Design and construction of a state of the art superflat floor

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By Jerry A. Holland and Wayne W. Walker hen selected by Du Pont to provide design and construction management services for a new Duluth, Georgia distribution center, we opted for a design that, in all ways feasible, would be state-of-the-art. This would better assure a design that accommodated the many specialized functions to be served by the suburban Atlanta distribution center. Because of time restraints, a fast-track method of design and construction was adopted.

Some of the unique features of the Du Pont facility's floor make it significantly different from typical warehouse floors, and even from many previously constructed critical-tolerance floors. The final design specified 116,000 ft2 (10,800 m2) with superflat floors, the design and construction of which incorporated the most advanced techniques available.

Some of the techniques were combined in ways never previously used, to our knowledge, to create floors of this type.

Why superflat?

Superflat floors were required to facilitate special wire-guided turret trucks operating in narrow-aisle high-rack warehouse areas. This extraordinary degree of flatness and levelness was dictated by the highlevel operation of the turret truck as well as the speed of the truck down the aisle. Even a slight difference in elevation of the floor 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 planned turret truck would have to be operated at less than maximum speed when traveling down the aisle on a typical bumpy warehouse floor, significantly reducing the warehouse productivity over the 25 to 50 year life of the facility.

Additional benefits of superflat floors include reduced lift-truck maintenance and improved driver performance and morale.

F-numbers Superflat floor specifications generally utilize F-numbers to specify minimum floor flatness and levelness.

F-numbers, which usually are determined by special measuring devices and statistical methods of analysis, are referenced in several ACI, ASTM, and CAN (Canadian standards) documents.

The FF number, an indicator of flatness or smoothness, is a measure of surface curvature over a 24 in. (61 cm) distance. The FL number, which indicates floor levelness, is a measure of floor slope over a distance of 10 ft (3.05 m). Narrow aisle superflat floors characteristically are specified in terms of an Fmin number in the actual wheel tracks of the lift truck to be used.

Superflat floors are defined as those with a minimum FF (flatness) number of 100 and FL (levelness) number of 50 to 100. The higher the F-number, the flatter and more level the floor. F-numbers are directly proportional; thus, a superflat floor with FF 100 is twice fl

as a floor with FF 50 and is eight times as flat as the average industrial floor, with an F-number of FF 10 to 15.

Coordinating design and construction For any architectural/engineering project, there must be close coordination from start to finish among the various design disciplines, construction manager, contractor, subcontractors, material suppliers, testing agencies, and owner. But even greater cooperation is necessary for a superflat project to be successful, and it was recognized from the beginning that the participants had to act as members of a close-knit team, free of any adversarial relationship. The success of the completed facility testifies to the high level of teamwork enjoyed on the Du Pont project.

Layout design Material-handling considerations controlled the design of the facility.

The high-rack layout dictated the locations of construction joints. The racks were placed back-to-back at the construction joints, and the narrow 6 ft (1.83 m) wide aisles were centered at the approximate centerline of the concrete strip placements.

Thus, longitudinal construction spacing was narrow, varying from 13 ft-0 in to 14 ft-6 in.

(4.0 to 4.4 m), as compared to typical strip widths of 15 ft-0 in to 30 ft-0 in. (4.6 to 9.2 m) or more for standard floors. This narrow strip width was chosen as it complemented the rack layout but was also Concrete International

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necessary to maintain consistent F-numbers of over 100.

No transverse construction or control (contraction) joints were located in the superflat strips for the following reasons: 1. F-numbers drop significantly immediately adjacent to any joints, primarily due to slab curl.

2. Gaps at the joints cause bumps when the lift trucks pass over them. The drop in F-numbers as just noted in combination with this bump can cause a rougher ride for the lift truck.

3. Joint spalling and similar problems would become more common because the lift truck wheels always run in the same tracks.

Post-tensioning design Superflat floors of the late 1970s and early 1980s used conventional reinforcement and were allowed to crack. Some of these early installations have proved adequate in spite of the cracks; however, others have not fared as well.

