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Foundation Concrete Pour
Checks
Make sure forming is solid
Make sure the forming is good and solid. Wet concrete exerts a considerable force on the forming so it is important to ensure it will not move in any significant way. The main mechanism in my foundation design for resisting the force against the foundation forms is the crushed rock round the outside.
Make sure rebar is securely tied
Wet concrete is heavy and can push rebar out of place if it is not well tied. Make sure the EPS pads (that stop the rebar damaging the waterproof membrane) are positioned and glued properly in place. Make sure all the vertical bits of rebar that go up into the walls are properly vertical.
Clean up
Obviously you don't want any trash in the footings or slab area, but it is also good to clean up all round the job site so that nothing gets in the way of the concrete pour. You need to also get out any leaves and pine needles that have blown into the foundation area. A good wet and dry vacuum cleaner is the best way of cleaning out the footing trenches.
Get water out
If it has rained (it will have done) after the waterproof membranes were installed then the footings will have filled up with water as there is no way for it to escape. You will need to use a wet and dry vacuum cleaner and also a submersible pump to get the water out.
Ensure good access
Concrete trucks are very large. You need to ensure that there is plenty of room for the concrete mixer to back up to where the building is. A concrete mixer truck is typically 12'6" high. The width of the truck is about 10 feet. They weigh 35 tons, so the roadway needs to be solid so they don't sink in. A pump boom truck needs much more space than a concrete truck. Even a small pump truck needs 20 foot of width to provide for its outrigger feet. The roadway does not need to be much more than say 12 feet wide, but the ground either side must be solid enough to take the outrigger feet. Pump trucks need a huge amount of vertical clearance (say 100 feet), so you need to ensure you can see the sky all the way along your access path and there must be no overhead power lines.
Make sure the concrete groove 2x4s are accurately set
In addition to making sure you have remembered to implement the key notch lumber, it is also important to ensure that they are accurately set to the right height as they are a useful reference to set the concrete height. Their height should be checked with a laser level and they should be well braced so they don't move up or down or side to side. They also of course need to accurately match with what will be the center of the walls. Check squareness by checking the diagonals are the same length.
Things to buy
Concrete vibrator
If you are planning to use concrete a lot then it is worth buying a concrete vibrator as they are expensive to keep hiring. Most concrete vibrators are expensive to buy, but I went with an economically priced one. Details can be found here .
You need to have a concrete vibrator and some kind of backup plan. If you only have one concrete vibrator then you need to think ahead of time what you will do if it breaks. One possibility is to check with a local tool hire place to make sure they have one available if you need to rush down to hire one if you vibrator fails. The best bet is to buy 2 concrete vibrators and that's what I chose to do.
Magnesium Hand Float
These are the hand tool of choice for flattening the top surface of the concrete. Purchasing details are here .
Long straight 2x4s
These are to use for screeding. Using 2x4s is lighter than using metal bars, but selecting very straight pieces is key. If using a 2x4 longer than about 8 feet then you will probably need a person at each end. Using 8 foot 2x4s is a good choice and can be used by one person.
Wheel barrows
This is if you are using the manual delivery method (which is strongly not advised). Even when using a pump truck it is good to have a couple of wheelbarrows standing by in case you need to go back and fill in some missed spots.
Safety equipment for workers
Need rubber gloves, rubber boots, safety glasses, and hard hats.
Laser levels
See later for details..
Calculate concrete quantity and implement plan for extra
You need to think in truck loads
Concrete is delivered by the truck load with 9 cubic yards per truck. The better ready-mix concrete companies will allow you to specify a part truck load for the last truck after you have taken one or more full truck loads.
If your concrete company will not do part loads or if there is too much of a penalty fee for part loads, you may find that you have part of a truck load left over after filling the foundations. If you need to take delivery of the entire truck load, you will need to implement an auxiliary place to put it. Making a concrete patio is one good answer.
Calculate the quantity
You need to accurately figure out the volume of concrete needed for the footings and the slab. Split up the foundations into a series of rectangular blocks and add up the amount in each block. You can either do this from drawings or can walk around in the foundations with a tape measure. Note that the dimensions are the inner dimension after all the polystyrene has been installed. For each rectangular block, in feet and decimals of feet, measure the length times the width, then multiply it by the depth. This will give the total cubic feet of concrete required. To convert this number to cubic yards, divide it by 27. The rebar will reduce the amount of concrete slightly but not by much. Here's the calculation for the foundations for my small evaluation building.
