New Rapid-Drying Concrete Addresses Floor Covering Failures
Preventing Moisture-Related Problems in Concrete Floors
Continuing Education
Use the following learning objectives to focus your study while reading this month’s Continuing Education article.
Learning Objectives - After reading this article, you will be able to:
- Summarize the health hazards, flooring damage and other problems that can result from high levels of moisture in concrete floor slabs.
- Identify current methods for mitigating high moisture levels in concrete slabs.
- Describe a preemptive approach of using rapid drying concrete mix to solve moisture problems.
- Discuss the occupant health and project management advantages of using rapid drying concrete mix.
Moisture-related problems with flooring materials and coatings are today one of the most common and costly of construction issues. The direct and indirect costs associated with these problems are estimated to be in the hundreds of millions of dollars each year in the United States alone. The affects of high moisture levels in a concrete sub-floor can not only lead to failure of the flooring system but can also lead to costly construction delays, indoor air quality issues and the legal disputes that follow when these problems develop.
Moisture in the Slab
Water is an integral component of concrete and is necessary to properly hydrate the cement particles in the mixture. However, water beyond that which is necessary for hydration is added to concrete to create a mixture of a workable consistency. The additional water is known as; “free water” or “water of convenience.” Once the mix is poured and set, free water is released from the slab in the form of moisture vapor during the drying process.
Another source of moisture that can affect concrete slabs-on-ground is water in the earth beneath the slab. This moisture from the water table is constantly migrating upward through soils, in vapor form, until it reaches the underside of the slab. Ideally rising moisture is stopped at this point as it contacts a low-permeance vapor retarder in direct contact with the underside of the slab. However if adequate moisture protection is not in place below the slab, moisture in the concrete will increase over time.
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High levels of moisture in concrete slabs can lead to flooring failures and health hazards. Image provided by U.S. Concrete |
Floor covering failures
If flooring materials are installed on a slab before it has reached an acceptable level of dryness, moisture can either prevent adhesives from properly curing or lead to the breakdown of adhesives due to the development of a saturated high pH environment at the surface of a slab. Adhesive degradation, blistering, disbondment, adhesive bleed and the expansion of flooring materials are all conditions that can be the result of excess moisture in the slab.
Historically contractors have either been forced to wait an extended period of time for the concrete to dry naturally to an acceptable level, or incur the cost and disruption of installing a topical moisture mitigation system. In either case the costs are significant and can delay the project completion beyond what may be acceptable.
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Elevated moisture levels in a concrete floor slab can result in indoor air quality issues and costly flooring damage. Photos provided by Craig & William Lepito |
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Building occupant health problems
Another moisture-related issue that can be even more significant than the considerable cost of a flooring problem is the affect that moisture in and from the slab can have on indoor air quality—or more precisely, the critical role of moisture in the growth of mold.
Since the concept of Sick Building Syndrome first hit the headlines in the mid 1980s, numerous studies worldwide have investigated damp building related illnesses that include a myriad of respiratory, immunological and neurological symptoms. Multi-million dollar toxic mold settlements in the 1990s drew public attention to the issue while academics focused on analyses of mold growth on building materials and measurements of airborne particulate matter.
More recently researchers have assessed occupant exposure to airborne mycotoxins—the secondary metabolites produced by molds that are capable of causing disease and death in humans—and gained new insight into their biomechanisms No longer assumed or suspected to be the cause of health problems, mycotoxins from mold are a proven potential health hazard, Since mycotoxins thrive and multiply in humid conditions, the continued presence of moisture itself may be considered as a potential health hazard.
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Mold containing mycotoxins can thrive beneath or within flooring materials when a high moisture condition is present in a concrete sub-floor. Photo courtesy of Hank Bruflodt |
VOC-forced changes in adhesive formulations
The drive to produce more environmentally safe building materials added to the challenge of successfully installing moisture-sensitive flooring materials over concrete sub-floors. In the 1990s national limits were established as to how much volatile organic compound (VOC) could be used in paints, coatings and adhesives. In some ways water-based adhesives are superior to their solvent-based predecessors. However since the change there has been a noticeable increase in the number of flooring issues.
| Overview of Problems Resulting from High Levels of Moisture in Conventional Concrete Slabs |
New rapid drying concrete technology addresses moisture emission issues from the beginning and at the source of the problem in the slab. Its characteristics include:
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Testing Moisture Emissions
Most U.S. manufacturers of floor coverings, adhesives, and resinous coatings require that moisture tests be performed prior to the installation of their materials and that the tested levels are at or below limits considered to be safe for the installation of their materials.
