The Atlanta Bonded Warehouse Corp. (ABW), Kennesaw, Ga., has always tried to improve its facilities and operations with each new warehouse or expansion. ABW believes in putting together the best team possible and working closely with them, rather than having the all-too-common adversarial relationship that is so detrimental to quality. Thus, for the addition of Cooler Rooms 6 and 7 to the 2500 Building in Kennesaw, ABW and the engineers worked closely on a design that would be state of the art, feature cutting-edge technology wherever possible, and be as constructible as feasible. Furthermore, input was received and incorporated from the contractors TPI, Acworth, Ga., and Precision Concrete Construction, Alpharetta, Ga. Although the project had many special features, the unique light-reflective floor with no cracks and only one open construction joint was the feature that set it apart from other facilities of its type.
Because food products would be handled, ABW wanted the facility's interior to be very cleanable and bright, with optimal lighting that minimized power demands. The company also desired the floor to be extremely durable, with joints and cracks minimized or eliminated. The design of the 180,000-square-foot floor included joints as far apart as 380 feet, and a light-reflective, post-tensioned, very flat/level surface. The floor was constructed in five major placements with four major construction joints, three of which were held tight by PT strands. In fact, the largest concrete placement (more than 43,000 square feet with no joints) was a world record for this type of floor.
A light-reflective (white) floor gives a much brighter, more evenly lighted facility. With tall racks (such as those of ABW) and a normal gray floor, even high levels of lighting cannot provide uniform and optimum illumination. Lift truck drivers are quicker and more efficient with ideal lighting. Furthermore, employee morale improves when work is conducted in a bright, pleasing environment. At food-handling facilities, such as at ABW, a well-maintained white floor exudes an aura of cleanliness and sanitary conditions. The high reflectivity of the floor requires fewer light fixtures than a typical gray floor, but still features better and more uniform lighting. The fewer fixtures also results in a substantial power savings over the life of the facility. This is especially the case with the ABW facility because the rooms were designed to have an operating temperature of 35º F and additional fixtures would have produced more heat load.
Because the facility was to handle food products, ABW wished to avoid the joint and crack problems that plague most distribution centers so the engineers recommended a two-way PT floor system with only construction joints, which were kept closed by the PT. There were no sawed contraction joints. If done properly, there would be no significant cracking (in fact there has been none in the 11 years since the floors were built).
Why very flat/level?
Very flat/level floors facilitate special high-lift, fast-lift trucks operating in high-rack warehouse areas. The specified tolerances include FF45 and FL30 overall, with local values of FF30 and FL20. This degree of flatness and levelness is dictated by the high lifts of the truck as well as its speed down the aisle. A slight difference in floor elevation beneath the truck's wheels could cause a substantial deviation from the proper relationship of the truck and upper rack locations, potentially causing operating difficulties. The typical bumpy warehouse floor would cause a lift truck to run slower down the aisle, reducing productivity more than the 25- to 50-year life of the facility. Additional benefits of very flat/level floors include reduced lift truck maintenance and improved driver performance and morale.
Material handling considerations control the design of the facility, and the high rack layout dictates the location of construction joints. Racks typically are placed back to back at construction joints to keep them out of the traffic aisles. Longitudinal construction joint spacing spanned as wide as 113 feet. This wide strip width and heavy hardener application rate made it a challenge to maintain consistent flatness much greater than specified, but it was achieved.
No transverse construction or contraction joints were located in the slab strips for the following reasons:
- No contraction/control joints are needed in a post-tensioned slab.
- Flatness tends to drop adjacent to any joints, primarily due to slab curl.
- Gaps at the joints cause bumps when lift trucks pass over them.
- The drop in flatness as previously noted, in combination with these bumps, causes a rougher ride and more lift truck maintenance.
- Joint spalling and similar problems become more common.
The 380-foot-long strip placements were post-tensioned to minimize possible cracking. The unique design and computer program developed by the engineers allowed relatively thin slabs to be used—6 inches thick rather than the 12 inches required by a conventional design and slab type—despite the heavy rack loads and lift trucks. The strips were tensioned in both directions in order to hold the joints tight and provide extra load-carrying capacity. The number of tendons was reduced by using two sheets of polyethylene and a very trimable, stable base material to lower the coefficient of friction.
Post-tensioning was done in three stages: The initial tensioning occurred early in the morning after placement to prevent early cracking. The amount of initial tensioning was based on field concrete cylinders taken from the last two trucks and tested early the same morning as the PT. The second tensioning happened early the second morning, based on other field cylinders. Final tensioning was completed as soon as the field concrete cylinder compressive strength reached approximately 2500 psi.
Joint design and movement
Large post-tensioned slab movements were expected, primarily due to shrinkage in a cold, low humidity environment; short-term elastic shortening immediately after the post-tensioning; and creep, or long-term shortening as a result of the post-tensioning stresses applied over a lengthy period of time. These movements are significant over a three-year period, but proceed at an ever decreasing rate during that time.
