The science of construction of large retail space floor slabs continues to develop at a fast pace.
The science of construction of large retail space floor slabs continues to develop at a fast pace.

Within the past six years, advancements in concrete construction have come about more quickly than for any other construction material. This is especially true for commercial and industrial slab-on-ground floor construction. It's the result of the owners' willingness to look beyond the initial cost of construction and focus on durability and maintenance over time; the specifiers and consultants who study how to improve the performance of floors; the contractors who revisit their projects over time to improve their concrete mixes and craft skills; and the manufacturers who develop new equipment able to produce higher quality floors. For contractors, the added incentive lies in the reality that they sometimes are held liable for making repairs on poorly designed floors.

Today green and sustainable construction is becoming the norm. Owners such as Wal-Mart want the public to know that they are doing their part to help reduce emissions of greenhouse gases into the atmosphere by using recycled materials, reducing energy costs, and spending less money on maintenance. Sometimes this results in higher initial construction costs, such as paying for very hard-troweled finishes. Even developers, who build and own structures briefly, are beginning to understand the marketing benefits of more durable and sustainable construction.

Of all the building elements, floors take the most abuse and cause the greatest disruption when problems arise. As a result, floors have become a focus for improvement and changes are occurring.

Owners influence floor construction

The trend toward exposed concrete floor surfaces replaces products such as carpet, vinyl tile, and quarry tile that add to the cost of construction, increase maintenance costs, and can increase insurance liabilities. Sometimes owners include decorative finishes, such as integral concrete coloring, chemical staining, and diamond polishing. They also are specifying gloss numbers to quantify levels of reflectance that can reduce lighting costs and ensure customer appeal. As always, they want floors without cracks and curled joints that cause problems for forklifts and store fixtures, and result in wavy appearances. Some of these expectations are unrealistic—all concrete cracks and all joints curl. Joe Neuber, president of Neuber Concrete, Kimberton, Pa., says he's noticing a trend toward buildings with less square footage and higher volumes. This means that aisle widths are decreasing while storage racks are getting taller, this problem.” When additional products aren't used for a floor surface, the material and the energy to produce them is saved.

The concrete shown here is a well-graded mix with top size aggregate of 1-1/2inches. These mixes make it possible to reduce the portland cement content, also resulting in less shrinkage and curling at the edges of a panel.
Neuber Concrete The concrete shown here is a well-graded mix with top size aggregate of 1-1/2inches. These mixes make it possible to reduce the portland cement content, also resulting in less shrinkage and curling at the edges of a panel.

Owners of retail stores want to reduce their lighting costs and floors with glossy surfaces offer that opportunity. Some retail facilities save as much as 40% on lighting bills.

It's widely known that a fair amount of carbon dioxide (a greenhouse gas) is produced in the kilning of portland cement. Wal-Mart and some other owners are specifying replacements of portland cement with as much as 20% fly ash or slag for concrete floors. These pozzolans are waste products from other industries so the only energy commitment is the trucking needed to bring them to ready-mix producers. Properties of the concrete also are enhanced when using fly ash or slag in the process.

Neuber says that crushed recycled concrete from 1-inch top-size aggregate down to finings is being used more often as compactable subgrade material. He uses it wherever he can because unhydrated material in the aggregate sets and provides a more stable working platform during concrete placement. But he stresses the need to properly compact it.

The nature of floor finishes

Owners want flatter floors, harder troweled, and more durable floor finishes. Thanks to the increased abilities of contractors and the constant development of tools and equipment to perform the work, these goals are being accomplished without significantly changing installation costs.

With concrete as the wearing surface, there's a trend to color floors using integral color, chemical stains, and water-based stains. Diamond polished finishes are standard as well. Owners know that customers like shiny floors so this becomes part of the building's aesthetics.

Wet curing for seven days is becoming more standard for retail stores. The slab is saturated with water and covered with plastic cover materials that hold water uniformly in position during the curing cycle. Floors develop much better resistance to abrasion as a result.
PNA Construction Technologies Wet curing for seven days is becoming more standard for retail stores. The slab is saturated with water and covered with plastic cover materials that hold water uniformly in position during the curing cycle. Floors develop much better resistance to abrasion as a result.

Some owners are willing to pay extra for very hard-troweled floors because they are more abrasion resistant and last longer. In order to preserve the finish during the polishing process, some owner specs avoid using diamond grinders and instead require the use of floor maintenance machines with diamond-embedded strip pads to attain the proper gloss. Afterward, owners maintain the floors with the same equipment—polishing the floor as abrasive foot traffic wears the surface and reduces the gloss.