North footing (full length and height): 205 x 28 x 40.5 = 232,470 cu in = 134 cu ft = 4.98 cu yards
South footing (full length and height): 205 x 28 x 40.5 = 232,470 cu in = 134 cu ft = 4.98 cu yards
West footing (reduced length, full height): 83 x 28 x 40.5 = 94,122 cu in = 54.5 cu ft = 2.02 cu yards
East footing (reduced length, full height): 83 x 28 x 40.5 = 94,122 cu in = 54.5 cu ft = 2.02 cu yards
Slab area (not counting footings): 132 x 83 x 9.95 = 101,343 cu in = 59 cu ft = 2.17 cu yards
Total foundation: 16.17 cubic yards
Amount left over after 2 trucks (9 cu yards per truck): 1.83 cubic yards
Area of 4" thick concrete patio: 148 sq ft If width is 12 feet, then that's a length of 12 feet.
Backup plan
It's a good idea is to have an alternate concrete project such as a driveway or patio prepared in case a hitch occurs with the pour. You want to ensure the concrete is not wasted. For my main house, the corner blocks on the downhill side are a useful place to put extra concrete.
Forming a concrete patio
You should have the patio area formed before the concrete truck arrives. Try to make the design of the patio forming such that you can adjust the area depending on how much concrete is left over.
An acceptable design of concrete patio is 3.5" of crushed rock, then 4" of concrete on top with rebar in it. You can form this using 2x8 lumber (which has a width of 7.5"). At a push for a small patio you can use 2x6s (which are 5.5" wide). Clear the area to get rid of organics and top soil. Stake, and screw the lumber to form the sides. The area formed should constitute your upper estimate of how much concrete will be left over. Ideally implement a movable divider piece of 2x6 within the frame so you can adjust how big an area is filled with concrete.
Use #4 rebar with a 16" pitch in both directions. Support it using about half way up using concrete chairs. Over any disturbed soil, use additional rebar. Make sure any disturbed soil is thoroughly compacted.
Bench furniture
If you just have a small amount of concrete left over then you can also make some concrete benches. The legs of the bench can be done by filling buckets with concrete. The top of the bench can be a piece of 2x12 lumber or you can form a 3.5" thick rectangle to make a bench top or table top.
Get foundation setup inspected by building inspector
Building code requires that the foundation setup be signed off BEFORE any concrete is poured. This will be done by the building inspector. The health department inspector should have already signed off if there is sewer plumbing under the slab (but make sure this has happened).
Do not pour concrete before these checks have been done!
Arrange concrete delivery system (Footings and slab)
You will likely need a boom pump truck to place the concrete in the foundations. Line pumps are a non-preferred method. Line pumps use pipes and hoses to move the concrete from the truck to the placement location, but require a lot of labor to move the hoses around while in use. A boom truck is like a long arm crane and has a tube all the way up and all the way down to deliver concrete from above just where it's needed. A boom pump costs a little more than a line pump but it's well worth it, particularly as you will save on labor. Pumping concrete for the foundations is easier than with the walls that are higher up, so you can potentially use a line pump for the foundations, but the walls definitely need a boom. Expect to pay about $500 for a three hour session with a boom pump truck.
For the foundations it might theoretically be ok to just back the concrete truck up to near the site and then run a sloping pipe that can be moved around. Note that the feed needs to be moved about rather than just dumping in one place. Dumping in one place causes the mix to separate and be uneven consistency. Concrete trucks have a U shaped chute that comes out at about a 5 or 6 foot height. If you are rigging up your own pipe system, you need some kind of funnel catching arrangement at the start of your pipe arrangement.
Particularly when pouring concrete into ICF blocks, but even when pouring into foundations, it is best to reduce the force with which the concrete exits the hose. It used to be possible to use a reducing device on the end of the pipe but this has now been outlawed for safety reasons by OSHI. Apparently the ungrounded metal ends caused problems. You are still allowed to use a slight reducer just before the flexible pipe end (because it has proper grounding via the metal boom), and this is a good idea. Talk to the pump truck driver to get him to fit this smaller bore pipe. When pumping it is also a good idea to try to kink the end of the pipe yourself to reduce the delivery force.
You will be paying for the boom truck by the hour so coordinating its time on site with the concrete deliveries is critical. You want the first concrete truck scheduled to be 15 minutes later than the boom truck and the next concrete truck 1 hour later. In a 3 hour rental you only actually get about 1hour 45 minutes of pumping because of the 30 minute setup and the 45 minutes of cleanout. That equates to two concrete truck loads (assuming 1 hour between trucks).
