|Concrete Calculator >>|
Six steps to properly prepare and place an exterior concrete surface
There are six steps to follow to achieve a high quality, durable exterior concrete project. The following are six crucial steps that all go hand in hand with each other. All six steps are required to obtain a professional, durable exterior concrete project. It is not acceptable to follow some or most of the steps as listed. Anything less than following all six steps could result in an unsatisfactory project. We recommend that you review the following six steps with your contractor and discuss these procedures before you sign a contract or begin a concrete construction project.
CREATE A PROPER SUBGRADE:
- The subgrade is the existing soil after topsoil is removed. All soft soil that cannot be uniformly compacted must be removed.
- The subgrade needs to be a consistent and uniform thickness of compacted material. Suitable materials include compactable gravel, sand or fractured stone.
- Slope the subgrade away from structures at 1/4" per lineal foot [same as the concrete slab] to ensure surface water drainage.
- Do not place concrete on a frozen or wet subgrade.
- Plan for water discharge from roofs, rain gutters and downspouts. Rain gutters and down spouts should direct water away from buildings and off of concrete surfaces. Downspouts should be connected to drain pipe underground and pitched to a suitable area or connected to a municipal storm sewer. It is desirable to maintain a dry subgrade under the concrete slab.
INSURE A PROPER SLAB DESIGN:
- A concrete thickness of 4" is suitable for most car and pick-up truck traffic. Heavier traffic use will require additional thickness or reinforcement. Concrete at 5" thickness will double the tensile strength of concrete when compared to 4" concrete slab.
- Control joints should be placed as soon as possible with a minimum depth of 1/4 the slab thickness - never less than 1 inch.
- Control joints should be spaced 2- 2 1/2 times the thickness of the concrete converted to feet. Control joints in a 4" thick concrete slab should be spaced consistently at 8 -10 feet.
4" thick x 2 = 8 feet
4" thick x 2 1/2 = 10 feet
- Control joints should not be spaced greater than 15 feet.
- Aspect ratio of joints should be maximum 1:1.5 length to width ratio
- As with the subgrade, slope the concrete surface away from structures 1/4" per lineal foot.
- Isolation joints are required at existing slabs, structures, and fixed objects. Isolation joints should run the full depth of the concrete slab to isolate the fixed object from the concrete slab.
USE THE PROPER CONCRETE MIX DESIGN:
- The concrete mix should be designed to obtain 4500 psi @ 28 days [compressive strength]
- The concrete mix should have a maximum water/cementitious ratio of .45
- The concrete mix must be an air entrained concrete mix. The air content of the concrete when placed should be 6 percent, plus or minus 1 1/2 percent.
- The concrete slump should be approximately 4 inches, plus or minus 1 inch. [Assuming the customer does not request slump altering admixtures]
- The concrete mix should include stone and sand that meet the current Wisconsin Department of Transportation specifications on concrete aggregates. Ask about a low chert stone mix option if your project will include decorative concrete.
Note: The air content and strength of the mix will be affected by any re-tempering with water added to the concrete mix at the jobsite. Avoid excess water in the concrete mix- do not add additional water to the mix unless it is needed to properly consolidate and place the concrete.
PROPER PLACING AND FINISHING OF CONCRETE
- Place the concrete as near to the final position as possible when discharging.
- Use only proper concrete tools when placing concrete that do not separate or segregate the mix. Use flat shovels and concrete cum-a-longs. Do not use gravel shovels or garden rakes.
- All concrete should be unloaded and placed within 90 minutes from the time the truck was loaded. Hot-weather concrete conditions call for a shorter placement time limit.
- No finishing operations shall be performed when there is moisture or bleed water on the concrete surface.
- Finishing operations for most exterior concrete slabs are limited to:
- Screeding or strike-off of the concrete surface
- Bull floating prior to the appearance of bleed water on the surface
- A waiting period for the water sheen to disappear from the concrete surface
- Edging and/or jointing
- Broom texture finish
- Protection of the concrete from the hot sun, high winds, or freezing temperatures is required until the concrete has gained adequate strength and is properly cured.
PROPER CURING AND SEALING OF CONCRETE
- The concrete must be cured immediately after finishing. Curing greatly increases the strength and durability of the concrete surface. Proper curing increases the resistance to freeze-thaw cycles as well as providing some protection from lawn fertilizers and de-icing chemicals.
- Curing the concrete is a process where a satisfactory moisture content and temperature must be maintained for a set period of time.
- There are several acceptable methods of curing; however the most common method is the use of a concrete cure and seal product applied at the manufacturer's recommendation. VanDerVart Concrete Products has several types of curing compounds and curing products. Contact us to review the options and determine the best option for your project.
Note: In the spring and fall of the year, the temperature often falls below 50 degrees. Use Cold Weather Concrete practices to maintain the concrete temperature and cure the concrete properly. Concrete requires 50 degrees at minimum to gain strength- this is a chemical reaction that is temperature sensitive.
PROPER AFTERCARE OF CONCRETE
- Keep standard automotive vehicles off of freshly placed concrete for at least 5 days. In most cases, trucks or heavy load vehicles require a minimum of 14 days before using the concrete. In cool or cold weather conditions, a period in extended term may be required.
- Freshly placed concrete should be sealed after allowing 28 days to air dry. Consider cleaning and resealing aged concrete every several years or as wear in high traffic areas begins to show. Follow the manufacturer's recommendations for the coverage rates to seal or reseal concrete.
- Sealing is a process where a protective coating or penetrating water repellant material is applied to keep moisture and contaminates out of the concrete. This step can ensure years of attractive, trouble free and durable concrete. Follow the manufactures recommendations for preparation and application.
- Sawed control joints in the concrete should be filled with a flexible caulk or joint filler after sealing. This process prevents water and deicing products from entering the concrete at the control joints.
- Rain gutters and down spouts should direct water away from buildings and off of concrete surfaces. Downspouts should be connected to drain pipe underground and pitched to a suitable area or connected to a municipal storm sewer. It is desirable to maintain a dry subgrade under the concrete slab. If water ponds adjacent to concrete or undermines the concrete, the concrete subgrade may settle or heave causing cracking of the concrete. If water collects under the concrete during the fall and winter season, significant frost heave will likely occur during the winters of Wisconsin.
- Prevent snow and ice from accumulating on the concrete surface especially during the first winter. Sealing the concrete will help to protect the concrete surface.
- Don't allow the use of de-icing chemicals on your concrete, especially the first winter. Sand is a good anti-slip product that can be broadcast onto the concrete safely. Heated sand is available each winter season from all VanDerVart Concrete Products locations.
- Do not use anti-ice products such as ammonium sulfate, ammonium nitrate or magnesium chloride. These chemicals will chemically attack and destroy the concrete.
- Prevent lawn fertilizers from accidently being placed on concrete. Some fertilizer ingredients contain chemicals that will stain or chemically attack the concrete.
Following all of these six steps will insure a high quality, durable exterior concrete project. Please request your contractor to order ready-mixed concrete and concrete products from VanDerVart Concrete Products. We have an experienced staff of professionals to ensure you receive high quality products and service. Our staff is available to assist you in the selection of aftercare products to protect your concrete.
The Staff and Management of VanDerVart Concrete Products
Contact one of our sales representatives at:
920-898-5771 (New Holstein)
or 920-683-5580 (Manitowoc).