Building owners also are hiring expert consultants to train crews and be onsite during concrete placements. More specifications require a percentage of finishers on a crew to complete the American Concrete Institute finisher certification program to ensure they understand concrete basics and have the needed skills to properly finish concrete.

The ability of concrete contractors to routinely increase floor flatness and levelness has changed dramatically in the past five years. This is in large part due to the development of equipment, especially riding trowels. So owners now specify more floors with FF requirements between 50 and 60.

Equipment and tool developments

Many developments in the past five years make work easier and better. Here are some examples.

Laser screeds. Their ability to screed very high FF floors continues to improve. Neuber waits to start finishing operations until the concrete is almost too hard to work before the first pass with pan floats in order to preserve flatness.

Finishing machines. Lampasona and Neuber agree that ride-on trowels make a big difference. Ride-ons are getting bigger (up to 3050 pounds) and better. Popular models currently have 4-foot or 5-foot-diameter rotors with up to 2 feet of space between rotors to prevent “ridges” or “windrows” from doubling up. They currently feature five or six blades per rotor to provide better support for pan floats and to achieve flatter finishes. Hydraulic steering provides longer life for the machine and less operator fatigue. They are powered by diesel or gasoline engines up to 100 hp. Look for them to get even bigger because they produce flatter floors.

Plastic blades. Installing burned finishes with steel blades darkens floors, causes problems for colored finishes, and reduces the reflectivity of plain concrete floors. At first contractors installing colored floors switched to plastic blades after the first few passes with steel blades but now contractors are using them on plain concrete as well; and some owners are specifying that they be used. The recent introduction of composite plastic pan floats allows contractors to start the finishing process earlier and achieve flatter finishes.

Vacuums for control joint cutting saws. Without a vacuum, early-entry saws collect dust between the saw's skid plate and the concrete, scratching the fresh concrete 4 inches on either side of the cut. This blemish compromises the appearance of finished work. Vacuuming the dust away as it's created solves the problem.

Placing equipment. Using laser-guided placing equipment to provide the right amount of concrete for a laser screed to strike the final grade improves FF numbers.

Laser-controlled grading. Machine control systems that pick up signals from laser levels provide much more accurate subgrade elevation control under floor slabs, which in turn ensures even thickness floors. Contractors increasingly are adding controllers to their machines. Grade is being rechecked with laser-guided equipment during placement or to remove ruts left from concrete trucks and laser screeds.

Concrete mix design trends

Using top aggregate sizes of 1½ inches for floor slabs 5 inches thick and greater has become the norm. But Neuber says he now uses top aggregate sizes of 2½ inches whenever projects allow because shrinkage is reduced further. When 2½-inch sizes are added to a well-graded aggregate concrete mix, aggregate surface areas are minimized and therefore the amount of cement paste to coat the surfaces. Neuber says mixes using 520 pounds of cementitious material are common and more specifiers are replacing some of that with fly ash or slag. Lampasona adds that water-cement (w/c) ratios tend to range from 0.47 to 0.55 with 0.52 being the trend. The goal is to minimize shrinkage and curling over time. When combined with a close joint spacing—no more than 15 feet in a 6-inch slab-load transfer devices in low shrinkage concrete mixes minimize joint deterioration, resulting in more durable floors. Contractors are in a good position to know how their mixes perform because they can return to their projects years later to see the long-term results.

Figure 1, Materials distribution.
Neuber Concrete Figure 1, Materials distribution.

Good concrete is compromised when improperly cured. The old curing method for floors involved covering it with plastic sheeting to help retain moisture. This process is being replaced with wet curing for the first seven days after placement. Contractors flood water on newly finished floors and place wet curing covers made from plastic fabric with a polyethylene backing over the water to ensure a saturated environment. When the covers are removed, floors are scrubbed and cleaned immediately to remove hydration byproducts that could leave markings and stains. The resulting floors have more durable surfaces and abrasion resistance, and are aesthetically pleasing.

What owners receive

The owners of retail facilities are thinking more about how to achieve longer life and durability in their floors now. They want to save more on energy costs and maintenance as well. Through the combined efforts of contractors and engineers, owners are receiving floors they can be proud of—flatter than ever, elegant decorative finishes, gloss numbers that reduce lighting costs, and with minimal shrinkage, less cracking, and reduced curling.