Craze cracking forms along the surface of a concrete slab. Photo by http://www.shutterstock.com/phoonperm.

Watching a concrete pour and finishing is a fascinating but nerve-wracking process. All the planning, design, and research you have done as a designer comes down to a few unforgiving hours (if that) that determine whether your vision is fulfilled. Although it may feel out of your hands, there are things you can do as a designer to help ensure the concrete elements are successful.

Concrete cracks. That is in its nature. As concrete hydrates (the chemical process where compounds in the cement form bonds with water and harden the concrete), it shrinks. As it shrinks, cracks will form where the concrete is weakest. These cracks happen at the micro and macro (think visible) levels.

There are two basic types of macro-level cracking: crazing and structural. Craze cracking occurs at the surface and looks like a series of fine veins, like an alligator’s skin. It occurs when the concrete’s surface dries faster than the center of the slab, and the trapped moisture causes the concrete to flex. Though it does not affect the structural integrity of the slab, it is unsightly and can result in surface spalling that eventually requires replacement. Crazing often results from improper conditions and curing methods during placement; it was either too hot, windy, and/or dry when the concrete was installed. Good specifications, good quality control on site, and proper curing methods will prevent this type of cracking from ever happening.

Structural cracks carry through a section or the entire profile of a slab. They can represent an unsightly yet stable installation in some situations, and in others, a deficiency that must be corrected immediately for people’s health, welfare, and safety. The structural engineer can help designers understand whether structural cracking is cosmetic or more serious.

Incised shapes can create structural cracks along the edge of the slab. Photo by John Payne, ASLA.

Structural cracks happen primarily for three reasons. First, concrete cracks where the subgrade has settled and created a weakness under the slab, which is also known as settlement cracking. If the subgrade does not provide uniform support under a slab, downward force in one area creates a pulling or tensile force along the edge of the depression, creating cracks. Poor compaction, using the wrong material as subgrade, or erosion of the subgrade prior to concrete being poured (or any combination of the three) are common causes of subgrade settling. When on site, it’s important to look for signs that the base has been compacted in a uniform way. Have compaction tests been done? If so, were they satisfactory? If it is an open-graded stone, was it compacted according to the specs?

Second, a concrete slab will crack where the concrete slab is tied to an immovable object, such as a building, drainage structure, or a footing. Slabs will move as the subgrade expands and contracts with temperature changes, but it usually happens in a uniform manner across the plane. If the slab is tied to a structure, tensile force between the two will cause the slab to crack. Check for isolation joints at these interfaces before pouring.

Third, concrete cracks can happen where the edge of the concrete slab changes direction—for example, where the concrete edge follows the complicated facade of a building, there is a leave-out area in the slab, or a slab is poured around a column. These incised shapes create stress in the slab, and cracks can radiate from that point into the slab.

Designers can influence where and how much concrete cracks. There are three basic approaches to addressing cracks in horizontal concrete: specifications and quality assurance, jointing, and reinforcement. These measures can and should be used in combination with one another, depending on the circumstance.

Many factors should drive the design concrete mix specified for each job. Don’t think a “one-mix-fits-all” approach will suffice. There are many factors to consider when specifying concrete, but three will specifically affect reducing concrete cracking.

Water-to-cement ratio. The more water is introduced into a mix, the more fluid it is, which is important for complicated formwork. However, more water reduces the compressive strength of the concrete and will increase the likelihood of cracking.

 Aggregate type and size. Specify only low-shrinkage aggregate. Maximum aggregate size should be no more than one-third of the thickness of the slab, or three-quarters of the distance between rebar and the form, also known as cover distance.

Materials sourcing. Specify a single source for all concrete components. This will go a long way in ensuring consistency between concrete loads. Specify sole-source components to help maintain consistent color and cure. Make sure the load ticket is scrutinized prior to placement; the same plant may use different stockpiles of components that may vary in critical qualities. Even the water-to-cement ratio can vary significantly between trucks.

This sawcut contraction joint does not connect to the outer corner of the slab, resulting in uncontrolled stress and cracking. Photo by John Payne, ASLA.

Installing joints in concrete does not prevent cracking, but the joints encourage the concrete to crack where you want it to and relieve expansion stresses that lead to cracking. There are various types of joints, and jointing terminology is often misused, whether in the design studio or on the construction site. The following are some of the most widely used joints to control cracking.

A construction joint is a joint where construction progress has ended or where different pours will take place as part of the construction sequence. These can be doweled together to eliminate any uplift from one slab to the other that may create a tripping hazard.

A contraction joint, also referred to as a control joint, is a sawcut or tooled joint used to control where the cracking is going to occur by creating a weakened plane along the axis of the joint. It is critical that if sawcut, joints are installed as soon as possible after the concrete pour, generally six to 18 hours after initial placement. If you wait longer, cracks will start to develop in the slab. This rule is one of the most-often violated in placing concrete, yet it goes the furthest to reduce unwanted cracking. Contraction joints should be a minimum of one-quarter the depth of the thickness of the concrete, a depth that is often not achieved, especially when joints are tooled into the still-wet surface. When you are observing the pour, take the time to observe how the jointing is being done and ask questions.