Historically the most common test required by manufacturers has been the measurement of the Moisture Vapor Emission Rate (MVER) by the calcium chloride method described in ASTM F 1869. However, as the limitations of this test method have become better known, most manufacturers are now requiring that the concrete’s internal relative humidity level be tested in accordance with ASTM F 2170. These two test methods measure moisture in, or from a different portion of the slab. The MVER test measures moisture emitting from the top ½ in. to ¾ in. of the slab only. Measuring the concrete’s internal relative humidity helps to determine what the moisture level in the slab will be once the flooring materials are installed. It is important to understand that the results of either test can only be considered meaningful if a low-permeance vapor retarder is in-place directly below the slab. (For further reading on testing see Moisture Testing of Concrete Slabs: When 3 lbs is not 3 lbs, by Peter Craig and George Donnelly)
Testing the Concrete Surface pH level
Most flooring and adhesive manufacturers also require that a pH test of the concrete surface be performed.
Soluble alkali salts are naturally present in concrete. However if there is sufficient moisture within the concrete to place these salts into solution, the pH level at the surface of the slab will rise to levels that can lead to the breakdown of adhesives and flooring materials. What is important to understand is that to perform a pH test one must introduce liquid water. If there is not sufficient water in the concrete to create a solution after the flooring system is installed, pH will not be an issue regardless of what the test indicated. In short, getting rid of the water and protecting the slab from moisture entering from below, will ensure that pH will not be an issue.
Moisture Mitigating Strategies
In the past, most projects have delayed implementation of a moisture mitigation strategy until it is certain that the treatment will be needed. Using this approach, the moisture level of the slabs is tested a few weeks before the flooring is scheduled to be installed. If the moisture levels are found to be acceptable, the moisture mitigation system is not installed. If, however, the moisture levels are not found to be acceptable, the system is installed. The biggest problem with this approach is that more often than not the floors are not found to be at an acceptable moisture level. Implementation of the mitigation system at this late stage significantly increases the cost and is a disruption to other trades.
As moisture-related flooring problems increased over the years so have attempts to treat or mitigate excess moisture in a concrete sub-floor.
The following approaches are currently being used to mitigate high levels of moisture in concrete sub-floors.
Accelerated Drying
In many parts of the country the ambient environment is not conducive to natural drying of a slab in a timely manner. One approach that has proven effective on some projects is to alter the ambient conditions using equipment that can raise the interior air temperature while at the same time lower the ambient relative humidity. This process is not inexpensive and still requires time. This method also works best when surface of the slab is ground or shotblasted open to aid in the release of moisture.
Topical treatments
Current topical treatments for moisture mitigation fall into the following categories: Reactive Penetrants, Moisture Mitigation Coatings, Modified Cementitious Overlays, Alternative Finishes
Reactive Penetrants
There are many companies promoting the use of sodium, potassium or lithium silicate as a solution to slab moisture problems. Some of these products are applied immediately following final finish of the concrete while the application of others is delayed. In all cases these fluid applied products must find sufficient calcium hydroxide in the concrete to complete a reaction. Some of these silicate-based treatments have proven to be very good surface densifiers for industrial warehouse slabs. However, their use as a moisture mitigation strategy cannot be validated by any test recognized by the flooring industry.
Moisture Mitigation Coatings
There are scores of topical coating materials being promoted for moisture mitigation of concrete slabs. These products include acrylics, hybrid epoxies and urethanes. While none of these systems have a perfect track record, to-date the best of these systems have proven to be the most successful approach to topical moisture mitigation.
Wally Johnson, U.S. Concrete’s vice president of sales and marketing points out that the main concern with moisture mitigation coatings is that a high level of moisture will remain present within the concrete. This can lead to the development of osmotic cells beneath or within the coating, or fuel an alkali-silica reaction (ASR) or near-surface alkali reaction (NSAR) if the aggregate materials are susceptible to such.
