Steel-Fiber-Reinforced Concrete Floor Systems

Concrete by itself is naturally very brittle, lacking ductility when tension or impact loads are imposed. Traditionally concrete has been successfully reinforced with steel bars and/or welded wire fabric set into the concrete where analysis indicates high tensile stress or high impact loads. Even well designed bar-reinforced concrete systems will manifest cracking over time.

Today a new concrete reinforcing system is evolving that attains superior crack control and very impressive impact resistance. The system entails the introduction of randomly distributed, engineered steel fibers into the concrete mix. This results in a hardened concrete product that offers a much more flexible composite system. The new system is equally suited to all types of concrete flatwork and to precast wall panels. Since the concrete mixture is fiber reinforced throughout its section in multiple directions, cracks developing in any specific direction have nowhere to go. Assuming normal design loading, cracks may appear; however, their width and length will be markedly limited.

The Austin Company has designed and engineered a manufacturing facility for the new Boeing Delta IV rocket. The new facility uses this steel-fiber-reinforced concrete system to great advantage in concrete floor slabs throughout. Stringent Boeing requirements for superflat floors with the added capabilities of impact resistance and flexural strength made the fiber- impregnated concrete system a natural candidate.

The Delta IV, Evolved Expendable Launch Vehicle (EELV), factory was completed in late December 1999 following a 2-year construction schedule. The facilities are located in the Mallard Fox Creek Industrial Park in Decatur, Alabama. This location is just one mile from the Tennessee River - the highway selected for transporting the 165-foot-long, 16.7-foot-diameter first stage, or common booster core, from the factory to the launch site in Cape Canaveral, Florida.

There are four buildings in this $450 million complex. The 1,500,000-square-foot main factory structure consumed approximately 20,000 tons of structural steel and 100,000 cubic yards of concrete. It is approximately one-half mile long and one-quarter mile wide. Like the rocket itself, the rocket factory was designed for maximum efficiency and flexibility, and, therefore, it has very large open and unencumbered floors.

The original design called for an 8-inch-thick concrete slab-on-grade throughout the factory. As part of the value engineering process, it was decided that reducing slab thickness in certain areas might yield significant savings. Boeing realized that they would be giving up some future flexibility but decided to accept that eventuality. Many of the various manufacturing areas are located permanently anchored by their very expensive and heavily founded equipment. Flexibility ceased to be a meaningful requirement. Because the floor slabs were to be fiber reinforced and because a number of floor areas were to be dedicated to light manufacturing and assembly activities, the decision was taken to reduce slab thicknesses.

Impact loading was the arbiter of slab thickness in most areas. The areas that were producing and then transporting heavy fuel tank parts by 20-ton high-overhead bridge cranes got the 8-inch slabs. Manufacturing staff and engineering offices were located on 5-inch-thick slabs away from the bridge cranes. Floors that were going to experience only moving loads from rubber-wheeled, dolly-transported finished fuel tanks were set at 6 inches thick.

Design and Construction Schedule:

One of the most challenging aspects of the entire project was the difficult construction schedule:

2-years from groundbreaking to full occupancy. Construction began in November 1997 and was completed in late 1999. The steel-fiber-reinforced concrete floor systems construction schedule was only three months. Preliminary design layouts for the new factory were being developed at a time when detailed design of the Boeing Delta IV was just getting under way. The short schedule dictated that the design of the floor systems be compatible with every conceivable load combination from the many different large tools and rocket components carriers, including the railroad system and various vehicular wheel loadings. The many different large tools and EELV components carriers imposed multiple loading locations and combinations on the Focused Factory floor systems.

In analyzing the stresses generated within the steel-fiber-reinforced slab-on-grade, analytical methods were used. Stresses from point loads were analyzed for specialized equipment support racks and transport vehicles to determine the thickness requirements of the steel-fiber-reinforced concrete slab. The restrained shrinkage and curling stresses anticipated for a given distance between floor joints was also a determining factor in the fiber dosage recommendations.