Therefore, the strips were posttensioned to minimize possible transverse cracking in Du Pont's long strip placements (over 250 ft [76 m] long in some locations). This post-tensioning allowed relatively thin slabs to be used, despite the heavy rack loads and lift trucks.

Most of the placement strips were tensioned longitudinally only. One area, however, was tensioned in both directions because the area rack layout had not been finalized.

Post-tensioning was to be done in two stages. The initial tensioning was done early the morning after April 1991

Turret truck in operation on a 250 ft (76 m) long superflat strip with no transverse joints in narrow-aisle high-rack warehouse. The aisle is 6 ft (1.82 m) wide, with storage approximately 40 ft (12.2 m) high. Note driver just below raised load. that strip's placement to prevent early cracking. The amount of initial tensioning was based on concrete cylinder tests done early that same morning. Final tensioning was to be completed when the concrete compressive strength reached 2400 to 3000 psi (16.6 to 21.0 MPa). The exceptionally long strip lengths required that the tendons be pulled from both ends.

The transverse post-tensioning

required the following steps: 1. Three to four slab strips placed in the middle of the area were post-tensioned longitudinally in the same manner as other areas.

2. These initial strips were posttensioned in the transverse direction with intermediate anchorages; the tendons continued outward to the boundaries of the area.

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3. Working outward from the initially placed middle strips, other strips were placed and posttensioned longitudinally, then transversely to the initial middle strips until the whole area was completed.

Joint design and accommodation of movement Large post-tensioned slab movements were expected, primarily due to shrinkage, shortterm elastic shortening due to the post-tensioning, and creep (longterm shortening as a result of the post-tensioning stresses applied over a lengthy period of time).

These movements become significant over a time span of 18 months or more, though they proceed at an ever-decreasing rate during that period.

Differential movement between adjacent slabs is also expected to occur due to different concrete strengths, varying times of placement and post-tensioning, different rates of shrinkage, and thermal differences. As an example, high concrete temperatures caused by the heat of cement hydration can cause the newest slab placement to grow or offset shrinkage while the older adjacent slab panel would be shrinking due to cooler night temperatures and normal drying shrinkage. The design problems caused by these expected movements had to be solved. Du Pont's requirement that all joints be fully sealed further complicated the design. Solutions to these movement/sealing problems follow.

Slab keys and aggregate interlock are not positive means of providing shear transfer between adjacent slab panels at typical joint spacings; therefore, smooth steel dowels were required.

However, the large movement expected of adjacent slabs required lightly greased, square dowels with cushioning side pads to be specified in all locations requiring positive shear transfer.

These padded, square dowels allowed longitudinal and horizontal differential movement between adjacent slabs, but transferred vertical loads. Close dowel alignment tolerances and 24

Smooth square dowels with cushioning side pads to allow longitudinal and horizontal differential movement between adjacent slabs but transfer vertical loads.

Note posttensioning tendons penetrating side other strict requirements were specified to minimize the possibility of dowel binding and the resulting slab problems.

Wherever possible, isolation joints were used to allow both vertical and horizontal differential movement. Typical locations were at the precast concrete wall/slab interfaces and around interior columns. So much movement was expected at some interior column locations that 1 in. (25 mm) thick premolded joint filler was used to wrap the columns. Polyethylene foam plank was used rather than the typical asphalt-impregnated fiber filler because the fiber filler was too strong to deform adequately.

Floor joint sealant selection was a compromise because all joints had to be sealed, accommodate large movements, and stand up to the normal abuse of a warehouse floor environment.

A silicone sealant, while accommodating the largest movement, would not be durable enough for the environment. A semirigid epoxy would be much tougher, but would not allow sufficient movement without failing.

An elastomeric polyurethane was chosen as the most practical sealant to meet the specific requirements.

Joints transverse to the posttensioning open so much that they tend to experience rapid, severe spalling when subjected to repeated lift truck traffic. Because elastomeric joint sealant manufacturers recommend a minimum sealed joint width of Âź in. (6 mm), even joints not transverse to the post-tensioning would be subject to spalling at heavy traffic locations.

To minimize

joint spalling, a proprietary epoxyarmored joint system was specified to provide extremely strong joint edges and yet allow joints to open without restraint.

