Frequently Asked Questions
Dusting is the formation of loose powder on the top surface of placed concrete. The surface can powder under any kind of traffic, and can easily be scratched by a finger nail or even sweeping. A concrete floor dusts under traffic because the wearing surface has become weak due to improper concrete finishing practices.
Dusting can be prevented by following a proper concrete placing practices: use moderate slump between 3 and 5 inches, do not start finishing operations while there is concrete bleed water on the surface, don’t sprinkle cement or water on concrete prior to or during finishing operations, make sure there is adequate ventilation for exhaust gases if placing in an enclosed area, and lastly use adequate curing to ensure concrete surface retains moisture within the first 3-7 days, especially in freezing conditions.
Scaling is the flaking or peeling of a finished surface of hardened concrete as a result of exposure to freeze/thaw cycles. Occasionally concrete can scale in the absence of such cycles, but that is usually due to improper placing practices such as: early use of a steel trowel, or finishing while bleed water is on the surface.
If you are placing concrete in an area that is exposed to freezing and thawing cycles (areas that experience snow fall, or have regular temperature drops below 32 degrees F) use air entrained admixtures, use moderate slump concrete 3 to 5 inches, do not add excessive amounts of water on the jobsite, perform finishing operations after bleed water has dissipated, properly cure the surface, and avoid using deicer chemicals during the first winter after the concrete has been placed.
All concrete has a tendency to crack and it is not possible to produce completely crack-free concrete. Cracking can be reduced and controlled if proper concreting practices are followed such as: using a concrete mix design to handle anticipated loads, protect and cure concrete properly, place and finish using proper concrete practices (moderate slump 3”-5”, finishing after bleed water is dissipated, protect the surface with curing, don’t sprinkle cement to dry up bleed water, etc.), when placing on slab on grade prepare a stable uniform compact grade, wet the surface of dry grades that absorb water, and provide for proper contraction and isolation joints. Lastly, 7/11 uses aggregates that meet 0.040 shrinkage specifications without having to use shrinkage reducing admixtures. We are the low shrink leader in the valley, and because of this, our concrete products have a lower rate of cracking than all of the other suppliers in the area.
Absolutely not. With 7/11’s concrete color system and a skilled decorative concrete contractor, concrete can take on almost any shape, pattern, color, or texture, in both exterior and interior applications. The number of special colors and finishes for concrete pavements and slabs is almost limitless. Color is a great way to customize a project, or add a unique aspect to a yard or driveway. In addition, colored concrete reduces unwanted surface glare. It is a remarkable and cost efficient customization tool.
Concrete Joints are pre-planned cracks in concrete structures. Concrete expands and shrinks with changes in moisture and temperature changes. The overall tendency of concrete is to shrink and because of this cracking at an early age usually occurs. Irregular cracks are visually unappealing and are difficult to maintain. Usually cracking does not affect the overall structural integrity of concrete; the concrete will still maintain its desired compressive strength. Joints all a contractor to plan where cracking will occur, and thus reduce displeasing visual concrete structures. Joints in slabs can be created by forming, sawing, tooling, and inclusion of joint formers.
Cracks in concrete cannot be prevented entirely, but they can be controlled and minimized by properly designed joints. Joints provide relief from the tensile stresses, are easy to maintain, and are less unpleasant visually in concrete structures than uncontrolled and/or irregular cracking.
Concrete Jointing is a significant part of general proper concrete placing practices. Joints must be carefully planned, designed, and constructed to avoid uncontrolled, irregular concrete cracking. The following guidelines should ALWAYS be followed when planning joints for concrete slabs.
- The maximum joint spacing should be between 24 to 36 times the thickness of the slab. Example: If you are placing a 6 inch concrete slab, then the space between joints should be between 12 feet and no more than 18 feet.
- Joint panels should be made square, or as near as possible where practicable. The Length of joint spacing should not exceed 1.5 times the Width.
- The joint groove should be no less than ¼ the thickness of the slab. Example: If you are pouring a 6 inch slab, then the joint should be cut to at least a depth of 1.5 inches.
- For conventional saw-cut joints, the cuts should be made typically 4 to 12 hours after the concrete has been finished.
- For all types of joints, proper concreting practices should be followed: planning the exact location of all joints, and include the timing of contraction joint sawing before placing concrete.
Plastic shrinkage cracks are caused by a quick loss of water on the top surface of concrete before it can set. The condition becomes critical when the rate of evaporation on the surface exceeds the rate at which bleed water can travel through the concrete to replace the lost water on the surface. Plastic shrinkage cracks appear on top of fresh concrete soon after it is placed and while it is still plastic. They are cracks that usually run parallel to each other. Conditions that give rise to high surface evaporation rates are wind (excess of 5mph), low relative humidity, or high ambient or concrete temperatures. If these ambient conditions exist, take not and follow general proper concrete placing practices. While plastic shrinkage cracking is visually unappealing, they rarely affect concrete’s structural strength.
Plastic shrinkage cracks can be prevented by noting adverse ambient conditions, such as high ambient temperatures, low humidity, or wind, prior to pouring and planning ahead. In such conditions always remember to plan to: dampen the subgrade and forms, prevent surface moisture loss by using fog sprays or windbreaks (as simple as a tarp covered fence will do), covering finished concrete with wet burlap or polyethylene sheets, using cooler concrete in high temperature areas (7/11 offers mix solutions to keep concrete temperature cool in poor ambient conditions), and lastly cure the surface as soon as finishing has been completed.