An isolation joint is used to isolate different materials or elements from your concrete slab; as mentioned before, you don’t want to tie your slab to something that will remain in place. The joints should have expansion joint material placed/secured on the isolated element prior to the concrete pour. These are then usually sealed to prevent any debris from entering the joint.

Even after years of use, well-planned and installed jointing can enhance a design. Photo by James Dudley.

An expansion joint provides space for thermal expansion of your concrete flatwork. Like an isolation joint, it is usually installed with some sort of expansion material and either has sealant on top to prevent debris from entering or is left open. If left open, it will require periodic cleaning, as material entering the joint makes it rigid and unable to allow the slab to expand without cracking. In practice, it is constructed like an isolation joint and can be detailed to make it indistinguishable from a contraction joint.

A decorative sawcut joint is not technically used to control cracking; however, it is a design feature frequently used to create patterns within your slab. These joints can look the same as expansion and sawcut contraction joints as described above if you have the correct details.

Think about how the jointing pattern can alleviate the stresses placed on concrete that were mentioned earlier. Creating guidelines that allow the contractor some discretion as to where to locate the joints in relation to field conditions while still preserving the design intent—rather than hard-and-fast jointing rules—can give the contractor the ability to better synchronize the jointing with likely problem areas. An example is a note reading “Align expansion joints with column centers; joint spacing not to exceed four feet or be less than 3.5 feet,” rather than a specific repeating dimension that will result in the same joints offset from the columns, which is much more visually distracting compared to a slight variation in joint spacing. Think about what the client or visitor’s eye will be drawn to—will it pick up a small change in the joint spacing, or will it be drawn to the relationship with another landscape element such as a planter or the change in direction on a building facade? Finally, publications from the American Concrete Institute can give helpful guidance, as can a trusted concrete contractor.

A concrete jointing plan helps ensure a successful installation. Image by James Dudley.

One of our favorite structural engineers, a legendary old pro, had a favorite response to questions about concrete strength and cracking: “Throw some more steel in it!” He was right in that reinforcing concrete improves its tensile strength, but it will not eliminate cracking, only limit the size of the cracks. The most common and familiar product for reinforcing concrete is deformed rebar (the bars have ridges cast into them); anyone in the design and construction world would recognize this product. Steel bar comes in different radii and can be placed with different spacing, which is directly related to the thickness of the slab and the loads that you expect the slab to bear. The engineer on the team is the best source regarding the size, spacing, lap distances, and coverage for your installation.

There are a couple of critical design and installation considerations to be addressed for rebar reinforcement to be successful. First, rebar must be placed properly within the profile for the slab to be effective. In a standard four-inch-thick pedestrian sidewalk slab, rebar should be placed in the middle of the slab. If your slab is thicker, reinforcing bars should be placed in the upper third of the slab. Rebar chairs (small supports) are used to keep rebar at its designed depth and should be spaced so your rebar does not bow between them. Never place rebar on the subgrade or hook or pull it into the slab during the concrete pour, as there is no way for the contractor to gauge how high or low the reinforcement is embedded in the slab. If it is too close to the surface or bottom, moisture will reach the rebar and it will rust, which can degrade the surrounding concrete. Epoxy-coated rebar is popular; however, moisture can still corrode the rebar if the coating material has been compromised. Galvanized rebar is more immune to corrosion, but it is much more expensive than regular rebar.

Rebar reinforcement is placed according to the plan. Photo by James Dudley.

Second, proper rebar sizing and spacing are critical to allow the concrete to resist stresses and to help avoid cracking. Engineers should provide the size and spacing of rebar in concrete slabs, or installation of incorrect size or spacing could lead to significant reductions in overall strength. For example, according to the professional engineer Mel Marshall writing in Precast Inc., “Placing #5 rebar correctly at 4-in. spacing provides a steel area of 0.93 sq in., whereas placing the same bars incorrectly at 5-in. spacing will reduce the steel area provided to only 0.74 sq in.—20% weaker!” Rebar shop drawings must be required and vigorously reviewed before installation. In the field, the quality control inspector needs to verify that rebar is correctly sized, spaced, and installed before concrete pouring. Tearing out incorrectly reinforced concrete will be costly, and delayed scheduling can be a significant emotional event for everyone involved.

There are a number of reinforcing alternatives available. Synthetic fibers, steel fibers, basalt reinforcement, bamboo, and fiberglass strand reinforcement are all alternatives to classic steel reinforcement. Some are new, some have been available for longer periods of time but are not widely accepted, and others are not widely used. Reasons for this include building code restrictions, cost, and overall familiarity of the product by the trades. Many of these alternatives appear promising, and we look forward to those potential options in the near future.

Concrete cracking in slabs can be controlled or minimized by a combination of good design and specifying, high-quality and consistent material sourcing, and proper subgrade preparation and installation methods. As with any built project, the key to success is teamwork and good communication among the entire project team, from early design through installation.

John Payne, ASLA, is a principal at SiteWorks in New York City. James Dudley is an assistant superintendent for a development, general contracting, and construction management firm.

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The use of reinforced welded mesh can make the project construction faster. As long as the reinforced welded mesh is laid as required, the concrete can be poured, eliminating the need for on-site cutting of the reinforcement, placing them one by one, and tying up.