Floor Slab on Ground
Design Information Sheet
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Frequently Asked Questions

Q What is steel fiber reinforced concrete?

A Steel fibers are defined in ASTM A820 as pieces of smooth or deformed fibers that are sufficiently small to be dispersed at random in a concrete mixture. The discussion of steel fiber reinforced concrete in ACI 360 states that “steel fibers have a higher elastic modulus and tensile strength than the surrounding concrete. In addition, many types of steel fibers are deformed to optimize anchorage in the concrete. These attributes allow steel fibers to bridge cracks that develop in the hardened state and redistribute the accumulated stress caused by applied loads and shrinkage”

Q Can steel fibers be added at the ready mix plant?

A Yes, introduce steel fibers after all other ingredients are already in the truck. Set the mixer on charging speed and add fibers at a rate not to exceed 100 pounds per minute. Mix for a minimum of 40 revolutions at charging speed.

Q How much mixing time is required when adding steel reinforcement to a ready mix truck?

A It is recommended to continue mixing at the highest drum speed for 4 to 5 minutes after all of the steel fibers are added to the truck or about 70 revolutions of the drum.

Q Can steel fibers be added to any mix?

A Yes, steel fibers can be used in concrete, mortar, and plaster mixes. Generally harsh mixtures containing less than 40% sand by volume, can create mixing and dispersion problems for steel fibers. Steel fibers have been used in concrete mixes ranging in compressive strengths from 2,500 psi to 20,000 psi. If special cements or admixtures are used, a preliminary test is recommended. imix XS mixes are engineered to provide optimum performance.

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Q Will steel fibers ball up in the mix?

A With a properly designed mix and proper attention to the loading protocol that has been established for the imix XS mixes, balling and clumping of steel reinforcement is not an issue.

Q How will steel reinforcement affect my concrete design?

A Steel reinforced mix designs are similar to those commonly used for plain concrete mixes. The imix XS family of mixes meet the recommended aggregate gradations and mix proportions provided in ACI 544. Using the largest practical top size aggregate and a well graded combined aggregate blend as apposed to a gap-graded blend can minimize shrinkage. Steel reinforcement may cause a reduction in slump due to their stiffness. This does not necessarily equal a reduction in workability.

Q Does steel reinforcement affect the concrete slump?

A Yes, a high dosage rate of fiber reinforcement will reduce the apparent slump by 1 to 3 inches. The imix XS family of mixes are designed to optimize performance using steel reinforcement as a component in the mix design. Use of vibratory screeds facilitate steel reinforcement placement with low slump concrete.

Q Can steel reinforced concrete be pumped?

A Yes, there is a potential slump loss of 1 to 3 inches through the hose depending upon the dosage rate for the reinforcement, ambient temperature. And hose length. The imix XS family of mixes have been designed to enhance workability and ease of flow through the pump lines.

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Q Will steel reinforcement protrude on the surface?

A No, proper placement techniques using a vibratory screed in conjunction with highway straight edge and power float finishers allow the reinforcement to lay flat.

Q Will steel reinforcement rust?

A For interior applications, no. Even with minimal random corrosion of individual fibers at the surface, staining of the concrete does not occur. Surface reinforcement typically appears bright and shiny in normal environmental conditions.

Q Are there any safety hazards for finishers?

A No, with good jobsite safety practices in place steel reinforcement will not impose any safety concerns. Workers handling bags while loading steel reinforcement wear gloves and eye protection.

Q Is steel reinforcement compatible with curing compounds?

A Yes, commonly use curing compounds produced by major manufacturers are successfully used on steel reinforced concrete floors. Always consult the manufacturer for specific information.

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Q Can abrasion resistant toppings and liquid sealants be applied over steel reinforced concrete floors?

A Yes, steel reinforced concrete is compatible with dry shake and spray on topping systems.

Q Will fiber reinforced concrete damage my power trowel?

A No, power blade trowels and disc floaters are commonly used to provide a flat and level surface without excessive wear to the blades.

Q How do I cut joints in a steel reinforced concrete slab?

A Early entry saws are commonly used to cut contraction joints. Cut the joints to 1/3 slab depth, if possible, per ACI 544.3R guidelines. Generally limit joint spacing to 1.5 to 2.5 times the slab thickness in feet (eg 8” thick X 2.5=20). Wet cut saws may be used to gain greater joint depth.

Q How will steel reinforced concrete affect flatness and levelness rating?

A Flatness and levelness are generally not affected by dosages up to 70 pounds per cubic yard.

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Q How can I quantify the difference between various steel fibers?

A The residual strength factor (see below) is the most common factor used to differentiate steel fiber types. Steel performance is a function of tensile strength, aspect ratio, and anchorage.

Q What is ,meant by residual strength?

A Residual strength values , or R-values, describe the level of flexural load capacity as a percentage of the ultimate load of the non reinforced concrete. ACI 208 criteria requires that a minimum of 80 psi strength gain be reached to achieve temperature and shrinkage crack control (see Stork Report). The imix XS steel reinforcement provides 139 psi which is greater than 1.5 times the minimum specification.

Q Will steel reinforced concrete wear forms or tools more than plain concrete?

A Steel reinforced concrete is no more abrasive than concrete that contains coarse aggregates and sand.

Q Can steel reinforced concrete replace wire mesh or rebar in slabs on grade?

A Yes, steel reinforced concrete can replace welded wire mesh and rebar in slabs on grade. The imix XS product can be equivalent substitution or by calculation using the applied load and sub grade values.

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Q Is steel reinforced concrete cost competitive with rebar or wire mesh reinforcement?

A If you take into account all costs related to safety, labor, storage, quantity of rebar or wire mesh, and slab thickness, significant savings can be achieved.

Q What is the benefit of using steel reinforced concrete over rebar or wire mesh?

A In reality rebar and wire mesh are rarely installed properly. The reinforcement in imix XS is distributed three dimensionally, therefore the reinforcement is always in the right location to provide strength durability and toughness.

Q Is there a minimum thickness of steel reinforced concrete?

A steel reinforced concrete has been placed successfully as thin as 2”.

Q What is the benefit of using steel reinforced concrete compared to micro fibers?

A Steel reinforcement is not meant to be a replacement for micro fibers. Micro fibers are added to concrete for plastic shrinkage crack control and provide a mechanism that increases the concrete’s tensile capacity in the plastic state. Steel reinforced concrete was not designed to control plastic shrinkage cracks.

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Q Can steel reinforced concrete be used in exterior slabs?

A Yes, there are numerous exterior slab and paving projects using steel reinforced concrete.

Q Can steel reinforced concrete be used on suspended slabs?

A Steel reinforced concrete can be used to replace shrinkage and temperature steel in composite metal deck suspended slabs. The steel deck institute design manual allows the substitution of steel reinforced concrete for wire mesh in this application.

Q Should there be an engineering plan when using steel reinforced concrete, if so what type of application?

A Steel reinforced concrete are routinely used in slabs on ground to control shrinkage and temperature cracking without specified engineering documents. When reinforced concrete is needed to resist loads defined in building codes for structures such as foundations, footings, walls or floors, engineering documents are required to specify where the steel reinforced concrete is allowed.

Q Can steel reinforced concrete be used in poured walls to replace rebar?

A Technically steel reinforced can replace rebar used for temperature and shrinkage crack control in walls. This is typically the horizontal bars. However, special approvals are needed to replace any rebar that is required by the building code.

Q Can steel reinforced concrete be used in footings and slabs?

A Yes, steel reinforced concrete can replace rebar used to control shrinkage and temperature cracking in footings and slabs. See ICC-ES report ESR 3226 available at www.ICC-es-org

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