The goal today for high-performance industrial floors is to place joints at mid bay and column lines or only at the column lines and to eliminate cracks and curling. The new Grainger Northeast Distribution Center in Bordentown, N.J., slated to go into full operation in 2017, is an example of just such a floor. At the 12-month walk-through on this 1.4-million-square foot building with joints spaced at mid-bay and column lines (30 feet), only one 4-foot-long crack was found in the entire warehouse. The floor profile at 12 months is the profile that the owner can expect to have for the life of the building so this floor clearly met the owner’s objectives.
The success of this floor can be attributed to the collaboration and cooperation among the owner, developer, general contractor, owner’s slab-on-grade consultant, structural engineer, concrete subcontractor, batch plant, testing agency, and material supplier who all shared the same goal of producing a high-performance floor. All parties attended a pre-slab meeting with a proper agenda and minutes; follow-up meetings were held to maintain focus.
The low-shrinkage, crack-free mix (per cubic yard)
|Cement - 530 lbs||Macro Synthetic Fiber: Tuf-Strand SF - 4 lbs|
|Sand - 1,240 lbs||Air Content - 2.5%|
|Coarse Aggregate #67 - 1,750 lbs||water-cement ratio - 0.49|
|Coarse Aggregate #8 - 450 lbs||Slump - 7 inches|
|Water - 261 lbs||Shrinkage @ 28 days - 0.032%|
|High Range Water Reducer: Eucon 37 - 74 oz|
A mockup test slab was placed and finished on site. The contractor constructed additional test slabs until all mix, placement, and finishing results satisfied all the team members. One important requirement was that the mix have normal setting time; delayed setting concrete can lead to blisters and delamination.
For extended-joint floors like this one, where the owner wanted to reduce the number of saw-cut contraction joints, a low-shrinkage mix was mandatory to ensure minimal cracking and curling. A key element was testing the proposed mix design for shrinkage. Minute variations of the proposed mix design were also tested and the optimal mix design with the best performance characteristics was chosen. The final mix had low shrinkage (0.032% @ 28 days) and incorporated 4 pounds per cubic yard of Tuf-Strand SF macro-synthetic fiber.
The specification for this 7-inch-thick slab was consistent with the current recommendations of ACI Committee 302, Guide for Concrete Floor and Slab Construction. This outstanding floor is the result of clear and current requirements for a long-term, high-performance industrial floor: excellent collaboration and common goals between the owner, engineers, contractor, and supplier team, and a low shrinkage, macro-synthetic fiber mix that allowed joints to be placed only at mid bay and column lines.
The floor profile and mix requirements and results
|Flexural Strength: psi @ 28 days||w-c||FF||FL||Compressive Strength: psi @ 28 days|
|Specified||750||0.50 - 0.52||45||35||4,500|
William S. Phelan is senior vice president of marketing & technical services for the Euclid Chemical Co.