Concrete mix requirements Some requirements for the concrete mix had never been used for superflat floors before, while others had been used only rarely.

Some of these mix requirements follow; 1. Perhaps the most important factor may have been the more uniform than normal aggregate particle size distribution requirements.

The engineer/architect specified that the total percentage of fine and coarse aggregates retained on any one sieve be a minimum of 8 and a maximum of 18 percent of the total combined aggregates (many mixes have ranges from 1 to over 25 percent), excluding the 1 % in.

(37.5 mm) and No. 100 (0.15 mm) sieves. These percentages were not easy to meet due to quarry and concrete plant restrictions.

However, the engineer/architect and the concrete supplier collaborated in determining a blend of aggregate size groups to meet the job specifications while remaining reasonably economical and allowing for proper quality control.

The middle aggregate sizes No. 4, 8, and 16 (4.75, 2.36, and 1.18 mm) sieve sizes - are normally used in asphalt and other applications, thus the amount available for concrete is the minimum possible. Usually two or three different size groupings of coarse aggregate are needed to meet these mix require merits.

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However, the only plant in proximity had a maximum of two bins available for coarse aggregate, with one of these bins holding No. 57 stone (1 in to 4 sieve [25.0 to 4.75 mm]) for other customers. No. 4 (1½ to 3/4 in. [37.5 to 19.0 mm]) stone was specified. No. 89 (3/8 to No. 16 sieve [9.5 to 1.18 mm]) stone was required to provide the middle sizes previously noted. No. 57 stone was the third coarse aggregate group included.

Normally this combination would require a minimum of three bins, but only two bins were available, with one already dedicated to one size. The engineer/architect suggested preblending the No. 4 stone with the No. 89 stone at the quarry and employing close quality control measures to minimize the segregation that would occur with normal procedures. The concrete supplier's quality control procedures eliminated aggregate segregation as a major problem.

Although many aggregate blends were considered, the best contained two middle sizes that were slightly less than 7 percent. This blend was considered to be the best that could be obtained with commercially available aggregates in the area, and the specification deviation was allowed; the subsequent very successful results of the mix showed this compromise was justified.

2. The specified aggregate was to have a high percentage of largesize stone. A nominal size of No. 4 (1 1 /2 to 3 / in. [37.5 to 19.0 mm]) stone was specified as the largest size, rather than the No. 5

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(1 to 1 /2 in. [25.0 to 12.5 mm]) or No. 6 (3/4 to 3/8 in. [19.0 to 9.5 mm]) stone typically used in the Atlanta area. These large sizes, combined with the middle sizes noted previously, made the mix a very "stony" but very workable one that minimized the quantity of cement paste, with the advantages noted in the following.

3. Natural sand, rather than the manufactured sand from rock crushing operations, was required for enhanced finishability and reduced water and cement requirements. Finishability is one of the most important factors for a superflat slab mix because the (top left) Conformance to superflat surface must be restraightened tolerances begins with the forms. many times.

Note bevelled and closely planed 4. Type II or Type I-I1 (Type I top surface. with a tricalcium aluminate content of 8 percent or less) cement was (top right) The special concrete specified for reduced shrinkage and mix was extremely pumpable, lower heat of hydration, as workable, and cohesive in spite of having 1½ in. (37.5 mm) compared to the normally used aggregate, crushed aggregates, and Type I. no admixtures. Note two-way post5. The maximum concrete tensioning tendons. temperature at placement was to be 85 F (29 C), which increased the (above) Using a 12 ft (3.66 m) wide highway "window of finishability." This was straightedge can improve F F especially important because slab values by 30 to 50 percent. placements were in the summer, with daily air temperatures of 90 to SUPPLEMENT SUPERFLAT FLOOR PAGE 5 OF 9

100 F (32 to 38 C). Aggregate stockpile sprinkling, early morning placements, and occasional use of ice on especially hot days facilitated this.

