Everyone's juices start flowing when you start talking about big concrete placements— 40,000 square feet and bigger, or larger than 1000 cubic yards per day. Watching concrete hit the ground at over a hundred yards per hour can be just plain exciting to a concrete guy. But after it's all over and the slab is sawed and cured, are we producing a better product?
Why large placements?
- Large placements often (but not always) mean lower unit costs. And the push to do them is often led by concrete contractors. If we are honest with ourselves, company ego certainly plays a part. Everybody wants to be known as the company that can put a lot of concrete on the ground in a hurry.
- The equipment, mix designs, and technology have certainly made the mechanical aspect of up to 100,000-SF placements doable. Laser screeds, ride-on trowel machines, early-entry saws, and improved mix designs have increased production rates enabling these “super size” placements to be done in a 24-hour period.
- Schedule—yes, clients love larger placements, fewer days to complete the project. One-million-square-foot buildings that use to take 40 placements are now being completed in 25 placements, 40% faster. On many large distribution center and warehouse projects, the roof and the floor completion are critical factors in allowing the other trades to move forward.
- There are fewer construction joints in the floor. The greatest deviation in finished floor elevations in any particular building is probably at the construction joints. Fewer construction joints also means less hand finishing which results in a more consistent, uniform slab finish.
What's wrong with large placements?
- Greater productivity doesn't always translate to greater efficiency. Concrete placement rates of approximately 125 to 150 cubic yards an hour is about the practical limit for a typical batch plant. A 60,000-square-foot 6-inch slab on grade requires 1100 cubic yards, which at the maximum production rate for the batch plant will require seven to nine hours to mix and deliver. We strive for a finishing window of eight to 12 hours in a well designed slab mix, so one needs 16 to 20 hours to place and finish the slab, not counting the time needed to saw and cure. Unless you have crews working staggered shifts, the overtime costs will negate the gains in productivity.
- Concrete testing becomes an issue with larger placements for several reasons. The logistics of making cylinders, measuring slump, and taking air tests for a thousand-plus cubic yard placement is daunting. Placing and testing this much concrete is pressure filled. Too frequently lab technicians are viewed as a hindrance to the overall effort, and they don't have the time they need to perform the required field tests. So what starts out to be a collaborative team effort between the testing company and the concrete contractor becomes instead an adversarial one.
- The overall quality of slab placements in excess of 40,000 to 50,000 SF is lower than for placements of smaller size. Once you decide to “super size” floor placements, the emphasis shifts to achieving higher production rates, with less emphasis on quality. It's not that quality must suffer on large placement; it's that issues involving inadequate planning, equipment failures, and manpower problems can take precedence and compromise the results.
- Larger daily slab placements increase the probability of accidents and injury to workers. A tremendous number of concrete trucks, equipment, and personnel operate in a limited space for an extended period of time. Fatigue and the pressure to place concrete as quickly as possible can lead to poor decision making and inattention to the many details.
Suggestions for large slab placement success
- Definitely have a pre-construcrion meeting to be attended by all the principals: ready-mix producers, the concrete contractor, testing lab, owner's representative, structural engineer, and all other pertinent parries. Review thoroughly all aspects of the upcoming placement: manpower, equipment, truck routes and staging areas, testing requirements and contingencies.
- Schedule workers so that the right number of people is in the right place at the right time. Try to keep the workday under 10 hours for any individual. Using a “stick man drawing” can help the planning process. This involves sketching on a piece of paper the placement configuration and then filling in the actual sequence of events for placing the slab. Position the concrete trucks, screeding equipment, trowel machines, and the workers required for each function. It is surprising how much detail is brought to mind as you fill in the sketch.
- Plan for the “what ifs.” Try to visualize everything that can go wrong and plan your contingencies. Equipment breakdowns, manpower shortages, problems with the mix design, and batch plant breakdowns are some of the more common problems that you need to consider.
Can we make large placements? Yes, large placements are regularly successfully completed—meeting both the requirements of the specification and making the client satisfied. But as an industry should we be promoting large individual placements in excess of 40,000 to 50,000 SF? Probably not. In most regions and markets across the country there are only a handful of concrete contractors who have the equipment, personnel, and management expertise to successfully complete these placements.
The type of placement also is a determining factor as to whether or not to “super size.” Issues such as whether the slab is exposed to the elements, specifications for flatness, mix design criteria, and whether site conditions are conducive to rapid concrete placement have to be considered. Individual onetime large placements are much easier to plan and have a greater chance of success than trying to place consecutive large placements.
— Chad S. White is the owner of WL Contracting, Fort Scott, Kansas, and specializes in industrial floor construction.