Dry pour - does it work?

[Tigercubrider]

I struggle to understand how it would work, but there are many videos online where Americans (mainly) “dry pour” concrete pads/slabs.

Obviously, I know that you can use paper or hessian bags to shore up riverbanks, but if the concrete can reliably be formed just by “watering” a contained layer of concrete dry mix, why did the cement mixer get invented?

I may build a shed/workshop and toyed with barrowmix. Then thought about a full diy slab by me and possibly a mate.
But does dry pour work?

I have watched videos featuring the “mud mixer” but can”t find a local uk hire source but presume the forced mixer type does similar?

 

[stuart45]

There is a video comparing the strengths of the wet and dry mixes. The wet was a lot stronger after 90 days. Also in the USA, the ready mix bags are relatively cheap, whereas here it's a lot cheaper to buy the all in ballast and cement separately.

 

[JohnD]

I think it is popular for fenceposts here, but not as strong or hard as proper concrete can be.

I've always assumed it was used by people who didn't know how to mix it.

 

[Ivor Windybottom]

It's more expensive, as to make premix you need to dry the sand, which takes lots of energy.

If mixing yourself then you need a mixer for dry/damp mix with wet sand. In which case you may as well squirt some more water in from the hosepipe and get a much better mixed and workable concrete.

Wet mix is also easier to work with. Dry mix needs to be compacted to force the air out. If you add water to dry mix while mixing it shrinks in volume, as the extra weight of the water squeezes out the air. Add more, end up with less, but it's denser.

Also dry mixing is a health and mess hazard, as you'll end up with cement powder puffing about everywhere. When wet mixing you add water before and after the cement, so only get a few seconds of dry powder blowing about.

Cement absolutely needs water to set, it's a requirement. So dry mix left outside may take a long time to find enough moisture to set. If you water it or it rains after you'll end up with all the sand and cement at the bottom and nothing but pebbles on top, which would be really weak and a terrible surface.

 

[stevie888]

You won't get decent concrete unless you use the appropriate amount of water and mix it thoroughly. The water is as an essential part of the hydration process as the cement and it needs to be present throughout.

Too much water weakens the fibal product. Hydration requires about 5l of water per 25kg bag of cement. We add wore to lubricate the mix to ease placing but this excess has to go somewhere onve the concrete is in place