There are many opinions floating around as to the benefits, or lack thereof, of reinforcement in slabs on ground. Not all reinforcement works the same way. To be able to understand the potential benefits and negatives of any particular reinforcement system, one has to understand how that system theoretically works and also what happens in the real world. The purpose of this article is to discuss some of these reinforcement systems, plus what they will and will not do.
Steel Reinforcing Bars and Welded Wire Reinforcement
Concrete is very strong when it is squeezed in compression but very weak when it is being pulled apart in tension. A good rule of thumb is that it is about 10 times as strong in compression as it is in direct tension. Thus, whenever you see a crack in a slab on ground, it is due to it having more tensile stress applied to it (from linear shrinkage, restraints to that shrinkage, curling, loads, etc.) than its tensile strength. Steel reinforcing bars and welded wire reinforcement are very strong in tension, have similar thermal expansion and contraction properties to those of the concrete, and thus can handle high-tension stresses while the concrete can take substantial compressive stresses.
One important concept is that typically used reinforcing (post-tensioning tendons and shrinkage-compensating concrete reinforcement are the exceptions) will not prevent the concrete from cracking. The reason for this is that the reinforcing cannot begin to start resisting significant tension until the concrete cracks. Until that point, it is mostly inactive inside your slab. Properly sized and located reinforcing will keep cracks reasonably tight and serviceable, if they occur, but will not prevent them. Furthermore, the great majority of reinforced concrete designs that have been reviewed for slabs on ground do not have enough reinforcing to actually increase the slab's load-carrying capacity above that of an unreinforced slab. Thus, unless the reinforcing is being used for other purposes (such as the “long dowel/enhanced aggregate interlock” concept noted later in this article), it is typically somewhat expensive insurance for a cracking problem that may never occur if other appropriate procedures are followed, such as proper joint spacing, dowels at joints, consistent slab thickness tolerance control, good base control, and low shrinkage mix design.
Many people believe that slabs on ground typically should have some reinforcing, but most slabs in North America are unreinforced concrete and perform well. If reinforcing is utilized, the amount that should be used depends on what is to be accomplished. The percentage of reinforcing refers to the cross-sectional area of the steel for a given width of slab divided by the cross-sectional area of the slab area considered. For example, if a 6-inch-thick slab is used with #3 rebar at 18 inches on center, the percentage of steel for a 12-inch width would be:
(0.11 in.2)(12 in./18 in.)(100)/(6 in.) (12 in.) = 0.10%
For enough reinforcing to accomplish enhanced aggregate interlock, the American Concrete Institute (ACI) Committee 360, Design of Slabs on Ground noted that designs using 0.10% deformed reinforcement through the contraction joints have been used successfully. Reinforcement amounts much less than 0.10% have not provided dependable load transfer; and much more than this has caused excessive out-of-joint cracking. This deformed reinforcement is an alternative to smooth steel dowels, and slab expert Eldon Tipping has coined the term “long dowels” for this concept. By continuing the reinforcement through the contraction joint, the cracks that form below the sawcuts will be tighter than they would otherwise be. Thus, the reinforcing is supposed to enhance aggregate interlock, which normally cannot be depended upon for long-term load transfer of repetitive loads if the crack is 0.025 to 0.035 inches or wider, per Portland Cement Association's research. #3 reinforcing bars at 16 or 18 inches on center are the most common reinforcing schemes used on slabs constructed with a laser screed. This is due to being able to drive the concrete trucks and laser screed over them as they lay on the base and then chairing them up just ahead of the concrete placement, as the workers stand between the bars. Generally, the reinforcing is located a third to a half of the slab depth from the top so that the sawcut will not cut the reinforcement. The availability and use of early-entry saws has made this method even more dependable because the sawcuts must be made as soon as feasible.
