Having never even mixed concrete for a fence post, I did some deep diving and learned the following.
Concrete is a mixture of cement (typically Portland cement), coarse aggregate (gravel), fine aggregate (sand), and water.
The choice of aggregates affects the workability and strength of the concrete, but it's primary purpose is as a cost reducing filler. The compressive strength of the coarse aggregate (e.g. basalt) is much higher than that of the finished concrete, but it is a lot cheaper. Its inclusion allows for less cement usage, the most expensive component of the concrete mix. Collectively the aggregates make up 60-70 percent of the concrete's volume.
Racing the clock
When water is added to cement, it sets by a complex process known as hydration. The process proceeds at a rate that depends on various factors, but in general the mixture is only workable for a couple of hours.
You've probably seen concrete mixing trucks like this:
The cement, aggregate and water is added to the truck as it leaves the supplier and it mixes on the way to the site. Australian and US standards dictate that the concrete should be used within 90 minutes of the truck leaving the plant (i.e the addition of water). For this reason, concrete must be ordered from a supplier that is relatively close to the location of use.
Concrete is specified by the following parameters:
This is the concrete's compressive strength. Standard grades range from 20 to 50 MPa.
We're ordering 25 MPa.
Slump is the consistency, or wetness of the concrete. It's referred to as "slump" because it's measured by putting the concrete into a cone shape and measuring how much it slumps over a fixed interval. Concrete that is too wet loses strength while concrete that is too dry is very difficult to move around and work.
We're ordering 80mm slump, which is as low as we can go since we'll be pumping it with a concrete pump.
This is the size of the stones that are in the concrete. Ours is a 14-20mm mix.
There are many different admixtures that can be added to the concrete mix to vary it's properties. These include set retardants (to slow the rate of setting - useful in hot weather), accelerants (to speed setting - useful in cold weather), corrosion inhibitors, and many others.
We won't be using any admixtures.
So our order is for 25 MPa, 80mm slump, 14/20 aggregate mix concrete. But how much?
It's important to get the amount of concrete correct. Too little and it will be necessary to order another truck to complete the job. Too much and we need to pay the supplier to take some back.
The theoretical footing volume based on the design dimensions is 6.11 m3, but the actual volume depends on the forms that I've built. Careful measurement of those forms gave a volume of 6.26 m3. Some concrete will be lost during the pumping operation and so I added 10% and rounded up to 7 meters. We have some over-excavated areas around the pool edge and so I built a small extra form to hold the expected excess concrete.
This was my first time ordering concrete and it proved to be painless. After getting quotes from the two closest, large suppliers, I picked the cheapest and locked in a date.
No wheelbarrows please.
If you're concreting some fence posts or a small area, mixing your own concrete is viable and probably preferred. As the area grows, at some point it makes sense to order pre-mixed concrete from a truck.
Our 7 meters of concrete comprises:
2250 kilograms (~4960 lbs) of cement
4200 kilograms (~9260 lbs) of sand
8400 kilograms (~18520 lbs) of gravel
1200 liters (~317 gallons) of water
That's around 16 tonnes of materials.
Your average on-site concrete mixer will mix 50-100 liters of concrete per batch. For 7 cubic meters, that's 70-140 batches. Our footing needs to be poured in one go. Suffice to say, mixing ourselves is not an option.
Off-site mixing - or rather, arrival by truck - presents a new challenge: How to get the concrete to where it needs to be? The concrete truck has a long chute at the rear, which can deliver the concrete directly into the forms in some instances, but unfortunately not in this one. Wheelbarrows are commonly used for smaller amounts (or if you have an army of helpers available), but won't work for us given the distance involved and the fact that our concrete would fill around 160 wheelbarrows with 100 kilograms each.
The logical choice is a concrete pump. Knowing nothing about concrete pumping, I called a few operators and learned that there are two general classes of pump and that what I needed was a line pump.
A line pump is a truck with a hopper on the rear that the concrete is poured into and a long pipe/hose that runs along the ground. The other type of pump is a boom pump, where the hose is held off the ground by a large articulating arm connected to the truck.
The upside of a boom pump is that the hose drops down vertically from the powered arm and can be moved around onsite without worrying about access. The downside is that they have limited range. They are also more expensive. A boom pump that would extend over the two-storey house is not a realistic option.
