Modernising Roman Methods of Placing No or Low Slump Concrete

In recent years the concrete industry have been making concrete a bit like pea or pumpkin soup and this does not necessarily make economic or engineering sense for many applications.

That is not to say that self compacting concretes do not have their uses. They do. It is just that for many applications such as house slabs and pavement there are better ways of placing concrete with much less risks and at lower cost. Let me explain.

If concrete is made with the consistency of pea soup then pumps are needed to move it. This is great for high rise construction or getting concrete around complex reinforcing but costs more money and often results in inferior concrete. Duff Abrams law makes it clear that less water means less cement for the same strength. Less water also concentrates alkalis resulting in better pozzolanic reactions if we are trying to replace cement with flyash or other pozzolans as we should be because less cement means less emissions. Less cement can be used because of more effective pozzolanic reactions and a lower water binder ratio. The industry promote the use of flyash but don't seem to have realised that they are fundamentally incompatible with pea soup concretes. It should be mandatory for their technologists to study Roman concretes.

In ancient times the Romans and I have no doubt the Chinese placed their early concretes relatively dry. According to David Moore "The Romans mixed their components (wet lime and volcanic ash) in a mortar box with very little water to give a nearly dry composition; carried it to the job site in baskets placing it over a previously prepared layer of rock pieces; and then proceeded to pound the mortar into the rock layer. Vitruvius, the noted Roman architect (circa 20 BC) mentioned this process in his formulas for concrete, plus the fact that special tamping tools were used to build a cistern wall. Close packing of the molecular structure by tamping reduced the need of excess water, which is a source of voids and weakness. But also close packing produces more bonding gel than might be normally expected."[1]

Roman methods are immitated by rollar compacted concretes for structures like dams but have not been developed for house slabs. My point here is that we should be developing the methodology of the ancients for slabs, pavement, curb and gutter and the like to make greener better concretes and it may even be cheaper to do so. I have done experiments to prove it to myself and am satisfied that it works so it is about time the industry took a more active role in developing dry placement methods and putting in place the required infrastructure and logistical support.

A Low Slump Concrete Placement Case Study

Around January 2003, Mike Burdon, a friend of mine who was interested in my work on magnesium cements and the strap method of prestressing invited me to do a slab in Tasmania.

What an opportunity! Somebody else offering to pay for the work. I decided to test three theories at once. The use of around 8% magnesia in the mix to improve durability and finishability, the strap method of reinforcing and low slump placement techniques.

Laying Low Slump Concrete.

The first layer has been laid, vibrated and smashed down with a lot of feet and a large 4" X 2" beam (this article suggests a vibrating compactor). The second layer is being placed ready for the same treatment. The reinforcing in this picture is high tensile steel strap

Mike Burdon Working on the Finish of a Dry Placed Slab in the Normal Way

Note that it would have been easier with a vibrating compactor

The Final Finish of our Test Low Slump Concrete

A bull float and hand trowel was used. For a larger slab a rotary helicopter finisher would have made the job easier

A Suggested Method

Following on from my experience with the above and other experiments the following method is suggested.

You will need to be able to buy or produce low or no slump concrete.

The first problem to overcome is that conventional concrete trucks are not very good at producing and delivering low or no slump concrete. Assuming these major problem can be overcome (we are working on a mixer with a delivery system to do this) then hire a vibrating compactor ("wacker smacker") and helicopter finisher ("chopper") for the day.

Prepare the edge boxing carfully making sure it is level if you are pouring a level slab as you will be using it to work your levels to..

Have on hand rakes and a very straight piece of timber to go the width to the edge boxing so you can make sure the concrete it flat. The vibrating compactor is useful for this job.

You will need a camera and video recorder for your wife or girlfriend to record you epic work and a bit of beer or other refreshments for hangers on and to keep you going.

As you are smart enough to at first take on only a small job, two or three people should be plenty. If you can't afford a vibrating compactor then put on a keg as lots of dancing feet do the same thing for laying concrete as they do for making wine.The analogy is to Roman slaves with their tamping rods..

Start by placing a 75 mm layer of the dry mix over plastic, rake or screed it flat. This is easy provided the mix is not too wet - then it gets difficult so we are suggesting a drier mix than in the case study.. Alternatively and in any event vibrate it down with the compactor.

Then put your reinforcing down. In the case study we used high tensile strap which is the subject matter of a patent John Harrison had. You can use mesh. Either way lay it. No bar chairs or other supports are required as the first layer will the steel. Place the next layer on top and carefully rake, screed of vibrate it dead flat as before but to the boxing edge (remember how you were asked to care about the boxing. It's too late now if you didn't). Continue to vibrate it down all over with the vibrating compactor or your little army of hangers on until what little moisture there is starts to come up. If it goes down too far anywhere add a bit from a barrow in reserve. Be careful as this is an art.

When flat and vibrated down and just enough moisture has come to the surface to finish the slab then use the helicopter finisher. This will be much earlier than normally as the set is a physical set due to lack of water more than a chemical cement set. A little sand or quarry dust, cement and water can be added at this point if necessary although we did not need to in our case study.

Then when titivated to your satisfaction - leave it alone, cover over and keep the dog off. It works for me so I see no reason why it should not work for you.

The Proof's in the Costing?

So how does the slab we have just made stack up in terms of cost to conventional methods?

In the following hypothetical analysis let's assume 5 cubic metres of 25 mpa concrete costing just a little less is required for the low or no slump slab and compare costs.

Dry Placed Concrete		Wet Pre Mix Concrete
Prepare boxing	500	Prepare boxing	500
Order mesh 2 Sheets F72 mesh (6,000 X 2,400mm)	200	Order mesh 2 Sheets F72 mesh (6,000 X 2,400mm)	200
Order plastic for under mesh	50	Order plastic for under mesh	50
Purchase dry mix concrete	800	Purchase wet mix conventional concrete	950
Hire 3 people 5 hours	375	Hire 3 people 8 hours	600
Hire wacker smacker	50	Hire concrete pump and operator 1 hour (+ travel cleanup etc.)	350
Hire power trowel	50	Hire power trowel	50
Total	$ 2025	Total	$ 2700

The above figures are a bit rubbery but are in the right ballpark. Low slump concrete should be cheaper because it should be possible to order the same strengh with a lot less cement and water and more fly ash and because a pump is not needed on the job. Because the mixture is dry placed it should be possible to finish it more or less straight away without having to wait for "first set" overcoming the issues builders have with the use of fly ash. Less people should be required on the job or everybody can go home earlier. (Most prefer the latter option!) The only extra cost is a vibrating compactor and these are commonly available as they are used for compacting gravel before laying pavers or for placing bitumen. The engineering payoff is much less cracking and other durability issues. The big pay off for the planet is that the whole process is much greener.

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