In some situations, it is desirable to eliminate contraction joints in large placements and use enough reinforcing to have many, very tight cracks that do not spall under wheel traffic and are not an aesthetic issue; a common example is a true “superflat” slab strip placement. To have this kind of performance, sometimes called a “joint-less” floor, at least 0.50% to 0.60% reinforcing must be used near the top of the slab. These cracks will be visible, thus the aesthetics of these cracks should be discussed with the owner. In the majority of larger projects, some doweled construction joints will be required to transition to a different slab type. These joints usually will open more than those with joints at typical spacings of 10 to 15 feet. Thus, if there will be significant wheel traffic, consideration should be given to having a very good dowel system, such as plate dowels, at the construction joint and armoring the joint.
For 0.10% reinforcing, the slab joint spacing should be the same as for an un-reinforced slab. Guidance for joint spacing to minimize out-of-joint cracking for such slabs is given in ACI 360 and typically should be in the 10- to 15-foot range noted earlier. Extreme care should be taken if the decision is made to extend the joint spacing somewhat by increasing the reinforcing but not to the 0.50% to 0.60% appropriate for “joint-less” floors. The main reason for the extra care is that curling increases significantly with every 1-foot increase in joint spacing, thereby significantly increasing the chances for out-of-joint cracking of unacceptable widths and joint problems.
Many opinions have been voiced regarding the best vertical location for a single layer of reinforcing for slabs on ground.
Some think it should be in the lower portion of the slab due to tension in the bottom of the slab when concentrated loads were to be applied. Others feel it should be in the middle in order to provide some tensile resistance for flexural tension either in the top or the bottom of the slab. However, it is best to design the bottom of the slab as unreinforced and locate the reinforcement in the upper part of the slab.
Locating the reinforcement in the upper part of the slab is best when trying to control the visible crack widths due to the loading, curling, and base friction. Slab curling produces a significant tension stress in the top of all normal concrete slabs; if cracks do occur they are V-shaped with the widest portion at the top of the slab. Thus, the higher the reinforcement, the tighter it will hold any cracks running perpendicular to the direction of the reinforcement. However, if the reinforcing is too high it can cause plastic settlement cracks, which run directly over the top and parallel to each bar or wire. So, if the bars are spaced at 12 inches on center and relatively straight cracks are observed every 12 inches, this type of cracking has occurred. The chances for plastic settlement cracks increase as one or more of the following occurs: reinforcing diameter increases, concrete cover decreases, reinforcing temperature increases typically from sunlight, concrete bleed rate increases, reinforcing movement while the concrete is still plastic, or anything that increases moisture evaporation rate from the slab surface, such as higher concrete or ambient temperatures, higher wind speed, or lower humidity.
Steel fibers have been available in the U.S. since the mid-1970s. Type 1 fiber is made of drawn wire of various geometries and Type 2 utilizes slit sheet steel. As with steel bar and wire reinforcement, steel fibers will not prevent cracks but can keep cracks, if they occur, reasonably tight if a sufficient amount of fiber and an appropriate joint spacing are used. If there is a sufficient quantity for a particular situation—based on slab usage, joint spacing, concrete shrinkage potential, etc.—the post-crack load-carrying ability of steel fibers can be very beneficial. However, if the cracks become wide enough to spall, this can be major problem. Thus, as with other types of reinforcing, the fiber dosage must be carefully considered with regards to the particular situation.
If steel fibers are to be used for long-term enhanced aggregate interlock and the joint spacing is to be from 10 to 15 feet, the minimum amount of fiber considered for concrete with typical shrinkage properties is 40 pounds per cubic yard. If the concrete is expected to be high shrinkage, the joint spacing should be at the low end of the range and/or the fiber dosage should be higher. As with steel bar or wire reinforcing, care must be taken if the joint spacing is extended beyond this specification. For longer joint spacings, at least 75 pounds per cubic yard is recommended.
The fibers decrease the slump of the concrete, but this can be compensated for by proper mix materials and proportioning. Generally, the same things that make a good mix without fibers will make one with them. At 40 pounds per cubic yard or more, a good midrange or high-range water reducer (the latter at a low dosage) can be very helpful and is necessary as fiber dosages increase.