When it comes to concrete, random cracking is unsightly. But, due to the fact that concrete shrinks, it is going to crack. Our best defense for concrete cracking is to try to control where the cracks occur.

Cracks will appear at any time and place where the stress within the concrete to pull apart exceeds the strength of the concrete to hold itself together. Concrete will shrink at a rate of approximately 1/8 inch per 20 feet, and while it shrinks it will crack randomly. We can control this random cracking by pre-determined location of joints in the concrete.

Some forms of joints are:

1. ISOLATION JOINTS (also called expansion joints) – These are placed wherever complete separation between the floor and adjoining concrete is needed to allow them to move independently without damage. These isolation joints are often made with an asphalt-based cork-like material.
2. CONSTRUCTION JOINTS – These are placed in a slab where concrete operations are concluded for the day. However, if the concrete operation is interrupted long enough that a cold joint may appear, you should install a construction joint regardless of planned joint layout.
3. CONTROL JOINTS(also called contraction joints) – These are intended to create straight lines of weakness in the concrete that the cracks will occur along. They should be created to a depth of one forth the slab’s thickness with a maximum joint spacing of 24 to 36 times the thickness of the slab.

   Control Joints can be formed with a jointer, also called a groover. They have a cutting edge or bit that makes narrow grooves in the slab. For a 4-inch slab the depth of the cutting edge should be one inch, for a 6-inch slab the edge depth should be 1.5 in, and for an 8-inch slab the edge depth should be 2 inches. The radius at the top of the groove should be 1/8 in. for floors and 1/4 to 1/2 in. for sidewalks, driveways or patios. Shallow groover tools should only be used for decorative applications.

4. SAW JOINTS – These are control joints that are sawed in, straight as possible, clean lines about 1/4 the thickness of the slab. Sawing should be done as soon as the concrete is strong enough to resist tearing or other damage by the saw blade. A slight raveling of the sawed edge is acceptable. It is important not to delay sawing too long because the concrete may crack before it is sawed, or it will crack ahead of the saw blade. Saw joints are usually preferred over joints with a groover tool on floors that will carry forklift traffic or other industrial vehicles.

Cracks in concrete cannot be entirely prevented, but they can be minimized and controlled with properly designed joints.

The National Ready Mixed Concrete Association (NRMCA) defines scaling as local flaking or peeling of a finished surface of hardened concrete as a result of exposure to freezing and thawing. Scaling will normally begin as a small area but can expand to cover large areas.

Although it is likely that scaling will never be eliminated due to the many variables that contribute to it, the Portland Cement Association (PCA), NRMCA and ACI International have developed the following guidelines to help minimize scaling.

Here are some tips to reduce scaling:

1. Use an air-entrained, low water/cement ratio (0.45 or less w/c, as delivered), moderate slump concrete (not exceeding 5 inches) with a 28-day compressive strength potential of at least 4000 psi. Except when absolutely necessary, do not retemper concrete before placement.
2. Avoid finishing practices that reduce or eliminate the air-entrained voids in the wearing surface layer, or that result relative to the bulk concrete at lower depths. Do not perform any finishing operations with water present on the slab surface. Delay finishing until all bleed water has risen to, and disappeared from, the surface.
3. Provide adequate curing for the concrete.
4. Avoid late season concrete placement, where concrete can experience freezing conditions and/or exposure to deicing salts before (a) the concrete has reached 4000 psi, and/or (b) the slab has had at least 30 days of air drying.
5. Protect first year concrete from the harsh winter environment. Prevent newly placed concrete from becoming saturated with water prior to freeze and thaw cycles during the winter months by applying a silane or siloxane-based breathable concrete sealer (do follow the manufacturer’s application recommendations). Do not use deicing salts in the first year after placing the concrete. Use clean sand for traction. When conditions permit, hose off salt accumulations deposited by vehicles on newly placed drivways, approaches, and garage slabs. Never use ammonium sulfate or ammonium nitrate as a deicer; these are chemically aggressive and destroy concrete surfaces.
6. Provide proper drainage.Poor drainage permits water or deicer and water to stay on the concrete surface for extended periods, increasing the severity of exposure for the concrete and promoting scaling, especially in driveway, sidewalk, and curb and gutter applications.

   Adequate curing is a must for the development of strength, water-tightness and durability in hardened concrete. It is very important to provide and maintain satisfactory moisture and temperature conditions immediately after concrete placement and finishing, and for as long as practical. As a general rule, the length of the curing period for concrete flatwork in temperatures above 40 degrees F should be a minimum of 7 days, or the time necessary for the concrete to reach 70% of its specified compressive strength. The later in the season it gets, the longer it will take to reach these points.

1. Use proven recommendations for adjusting concrete proportions, such as water reducing agents. Set retarding admixtures, and Fly ash.
2. Advanced timing and scheduling, to avoid delays between trucks. Trucks should be able to discharge immediately and adequate personnel should be available to place and finish the concrete.
3. When possible, deliveries should be avoided during the hottest part of the day.
4. Limit the addition of water at the job site. You should add no more than 2 to 3 gallons of water per cubic yard.
5. Slabs on grade should not be placed on polyethylene sheeting. If a vapor barrier is a must then a damp bed of compactible granular fill material should be placed over the grade.
6. Wind breaks and shade should be used if at all possible.
7. Finish as soon as the sheen has left the surface of the concrete.

The National Ready Mixed Concrete Association (NRMCA) defines hot weather concreting as anytime during high temperatures in which special precautions need to be taken to ensure proper handling, placing, finishing and curing concrete. During the summer months you will most generally experience problems that require precautions being taken for hot weather concreting, but the associated climatic factors of high winds and dry air can occur at any time. These conditions often produce a rapid rate of evaporation of moisture from the surface of the concrete and an accelerated set time among other problems.

Hot weather is a major factor to consider when planning a concrete project. Too often it is assumed that not much can be done to combat hot and dry conditions. Many times the addition of large amounts of water is used to fight off rapid set times or to ease the placing and finishing process. These assumptions and practices often times prove to be detrimental to the concrete.

Problems that can occur due to high temperature, high wind and low humidity are:

• Increase slump loss, making concrete more difficult to place and finish.
• Accelerated set time, decreasing the amount of finish time.
• Increasing the potential for plastic and drying shrinkage cracking.
• Decreased air content.
• Reduced strength if water is added to offset slump loss.
• Thermal cracking may result from a rapid drop in concrete temperature when placed on a hot day followed by a cool night.