# Forum Home Renovation Concreting  Slump...does it matter?

## woodchip

Gday,
The first thing concrete truck drivers do is water down the concrete when they get to a site, is this required? Is it because the pump operator wants it easier to pump? & does having for example a 120 slump instead of a 80 make the concrete noticibly weaker? 
thanks 
cheers

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## Belair_Boy

Good Question 
Adding water to a concrete mix makes it weaker.
When you order ready mix in a truck you specify the slum and strength required and the batching plant will add the correct amount of cement to give that strength at that slump.
Say you order 120mm slump but when it arrives it is at 80 slump, then adding water to get 120 is OK but if you ordered 80mm slump at 32MPa and add water to get 120mm slum you will no longer have 32MPa concrete.
The ratio of water to cement will give you specific strength and so higher slump (more water) means more cement and therefore more expensive for any given MPa.
If the pump operator waters the concrete down to make it easier to pump and it ends up at a higher slump than ordered then it will be weaker.
You can't increase the slump by adding water without lowering the strength, unless you add more cement. 
I hope this is clear :Smilie:

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## Bloss

Concrete companies and their drivers know what they are doing - you will be delivered what you have ordered and that includes any allowances for water to improve the flow from the truck. The amounts added are relatively small in any case. Batching plants nowadays are largely automated so a computer determines what gets prepped and pre-mixed and loaded into the trucks. If it's really critical to get concrete at a particular strength then one must do as is done for large structural pours - that involves capturing standard samples from each batch - labelling, curing and testing at the same time under the same conditions. Few residential uses are so critical - the specced mix has more than sufficient safety factor built in so even if slightly weakened it will still be well over the engineering demands needed.

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## thetrooper

In the instance of pumping the conc the pump guys like it at a certain slump to make it easy to get through the system. when i was making concrete panels would could pour it any where from 80- 200 slump depending on the job. but having said that you can add a water reducer  or in our case we used hot water all yr round (20-28deg)
question is what are you wanting to do or is it just a general question ?     
cheers 
scott

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## intertd6

Yes water weakens the concrete & it makes the finished concrete less durable, usually on a high spec project drivers are not allowed to add water unless directed to by an engineer, the slump test is mainly a test for consistency of water in the mix, not the strength. A load being tested at 80mm slump for the required 120mm slump would be rejected & sent away. This all gets thrown in the bin for low spec jobs like driveways, footings & house slabs on ground. For a mix being too rich in water the rest of the ingredients should be added in the right ratios untill the slump is correct, not just cement alone.
regards inter

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## woodchip

Thank you for all the replies, good explanations, that makes it a lot clearer :2thumbsup:     

> question is what are you wanting to do or is it just a general question ?     
> cheers 
> scott

  In response to the above questions, recently did a house slab on ground, ordered 80 slump, noticed the truck driver adding water to the mix, because the pump operator told him he wanted it at 120, I told the concrete driver to ease up on the water...but it was clear I didnt really know what I was talking about, so want to be prepared next time. 
So by the sound of it, I should not let concrete drivers add water(to acheive anything other than the ordered slump), but if I miss it or am too busy doing something else, there is a fairly wide margin for tolerances for low end applications ie, house slabs, driveways, patios etc 
cheers

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## Gaza

you can get palastiers added at the plant that allow for it to be pumped easier

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## thetrooper

> you can get palastiers added at the plant that allow for it to be pumped easier

   
hehe add hot water, water reducer & a extra 90kg of cement per cubic metere and you have a time bomb at times !

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## intertd6

If hot water is added & is over 50' C the concrete can flash set. 
regards inter

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