FEATURE ARTICLE, MAY 2008

THE BOTTOM LINE ON CONCRETE AND FLOORING
Paying attention during the construction process to what’s underfoot can increase quality and long-term profitability for building owners.
Greg Freehauf and Jerry Webber

Patience is a virtue, we’re often told as children. The problem is that, in the construction industry, fast-track schedules and tight budgets begin to chip away at our dedication to this virtue. However, all too many industrial developers often discover themselves shaking their heads, looking at an uneven floor or separating tiles, and wishing they had taken a patient approach to concrete floor construction. In many instances, developers can save themselves steep repair costs by choosing the flooring construction process, even if it adds time to the schedule.

Before ceiling heights began to soar beyond 28 feet in warehouse construction (40-foot clear heights are now not uncommon), modest amounts of concrete floor curling and even some noticeable shrinkage were mere nuisances, not operational hazards. However, today, these common floor inconsistencies can become operational obstacles in warehouse environments where high-stack racking systems are used. A floor curl of just 1/16 of an inch can result in several inches of load tipping at the highest level of a load on a high-mast forklift truck. Imagine the implications to a building user, should the floor slow down forklift traffic, or worse yet, create the possibility of loads falling from the forklift, and the potential liabilities that could ensue.

Luckily, as ceiling heights and racking systems have climbed, technological developments under our feet have kept up with the times. There are processes that can help developers avoid curling and reduce shrinkage, but all those involved in the warehouse development process need to understand that it takes an investment of time and money upfront to reduce the possibility of shrinkage and curling after the building opens.

If you are an industrial developer or potential investor in a warehouse building, you should ask your design/build or general contractor what it is doing to prevent floor curling and shrinkage in the warehouse, as well as how it can ensure that the finished areas of the office portions of the building are protected from ground moisture and vapors. While the processes being implemented today to avoid these factors may require additional time and labor costs from the pro-forma, the results are well worth the investment. As with so many things in life, the time and money spent initially to avoid the problem will be far less than the eventual cost of repairs.

Defining Curling and Shrinkage

A concrete slab that loses the majority of water from only its top side will find that the top side shrinks more than the bottom, causing curling. If the top shrinks more than the bottom, the slab edges will lift off the sub-grade.

Shrinkage refers to the natural shrinking of the cement as it dries. While the mixture will always undergo a certain amount of shrinkage, more shrinkage than expected will result in gaps between slabs and unwanted breaks in the wearing surface of the floor.

Design/build contractors generally work with their specialized subcontractors to assess each unique building project, and identify the best processes for that situation. TCB Design Build turns to Tom Morris, vice president of Testing Service Corporation (TSC), for in-depth information and solutions to consider for flooring, and have worked with TSC to test the following solutions in the field.

A Concrete Recipe to Avoid Warehouse Floor Shrinkage

The primary method used to avoid shrinkage is to use a carefully controlled concrete mix, much like sticking to a recipe when cooking. Concrete is comprised primarily of Portland cement, water and aggregates. The cement and water form a paste that holds the aggregates together. As the paste hardens, moisture loss volume change takes place, resulting in shrinkage, cracking and curling of slabs. The aggregates are composed of stone and sand, and because of this, do not shrink. Therefore, the concrete should be proportioned so that the amount of paste — the portion susceptible to shrinkage — is minimized.

Paste volume need only be sufficient to entirely coat the aggregates, fill the voids between the aggregate and to allow the finisher to completely close the surface of the slab. Paste volume is minimized through use of a component of 1.5-inch size coarse aggregate, in effect substituting solid rock for less desirable, shrinkage prone cement paste.

In addition to providing a component of 1.5-inch size coarse aggregate, shape and the combined aggregate gradation need consideration to achieve dense aggregate packing and a workable mixture. The objective is to achieve a mixture that uses as much sand and stone as possible, since those materials do not shrink, but remains glued together.

Water should be sufficient to produce at most a 4-inch slump, prior to addition of water reducing admixtures. The American Concrete Institute indicates a desirable range of water-to-cement ratios would be in the range of 0.47 to 0.53 or higher.

It should be noted that concrete mixture designs for floor slabs on grade should not be based on achieving high 28-day compression strengths or low water to cement ratios. A 28-day compression strength of 3,500 pounds per square inch is more than adequate for warehouse/distribution center floor slabs on grade. High compression strengths lead to high cement contents and increased potential for shrinkage.

