Pouring A Concrete Footing
Pouring a concrete footing for a good, solid foundation is the goal here.
STEP 1: First you must figure how much concrete you will need by multiplying the width of the trench by the height of the intended pour. Multiply this by the total length of footings you have. Then divide this by 27 to end up with cubic yards of concrete needed.
Say the concrete footing is 12 inches wide by 12 inches deep (don't worry about variations, just use the average). If we convert this to feet we have 1 foot by 1 foot. So, we will need 1 cubic foot per linear foot of footing. If you have 218 total feet of trench, you will need 218 cubic feet of concrete. There are 27 cubic feet in a cubic yard (concrete is sold by the cubic yard) so we divide 218 by 27 to end up with 8.07 yards of concrete. Order 9 yards at least.
If your concrete footing is 18 inches by 18 inches, (1.5 cubic feet by 1.5 cubic feet) you would have 2.25 cubic feet of concrete per linear foot of trench.
Concrete is ordered with a “slump” which indicates how runny the concrete is. Adding water to the mixture can weaken the concrete and while the driver is usually willing to add some, he may not add very much without a signed waiver. Generally, the runnier the concrete is, the easier to work it is, because it flows more like water. However, the runnier it is, the more prone to blowing out forms and bulkheads it is, because it runs like water. Generally for a concrete footing, when you can reach all sides of the house, use a slump of 4 or 5. This is fairly stiff, while allowing you to work it with shovels.
Word to the Wise: Most concrete trucks have a 9 or 10 yard capacity so you can order a full truck with a “call back”. What this means is you will get a full truck and can pour it all out, then figure up again how many feet of trench remain un-poured and “call back” to order the remainder you need to finish the job pouring your concrete footing. This allows you to be more accurate with the final amount, but always order a little more that you think you need. If you are pouring a lot of concrete you will want to space the trucks out with a certain time delay for each load. This delay will be based on how much help you have placing the concrete. You do not want to have four or five trucks parked on site waiting for you to get to them, as there is only a four hour window to work in. Once the concrete is in the truck, you have four hours to get it placed. They will literally pour it out anywhere to keep it from setting up in the truck.
Concrete hardens as the result of a chemical reaction – not because it “dries out”. The setting time can be influenced by several factors. If it is really cold, concrete will set very slowly. In fact, if you are pouring a slab in freezing weather, you will need to protect the concrete with insulating blankets while it cures. You can add calcium to speed concrete in curing. You can also add a chemical to slow it curing. Your concrete supplier will have an outside sales rep. Use him to advise you and if possible have him visit your job before the pour.
Word to the Wise: The chemicals in concrete are caustic and will give you serious chemical burns if you allow the cement to stay on your skin. I had a friend who didn’t know this, and he kept wiping the cement off his hands, all day long, on the front of his pants legs. He ended up with second degree chemical burns on his legs! If you contact the cement with bare skin, wash it off completely within a few minutes. Also, don't make the mistake of trying to order the concrete too close. If you come up a half yard short and have to order again, you are adding a couple of hours to your job time, and they may charge you an additional fee to bring you another half yard. Order at least a half yard more than you think you need. Even better, add a yard or more. The extra $100 is well worth it. Use any extra to pour HVAC pads, porch stoops or pot holes.
There are two types of concrete truck – rear discharge and front discharge. If possible, ask for a front discharge truck. There are several benefits, but mainly the front discharge truck is much easier to maneuver around the job site, the driver can actually see where he is going (meaning maybe he won't run over your kickers) and the driver has control over the concrete chute. He can position it where it needs to be to place the concrete, and he is probably better at it that you are.
Step 2: If you have a steep slope with lots of bulkheads in your concrete footing, you need to start on the lowest area and work up. Pour the concrete deep enough to fill up to the bottom of bulkheads and then stop. You need to let the first concrete stiffen some before you pour the step higher because the weight of the concrete will push the lower concrete out and give you fits. If your ground is level, no problems.
As you place the concrete, try not to overfill any one area. Concrete is heavy! You do not want to shovel it. If you get more in one spot than you need, drag it along the trench to move it.
STEP 3: Using a flat point shovel, pat the concrete in the trench and find the rebar stakes that indicate the finish height. As you are pouring your concrete footing, don't worry too much about how pretty the concrete is. First just get it placed, and roughing it in with the shovel is fine. Find all your grade stakes to be certain you do not over fill the trench. If you must err, make it a little lower than perfect rather than a little higher than perfect.
When you fill up the trenches against step bulkheads, the bulkhead may lean over a little. As long as it holds, a little lean won't hurt anything.
STEP 4: Once all the concrete is placed, or while waiting on subsequent trucks, take a hand float and work the concrete footing. Smooth it out and pat it down. Footings are not really critical for finish, so don't worry too much about how pretty it is. If it is the right height and runs consistent, you will be fine. There should not be any up and down areas.
STEP 5: If you are going to erect perimeter walls higher than 6 courses of blocks in your concrete footing, you must place some vertical rebar in the footing to capture the bottom of the blocks. Cut these rebar pieces about three feet long. After the footings are poured, stand the rebar in the footing near the corner, about four inches from the foundation edge. Continue placing these standing rods every four feet where the block wall will be high. These rebar will secure the base of the wall to the footing when you pour the wall with concrete
STEP 6: Be sure to wash any tools off before the concrete sets up on them.