Stitching is applicable for a number of situations where slab strengthening is required. Among these are:
• Strengthening longitudinal cracks in slabs to prevent slab migration
• Mitigating omission of tie bars due to construction error
• Restoring the tensile strength across cracks in swimming pools to stabilize the structure
The first reported attempt to stitch cracked concrete was performed by the Corps of Engineers, Waterways Experiment Station. The Corps performed a strengthening study in 1971. At the end of the test, the cross-stitched joint withstood the repeated loading even better than the surrounding reinforced concrete. The report concluded that stitching is viable and effective. The first known use of concrete stitching on a US highway was in 1985 by the Utah Department of Transportation. Engineers used concrete stitching to strengthen uncontrolled cracks on a new section of I-70 through the mountains of central Utah. A review of the I-70 project in February 2000, after 15 years of service, found the project to be in generally good condition. The cross-stitch cracks performed well overall, preventing lane separation and minimizing the settlement of the slabs. Both of these reports outlined the testing of a variety of joint and crack strengthening methods and concluded that stitching is one of the best concrete strengthening techniques.
Concrete Stapling and Pining
In the past, contractors often glued metal “U” clips, pins and plates around the crack in pools. Some designed and installed their own appliances. One such device involved a steel plate with one stationary pin and one moveable pin for tightening. Although the use of concrete stapling and pining was commonly performed in the past, it seldom worked in the long run. Often times these gadgets were not properly engineered and worsening the crack. Time after time the fractures reappeared and leaks reemerged.
Anchor Stitching Technique
Anchor stitching across a cracked or fractured concrete slab provides a positive mechanical interconnection. The stitches are typically fabricated with a machined face and slot, welded to a high quality rebar hook, and installed in a niche below the concrete surface.
Which Method of Crack Repair Should I Choose?
Frequently, the use of short concrete staples with pins is unsuccessful and these fractures reappear. Contractors often evaluate the costs and use the method that provides the optimal combination of strength, installation time, and cost. Concrete anchor stitching is generally less expensive than concrete stapling and pinning, and stitching is generally preferred by contractors because fewer materials are required. Proven to successfully repair cracked concrete, engineers often specify concrete stitching as the recommended solution.
Anchor Stitching - The #1 Crack Repair Solution on the Market for Swimming Pools
The concrete anchor stitching technique involves the use of rebar, tensioned and anchored into solid concrete. Tensioning is achieved by tightening the anchor on the machined wedge applying force along the stitch. The stitch length should correspond to rebar spacing within the slab, bridging the crack and extending into the unaffected areas; spanning well into solid reinforced concrete. The stitch is tensioned by tightening down the anchor sufficiently, without causing the crack to migrate to another part of the slab. When stitches are anchored into the cement, loads are transferred into well intact reinforced areas. An engineered anchor stitch structural system is clearly the best solution to repair significant swimming pool cracks.
About the Author
Director of MW Engineering, Inc., and founder of SlabTite Structural Systems, Mark Weber PE, is a Licensed Professional Engineer that specializes in Mechanical and Civil Engineering. Our Civil Engineering and Consulting services include design of structural repairs, and fracture stabilization;
References: ACI 224.1R-93 “Causes, Evaluation and Repair of Cracks in Concrete Structures” reported by ACI Committee 224 in the Journal of the American Concrete Institute, May-June, 1984, (Reapproved 1998), American Concrete Institute, Farmington Hills, MI