For example, TCB Design Build used these ratios as a guideline to complete the concrete floor at 594 Territorial Drive in Bolingbrook, Illinois. A 96,978-square-foot speculative distribution center located on suburban Chicago’s I-55 distribution corridor, the building was designed to accommodate both large and small users. Currently in the final phases of construction, it was critical that the floor accommodate potential tenants that will almost certainly utilize high racking systems.

Curling: How to Avoid It

Depending on the time of year and the flatness objectives a developer is striving to achieve, companies should consider using a concrete curing process known as wet-curing. While it is not widely used for standard distribution centers, it is commonly used in freezer-cooler environments, and poses a unique advantage for properties that utilize tall forklifts and high rack systems. Wet-curing for large slabs will control moisture content, and help to ensure that the top layer and the bottom layer dry evenly. This even drying is critical; if the top dries faster than the bottom, curling occurs. If the top stays wet longer, then the drying is more equal. The wet-curing process involves spraying the top layer of the recently-laid concrete periodically as it dries, and covering it with specialized plastic sheeting. Slowing the evaporation rate keeps the top layer moist longer, so the drying occurs at an even pace from top to bottom.

TCB Design Build used the wet-curing process to ensure that the floors would not curl in its Continental Refrigerated Systems building, one of the first speculative freezer-cooler distribution centers constructed in the Midwest. A 513,000-square-foot freezer-cooler distribution center located on suburban Chicago’s Interstate 80 distribution corridor, the building was designed to store frozen and refrigerated foods, and needed to meet stringent user requirements for the racking and storage of products such as ice cream and frozen packaged foods, which are stacked nearly to the top of its 32-foot clear ceilings.

There are time and labor costs to this process. You should allow about 7 days for each section, so approximately 1 extra week after your last pour should be added to the construction schedule.

There are seasonal and climate conditions under which this process becomes even more important. The hotter the weather, the more rapid top-layer evaporation occurs, so it tends to be more important to deploy the wet-curing process during the summer months than in colder weather. Conversely, if humidity is high, there is less of an issue with evaporation than if the weather (or climate in general) is noticeably dry.

The alternative to wet-curing is to use a spray-on curing compound, which takes little more than an hour — clearly a time saver in the short-run. While this is the accepted industry standard, and can be tempting in a fast-track schedule situation, the payoff in the long-run may not be very good, as the process is not as universally effective as wet-curing. Developers should talk to their contractor about the pros and cons of each method on a case-by-case basis.

While TCB Design Build chose to use wet-curing in the Continental Refrigerated Services property, the company elected not to use it on another speculative building, 300 Central Avenue, in University Park, Illinois. For 300 Central, a 575,024-square-foot speculative distribution center, the timing of the construction (the floors were poured during a cooler season) and overall technical requirements of the construction meant that using the more time-consuming and costly process was not necessary. Recently completed, the floors show no signs of curling or shrinkage. Symptoms usually appear within 30 days of construction completion, and this building is well past that marker. The decision to use the wet-curing process or spray-on curing process should be done consciously, and with consideration to a multitude of factors.

Unwelcome Neighbors: Vapors and Ground Moisture

As important as it is to even out the drying process in warehouses, the office portions of these buildings benefit from vapor retardation, which decidedly encourages dryness — especially underneath carpeted areas. Office area slabs that have vapor sensitive floor coverings should be provided with a plastic water vapor retardant layer, placed directly below the concrete floor slab. This will help to avoid moisture seepage from the ground beneath the floor, which can cause carpet adhesives to malfunction, and also can cause problems with tile floors and carpet tiles.

The Bottom Line on Flooring

Patience and attention to detail will make the difference between a smooth, flat floor and a curling, concrete sea riddled with waves and shrunken slabs. By choosing to use time-consuming, yet successful, processes like wet-curing and vapor retardation during the construction process, developers can greatly reduce the possibility of the need for expensive repairs later.

Greg Freehauf is president and Jerry Webber is vice president of TCB Design Build, a Tinley Park, Illinois-based design/build company specializing in third-party construction of commercial real estate projects.

©2008 France Publications, Inc. Duplication or reproduction of this article not permitted without authorization from France Publications, Inc. For information on reprints of this article contact Barbara Sherer at (630) 554-6054.