Concrete Slab Preparation
Concrete slab foundations are the most common type of foundation in the Southwest. There are much flatter areas to build on in the Southwest and less need for basements. This changes somewhat in the higher elevations. You will begin to see more walkout basements and crawlspaces . Even so, many builders prefer the slab-on-grade foundation to other types because it is fairly simple.
A good subcontractor will layout the house and do the staking. Staking shows the backhoe operator exactly where he is supposed to dig your footings. Confer with your foundation contractor about the footprint of the home and make sure it does not encroach on required setbacks. The importance of making sure your footprint is in the right place cannot be stressed enough, especially when you are doing a poured foundation. You cannot move a slab. It must be torn up, reinspected and replaced.
Be certain to contact your local "Call Before You Dig" service so the utility companies can mark where their lines are running. This is a requirement before you begin any excavation. Nothing is more dangerous than hitting a gas line or electrical conduit that is running through your lot.
The backhoe operator will be able to follow the staking layout and dig all the footings for the slab foundation. Walk the footings with your contractor and verify depths and thickness.
Rebar is an important element of reinforced concrete and amounts are specified in the approved construction drawings. Depending on the type of loads you will be expecting the footer to carry, you will have more concrete and more rebar. It is important that the proper size of rebar is used and that it is lapped at the minimum requirements. Walk the completed footings the day before inspections with your foundation contractor.
The concrete contractor SHOULD be able to pour the next day. It most likely will be done in the early morning and it’s a good idea to be there. Many builders use 3rd party inspectors to check the concrete during pours. This is not a bad idea. They have a lot of expertise and will test the slump (how wet the concrete is) to make sure it is not watered down. They will also do tests for the overall PSI (Pound per Square Inch) strength of the concrete. There are certain minimums that must be met per the plans and local codes. It is vital to make sure when you award your bids that you include a clause requiring not only adherence to drawing specs, but also, to building codes. Protect yourself.
The Stem Wall is a vertical extension of the footer. The exterior framed walls sit on top of the stem wall, transferring the load of the whole house onto those elements. Can you see why the proper PSI is so important? Imagine the whole weight of a 2000 square foot house being supported by a strand of concrete 12” x 24” or less. It had better be up to spec. Stem walls can be set up with forms or built of block and filled with concrete and rebar. Again, this will be specified in the blueprints. It may be an advantage to go with the poured stem wall because it may mean one less subcontractor.
Stem wall and Rebar Inspection
Under Slab Plumbing
After the removal of the formboards and excess material, your foundation contractor will back fill against the stem wall. Usually they will put a dirt ramp in against the wall so they can drive a small tractor into the interior of the house. Now they will dig the trenched for the
. Your plumber has laid out his locations and set measurements for the trencher to follow. The proper fall of wastelines in very important. Over the years, builders have determined the optimum size of lines and degree of drainage necessary to insure proper flow of waste from the home to the sewer.
The plumber will probably take several days to lay their lines and shade them(protect them by packing sand or gravel around them). Once they are complete, the lines will be put under pressure. Some municipalities require it to be water, some air. Either way, the goal is to show that the lines are constructed properly and won’t leak. Make sure this is done prior to calling inspection.
Just a reminder, when interviewing plumbing contractors, ask for specific timelines for completion of job. It should not take more than a few days to lay the wastlines. If it begins to drag on, it is because they are putting your project on the back burner. Remind them of the commitments they gave in their contract proposal and that you expect them to honor it.
What will the building inspector be looking for? Sometimes for just one thing. The main concern of the inspector will be, “ Do these lines hold water pressure?” You can check this yourself in the morning by looking at the gauges that are set to measure the pressure in the lines. If they have moved significantly since the previous evening, there is a leak. They will usually also put a level on the lines to verify they have a proper flow to them and that they are properly shaded. It has been my experience that this is normally a slam dunk inspection. FIND CONTRACTOR AND SUBCONTRACTOR LISTINGS HERE.
