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Slab on Ground Joint Spacings

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When selecting contraction joint spacing to control shrinkage cracking, the following factors should usually be considered:

o Slab thickness
o Shrinkage potential of the concrete
o Base or subgrade friction
o Restraints to shrinkage
o Discontinuities such as footings, pits, equipment pads, and trenches
o Weather conditions (Temperature, wind, and humidity)
o Method and anticipated quality of concrete curing
o Amount and location of reinforcement

Although shrinkage cracking in slabs on ground begins at the top surface of the slab, it is primarily caused by restraint to shrinkage from subgrade friction. In thicker slabs, less restraint is transmitted to the top surface of the slab, so joints can usually be spaced further apart.  A commonly used rule of thumb is that joint spacing in unreinforced or "lightly reinforced" slabs should be between 24 and 36 times the slab thickness with a maximum spacing of 18 ft (5.5 m).  For example, joints in a 6-in. thick slab should be between 12 and 18 ft.  

These recommendations are based on PCA work done during the mid-1960s and they generally produce acceptable results.  It should be noted, however, that the original basis for the different spacings is probably not applicable for current slab on ground construction.  Whenever possible, actual construction and jobsite conditions should be considered when setting joint spacing, instead of relying on these somewhat arbitrary recommendations.

The original PCA guidelines were that joint spacings in slabs constructed with 4 to 6-in. slump concrete should not exceed 24 slab thicknesses (24t) for concrete which had a maximum coarse aggregate size of 3/4 in. and 30 slab thicknesses (30t) for concrete with larger aggregate.  If concrete with less than a 4-in. slump was used, joint spacing was limited to 36 slab thicknesses (36t), provided the slump reduction was due to the reduction of the amount of concrete mix water.

The intent of the PCA guidelines was to distinguish between "high shrinkage concrete" and "low shrinkage concrete". Although drying shrinkage is an inherent property of concrete, the shrinkage potential is typically reduced when larger coarse aggregate is used since mixes with larger coarse aggregate tend to require less paste.  Shrinkage of the cement paste as excess mixing water evaporates is the primary cause of drying shrinkage.  However, gradation of the aggregate is as important as, if not more important than, the largest aggregate size.  Various guidelines have been developed for aggregate gradations.  The overall objective is to produce a combined coarse and fine gradation that optimizes packing of the voids between particles.  

Similarly, the limits on slump were intended to distinguish between concretes based on their water to cement (w/c) ratio.  A concrete with a higher w/c ratio is likely to experience more moisture loss and thus more shrinkage.  In current slab on ground construction, however, high slump is typically achieved through the use of water reducing or superplasticizing admixtures.  A concrete with a 6-in. slump may actually have a lower w/c ratio than a concrete with a 4-in. slump.

In addition, the PCA guidelines do not address differences in subgrade friction and restraint to shrinkage.  Although these factors are difficult to quantify, they will have a direct impact on the amount of cracking.  If the subgrade friction is reduced to the point that there is very little restraint to shrinkage, the joint spacing becomes much less important and very wide joint spacings can be used, without any increase in cracking.  Although there will be drying shrinkage and movement due to temperature changes, the movement will be accommodated as wider joint openings.  As long as the joints are properly detailed for the expected opening, the performance of the floor will not be affected. If there are no joints, the movement will be accommodated by contraction at the slab edges.

In actual construction, restraint is unavoidable, even if the subgrade is very well prepared.  Since the 1960s, however, there has been an overall industry trend toward better subgrade preparation and thus less restraint.  This is partly a reflection of the equipment such as laser-guided screed boxes that is now available for grading, but it also reflects a growing awareness among both designers and contractors that proper subgrade preparation is essential in slab on ground construction. On a project where there is good control over subgrade tolerances, recommendations based on construction practices from the 1960s would tend to be conservative.


Gail Kelley