Last Updated on June 20, 2022 by Jose Villegas Jr.
What are Contraction Joints in Concrete Slabs?
Contraction joints in concrete slabs are weakened planes generated by tooling, saw cuts, or mechanical devices, that are utilized to attract and conceal restraint-to-shortening cracks. Contraction joints are also known as control joints.
Many concrete contractors don’t construct contraction joints in residential slabs-on-ground because they believe they aren’t required or may affect structural stability. Be careful if you skip joints to prevent getting blamed for cracks.
For starters, the American Concrete Institute (ACI) Residential Code Requirements for Structural Concrete (ACI 332-14) mandates contraction joints and includes a formula for determining joint spacing for plain and reinforced concrete with up to 0.5 percent reinforcement (Refer to Table 1 below). The majority of residential slabs have less than 0.1 percent reinforcement. (In a 4-inch thick slab, 0.1 percent reinforcement equals #3 bars in both directions at 24-inches.)
Second, although the Post-Tensioning Institute discovered that neither random cracks, which are widespread, nor contraction joints (also known as control joints) impair the structural performance, they are rarely used in post-tensioned (PT) slabs. Contraction joints, which are weakened planes generated by tooling, saw cuts, or mechanical devices, can be utilized to attract and conceal restraint-to-shortening cracks, according to the institution. Their location, which should be specified by the engineer-of-record, should avoid areas of excessive bending moment and shear.
Finally, even if contraction joints aren’t included in construction plans, they may be required. Almost certainly, the contract contains a provision requiring that all applicable codes be observed. The contractor is then responsible for submitting a request for information, which will notify the licensed design expert that joints are needed. This can help deflect blame for sporadic cracks while also protecting you from any construction litigation.
The layout of Non-Post-Tensioned Concrete Contraction Joints
Interior footings, re-entrant corners, and embedded components frequently obstruct evenly spaced, parallel, straight-line couplings. Joints can be angled or even curved, and spacing can vary. However, because the slab will be covered with flooring materials, the aesthetic is rarely important. The most essential thing is to keep random cracks to a minimum.
The slab’s connection to the perimeter footing is the initial consideration.
The slab-on-ground can be isolated or monolithic with the footing in plain and reinforced concrete. The first joint close to the footing, when separated, can have separation up to that shown in Table 1. When utilizing concrete with a maximum size aggregate of 3/4 inches, the first joint can be 11.5 feet away if the slab thickness is 4 inches.
| Slab thickness (inches) | Maximum aggregate size less than 3/4 inch | Maximum aggregate size 3/4 inch and larger |
| 3.5 | 8 feet | 10 feet |
| 4.5 | 10 feet | 13 feet |
| 5.5 | 12 feet | 15 feet |
More contraction joints will be required if the slab is cast monolithically with the perimeter footing. The first joint from the footing must have a spacing of half the spacing permitted in Table 1 according to ACI’s Guide to Design Detailing to Mitigate Cracking in this situation.
Load-bearing walls are supported by interior footings. It may seem reasonable to bury contraction joints beneath the wall, but a joint over an internal footing will not activate—that is, a crack will not develop the full depth of the slab and open during thermal contraction and drying shrinkage. The joint must be placed so that the slab does not extend above the footing. When the slab is cast monolithically, the joint spacing is the same as when the perimeter footing is measured from the footing’s outside border.
Another reason for varying spacing and curved or angled joints is that embedded components in the slab, such as anchor bolts and plumbing pipes, must be completely avoided by the joints.
Many re-entrant corners are seen on residential slabs, and they don’t always line up on opposite sides of the slab. To avoid an interior footing, corners can be joined by angled or even curved joints and should be intersected by at least one contraction joint. This isn’t always doable, though. Each panel bordered by joints must have a long side to the short side aspect ratio of less than or equal to 1.5, according to ACI. This criterion takes precedence over the intersecting joint criterion, and it frequently necessitates the use of extra joints. The outcome will be joints that are closely spaced in some residential slabs.
Re-entrant corners may need to be left without a joint in specific instances. At some corners, a crack may form, but it will be short and shallow.
The Layout of Contraction Joints in Post-Tensioned Concrete
Maximum moments in residential post-tensioned slabs are typically within 10 feet of the slab edge. Only minor moments, shears, and differential deflections exist in the slab’s center part, known as the dormant zone. As a result, contraction joints placed more than 10 feet from the edge and almost perpendicular to it will not affect structural capacity.
The engineer-of-record must specify the configuration of the contraction joints.
Contraction Joints Come in a Variety of Shapes and Sizes
Contraction joints, also known as control joints, are inserted in new concrete with grooving tools or mechanical inserts, or after the concrete has been set via saw cutting.
The joints that are put in new concrete are the most functional. Before any shrinkage occurs, they form a weaker plane, which can be caused by chemical reactions during setting, moisture loss in the initial hours and days following placement, and a drop in concrete temperature from the time of concrete hardening.
The next best joint is early-entry sawcuts, which can be put within hours of being placed.
Diamond blade saw cuts, whether wet or dry, are the least ideal since they are put after the concrete has grown sufficient strength to resist raveling. To provide sufficient strength, concrete is sometimes left to cure overnight, which can be too lengthy before joints are installed. Thermal contraction and, secondarily, drying shrinkage may have caused cracks to form.
For sidewalks, driveways, and garage slabs, grooving is the most common installation method. On small (residential) slabs, mechanical inserts such as zip strips can be used. For bigger slabs, early-entry saw cuts are preferred because the distance between slabs can be too great for grooving or mechanical insert joints.
A new mechanical-insert method was recently introduced, which produces a joint that isn’t as neat as early-entry saw cutting but is just as effective and costs about 75% less. A tool is physically moved forward across the slab to insert a folded strip of plastic tape into fresh concrete. When the concrete is extremely workable and vertical embedment and joint depth can be regulated, the process takes place before or after bull floating. Finishing procedures are not affected because the tape is put just below the surface. After thermal contraction and drying shrinkage, the final joint look is a rather straight crack.
Depth of the Contraction Joint in Concrete Slabs
One-fourth the slab thickness or a minimum of 1 inch, whichever is greater, is a standard requirement. Conventional wet or dry diamond blade saws must meet the one-fourth depth criteria. The depth criteria of 1 inch for slabs up to 9 inches thick is permissible when early-entry saws are employed, according to ACI 360 and 332. Because tooled and mechanical-insert joints are likewise early-entry contraction joints, this requirement also applies to them.
Responsible for Installing Contraction Joint
Random cracks are frequently blamed on concrete contractors. When the building designs contain a necessity for contraction joints, this will change. If the blueprints do not specify contraction joints, the residential concrete contractor or the builder should follow ACI 332’s instructions and install them or notify the engineer that they are needed.
tag: contraction joints in concrete slabs