Given the incredible weight of wet (and dry) concrete, the line pump uses rigid metal pipes between the truck and the job site, and then a flexible rubber hose for the last 10-15 meters. I'd estimate that the hose itself has an inner diameter of around 100mm. Some quick math reveals that the weight of the concrete in every meter of hose is around 19 kg (
pi*radius²*1m*2400kg). For the two guys dragging 10 meters of the hose around, it's brutal work.
One, two three
The process of concreting works something like this:
- Pour the concrete into the forms.
- Vibrate it to remove trapped air (the poured concrete may contain anything from 5-20% air by volume in the form of small bubbles, which reduces strength).
- Screed it to the correct level.
- Use a darby or bull float to level high spots and fill any gaps.
- Wait for it to set a little. During this time the solid particles of the concrete settle and some water (called bleed water) rises to the top. It's necessary to wait for this water to evaporate so as to avoid working it back in to the surface.
- Finish the surface using floats/trowels.
That's the theory anyway. Having never done any concrete work, it all seemed so simple.
I was lucky enough to have two friends available to assist. One to move the concrete around and one to vibrate, while I screeded and used the darby.
The day of the pour rolled around quickly. We had originally scheduled the pour for 2 pm, but the pumping crew got in touch early and let us know they'd be ready at 11:30 am. A quick call to the concrete supplier confirmed that they could deliver at midday and so everything quickly moved up a couple of hours, sacrificing my original plans to spend those hours putting in additional bracing in favor of expediency.
The pump arrived right on time and the guys set to work setting up the pipe. The heavily sloped site with little access on most sides presented them with some significant challenges. After much consideration, they decided to setup two fixed points on different edges of the site and switch the hose between them as the pour progressed.
This is not ergonomic work
The concrete truck followed shortly thereafter and before I knew it I was ankle deep in concrete and we were frantically moving, vibrating, screeding while the pumping crew delivered ever increasing amounts of concrete.
As Murphy would have predicted, the concrete vibrator died within minutes of beginning the job and my friend's vibrating role became one that needed to be completed by hand. A much slower and less effective method.
Given the lack of edge access to many of the footings, I spent the next 90 minutes ankle-deep in the concrete, stooped over, slowly working backwards with the screed and darby.
The decision to forgo the additional bracing proved problematic. The hydrostatic pressure of the wet concrete on the lower footing forms pushed them outward in one spot, resulting in the footing being around 10% wider than it should be in that area. This was not a problem per se, but the elevation of the top of the forms appeared to be affected also. A quick check with my laser showed some minor height variation (<10mm), but hopefully not enough to cause issue when it comes time to lay the bricks. The mortar under the first course should be able to account for the variation, although I write that having never laid a single brick in my life.
The importance of a decent cure
Despite the common misconception, concrete doesn't dry, it cures. As discussed earlier, the cement hydrates in the presence of water, slowly gaining strength in a complex reaction.
The presence of water is critical to this process. Once the water has gone, the curing ceases and the strength development of the concrete stops along with it. It's estimated that concrete that is protected from moisture loss and thus fully cured over a 28 day period is up to 100% stronger than concrete that is not.
It's therefore imperative to ensure that the concrete doesn't dry out before this process is (substantially) complete. The speed at which the concrete cures is very temperature dependent. It will cure more quickly at higher temperatures, but moisture loss will also be more rapid and so greater attention must be paid to ensuring that it is prevented. At very low temperatures, the curing process almost stops. Concrete that is cured at lower temperatures is generally stronger all other things being equal.
Just like your washing on the line, the rate of moisture loss is also very dependent on humidity and prevailing winds.
Curing techniques rely either on adding water or preventing the loss of it. Continuously adding generally requires more attention (unless you can flood it and get it underwater) than preventing it's loss, and so we chose the latter. We used a combination of tarpaulins, black builders' plastic, and toughened cling wrap to "seal" the surface of the footings. This in combination with keeping the forms in place prevents the evaporation of the water and allows curing to continue. We also ensured that the forms themselves and the entire area around them remained very wet.
As the temperature is getting too cold and the days too short for enjoyable work, I'll be breaking for winter. This in conjunction with my concern over a less than ideal vibration job led to the decision to leave it fully wrapped up with the forms on for a full 28 day cure period (vs the initially planned 7 days).
So we'll need to wait a while to get the forms off and see how it looks.