Despite our best efforts, concrete cracks frequently enough that anyone working with it should understand why and how to repair the cracks when necessary. In this three-part article we are exploring cracks in slabs only, not structural cracks or concrete disintegration such as from freeze-thaw action or alkali-silica reaction (ASR). Those situations certainly do crack the concrete, but those are different articles.
In Part 1 of this article, we explored cracks that form in plastic concrete—concrete that hasn’t yet achieved full strength. In this Part 2, we look at cracks that form in the hardened concrete. In Part 3 we will review the means to repair cracks.
Drying Shrinkage Cracking
The most common cracks in slabs are random drying shrinkage cracks. As concrete dries, it shrinks about 0.06% but the concrete’s tensile strength to resist this movement is only about 0.015%. The cracks form because of restraint—anything that prevents the slab from moving freely, such as the subgrade, creates restraint. If we could magically suspend the slab in midair or place it on a perfectly flat slip sheet that had zero friction, much of the cracking could be avoided, although there’s still internal restraint in larger panels so it might crack anyway.
How much cracking occurs is controlled by the amount of shrinkage—what Scott Tarr calls the shrinkage potential of the concrete. The shrinkage potential can be minimized by reducing the amount of water in the mix and by using larger, and low-shrinkage, aggregate. But, as Scott will point out during an interview at the WOC360 Virtual Industry Forum on February 4, the shrinkage potential of aggregate can be a local issue. You should have an idea of the shrinkage potential of the aggregate the producers in your area use.
One of the best ways to reduce shrinkage is to not add any more water to the concrete than absolutely necessary to get it placed—only a little bit of extra water can result in lots of extra cracks. Think of it as liquid cracks!
Shrinkage-reducing admixtures can help reduce cracks if you can’t reduce the shrinkage potential any other way. Good curing practices result in a stronger, crack-free concrete surface but, unfortunately, do not reduce the shrinkage in the matrix of the concrete and therefore do not reduce drying shrinkage cracks. You can’t keep the concrete wet forever—unless it’s under water.
In typical concrete placed in a strip, like a sidewalk, shrinkage cracks will form in the transverse direction about every 15 feet. This is why we put in contraction joints (sometimes called control joints), which are nothing but controlled cracks. The joint creates a thinner, and therefore weaker, cross section so that’s where it cracks. Properly tooled or sawed contraction joints, installed at the right time and at the right depth, will activate (crack) and reduce the tensile forces from the shrinkage so cracks don’t form in other places. There are, today, floors being placed without joints or with very widely spaced joints but those use some extra (higher cost) methods to handle shrinkage. Scott Tarr explains this in his WOC 36 Virtual Forum interview. Also see this article: Floors Without Joints.
For a typical concrete interior floor, contraction joints should be installed at the column lines, then space the intermediate joints equally between. The standard rule for the spacing of contraction joints in feet is the slab thickness in inches times 2 to 3—so for a 5-inch thick slab, the joints would be 10 to 15 feet apart. And try to keep the panels as square as possible (aspect ratio); no one side more than 1.5 times longer than the other.
Joints can be tooled in or sawed and the depth should be at least a quarter of the slab thickness or 1 inch minimum. Joints should be installed as soon as possible without messing up the slab surface—about 4 hours after finishing in hot weather and up to 12 hours in cold weather. For early-entry sawing (for example, Soffcut), the joints should 1 inch deep (never more than 1.25 inches) and installed as soon as the surface doesn’t ravel—from 1 to 4 hours after finishing. The timing on sawcut joints is critical—wait too long and the natural shrinkage will create its own joints!
Cracking Due Other Causes
Two other common causes of slab cracks are settlement of the soil support system and overloading. A proper subbase is essential for crack-free success. Start by understanding the entire soil support system. The subgrade is at the bottom—it is either the native soil or fill material. The subbase is on top of that and is, according to ACI 302.1 (Guide to Concrete Floor and Slab Construction) a layer on top of the subgrade composed of “compactible , easy to trim, granular fill that will remain stable and support construction traffic.”
Proper compaction is the key. Both the subgrade and subbase need to be compacted and the equipment used depends on the materials (see Subgrades and Subbases for Slabs).
Overloading is also common—often during construction when the slab may not have achieved its full strength. A concrete parking lot intended for passenger cars that ends up being driven on by heavy trucks, will often suffer overloading cracks.
Another common cause for cracks in slabs is curling. As the slab dries, the top dries more than the bottom and so the edges or corners of a slab panel will curl up and become unsupported by subbase. When traffic crosses that corner it can crack on a diagonal across the corner.
Completely crack-free slabs are unrealistic in most cases. ACI 302.1 recommends that the contractor advise every owner “that it is completely normal to expect some amount of cracking and curling.”
Many, perhaps most, cracks in slabs do not negatively affect the performance and trying to repair them will often create more problems than it solves. However, some cracks do need to be repaired so in Part 3 of this article, we will look at some ways to repair cracks in slabs
This article is adapted from a longer article in Concrete Construction, February 2019.