About Concrete

GENERAL INFORMATION ABOUT CONCRETE

SUBGRADE PREPARATION

Cracks, slab settlement and structural failure can often be traced to an inadequately prepared and poorly compacted subgrade. The subgrade on which a slab on ground is to be placed, should be well drained, of uniform bearing capacity, level or properly sloped, and free of sod, organic matter and frost.  The three major causes of non-uniform support are:

1. the presence of soft and hard spots

2. backfilling without adequate compaction and

3. expansive soils

Uniform support cannot be achieved by merely dumping granular material on a soft spot. To prevent bridging and settlement cracking, soft or mucky areas and hard spots (rocks) should be dug out and filled with soil similar to the rest of the subgrade or with granular material such as sand, gravel, or crushed stone. All fill materials must be compacted to provide the same uniform support as the rest of the subgrade. 

Planning:

• Layout and Elevations: The surface of the finished slab should slope a minimum of 1/8 inch per foot. A slope of 1/4 inch per foot is preferred. When run-off cannot be achieved, trench drains should be installed to eliminate standing water on finished slabs. 

Pre-planning of control joints is critical for proper layout and crack control. Cracks will form off inside (or re-entrant) corners. Proper planning can eliminate and/or mitigate random cracking. 

Preparation:

• Excavation: Don't excavate more than needed. Make sure all debris is disposed of properly and that there is ample room for a solid gravel base and the thickness of the slab. 

• Subgrade: Subgrades should provide uniform support throughout the slab. Uneven subgrades can lead to settlement cracking. New construction sites with soils of poor value should be compacted. A gravel base of 2-4 inches should be placed and mechanically compacted to a uniform and stable condition. Subgrade should be free of frost and water before concrete placing begins. A slightly damp subgrade (gravel) is preferable. 

• Forms: Setting forms properly and with attention to detail will provide straight lines and edges. Planning for proper lines begins with choosing straight and true lumber and form material, also properly jointing, nailing and staking the forms. Lumber has crowns and curves and heed must be taken when purchasing and placing forms for the slab. 

Joints: 

• Isolation joint material should be placed against any and all abutting structures such as curbs, foundations and walkways, also around post and piers. 

• Control Joint (Contraction Joint) locations should have been laid out in the planning stage and should be installed every 8-12 feet in a 4" slab keeping the maximum distance between joints at 24 to 36 times the slab thickness according to ACI. The resulting panels should be as nearly square as practical. Never make the long side more than 1-1/2 times as long as the short side. 

• Joint Depth: Jointing should be a minimum of 1/4 of the thickness of the slab or a minimum depth of 1 inch for a 4 inch slab. 

• Joint Timing:  Grooved joints should be done following bull floating and when the concrete has firmed up and bleed water has disappeared. Saw cutting these joints afterward may still be necessary to achieve the proper depth. Sawing control or contraction joints should be done 6-18 hours after placement of the concrete, although sawing as late as 24 hours may be successful under some conditions. Sawing should be done using chalk lines for accuracy and should begin as soon as raveling of the joints is no longer a risk. 

CURING IS KEY

Curing is one of the final but vital steps for a quality concrete product. Curing is ensuring that the concrete has enough moisture and the proper temperature to chemically react to bring your concrete to full strength. Curing greatly increases the strength and durability of concrete and its resistance to freezing and thawing and deicer salts. If your concrete is not cured properly, you may end up with only 50% of the desired strength at the surface, which may not be strong enough to last through the winter. There are several methods of curing. The most common is the application of a compound sprayed onto the surface of the concrete immediately after finishing that prevents the concrete from drying prematurely. Concrete needs 30 days to properly cure under normal conditions. Under colder or wintry conditions concrete needs a much longer time frame to reach its cured state. Proper steps need to be taken to cure the concrete and to keep the concrete from freezing. 

SEALING YOUR CONCRETE

Water repellent concrete sealers can help prevent surface defects from cold weather by keeping water from getting into the concrete slab. Applying a quality concrete sealer helps maximize protection and lengthens the time between applications. Cure and Seal sealers should be applied immediately after finishing procedures are completed. It also should be reapplied in 30 days for maximize protection. Always follow the manufactures directions carefully. 

USE CAUTION WITH DE-ICERS

While concrete is the most durable product available for your home, proper care is a requirement for long lasting beauty and wear. One of the most damaging things to a new concrete surface is the use of de-icers, especially the first winter. Concrete takes a while to reach full strength and while some de-icers such as salt do not chemically react with the concrete, they increase the number of freeze/thaw cycles the concrete must go through. This has the potential of damaging the concrete before if can reach maximum strength. NEVER use de-icers containing ammonium sulfate or ammonium nitrate. These chemicals are often packaged and sold as de-icers but they will rapidly cause disintegration as well. After the first winter, de-icing chemicals such as sodium chloride or calcium chloride may be used sparingly. Be cautious of products that claim to be "safe for use on concrete". Read the label to make sure that they do not contain ammonium sulfate or nitrates. 

FINISHED PRODUCT

You will get many years of great service from your concrete by taking the proper steps and making the right decisions from beginning to end. 

CONSUMER SEALING AND AFTERCARE SUMMARY

Keep vehicles off freshly placed concrete for at least 7 days. 

Freshly placed concrete should be sealed after allowing a minimum of 30 days to air dry. This is addition to the curing compound used on the first day. Consider re-sealing aged concrete every several years or as wear in high traffic areas begins to show.   

Sealing is a process where a protective coating or penetrating water repellent material is applied to keep moisture and contaminates out of the concrete. 

The use of down spouts can help ensure that drain water does NOT undermine the slab which can cause settlement cracks. 

Prevent snow and ice from accumulating on the concrete especially the first winter. Sand can always be used for traction. 

Never use de-icers containing fertilizer ingredients such as ammonium sulfate, ammonium nitrate, calcium chloride and magnesium chloride. These chemicals WILL ATTACK AND DESTROY CONCRETE BY CHEMICAL REACTION. NEVER USE THESE PRODUCTS!