Design Priorities in Industrial Floor Slabs
Little is actually known about the Roman Architect Vitruvius. We don’t even know the year he died. Some historians have suggested this implies that he was not very well known in his own time. But he did leave behind the only known Architectural treatise from classical antiquity. There were likely other comprehensive Architectural works that existed in the first century BC, but it is assumed that any that did exist were destroyed in the fire that consumed the Library of Alexandria in 48 BC; an event contemporary to Vitruvius. Even his book was virtually lost until rediscovered by a Florentine scholar in 1414.The treatise was originally called “De Architectura” and later named “The Ten Books of Architecture”. What we do know about the man is mostly inferred from this work, and it is an amazingly comprehensive description of Roman technology and actually a fascinating read. Here’s a quote from what he had to say about concrete, a Roman specialty:
“Hence, when the three substances, all formed on a similar principle by the force of fire, are mixed together, the water suddenly taken in makes them cohere, and the moisture quickly hardens them so that they set into a mass which neither the waves nor the force of the water can dissolve…..”
What Vitruvius is trying to explain here is something we now call “Pozzolanic Action”. The quote above is from a section of the book titled “Pozzolana”. Now ‘ol Vitruvius did not have the benefit of all of our accumulated scientific knowledge when trying to explain why concrete does what it does. After all, he was still in the age of all things being composed of Fire, Earth, Air and Water. So surely we now know all there is to know about concrete ………. right? Well, yes and no. While we have long ago described the chemical reaction that makes concrete harden, within the world of Commercial buildings, differences of opinion abound on what makes for the best design for concrete slabs, particularly Industrial slabs.
Arguably no other component of an Industrial building is more critical than its floor slab. The performance of the slab directly impacts the utility of the operations within the building and repairs are costly and disruptive. I won’t have the space to discuss all the aspects of slab design here, but I can talk about a few design priorities for industrial slabs and the current thinking of some of the people who design and build them.
Cracks forming in an industrial floor slab that “open up”, or unprotected floor joints, will rapidly deteriorate from the impact of forklift wheels and in very short order, pose not only a physical obstruction to forklift traffic, but a safety hazard. For this reason, a lot of attention is paid to the joints of an industrial floor slab and the minimization of cracking.
One Structural Engineer I spoke with recently said that the two most important elements for minimizing cracking of a slab are the water/mix ratio and the early cutting of control joints. Many of the industrial clients I have worked with have specifications prohibiting the adding of water to the concrete after it gets on site for exactly this reason. The same clients generally require that a slabs control joints be cut as soon as it is “walkable”.
A frequent disagreement among slab designers concerns the addition of what is commonly called “Fly Ash” to the concrete. There are actually a number of substances that can act the same way in the mix, but all essentially replace a percentage of the more expensive Portland cement. For some, this is reason enough not to allow the addition of Fly Ash to the concrete. However, these additives have been shown to have the aforementioned “pozzolanic action” similar to the Portland cement and Fly Ash increases the workability of the mix and reduces its cost, among other benefits. Over the last few years, I am seeing less and less concern about Fly Ash in slab mixes and its use is becoming more the norm.
Another often heard discussion is how best to reinforce industrial slabs on grade. One institutional client I worked with years ago felt it was worth it to place #3 rebar each way at 16 inches on center thru-out their industrial slabs. Most Engineers I talk with feel like some sort of steel reinforcing in slabs is good for helping to insure two things: One, the reinforcing can help keep any shrinkage cracks that do appear from getting wider and reduce overall curling of the surface as the concrete cures. Two, the reinforcing can help the slab span small voids in the subgrade. I’m not so sure about that one, since the reinforcing is never placed below the midpoint of the slab pour and often in the upper third. I’m not sure how it can contribute much to resisting a bending moment when in that position. Nevertheless, the trend seems to be that most industrial slabs have moved toward using 12×12 inch welded wire fabric, sometimes specifying a heavier gauge wire. I have done un-reinforced industrial slabs, but it’s rare.
Control joints are another priority for industrial slab designers. The joints need to be filled with a semi-rigid sealant of which there are numerous products. The key point is that this sealant protects the edges of the joint from chipping off under wheel loads. Small chips quickly become a major pothole under the constant pounding of forklift wheels. Control joints should be carefully planned with the right aspect ratio, (square), and an eye for “re-entrant”, or inside corners where cracks will propagate from.
There is one priority for slab design that has nothing to do with concrete – the sub-grade preparation. All the slab experts I have worked with over the years, have all stressed careful subgrade preparation and maintenance of that preparation throughout the pour.
So when focusing on the design of an industrial floor slab, consider the following as priorities to optimize for crack mitigation:
- Sub-Grade preparation
- Water to cement ratio
- Control joint timing/planning/sealing
There is some new thinking being done these days about slab design that emphasizes some aspects of the concrete that traditionally have not been seen as particularly important, as well as new ideas about reinforcing, aggregate size and control joints. One Engineer that I spoke with has been doing some heavy duty industrial slabs around the country that take a much different approach. In these designs, utilized in high traffic lanes, compressive strength of the mix is limited to 3500 psi and no control joints are specified.
Large reinforcing bars, tightly spaced in the longitudinal direction, keep the inevitable shrinkage cracking tightly bound and careful attention is paid to the flexural strength of the mix, not normally a specification of much concern in slab on grade design. In addition, much larger aggregate size is used, the idea being that this reduces the amount of “matrix” in the slab and thus has less shrinkage as it cures. The result is a very high performance slab with no joints to fail.
Interesting stuff… and I may do a separate post on just this approach in a future article. There is a lot to cover on this strategy and some more research is in order.
Roman concrete is known as some of the most durable ever poured, and interestingly, there was not a speck of Portland cement in it. The Romans used volcanic ash, “Pozzolana”, along with the other ingredients. As strong as their concrete formula was, even Vitruvius probably had to say to someone with a shrug way back in the day…………”Hey, concrete cracks”.
Next Post: “Build it and they will come” – Strategies and configurations for speculative Industrial buildings.