In addition to Portland Cement Associations Slab Thickness Design for Industrial Concrete Floors on Grade equations, a finite element analysis model of the steel-fiber-reinforced concrete slab-on-grade was also used in the design of the multiple loading systems. The software used was SAFE, Slab Analysis by the Finite Element Method, and the airport program, Concrete Thickness Design for Airport and Industrial Pavements. The slabs were designed using a modulus of subgrade of 150 pci.

The subgrade is probably one of the most important features of a good slab-on-grade. Preferably, the subgrade should exhibit uniform bearing. If any pumping or rutting of the subgrade is noticed during the proofrolling stages, the unsuitable material must be removed and replaced with soil meeting the design requirement. In areas where fill was required, it was compacted to 98% and tested according to ASTM D-698. A crushed stone aggregate base coarse per ADOT 825 Type B was used as main base for the fiber reinforced concrete slab-on-grade floor systems. Where the base thickness was greater than 6 inches, the base was placed in two uniform lifts. Each lift was compacted using a vibratory skid plate compactor with a minimum of two passes on each lift. Each lift was compacted to 98 % and was tested according to ASTM D-1557-91.

The Delta IV project required the development of completely new mix designs for the entire project. The floor systems used 35,350 cubic yards of 4,000-psi steel-fiber-reinforced concrete.

Master Builders supplied all the admixtures, eliminating adverse reactions between different admixtures. The steel fibers and manufactured sand along with natural sand were used in different mixes and in different combinations.

In order to meet the very aggressive schedule, the 1,500,000-square-foot floor systems had to be placed in a minimum amount of time. The steel-fiber-reinforced concrete had to be configured to be pumped, hand placed and placed by a laser screed.

The floor systems placement began in the winter and progressed into late spring of 1999. Varying weather conditions and temperatures had to be addressed in the mixes. Combinations of midrange water reducers with and without accelerators and hot water were used in varying dosages to achieve the desired placement rates and set times.

A minimum temperature of 65 degrees and a maximum temperature of 90 degrees Fahrenheit had to be maintained at all times. Ice, chilled and hot water were used to meet the temperature requirements.

Depending on temperature and fiber dosage, the following admixtures were used in different combinations:

· Water Reducers (MB Pozzolith 220-N)

· Midrange water reducers (MB Polyheed 997)

· Midrange water reducers with accelerators (MB Polyheed FC 100)

· Super-P (MB Rheobuild 1000)

· Hot water

· Accelerators (MB Pozzutec 20)

The slabs were placed in 40,000-square-foot modules. Fifteen Mack trucks with 10-1/2-yard MTM mixers were used to carry the concrete from the two computerized concrete batch plants that were used for the project. Batching, mixing, delivering and sampling of the steel fibers was done in accordance with ASTM C 94 and applicable portions of ACI 304.

ACI Committee Report 544, section 3 R specifies that the fibers may be introduced into the concrete at any time at the batch plant or job site. A conveyor system was installed exclusively to load the steel fibers required for the concrete mixture for the floor system, enhanced the efficiency of the floor systems construction schedule. In addition, a new geothermal water unit was installed at the site to provide hot/chilled water for the concrete batching operations.

Pinwheel joint patterns were used at the buildings steel columns to eliminate the diamond type isolation joints typically used in industrial floor systems. The pinwheel pattern also eliminated one construction step and significantly reduced the construction schedule.

Saw-cut control joints divided the floor system areas into rectangular or relatively square sections.

The joints needed to be one-third of the slab thickness to ensure that shrinkage cracks will occur only along the control joint. The width of cut was approximately 1/8 inch to enable the joints to be filled with a flexible epoxy.

Key formed construction joints were used on the sides and the end of each placement.

Eight months into construction, portions of the building were turned over to Boeing so that they might begin the process of test coating the rocket booster components. The scheduled manufacturing production flow for the first two fuel tanks (oxygen and hydrogen) established the entire production flow of the construction project.

