites more lower-level moisture to rise. This is called capillary action. However, contractors must also consider the relative humidity (RH) of air surrounding the slab. If highly humid, the newly-dried concrete surface may absorb moisture vapors from the air. This is called moisture movement, and it’s a constant process until floor coverings are installed or the slab surface is sealed. Also, the composite of the concrete mix can influence drying time. High water content slows concrete drying time, as can the mixed properties of cement and aggregate which are also used to form concrete. Regardless of the variables, they all come into play as concrete cures from its liquid form to its solid permanence.

The American Concrete Institute (ACI) has established a general drying guideline for concrete slabs. The following weather conditions are needed:

• Temperature: 70 degrees F or warmer
• Relative humidity: 40%
• Wind: 15 miles-per-hour (maximum)

Under these conditions, concrete will dry at an approximate rate of one month for every inch of slab thickness. If the slab is six inches thick, for example, the above weather would allow it to dry in six months. However, builders would double this dry time if the concrete slab is re-wetted at any point, or if weather conditions change appreciably. The ACI went further. It developed a white paper outlining the complexities of concrete moisture in an effort to assist the industry.

In Practice

Unfortunately, fast-track construction schedules pressure contractors into installing floor coverings over concrete which is not in “dry” condition. Owners drive the construction schedule (in most cases), so they need to learn that concrete dries of its own time if it is to work. A slab which fails to reach moisture equilibration creates ongoing costs needed to mitigate the moisture problem. This leads to a crucial understanding of the issue: the costs on ongoing flooring issues outweighed by the benefits of pushing forward anyway. An owner needs to permit the necessary time for the concrete to dry sufficiently.

Since flooring is the last part of installation, potential flooring problems surface in the project’s eleventh hour. Diligent floor covering contractors ensure that moisture content tests are conducted before laying a single tile on the floor. If tests indicate a “go,” they can proceed safely. If not, then liability emerges as an issue. Flooring sub-contractors often ask general project managers to sign off on flooring installation to relieve the installer of legal liability. Floor installers expect that the project manager’s signature will insulate them from the fallout of an improperly-installed floor. The general contractor is often blamed for making a bad slab, even though they are often the first to raise the subject of high moisture test results. So, general contractors may end up paying to fix a problem that was produced by impatience, not bad concrete or workmanship. In every floor failure, the owner of the facility would confess to have enacted precautions if they had understood the moisture risk. In practice, the first moisture problem is the builder’s final headache.

In Hospitals

Moisture-related floor covering is a natural process, not a human problem. In fact, the sociology of the problem is far more complicated than physics. Even with vapor retarders and other precautions, new slabs don’t dry quickly. Period. No country, no organization, and no industry is exempt from this universal truth. The answer? Building owners, architects, and contractors need to admit that moisture problems can occur in any building, including theirs. Those who design and own medical facilities have a serious responsibility to see that they are built to be functional. If moisture arises as an issue, they must agree to specify and install remediation systems. That cost can be absorbed into the overall building budget (which is often tweaked anyway). Owners who don’t budget for such scenarios end up with the need to repair the tainted floor at much greater expense later. Unrealistic expectation is a flaw in human character, not concrete.

Owners and builders are empowered to preempt moisture difficulties in numerous ways: construction schedules, concrete mix designs, placement specifications, building acclimation and floor covering sensitivities. Once detected, owners can ask architects to specify a viable and proven treatment. Then, general contractors can build extra remediation time into existing schedules. Too often, the person who replies, “We have never had a moisture problem” just didn’t know that such problems existed. In such a Catch-22, the building is erected before flooring problems surface a year later. It may take a year for the vested parties to discover the root of the problem and, perhaps, another year to make matters legal. No such situation holds the promise of cost-efficiency, safety and security. It is a circumstance in need of a concrete cure.


Knowing that different types of flooring have the ability to handle different levels of moisture is critical in making decisions. Here is a tool that should help with RH specifications. The physics of effective flooring installation are simple; the social issues are not. Time, economics and professionalism exert their own distinct influences on construction – from the owner down to the bottom of the sub-contractor list. Bad hospital flooring represents a high-stakes health variable. Shoddy installation impacts patients, administration, employees and families all over the nation. Medical facilities cannot afford to shut down, lay-off staff and move patients when bacteria and mold form in an inappropriately-moisturized floor. Luckily, a better knowledge of floor installation mechanics can form the cure to this age-old construction problem.

Further Reading:

American Concrete Institute, Committee 302; Guide for Concrete Slabs that Receive
Moisture-Sensitive Flooring Materials. Ellicott City, MD: American Sports Builders Association
(ASBA). Web (PDF); 2006.

Jason Spangler

Article by Jason Spangler

Jason Spangler has 25 years of experience in sales and sales management, and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as its Rapid RH Product Sales Manager.