Shrinkage and Curling of Concrete Slabs on Ground
Construction Materials Engineering
From ACI 360: “…it is completely normal to expect some amount of shrinkage and curling on every project…”
Curling is caused by Drying Shrinkage as well as by moisture or temperature differences across the thickness of the slab.
The likelihood of Curling increases as the top of the slabs cools or dries out and contracts more than the wetter bottom. Curling occurs as the top of the slab shrinks while the bottom is restrained. Drying Shrinkage is the reduction in concrete volume resulting from a loss of water from the concrete after the initial hardening. Normal concrete shrinks about 5/8 of an inch every 100 feet, or about 1/8 inch every 20 feet.
So, how can we reduce the potential for shrinkage and curling?
Curing – Adequate curing of slab particularly during early ages; followed by adequate measures to further reduce moisture loss from slab surfaces by using breathable coatings, sealers, etc.
Reinforcement – reinforcing steel & wire mesh should be placed in the top 1/3rd of the slab thickness. Steel or wire mesh in the bottom restrains the shrinkage in the bottom while the top shrinks, increasing the possibility of curling at the edges. It can be challenging to keep the reinforcement from falling down as concrete is placed. However it can be done.
Don’t Add Excess Water - Only a set amount of water is needed to hydrate all the Portland cement. Each concrete mix design specifies the maximum amount of water which should not be exceeded. Adding more water to the mix increases the volume prior to hardening, which in turn leads to increased shrinkage as the excess water evaporates. Take precautions to avoid excess bleeding.
Water-Reducing Admixtures - Water-reducing admixtures could be used to lower the water content.
Plasticizers – If a higher slump is needed, use plasticizers instead of additional water.
Aggregate Gradation –
Use a coarse aggregate gradation while still maintaining placeability, workability, finishability, and pumpability.
Aggregate Size – Use largest practical maximum aggregate size and/or highest practical coarse aggregate content. Coarser aggregate requires less paste to coat the exposed surface area of aggregate than the equal weight of smaller aggregates. Less paste means less Portland cement, less water, and shrinking and curling.
Contraction Joint Spacing – Contraction joints allow concrete to crack at specified locations to reduce random cracking, which is typically undesirable, that results from shrinkage during curing. Standards such as ACI and PCA provide guidance on the spacing of contraction joints. Good industry practice is to use joint spacing not exceeding 24 times the slab thickness.
Contraction Joint Depth – To be effective, contraction joints should be cut deep enough (often 1/4 to 1/3 the thickness of the slab) so that the joints are weak enough to propagate cracking at the joints.
Low Friction Subgrades – Sometimes it is possible to use low friction materials immediately below the slab that allow the slab to move as the concrete shrinks which can help the slab to stay together between the joints.
Avoid higher than necessary Portland cement content: Use Fly Ash or Slag as partial replacement. Fly ash and slag shrink significantly less than Portland cement.
Steel or Synthetic Fibers - Steel or synthetic fibers in the proper dosage can reduce the likelihood of cracking between contraction joints.
- Use Thicker Slabs or increase the thickness of slab at edges.
- Use a permeable (porous) dry (or almost dry) subbase material.
- Use of Shrinkage-compensating concrete.
ACI 302.1R (Guide for Concrete Floor and Slab Construction) serves as an excellent reference document that addresses Curling and Drying Shrinkage subjects in detail. Technical publications by PCA (Portland Cement Association), NRMCA (National Ready-Mix Concrete Association), and other relevant industry entities also offer great insights and valuable guidance on this topic. Concrete design and construction is a real team effort between the structural engineer, the general contractor, the slab subcontractor, the construction materials testing firm, and the ready mix supplier. Pre-pour meetings are very helpful to bring the many factors and roles together to produce quality concrete construction.
We hope this “Lessons Learned” is helpful to your future projects. ECS is a resource to call upon regarding mix design reviews, and concrete testing on your next project.
ECS Corporate Services, LLC