Modified Cementitious Overlays
There are several companies promoting the use of modified cementitious materials to mitigate concrete sub-floor moisture and pH. One major attraction to this approach is the idea that one can mitigate moisture issues and level the floor in one step. While the jury is not in on all of these materials, at least one major player withdrew its product from the U.S. market after experiencing a major failure.
Alternative Finishes
The high cost of dealing with slab moisture problems and construction delays has led to an increased use of alternative flooring such as bare, polished or stained concrete. While these finishes may be suitable for some facilities they are not suitable for many others.
Other Mitigation Approaches
Integral Admixtures
A number of companies are promoting the use of chemicals introduced into the concrete to mitigate moisture. While a number of these materials are capable of demonstrating waterproofing properties, the effectiveness of most are not able to be validated by test methods recognized by the flooring industry.
The Ideal Solution
The truth is that there is no company that offers a moisture mitigation system that has a perfect track record of success. When failure of a moisture mitigation system occurs, the cost of correction can be many times the original expense and may also cause the owner of the building to lose revenue. Repairing the repair is not a situation that anyone can afford.
The ideal solution to avoid slab-related moisture problems would be for the slab itself to dry naturally to the required level in an acceptable amount of time. Over the years a number of concrete experts have designed concrete mixtures that could dry quickly but these mixtures were difficult to place and finish and next to impossible to pump.
Today there is new promise that can help put an end to moisture-related flooring problems using concrete that consumes its free water internally and is easy to place, finish and pump. This new approach to rapid-dry concrete has taken years to develop and been validated both in laboratory and field conditions.
Rapid Drying Concrete
After more than six years of research, laboratory testing, and field verification, a new concrete solution has recently been developed that combines the benefit of rapid drying with placement and finishing characteristics similar to those for conventional concrete. Through a combination of unique chemistry and controlling the water of convenience the new portland cement-based concrete solution dries in a fraction of the time required by conventional concrete.
The significant advantage to this cost-effective concrete solution is its ability to stop slab moisture problems from the very beginning and avoid the many problems of moisture emission.
Equally significantly, it contributes to maintaining the health and safety of building occupants by addressing some of the root causes of damp building and illnesses
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With less water of convenience compared with a conventional slab, a rapid-drying slab employing new technology dries more quickly. Image provided by U.S. Concrete |
The new mixture can be placed using conventional equipment, such as concrete pumps, and it can be finished using standard equipment, including laser screeds, conventional vibratory screeds, pan floats, and power trowels.
MVER and RH levels
Field and lab tests have demonstrated that target levels of internal relative humidity (RH) are reached 45 days after building enclosure. Under high internal relative humidity conditions some conventional concrete will never reach those levels.
The new mixture also quickly and permanently meets the moisture requirements for most types of flooring materials.
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New rapid drying technology reached the target level of 80% relative humidity (RH) three times faster than 6 sacks (6sk) or 564 lbs of conventional concrete with w/c ratio of 0.43. Image provided by U.S. Concrete |
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MVER tests for new rapid drying technology mixes meet flooring manufacturers requirements and show lower rates than for the control mix. Image provided by U.S. Concrete |
Rewetting
Unlike conventional concrete mixtures whose drying clock has to be restarted after being rained on or re-wetted, rapid drying concrete is not seriously affected by re-wetting. The drying of rapid drying concrete actually accelerates after being rained on compared with conventional concrete.
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Side by side new rapid drying technology and conventional concrete were dried to an MVER of 3 lbs/1000 ft 2/24 hrs and “ponded” with a quarter inch of water for 24 hours. After soaking, the new concrete reached previous MVER levels in 16 days. Image provided by U.S. Concrete |
Curling
Because the new technology concrete relies on an internal drying mechanism rather than on surface evaporation to reduce internal moisture, the slab exhibits practically no curl or warping. Tests have demonstrated that the new mixture stabilizes shortly after construction whereas the degree of curl in conventional mixture continued to increase for months.
Reduced curling is a tremendous cost and time saving benefit for any tolerance critical situation.
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Curl specimens of the new rapid drying concrete and conventional concrete (w/c= 0.41) were tested with one end anchored after being cover cured for seven days. Image provided by U.S. Concrete |
Cost effective
The advantage of being able to design away slab moisture problems from the start of the project can translate into even greater cost savings when project delays are avoided and there is no need to disrupt other trades or activity to implement a late stage topical solution.