Backfill, Compaction and Preslab
The backfill of trenches with plastic waste lines is a dangerous business. Too often, excavators show little regard for plumbers work and dump big rocks and clots of dirt right down on top of the pipes. This has the potential to open up a leak under your house. No Bueno Amigo! Communicate your concern to the excavator and do your best to be on site during the process. This will go a long way toward preventing under slab leaks. The foundation contractor will then compact the dirt and level it. The preparation continues. If you look at a section drawing or detail in your Blueprint General Specifications, you will see exactly what will be going under your slab. Sometimes that includes a vapor barrier, sometimes insulation from the stem wall, but always, it includes a layer of gravel and rebar or reinforcement as required. Pipes that go through the slab must be protected with foam wrap. Interior load bearing walls must have footings so verify that they are in place, the proper depth and with, with required reinforcement. Double-check all areas as mentioned previously. Call the inspection when your concrete contractor says it’s ready.
The Inspection and the Final Pour
By now you are probably getting fairly well acquainted with your inspector. You can probably tell what are some of his or her hot button issues. Address those first. The only hitch in this inspection is the rebar or the forgotten interior footing. Be sure the laps are the correct length and tied and that all the chairs are standing. Clean debris from the pour area.
It is critical that you have a termite pretreat scheduled for the same day as the preslab inspection. Some municipalities require that it be done, but even if it isn't required by law, most people purchasing a home will want proof that it was done. It should cost between one to two hundred dollars but it pays off when it's time to sell your house.
If the concrete is poured in the summer time the crews should start VERRRY early, while it is still dark if possible. If the pour is in the winter, daylight is better and overnight protection( such as straw or blankets) is recommended to reduce cracking. If you plan on having wood floors, communicate that to your concrete foreman. That should be established before your bid. Preparing concrete for wood floors is much more difficult and exacting than the tolerances allowed for standard flooring. It costs more. It is always a good idea to have the 3rd party testing of the concrete as it is poured. All done. Now it’s time to frame. In most cases, the garage is poured either one to two days after the slab OR is poured after framing is complete. The foundation contractor may have to epoxy reinforcing dowels and rebar into the stemwall,since there will be an expansion joint. This helps secure the garage and the house slab.
I suggest it be done sooner, if possible. Finishing concrete when there are walls up is a difficult task. View some examples of slab foundations on our concrete images page.
A lot of owner builders are unfamiliar with the characteristics of concrete. This can lead to a false expectation of a final result when the slab or flatwork is poured.
Concrete expands and moves. It is a fact that cannot be overcome. Do not be disappointed if 2-3 days after your slab is poured, there are hairline cracks in your floor. It happens. That doesn't mean there is something wrong with your slab.
Because of the inevitable movement of concrete, cracking to some degree WILL occur. This cannot be prevented completely. One very effective way to reduce cracking is to have control joints or cut joints. The contractor can relieve some of the pressure on the slab by cutting a joint. The slab will naturally crack on this joint,lessening the cracks elsewhere in the slab.
Small cracks in the concrete should not impact the strength of the foundation. Remember, the slab itself is not what the house sits on. The footings support the house.
Because there is moisture in concrete, drastic changes in temperature can damage it, especially if it isn't protected early in the curing stage. High moisture content coupled with large temperature swings can cause expansion, cracking and chipping. It is important that your contractor has a plan in place to protect the concrete early on from cold weather.
For foundation preparation images click here. Sometimes a visual is needed to show what is really taking place. Check out Photobucket.com after you look at our image page. Do a broad search like "construction". You will be amazed.
Monolithic Slabs and Post Tension Cables
A monolithic slab is a variation of slab foundations that is poured all at once. The footing, the stem wall and the slab are once continuous piece. They are popular and sometimes required in earthquake zones. Often, like here in Las Vegas, builders are required to incorporate post tension cables into the slab to increase the tensile strength of the foundation. Post tension cables are put in similarly to rebar. The cable is actually greased and in a sleeve. There are anchors on one side of the cable that are embedded in the concrete after pour. Several days after the pour, A crew will come to the site with a machine and begin to stress the cables by pulling them tight. This causes a lift to occur that gives the slab itself higher strength.
Other OwnerBuilderOnline.com Concrete Pages
I think it's a good time to remind the owner builder that they are the project manager. As we get more specific about the steps in construction and details, we sometimes forget we have contracted licensed, experienced professionals in each area of construction. Know what's going on, be on the jobsite as often as possible. Take part in inspections whenever you can but don't become a wrench in your own machine. Framing is the next step in the building process.