I needed to mix around 20 cubic feet of concrete for setting some fence posts. After mixing the first four cubic feet with a spade in a wheel barrow I decided I needed an easier solution.
Without access to an electric or petrol concrete mixer mixing concrete by hand can be back-breaking work. Mixing by hand with a spade or shovel in a barrow or on the ground involves physically lifting and turning the sticky mix many times and uses a lot of energy and muscle power.
By contrast, this method uses a strong plastic tarpaulin sheet to 'roll' the mix around on the ground. The technique uses FAR less energy as the mixture is never physically lifted and the bulk of the weight remains on the ground at all times.
I should point out that this method of mixing is not my idea and that there are a number of purpose made concrete mixing sheets or bags on the market. Having not tried or heard of this method previously I was reluctant to lay out much cash on a potential white elephant and thought that an inexpensive tarp might be a good alternative. There are a few references to mixing concrete with tarpaulins on the web, but nothing of much detail that I could find. Hence this Instructable....
Step 1: What You Need ...
I bought this tarp off EBay for just UK £3.00 delivered. The cheapest 'purpose made' concrete mixing sheet I could find on the 'net was roughly £15 plus delivery. In addition I believe the cheaper commercial versions may be heavy gauge polythene as opposed to the woven fabric of the tarpaulin which adds greatly to the strength of the sheet.
As well as a tarpaulin you will need three lengths of strong, thin (roughly 10mm rope)
Step 2: Setting Up the Sheet ...
I unwrappd the 8 x 6 foot tarpaulin, then doubled it along its length to make a double thickness sheet 4 x 6 feet.
Tie a piece of rope through the lower and upper eyelets at each of the two 'loose' corners of the tarp, and the third bit of rope through the lower and upper eyelets in the middle of the 'loose' edge.
The tarpailin is hung by the ropes from a convenient sturdy support. I used my boat which had various handy cleats and stuff but the bumper of a truck, strong hooks attached to the wall of a garage or shed, a fence, tree etc could all potentially be pressed into service.
I found that the back edge of the tarpaulin needs to be approximately 2 feet off the ground.
I lay my tarp on an old, thick blanket as I was working on rough gravel chippings which would quickly have abraded and cut through the plastic sheet.
Step 3: Adding the Mix ...
I used 'all in ballast' for this mix. The ballast is a mix of gravel, small stones and sharp sand. No additional material is needed other than cement (and water obviously).
I used a mix ratio of 5 parts all in ballast to 1 part cement, which equated to two builder's buckets of ballast and just less than half a bucket of cement. These were dumped on the tarp and a quantity of water added.
I don't propose to go into the details of obtaining a perfect ratio of concrete ingredients in this Instructable. There is plenty of information about this to be found elsewhere on the web. i just made sure not to add too much water initially, then added more as required to get a mix of the required consistency for pouring into my post holes.
I think that 2 - 3 buckets of total dry ingredients is probably about optimum for this method (certainly for a tarpaulin of this size). Any more and it becomes heavier to mix and tends to spill over the edges of the sheet.
Step 4: Mixing It Up ....
Once the ingredients are on the tarpaulin it's time to mix. I generally roll the mix around a bit to incorporate the cement into the sand & gravel before I add water but that may not be necessary - I'm no expert.
After adding a bit of water, grasp one of the corners supported by rope and the adjacent, loose, folded corner and give each corner a sharp lift in turn, rolling the mix over on itself towards the other end of the tarp. It can be easier to lift each corner with both arms, taking care not to tip the mix off the edge of the sheet.
It is important to keep your back straight while doing this and to roll / lift the mix using the power of your legs. Grasp the appropriate corner of the tarp, bend your knees slightly keeping your back straight then stand upright, straightening the knees to lift the mix. It should tip forward, tumbing over itrself, very much like the action within a powered cement mixer.
Once you have moved the mix toward the other end of the tarp with a few alternate lifts of the corners of one end, change ends and work the mix back the other way. the mix will happily sit on the tarp while you change ends unless you add too much water or the mix is too liquid.
Continue mixing, adding water as necessary until the concrete is fully mixed and of the desired consistency. It should take no more than a couple of minutes with practice and requires surprisingly little effort - the hardest part is carrying the sand and gravel to the tarp !
Step 5: Pouring the Concrete ...
Once mixed the concrete may be transferred to a bucket or barrow by lifting the tarp, taking care not to allow the mix to spill over the back or sides of the tarp as you pour.
I was transporting my concrete to the post holes in the same builder's bucket I used to measure the ingredients. The two buckets of ballast, 1/3 bucket cement & water mix I used gave roughly two and a half buckets of mixed concrete and I found it much easier to manually shovel the first bucket load off the tarp. Thereafter the second bucket & a half were much more maneageble and easier to pour direct off the sheet.
Using this method I was able to mix all the concrete I needed with relative ease (remember, concrete is heavy and there will always be some physical effort required in mixing it up, even if only throwing dry ingredients into a mixer).
I can testify that this method is FAR easier than mixing concrete with a shovel. The equivalent of a barrowload of concrete may be thoroughly mixed in two minutes with little effort.
NOTE - If you have a willing helper then the ropes and support may be dispensed with completely and the concrete mixed with each of you holding two corners of the tarpaulin.
Happy Mixing !