6. Consistency of concrete at placement is a key factor. This was achieved with an on-site slump station, close cooperation in delivery and placement, minimizing or eliminating the use of admixtures (the initial mix had a small amount of a low-range water reducer, but it was immediately found to not be needed and only introduced another variable; thus, no admixtures were used for the bulk of the slab placements), close quality control, and other measures. The concrete supplier provided a quality control manager to be on-site for all superflat slab placements, and tight quality control procedures were maintained at the plant. Because the condition of the ready-mix truck drum blades is so important to proper mixing and consistency, all trucks proposed to be used were inspected; the identification numbers of all trucks passing the inspection were listed, and only these trucks were allowed on the site.

The following are advantages, as compared to a typical concrete mix, derived from these specified requirements: 1. Less water was required.

2. Less cement was required.

3. The mix was far more workable and finishable, in spite of high coarse aggregate content and the absence of admixtures.

4. The mix was far more pumpable, in spite of 1½ in. (37.5 mm) aggregate and absence of admixtures. No mix adjustment was required when changing from tailgate placements to pumped placements 400 ft (120 m) away.

5. Extremely high F-numbers were obtained.

6. More consistent concrete strengths were seen at all ages.

Cylinders were tested at 1, 7, and 28 days. The standard deviation for 1 day breaks was 281 psi (1.94 MPa), which is incredibly low for one day, especially considering the breaks

5

were actually made on cylinders less than 24 hours old.

7.

Higher 28-day concrete strengths were obtained for a given cement content. This was especially significant considering that Type I-II or II cement, which tends to gain strength at a slower rate than Type I for the first 28 days but catches up in the next 28 days, was specified.

8. There was very little bleeding and far less segregation than is usual. The coarse aggregate stayed very high in the slab.

9. There has been significantly less shrinkage, creep, and curl. The extra mix requirements added an additional 10 to 15 percent beyond the typical ready-mix concrete cost.

All parties involved with the project agreed that the extra cost was well worth it, and the concrete contractor stated that this extra cost was at least partially and possibly totally offset by the reduced labor required.

The following comments by key concrete mix participants are of special interest: 1. The floor flatness consultant, who has measured and observed the installation of several million ft2 of superflat slabs throughout the world, stated this was the best concrete mix he had ever seen. This was in spite of the fact that the crushed granite coarse aggregate used in Atlanta has long, slivered particles that cause a tendency towards harsh mixes.

2. The concrete contractor stated that this mix pumped and finished better than any other of the 20 million plus ft2 of slab they have placed.

3. The concrete supplier said a higher level of quality control than usual was required, but was extremely happy with the way the mix performed. There was no concern over low concrete strengths.

Bidding and preconstruction preparation Superflat floors require special expertise for both design and construction.

Few architectural/engineering firms have designed, and even fewer contractors have constructed, superflat floors. For this reason, a prequalified bidders list was prepared for the Du Pont slab package, using only contractors with a proven track record of consistently producing criticaltolerance floors. The successful bidder proved to be proficient and cooperative during the project.

The engineer/architect assisted the contractor by maximizing constructability and communication. Examples of this are scheduling slab installation so that it could be done after the building was closed in; specifying the mix design and ready-mix concrete quality control requirements as noted previously; locating the placement strip side construction joints on one side of the columns, rather than on the column centerline, so that only one placement would be inconvenienced instead of both; and holding regular meetings, starting with the pre-slab construction meeting.

The pre-slab construction meeting was crucial to getting the slab placements off to a good start.

The engineer/architect required the following representatives to be present because their cooperation and performance was critical to the project's success: 1. The concrete contractor's project manager and the field superintendents assigned to the project. A pumping subcontractor would ordinarily have been required, but the concrete contractor does all their own pumping, eliminating the need for this subcontractor.

2.

All pertinent concrete supplier representatives, including the concrete supplier's QA director, the mix designer, and the local concrete plant manager and QA personnel.

3. The office manager and field superintendent from the fabricator and installer of the post-tensioning materials.

4. Three representatives of the subgrade and concrete testing agency, including office, lab, and field personnel.

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5. The slab tolerance testing consultant.

6.

Several of the engineer/architect's representatives, including the construction management department manager and site representatives, structural group leader, project manager, and the inhouse structural consultant for the superflat floor.