Ensure you have all the necessary walkways
In a previous step you will have implemented wooden blocks on the slab area with wooden planks between them. Do a trial walk through of the pour, pretending that you are holding the end of the concrete boom hose and placing the concrete where it is needed. Make sure you have all the necessary walkways to do this. You might also find that a diagonal plank across the center is useful.
Implement a wash-out pond
The pump truck operator will want to have a source of water that they can suck up into the pump to clean it out. They will also want to have a place to dump the washed out concrete. A suitable pond can be implemented by digging a 6'x 6' hole that is about 2 feet deep. Line it with a polyethylene sheet (one continuous waterproof piece). Half fill it with water. You can also use the pond to wash off your concrete tools after the pour.
Arrange for the necessary people
You need plenty of folks to help get the concrete into all the right places and flatten the top surface. It's also worth having at least one person who has taken part in a concrete pour before.
It may be that the company supplying the concrete pumping can provide experienced people (at considerable cost), but typically the pump truck driver will want you to hold and direct the end of the delivery pipe. This is so that there is no liability on the concrete pump operator if anything goes wrong with your job. The pump truck driver will operate the up/down right/left controller for the boom, but they will do so under your direction.
Laser levels
Self leveling lasers are these days the best way of ensuring that you get the concrete top surface level. It is best to get a self leveling laser that shoots out a horizontal line over a wide angle. You can pay a lot for such a laser, but there are also good reasonably priced ones, such as the one that can be found here .
The laser can be positioned in a box that is placed on the slab area. It is designed to keep the beam about 1/8"-1/4" above what will be the surface of the concrete so you can readily see any high spots. The box gets removed after the concrete has set and the hole gets filled in later.
Multiple single point Laser Levels can be used instead if you don't have a self leveling wide beam laser. It's also not a bad idea to use a few low cost fixed lasers as a double check even if you do have a fancy laser. The single point lasers should be placed as needed to shine reference lines across what will be the top of the flat concrete. Given that they are low cost, it is worth checking that they really are shining a totally level beam. Position the beams so that they are about a 1/4 of an inch above where you want the concrete surface.
Check the weather
Having the right weather is important during the concrete pour. The temperature needs to be between 50 degrees F and 90 degrees F (ideally a max temp of 70 degrees F). There must be no rain and no threat of rain. If these weather requirements cannot be guaranteed then you should postpone the concrete delivery.
The higher the temperature, the faster the concrete will set, and very hot conditions will affect the ability of you and your workers to perform.
Order concrete
See the page here that describes the concrete mixture required and other ordering details. Getting the correct concrete mix is important so it is worth studying that page well. It is also vitally important to get the delivery times properly scheduled as you need to have the concrete poured within about 2 hours of it being mixed.
Pump the concrete
The foundations will require multiple concrete trucks worth of concrete. This gives the opportunity to vary the concrete mix for the top layer that will be the basement floor (ie the top of the slab). Only the last batch needs the polypropylene strands to reduce surface cracking and only the last batch needs lots of retarder. It is only the final pour on the foundations that needs screeding to create a nice level surface.
The concrete needs to be placed by the boom in approximately the location it is needed, rather than piled up and then moved to the right place. Moving the concrete too much may cause it to separate with the large aggregate falling to the bottom. Also avoid dropping the concrete from too much of a height or again the result may be separation.
Try to avoid directing the delivery pipe directly at the rebar as this can move the rebar out of its proper location. You won't be able to completely avoid this but hopefully you tied the rebar well so it won't be a problem. A good technique is to direct the concrete at a shovel that can take some of the force. Also try to form a bit of a kink in the flexible end of the hose to also help reduce the force.
Concrete can be placed in parallel sections as long as each subsequent section is placed before the prior section has begun to set up, or there will be cold joints between the two. On a relatively small foundation it is preferable to pour the concrete in horizontal layers with each successive truck load. Horizontal cold joints in foundations are not much of a problem.
When pouring the concrete using the delivery hose, make sure you get the amount of concrete right to get it up to the level required. You can judge this using the 2x4s that form the wall key notch. The concrete should come up to the top of these 2x4s. It is a bit of an incremental process because the concrete vibrator will likely cause the level to go down as the air pockets are eliminated.
Vibrating the concrete
This is done in parallel with the pouring of the concrete. A suitable low cost vibrator is shown below.