Differential movement between adjacent slab strips also was expected to occur due to different times of placement and post-tensioning, different rates of shrinkage, and thermal differences. Furthermore, because Rooms 6 and 7 were connected but offset from each other in the east-west direction, the movement due to the PT was in opposite directions about each axis.
To solve these movement/sealing problems, a number of steps were taken. The mating edges of all new slab strips were built as straight and plumb as possible. After stripping the side forms, the mating edges were covered with two coats of a high-solids curing compound, which reduced the sliding friction between adjacent slabs and improved curing at the critical joint locations. Short lengths of tight-fitting, soft pipe insulation were placed on the PT tendons next to the previously placed PT slab anchorages to allow the anchorage wedges to unseat and make differential slab movement possible. These steps allowed the PT slab strips to move differentially to each other without distress parallel to the joints but still have sufficient clamping force to transfer vertical load across the joints.
Wherever possible, isolation joints were used for vertical and horizontal differential movement. Typical locations were at the wall/slab interfaces and around interior columns and bollards. The column and bollard isolation block-outs were offset so when all movement took place over a two- to three-year period, the column or bollard was approximately centered in the hole (instead of starting out centered and ending up near one side). As much as 2 inches of movement was calculated. Polyethylene foam plank was used instead of asphalt-impregnated fiber filler because it was too stiff and would not deform adequately.
The number of open joints was minimized further by two techniques the engineers pioneered. All PT slabs must have a 3- to 4-foot-wide jacking strip between the tendon anchorages at the end of the PT slabs and previously built walls to allow jack access for tensioning the tendons. Later, the jacking strip is filled in; common practice is to have two open joints (one between the PT slab and the jacking strip and another between the jacking strip and the wall). However, on this project the engineers called for tendon tails (or other approved means) to tie the PT slab and jacking strip together so the PT slab would drag the strip along with it, thereby eliminating one open joint. This proved highly successful—although careful detailing and construction was necessary to prevent restraint and the resultant cracking from strip penetrations, such as sprinkler risers.
Individual, deep dock leveler pits typically have been used in the past, and their potential restraint to PT slab movement caused PT designers to end PT slabs several feet inward from the pits. This means placing a joint in the highest traffic areas that continually widens in the first three years, causing spalling and filler problems. Furthermore, individual pits have reentrant corners and other restraints causing the slab around them to crack, if not properly detailed. However, on this project, ABW and the engineers specified a continuous, shallow dock leveler pit with levelers that rotate to the vertical position when not in use. The lower, base slab was constructed before the upper (PT) slab was placed. A double slip sheet of polyethylene allowed the PT slab to slide over the base slab as the upper slab shortened. Holes were cut in the dock leveler support channel to allow the PT tendons to pass through (after coordinating their location with the leveler manufacturer's pivot and anchorage requirements). Thus, there is no slab joint inward of the dock levelers and no reentrant corners to crack.
Where joints were required between PT and new non-PT sections in traffic areas, a unique bar armored joint system was designed, along with square dowels and thick cushioning side pads to allow horizontal movement parallel and transverse to the joint while transferring vertical load. The engineers also designed special armored joints and dowels where significant movement was expected between new and existing slabs.
Concrete mix requirements
For the concrete mix, some unique requirements were specified by the engineers. The most important factor may have been the aggregate gradations that met requirements for coarseness factor, workabilty factor, power 0.45, and combined percentage retained on each sieve.
Preparation for concrete placement
Specified measures included surcharging the site and replacing wet silty soils with granular dry materials. Proper PT tendon locations were extremely important, including supports at the correct height and spacing. Mislocation due to improper installation or movement during concreting operations could cause cracking or blowouts during post-tensioning and affect levelness.
Concrete placement and finishing
The floor was designed for concrete tail-gating: lifting and placing the tendons on chairs just ahead of placement, striking-off and consolidation with a laser screed, and placing color hardener with a telescoping spreader. The mineral aggregate light-reflective hardener was applied at the heavy rate of 1½ pounds per square foot. Using wood bull floats minimized the chances of surface delamination. Long channel floats and 10- and 12-foot highway straightedges provided close surface tolerances. Pans were used on riding trowels to enhance floor flatness.
Monitoring flatness and levelness
The specified surface tolerances were greatly exceeded, with typical ranges of FF60–90 and FL35–45—two to three times better than typical industrial floors.
For 11 years, this floor has met or exceeded owner expectations. According to ABW, the flatness and lack of open joints and cracks practically eliminates floor maintenance, reduces lift truck maintenance, and minimizes complaints by drivers. The light-reflective slab reduces lighting by 80% compared to a typical gray floor, significantly reducing both first costs and life-cycle costs, and provides more uniform vertical and horizontal foot candles that use far less power. It's the ultimate sustainable floor; sometimes the greenest floor is a white one.
Cecil L. Bentley Sr., is a concrete construction consultant for Structural Services Inc. (SSI), Atlanta; Jerry A. Holland, PE, F.ACI, is director of design services for SSI; Frederick R. Keith, PE, is CEO of Atlanta Bonded Warehouse and Colonial Cartage Corp., Kennesaw, Ga.; and Wayne W. Walker, PE, is director of engineering services for SSI.