In the past it has been said, with some justification, that "Concrete slabs on ground are the Rodney Dangerfield of construction—they get no respect." Over the past few years, however, that statement has been become less and less true. In the field, contractors routinely produce floors that are initially twice as level and flat as floors constructed 15 years ago. The impetus for this improvement has been the introduction and widespread use of the F-number system and its ability to objectively evaluate the flatness and levelness of floors as they are constructed. The subsequent development of the laser screed and pan floats have permitted the placing and finishing of high-tolerance floors at production rates previously unachievable. In design offices, engineers are focusing on approaches that minimize unplanned cracking and provide superior joint performance. The several design choices available to the designer allow production of a slab on ground that is optimized for the needs of the owner.
ACI Committees 360, Design of Slabs on Ground, and 302, Construction of Concrete Floors, have been at the forefront of the effort to provide the industry with the tools to produce slabs that are flat and level, with a densely finished surface, stable joints, and limited cracking while being capable of supporting applied loads. In 2006, ACI Committee 302 completed work on a second document, "Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials."
The goals of ACI Committee 360 are to provide guidance for designers in planning, analyzing, specifying, and detailing slabs on ground, which will offer the expected serviceability under the anticipated loadings and conditions of use. ACI Committee 302 has focused on issues related to successful implementation of the designs developed by the designer. It should be noted that even the best design often requires reconciliation with the actual means, methods, materials, and site conditions.
ACI Committee 360
ACI 360R-06, "Design of Slabs-on-Ground," presents the designer with four basic design options:
- Unreinforced concrete slab
–Slabs reinforced to limit crack widths
–Slabs designed to minimize cracking
- Shrinkage-compensating concrete (SCC)
- Post-tensioned (PT)
- Structural slabs
These choices are each developed in specific chapters of the document, and are preceded by general considerations, soil support system requirements, and jointing options.
Any discussion of the slab-on-ground design options begins with the acknowledgement that ACI considers slabs on ground to be nonstructural. These slabs generally are modeled as an elastic slab supported by a field of linear springs. Stresses from applied loads can be calculated using this or similar models.
Slab-on-ground design should result in sufficient strength to support without distressing the loads applied to the finished surface. Because the concrete slab on ground is assumed to be continually supported, an adequate soilsupport system is critical to the slab's performance. Slabs on ground typically are designed as unreinforced sections. The moment and shear stresses are calculated in working stress and an appropriate safety factor against firstcracking (modulus of rupture) is applied to determine the required section modulus and thickness. The first three design choices all use this approach to develop a thickness solution. Under choice three, PT slabs evaluate the net tensile stress including the effective PT compression, while SCC allows extended joint spacing independent of slab thickness.
The reason for the stress-limiting approach to slab design is to avoid unplanned cracking. A normal property of concrete is that it shrinks as it loses the excess water required for initial placement. Completion of this drying process can take as long as two years. When movement in response to shrinkage is restrained, the concrete can crack. Reinforced concrete design assumes the concrete has cracked—the reinforcing is provided to resist stresses. Unplanned cracking of slabs on ground generally is contrary to the owner's expectations.
The second choice only provides reinforcing steel to limit crack opening widths. The presence of reinforcing does not prevent cracking. Such reinforcing has negligible effect prior to formation of cracks forming. The goal is tolimit crack widths when and if cracking occurs.
The use of reinforcement to limit crack width received significant attention from ACI Committee 360. In the previous edition of the document, the subgrade drag formula provided a ready solution for using reinforcement to extend the contraction joint spacing. This formula yielded very low reinforcing ratios, suggesting the use of 6x6x10/10 welded wire reinforcement. It considers only subgrade friction, but the industry now recognizes the primary role that warping (curling) plays in slab cracking.
The committee has concluded that there are currently no valid criteria for selecting reinforcement to extend the joint spacing in slabs on ground. Where the intent is to avoid unplanned cracking, joint spacing should be selectedon the basis of slab thickness only.
Figure 5.6 recommends joint spacing based on slab thickness and anticipated concrete mix shrinkage characteristics. Specific mix shrinkage data generally is not available. Without valid data, high shrinkage should be assumed.
Other Reinforcing Considerations
Reinforcement also can be provided to maintain a nominal moment strength if unintended cracking does occur. Another option is to provide significant continuous steel at a ratio of about 0.50%. Properly designed and located, continuous reinforcing should produce acceptable tight, fine cracking at close spacing, and contraction joints may be eliminated. Cracking produced by this design approach is intentional. Care should be taken by the designer toensure that the owner is aware that the slab is intended to have frequent, closely spaced cracks.
A new reinforcing alternative discussed in ACI 360R-06 provides relatively light reinforcing continuous through the contraction joints. This concept is introduced in the joint chapter as a method of stabilizing the contractionjoints, not to limit crack widths.
The issue of joint stability also received significant attention. Unreinforced slabs usually are divided into panels by contraction joints. Load transfer across contraction joints is accomplished by either aggregate interlock or load transfer devices, such as smooth steel dowels. Aggregate interlock can be lost if shrinkage is excessive or if all joints don't activate (by not cracking under every sawcut, for example), leading to accumulated movement at widely spaced joints.
Conventional wisdom has been that, when reinforcing is used, it should be discontinued at the contraction joints. The concern here was that the reinforcing could "blind" the slab to the presence of the saw joint and cracking would occur away from the sawcut. The use of light reinforcing, not more or less than 0.10%, is presented. This reinforcing, while continuous through the contraction joints, is provided only to hold the activated joints in effective contact for load transfer. If properly positioned, even light reinforcing would provide some resistance to crack widening. However, to reiterate, joint spacing should follow Figure 5.6.
The new chapter on fiber-reinforced slabs on ground provides useful insights on using fibers to enhance slab performance. Fine monofilament fibers (denier less than 100) are useful in controlling bleed water channel formationand plastic shrinkage. They have limited effect after the concrete takes initial set.
Macropolymeric fibers (denier greater than 1000) help control cracking due to drying shrinkage. Steel fibers can increase impact resistance and other properties.
In general, ACI Committee 360 believes that fibers should be treated as an enhancement. The notion that fibers contribute to ductility was resisted. Fibers typically fail by progressive bond failure. This may mimic tensile ductility, butwould not justify extending joint spacing or reducing slab thickness. Further research and development may build better confidence in this area.
The structural slab chapter, while brief, draws attention to the requirement that load bearing elements must bear on foundations. Slabs on ground can be designed as foundations, however, the design criteria would then be mandated by ACI Committee 318, Structural Concrete Building Codes' document "Building Code Requirements for Structural Concrete and Commentary." This can be a gray area and the chapter is intended to draw attention tothe issue and act as a placeholder for future elaboration.