Concrete - Will adding water weaken it ?

[Mick Leek]

Hi all.

OK i know its a bit late in the day to be asking this one but here goes. I am laying a floor slab tomorrow which will take about 6 meters on concrete.

The delivery truck can just reach the start of the slab so no problem there. The problem is i need to work the crete to the far end of the slab There is no way i am going to barrow 6 meters.

The plan was to get the driver to add a little more water to make the mix a little more workable. Will this weaken the final slab and should i avoid doing it ?

The building regs guy said 4" will do JUST but he advised 6" I have preped the subase 11" below the final level, Installed 4" of Expanded polystyrene insulation, leaving a slab depth of 7" with A142 reinforcing mesh

I was thinking that because he only wanted 4" and i have made provisions for 7" should i be ok to add a few more drops of water ?

Thanks all

[Tony McC]

Adding water changes the water:cement ratio and will weaken the 28-day strength. If you get the driver to add water when the wagon arrives, you'll be required to sign a waiver absolving the concrete co of any responsibility for the concrete. What should be done is to ask the batch plant to increase the slump to, say, 100mm, and they can alter the cement content accordingly so that the desired 28-day stength is assured and, more importantly, is guaranteed.

In summary: water added on site is done at your own risk; water added at the batch plant is their responsibility.

[Mick Leek]

Thanks Tony,

In the end i just took it as it came. We were lucky as in we couldnt get a big lorry on the drive so we had to receive the concrete in 2 loads (mini mix wagons)

We asked the driver to have a brew before he returned with the second load so it gave us 40 mins or so to work the first load to the rear of the slab.

Dont know what went wrong with the calculations though, When we ordered we gave the company the dimensions and the thicknes of the slab. They calculated that we needed 5.7 cubic meters, so we asked for 6 to make sure.

Lets just say that there wasnt 1 chipping spare and im glad i decided to round it upto 6 meters..

Anyway, the slab is down. How long should i wait now till i start building on it? I was hoping to start laying a few bricks on wednesday if the weather is good.

Mick.

[Mick Leek]

All systems go now....



It rained all night after we layed the floor and prevented me from floating it. It has also rained all morning.. There are little moon craters all over the surface now.. I have tried to smooth it the best i can but would it be better for me to leave it now and maybe give it a little self leveling once the walls are up ?

While im on the subject, When i purchased the insulation for under the floor, I obviously shopped around for the best price. I decided that Polyurathane insulation was way beyond my budget so i decided to get Polystyrene. Pros and cons are as follows,

Polyurathane: Only Need 65mm, Easy to work with, superior heat retension,

Polystyrene: need 100mm to get same benefit, easy to work with but leaves little ball's all over the place,

WHY oh WHY do building supply companies vary so much with prices ?

I purchased all the building materials ie: bricks, blocks, sand, cement from Travis Perkins ( guy seemed really nice so i decided to go for it although they were slightly more expensive) but when i went for insulation they were nearly double the price of a local back street company. In the end i got the insu from the back street place. Problem was that there packs were in 3 times 8x4 sheets. It ended up that i was one sheet short. They wouldnt split a pack, so i went to travis perkins to get one. they sold me a sheet and it was more than double the price.

When i challenged them about this and showed them the receipt from the other place, one guy walked to the guy who was serving me and winked at him. He then said, "we can match that price cause we bought it cheaper from Bla Bla Bla down the road." The guy who was serving me then said "Oh yes so we did"

Is this normal in the building supply market ? I have now changed my mind with regards travis perkins. I will leave it up to you to judge them, but i certainly wont be using them again. Not when they can suddenly reduce there prices by 60% because i had an issue. It makes me wonder what they could have supplied me all the building materials for in the first place.


Mick

[danensis]

Heh, heh heh, asking Tony about TP is like waving a red rag in front of a bull.

[Tony McC]

Mark-up for many materials supplied via BMs, especially the larger clowns such as Travis Bloody Perkins and Sodding Jewsons is often in the region of 100-120%, so dropping their price by 60% is no great act of generosity on their part, believe me!

The smaller, independent merchants are usually much keener, especially for DIYers and small builders. The larger nationals reserve their best prices for their BIG customers and rely on the smaller contractors and DIYers to subsidise this practice. That, to me, is bloody immoral and is why I avoid giving business to these usurers whenever possible.