The most important aspects of the project were discussed, coordinated, and finalized where possible at this preconstruction meeting. The success of the meeting, and the ultimate success of the project, hinged on the spirit of cooperation and the fact that attending personnel would eventually be directly involved in the project and had the authority to control their part of the work. For too many projects, there is no formal preconstruction meeting or the attendees are either at too high or too low a level to implement the measures discussed at the meeting.

It was decided to install one or two test slabs in a nonsuperflat area.

The engineer/architect believes that even the best, most experienced contractors should routinely install test slabs and demonstrate an ability to satisfy specified tolerances and other requirements with the environment and particular crew, procedures, and equipment to be used on the final work. Only the first test slab was required because of its success.

Preparing for concrete placement A good subgrade, important for all slabs-on-grade, is especially critical for superflat floors because of the close tolerances and heavy rack loads. Typically, the only locations where joints are dowelled are those where lift truck traffic occurs. The construction joints where the racks back up to each other are not usually dowelled except at the cross aisles; thus, a strong, uniform subgrade is imperative. The Du Pont subgrade preparation was performed during a very wet winter and spring, which caused severe soil April 1991

problems. All possible measures were taken to ensure the subgrade was of top quality, although they involved extra cost and the possibility of significant delays to the project. These measures included laser grading and replacing large Quantities of wet silty soils with more granular dry materials.

Side forms are crucial elements of a successful superflat slab and affect most aspects of slab construction. If the tops of the forms are not sufficiently true, even the best finishers cannot meet the desired F-numbers, and this is especially so for FL values. The concrete contractor took several measures to ensure quality forms.

The top edges of the wood forms were planed and then bevelled in the shop, leaving a flat top surface ½ to 3/4 in. (12 to 18 mm) wide, before shipment to the site. The forms were handled very carefully to prevent damage and were set level and plumb, (plumbness is more important than most people suspect).

A 10 ft (3.05 m) straightedge was placed on the installed forms and, if any daylight could be seen, a power hand-plane was used to true the top surface. Then the forms were adjusted as necessary. During concrete placement, the elevation of the forms was constantly checked and, if necessary, readjusted. The resulting high FL values indicate the care taken with the forms.

(top) Profileograph produces a continuous graph for an entire slab placement that reveals at a glance if the slab is in or out of tolerance. (bottom) Profileograph is set up to track the exact path the lift truck's wheels will follow down the narrow superflat aisles.

Proper post-tensioning tendon location was extremely important.

Tendon supports had to be the correct height, spaced closely enough, 27

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Slab with F F value of 274 shows workers' reflections two weeks after it was placed.

and adequately secure the tendons. from the aisle centerline, to keep Mislocation due to improper from having a tendon too close to installation or movement during the wire. If in close proximity, the concreting operations could cause tendon could affect the lift. truck cracking or blowouts during post- operation. tensioning and affect levelness.

Because the lift trucks would be Concrete placement and wire-guided by a wire installed later fishing in a sawcut slot down the middle of the aisle, an even number of As already noted, some of the longitudinal tendons had been concrete was tailgated but most was required, along with a minimum clearance E.I. Du Pont De Nemours and Owner: Company, Wilmington, Del. Engineer/architect: Construction manager:

Concrete and pumping contractor: Post-tensioning subcontractor: Concrete slab supplier: tolerance testing: Concrete and subgrade testing: 7

pumped. Great care was taken to keep the concrete placement going at a consistent rate, as even a 15 or 20 minute interruption can cause quite noticeable elevation differences that show up during tolerance testing.

The concrete was consolidated by a vibratory screed and poker vibrators around the post-tensioning anchorages. A vibratory screed is very beneficial in achieving proper concrete consolidation and can affect the F L values; however, it does not significantly improve F F values and actually induces slight undulations in the surface that are corrected by the later finishing procedures.

Good concrete consolidation behind the posttensioning anchorages is extremely important to prevent blowouts and movements.

