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