Vibrating will get the concrete to flow into all the nooks and crannies. A vibrator will typically vibrate a 3 foot diameter sphere in one dip into the concrete. The vibrating should be done as soon as the concrete is placed, ie when it is still very fresh. The vibrator should be dipped into the concrete and withdrawn vertically after 5 seconds. More than 5 seconds can cause separation, ie the larger aggregates sinking to the bottom. Do not leave it in too long and do not drag it around in the concrete.
Try to avoid vibrating the rebar as this will reduce the bonding between the concrete and the rebar. Also try to avoid touching the vibrator head on the foundation walls as that may hurt the waterproof membrane if a sharp rock is vibrated into it.
When the concrete is poured in layers, you should penetrate the join between the layers with the end of the vibrator as this avoids a cold joint.
Remove outer edge jigs
Around the outside of the foundation forming there are jigs made out of 2x4 to keep the polystyrene up against the outer upper Form-a-drain. You should remove these just before the concrete gets to this level. In practice that means just before the last truck load of concrete. The pressure of the lower concrete will keep the EPS up against the Form-a-drain.
Flattening the concrete
Experienced professional concrete workers try to achieve a good quality surface finish on the concrete. This is obviously a good thing to strive for. Personally I try to decouple the structural aspects of the concrete from the aesthetics. I do this by installing a flooring system over the concrete floor once the house is structurally complete. Depending on the room, I either use self leveling cement, or porcelain tiles, or wood flooring. This hides any imperfections in the surface of the structural concrete. Note that in my design the radiant heating pipes do not go in the structural concrete of the floors. The radiant pipes are in the flooring system.
Even when you will be laying flooring over the top, try to get the surface of the structural concrete finished to a reasonable quality so it can be used as a bare concrete floor if that's what you decide to have temporarily before doing the proper floor. It is also much easier to lay the flooring system if the concrete is properly level and flat.
Cleanup around key notch lumber
As part of the concrete pour you need to clean up around the key notch lumber to make removal possible. While the concrete is still wet you need to clean off any concrete that is over the top of the 2x4s. Cut along the edges of the lumber while the concrete is still wet.
Ideally form the concrete to ensure removal of the key notch lumber will be easier.
Possibly make control cuts in concrete
Traditional thinking is that if the concrete of the slab floor is going to be visible, ie not covered by your flooring system, then it is worth cutting groves in the floor to control where the cracks occur. You should expect that there will always be cracks in concrete and the control cuts don't stop the cracking, they just influence where the cracks occur. About 8 foot squares should be ok. Make it look nice by making it accurate so they are parallel to the walls.
If you will be covering the structural concrete with flooring such as wood or tile or some self leveling finishing concrete then you can decide not to bother with the control cuts. Just let the concrete form cracks wherever it wants. Personally I do not bother with control cuts.
Control the curing of the concrete
For proper curing it is necessary to control the rate of evaporation of the water from the concrete (and avoid water temperature extremes such as freezing). Need to keep sun off it as sun will cause more evaporation of water.
You need to keep the slab consistently wet. One technique is to lay polyethylene sheet over the slab and soak the entire area with a garden hose and/or sprinkler. The water will go under the polyethylene sheet and get trapped there. Don't use the polyethylene sheeting that is round the outside ready to go up the walls because it will likely be covered in mud and also pulling on it may disturb the partially solidified concrete.
Another good method is to pull the Fab-form over at the edges so it forms a bit of a ridge so that the whole slab is kept under about half an inch of water. This is the method I personally prefer.
Photos
The following photos show the complete concrete pour sequence from start to finish.
Everything ready
Pump truck setup
First concrete truck load
Pumping first load
Pumping second load
Pump cleanout
Rough Flattening
Laser flattening
Cutting round key notches
All done
Remove the blocks and planks that are the walkway system
The wooden blocks that support the walkway planks need to be removed once the concrete has set. You will be left with square holes in the concrete slab which will be filled in on a later concrete pour (eg when pouring the walls). Also remove the box (or boxes) used to hold the laser level. See note below about wood swelling when wet.
Remove key notch lumber sticks
The 2x4 sticks fitted on the vertical rebar need to be removed when the concrete is well hardened. Do not try to pull them out until the concrete is fairly well hardened or it could pull the rebar up within the concrete. Wait about a week.
You may well find that it is easier to get the wood out after the concrete has cured and you are no longer keeping it wet. This is because the wood will swell up when it is wet.
To get the key notch lumber out you will need to drill holes in a line to form a cut between the rebar.
Then use a crowbar to lever up each piece and lift it out. Make sure you do not damage the rebar.
This is what it should look like once the key notch lumber has been removed...