Steel Fibers Reinforcement:

Steel-fiber-reinforced concrete is a state-of-the-art composite material made of hydraulic cements, fine or fine and coarse aggregate, and a dispersion of discontinuous steel fibers. It may contain pozzolans and additives commonly used with conventional concrete.

The addition of these fibers can provide improvements of the engineering properties of the concrete. Impact strength, toughness (post-crack ductility) are some of the properties that are greatly improving the concrete slabs-on-grade. The ability to resist cracking and material disintegration, as well as fatigue resistance is also enhanced.

The addition of the steel fibers to the concrete mix does not significantly increase compressive strength but does increase the compressive strain at ultimate load. The changes in mixture proportions to accommodate the steel fibers do have a significant influence on the compressive strength.

Over 1,700,000 pounds of steel fibers were used. Xorex 2-inch steel fibers were used in dosage rates from 35 to 55 pounds per cubic yard of concrete.

The steel fibers are manufactured specifically for concrete reinforcement by deformation process in corrugating cold drawn steel wire segments. The material is in accordance with ASTM A 820, Type I, low carbon, cold drawn, deformed steel wire, with a minimum ultimate tensile strength of 120,000 psi (827 Mpa). The steel fibers with a length of 2 inches have an average equivalent diameter of 0.040 inch and average aspect ratio = 50. In addition, the fibers are corrugated the full length for increased mechanical anchorage.

The steel fiber is an alternative to conventionally reinforced slabs-on-grade as it provides superior performance as well as elimination of conventional reinforcement, faster placement of slab, lower life cycle costs and reduction in labor costs.

This is an example of the new building environment pressing the construction industry to new levels of value-added performance, resulting in superior solutions.

Floor Flatness and Levelness Testing

The steel-fiber-reinforced concrete slab-on-grade floor system was tested for flatness and levelness.
Testing was done in accordance with ASTM E 1155. This is a standard test method for determining floor flatness (FF) and floor levelness (FL) using the F-number system.
The FF and FL numbers are dimensionless numbers that are statistical analyses of the profile of the concrete surface. They are an indication of how flat and level a concrete slab is. The American Concrete Institute (ACI) recommends that flat floors have overall FF and FL numbers of 30 and 20 respectively. The Focused Factory specifications required FF and FL numbers of 50 and 30 respectively to meet the floor systems requirements.

The test was performed with a Model 9401 F-Meter. The F-Meter is effective and efficient and simple to operate. It is pulled by the operator and takes continuous measurements as it is being pulled. By using an F-Meter, testing could be done at a rate of approximately 500 square feet per minute. Other F-Number testing instruments are often up to 10 to 20 times slower (when using a face profiler). The floor flatness and levelness test was performed 24 hours after concrete placement covering an area of approximately 10,000 square feet.

The Project Team

The Austin-Alberici design-build team consisted of The Austin Company, headquartered in Cleveland, Ohio, and J.S. Alberici Construction Co., Inc. of St. Louis, Missouri. The Austin Companys regional office based in Irvine, California, provided the design services and led the detailed design effort for the steel-fiber-reinforced concrete slab-on-grade floor systems. Only one concrete contractor was involved: Baker Concrete Construction, Inc. of Monroe, Ohio. The Xorex steel fibers were provided by Synthetic Industries, Inc. of Hanover Park, Illinois. DCA Ready Mix Inc. of Decatur, Alabama, provided the steel-fiber-reinforced concrete. The concrete floor system flatness and levelness testing was performed by Baker Concrete and Qore Inc. of Decatur, Alabama. The aggressive design-build schedule required close coordination among the different team members. Communications were greatly enhanced by the use of Lotus Notes-based e-mail and the use of an extranet. The team managed to finish the construction of the floor system on schedule, within the project budget and with an excellent quality and safety record. oncrete by itself is naturally very brittle, lacking ductility when tension or impact loads are imposed. Traditionally concrete has been successfully reinforced with steel bars and/or welded wire fabric set into the concrete where analysis indicates high tensile stress or high impact loads. Even well designed bar-reinforced concrete systems will manifest cracking over time.