F F , values are affected primarily by finishing operations after the initial strike-off; thus, the tools and procedures used for finishing the slabs played a major role in meeting superflat tolerances. Bullfloats were not used because many times they create more problems than they solve; 10 and 12 ft highway straightedges were used instead. By substituting highway straightedges in the finishing operations, finishers can increase F F values by 30 to 50 percent. The straightedges were used to cut down high spots and fill in low spots and were used over and over throughout the finishing operations until the floor surfaces were too hard to be affected by them. Special power floating and trowelling techniques were used to maximize flatness.

Wayne Walker & Jerry Holland Lockwood Greene Engineers, Inc., Planners/Engineers/Architects/ Managers, Atlanta, Ga. Tri-Bentley Constructors, Inc., Marietta, Ga. VSL Corporation, Atlanta, Ga. Williams Brothers, Atlanta, Ga. Face Construction Technologies, Inc., Norfolk, Va. Law Engineering and Testing, Atlanta, Ga.

Monitoring flatness and levelness As soon as the final trowel pass was made, the surface tolerances were tested with a Profileograph and Dipstick, allowing the contractor immediate feedback so that any changes could be made for improvement.

Many people mistakenly believe the only purpose of slab Concrete International

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tolerance testing is to see if a contractor has met the specified requirements so they can penalize him if he did not. However, timely testing serves as a check for the contractor as well to indicate needed adjustments.

The Profileograph provides a continuous graph of the floor profile in the actual wheel tracks of the lift truck to be used; out-oftolerance areas can be seen readily on the graph and marked on the floor for correction. (There was no out-of-tolerance work on this project.) The Dipstick is used to determine the FF and FL values.

Testing was completed very quickly, and the specified high solids curing compound was applied immediately afterwards.

To put these values into the proper perspective, the average F-numbers for this project are approximately 10 times flatter and more level than the majority of the industrial slabs-on-ground now existing; the highest FF number obtained was between 25 and 30 times better.

In fact, the F-numbers were so high for this project that the concrete contractor received a Golden Trowel award given by Face Construction Technologies at the 1991 World of Concrete Exposition in Las Vegas for the flattest superflat floors in 1990.

Many superflat floors have required some, and occasionally extensive, grinding for out-oftolerance areas. No grinding was required for this project.

The future Production Because of the superflat tolerances and the fact that the facility consisted of four completely separate and different areas, slab production was significantly lower than for a typical large floor. Daily production of two to three placement strips typically varied from 6000 to 10,000 ft2 (550 to 950 m2). The extra care and reduction in productivity pushed labor costs 75 to 100 percent higher than for conventional slab work.

The increase in material costs were only 3 to 5 percent of the total material, excluding the extra costs of the special concrete mix, which was at least partially offset by the labor savings noted earlier.

Results The Du Pont superflat floor has been very successful and has met or exceeded owner expectations.

The F-numbers for the entire project averaged FF 141 (41 percent flatter than specified) and FL 110 (47 percent more level than specified). One placement had an FF of 274 (2.74 times better than specified), which probably makes it one of the three fattest floor placements ever recorded.

April 1991

Although superflat floors account for only 0.1 percent of the industrial floors produced in the United States and Canada at the present time, this percentage is rapidly increasing, and they may well become commonplace in the warehouse/distribution centers built in the future. As more and more design firms and contractors get involved in these projects, they must keep abreast of the state-ofthe-art techniques necessary to produce successful superflat floors.

Selected for reader interest by the editors.

Note:

This article has been updated since its first publication in April 1991 ACI member Wayne W. Walker is the Director of Engineering Services at Structural Services, Inc., Atlanta, Ga. He has been a speaker at ACI seminars, Chair of ACI Committee 360, Design of Slabs on Ground, and is a member of ACI Committee 302, Construction of Concrete Floors. He has also published other papers and has developed many computer programs to analyze and design slabs on ground.

ACI Fellow Jerry A. Holland is the Director of Design Services at Structural Services, Inc., Atlanta, Ga. He has over 35 years of experience in design, construction, and troubleshooting concrete materials and structures.

He is a member of ACI Committees 360, Design of Slabs on Ground; 223, ShrinkageCompensating Concrete; 301, Concrete Specifications; 302, Construction of Concrete Floors: 350, Environmental Engineering Concrete Structures; 325, Concrete Pavements.

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