Other potential cost savings with the new rapid-drying concrete mixture include:
- Costs associated with addressing sick building syndrome and air quality issues within the building.
- Remediation costs for mold and mildew damage;
- Corrections to failed flooring.
- Investigations and testing required to resolve disputes.
- Construction delays and cost overruns caused by unplanned extended drying times.
- Costly business interruptions if flooring has to be treated, repaired or replaced.
- Liabilities associated with flooring failures and occupant illnesses.
Applications
The new technology concrete mixture may be used for any project using concrete. It is especially recommended for use in schools, hospitals, biotechnology facilities, electronics facilities, data centers, and retail stores.
| Characteristics of New Rapid Drying Concrete |
New rapid drying concrete technology addresses moisture emission issues from the beginning and at the source of the problem in the slab. Its characteristics include:
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Installation Requirements For Rapid Drying Cconcrete
Design professionals should note the following installation requirements for rapid drying concrete. They are based on findings of lab and on-site testing:
Geotechnical considerations
Step 1. If the underside of the slab is at risk of water-in-contact, use appropriate design considerations for a hydraulic structure including effective drainage and waterproofing.
Where possible install a layer of fill material capable of serving as a capillary break such as coarse gravel or crushed stone.
Sub-slab moisture protection
Step 2. A low-permeance vapor retarder should be installed directly in contact with the concrete. The below-slab vapor retarder should conform to the minimum requirements of ASTM E1745 Class A, with a water-vapor permeance reduced to not exceed 0.01 perm (a measure of the rate of transfer of water vapor through a material permeability test) after conditioning.
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Photo provided by Stego Industries |
Concrete considerations
Step 3. Use a self-dessicating, rapid dry concrete mix. Insert continuous rebar (optional). Finish concrete to a smooth, but non-burnished finish free of trowel marks or ridges.
Curing
Step 4 Curing should be accomplished by cover curing the finished slab for 72 hours.
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Photo provided by Peter Craig |
Surface preparation
Step 5. Remove all traces of oil or wax-based sweeping compounds
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Photo provided by Peter Craig |
Testing
Step 6. Post-installation moisture testing is best accomplished through the measurement of the internal relative humidity of the slab in accordance with in-situ RH% test (ASTM F2170.) and MVER testing by the calcium chloride method (ASTM F1869) RH% testing if required. Calcium chloride testing alone does not provide sufficient information to reliably determine the moisture-related suitability of a concrete sub-floor.
It should be noted that a concrete slab will not test to a 3 lb MVER unless the relative humidity is below 40%.
| CASE STUDY ONE | ||
Kenton Rogers, Senior Project Engineer, XL Construction, Milpitas, CA, chose rapid drying concrete for a facility designed to house sensitive equipment that required a controlled environment. “In my experience, I have never had a flooring failure in a job before, because typically we spend a lot of money on topical barriers. For this project we were looking to do a more cost effective and reliable solution that focused on prevention. Rapid drying concrete allowed us to erect steel a day and a half after the concrete was poured. There was a huge schedule gain during the project because we didn't have to apply topical solutions. Another huge benefit, which was a great selling point to the owner, was that if they decided to do remodeling in the future, they would not have to re-apply a moisture barrier." |
| CASE STUDY TWO | ||
Seeking a timely turnaround for a schedule driven project, Nick Dolci, Estimator /Project Manager, Joseph J. Albanese. Santa Clara, CA., used rapid drying concrete for the first time. “The biggest benefit was that all of the moisture comes out of the mix itself, it's all completely used within the hydration process. The other benefit is the fact that you can apply the epoxy flooring much quicker. Using the product is not much different than applying a traditional concrete mix. The pumping was pretty normal at 85 cu yds an hour. The schedule savings are huge for clients who are looking to have their project turned around as quickly as possible.” |
Conclusion
High levels of moisture within a concrete slab can severely impact the health of a building’s occupants and the success or failure of its flooring systems. Moisture-related slab issues currently cost hundreds of millions of dollars each year to correct. A new rapid drying concrete has recently come on the market that solves moisture problems before they arise and eliminates the need for topical moisture mitigation treatments. Design professionals should find this new technology a welcome cost effective solution to moisture-related health hazards and flooring failures that have long beset projects of all types nationwide.
