Slab-on-grade foundations are concrete slabs poured directly on the ground with no space between the ground and the concrete. This most often occurs in areas where the ground is not subject to freezing and therefore not likely to heave. There are derivations of this where frost underneath the structure is controlled with foam insulation or hydronic (hot-water) heating systems incorporated into the slab to prevent the ground below from freezing. However, it is usually not typically economically favorable to use the slab-on-grade in very cold climates. There is at type called a Frost protected Slab that is standard in Sweden and is gaining acceptance here.
The advantages of slab-on-grade are that it is a relative inexpensive form of residential foundation, they are very sturdy when properly designed and constructed and helps obstruct termites from entering the house. When properly constructed they offer very few spaces in the foundation itself for termites to intrude into the structure from below. This assumes that all pipe penetrations are properly sealed with stainless steel wool or adequate caulk to prevent their passage. It also reduces the amount of perimeter crack that is vulnerable to infestation making treatment easier in the event that termites do get into the house.
You must plan carefully for utility penetrations through the slab for water, electric, gas, telecommunications and any other utilities. This is often times overlooked, especially because we fail to account for changes in technology.
When planning a slab-on-grade foundation, you must bring a conduit from underneath to an accessible place outside for water. Most plumbers will direct bury the line or embed the copper lines directly in the slab. This is not a good practice because it makes repair costly and difficult if something goes wrong with the water line. If you install a 2′ PVC conduit for the water line, a new one can be pulled in later if you need to. This will also prevent the copper or galvanized water line from degrading from contact with bare concrete.
The same procedure needs to be followed for cable and telephone. You can put both services in the same conduit in this case so one 2′ conduit should adequately suffice in this case. Electrical services must enter a disconnect on the exterior of the house so it is not typical that you will need to provide a conduit under the slab for this. In all these cases, the conduit should terminate in one central location in the house such as a utility closet or garage.
Slab-on-grade design calls for a slab thickness a minimum of 4″ thickening to a minimum of 6″ within about 16″ of the perimeter for additional reinforcing steel to support exterior wall loads. If there is to be a interior bearing wall, the engineer will also call for this area to be thickened and more reinforcing to compensate for these loads.
Perimeter footers and interior footers or rafts are typically reinforced with steel bars called re bar. This adds tremendous strength to the concrete by compensating for the concretes poor tensile strength. By combining steel with concrete, you combine two incredibly strong materials in a way that compensates for the weak characteristics of each.
You will also see what is called WWF (welded wire fabric) also called as reinforcing in concrete but its true purpose is to distribute temperature changes in the concrete more evenly to prevent cracking, the proper term is controlling cracking. I say prevent cracking, but you will still have cracking in concrete, it is the nature of concrete to crack, so another step taken to minimize the effect of cracking is the process of placing control joints at specified intervals to guide the cracks to the bottom of the crack where they will not be seen and to minimize the travel of any one crack. If you see minor cracks in the bottom of the control joint, do not panic, it is normal. The design and placement is very important to the integrity of the slab. Discuss this in detail with your selected concrete sub to make sure adequate control joints are placed in the slab.
Proper Construction Means Proper Curing
The key to a good slab-on-grade foundation that has been properly designed is to allow time to wet cure the concrete. Concrete is poured as liquid, but begins to harden immediately. It is usually hard enough to walk on in a day. It should not be built on for a minimum for seven days and you must take care that the temperatures are not too cold or too hot when you pour it. The benefits of proper curing are:
Increased strength gain
Increased abrasion resistance on the surface
Less permeable concrete with increased resistance to chloride penetration and freeze/thaw damage
Increased resistance to early cracking–slabs gain strength before drying out and have more resistance to shrinkage forces.
So what is a proper method of curing. Well in warm climates, it entails keeping the concrete damp for a minimum of 7 days. This is accomplished by spraying water on the slab every day (twice a day if needed) spraying the concrete and covering it with plastic sheeting to retain the water within or using some other medium (kept wet) such as sand, straw, or blankets to do this function for at least 7 days, the longer the better. With proper planning this will be possible although if you are working with a builder, he will probably want to start framing as quick as possible after the slab is poured. Do yourself a favor and allow the 7 days for proper curing and make sure that adequate methods are employed to retain moisture in the slab during the process.
The last 2 important considerations are temperature when pouring. Concrete generates tremendous heat during the curing stage. If it is poured at very high temperatures (over 95 ‘F) you must take special precautions to mitigate the heat, talk with your contractor if you suspect you will have this condition to make sure he is prepared to take the proper steps to protect the concrete. Concomitantly, you should not pour concrete when the temperature is below 20 ‘F unless you are prepared to maintain heat on the slab until it has fully cured. This will be very expensive to do so it is best to not pour when temperatures are going to be below freezing for extended periods. You can do it , but you must make sure to maintain the internal temperature of the concrete at no less than 55 ‘F.
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Source by Randy Covington
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