C25/30 concrete mix has failed test

[Spanglish]

I am down in Spain trying to lay a foundation for a house extension.

The building law in Spain requires that all structural concrete is sampled and tested. With this in mind we made a couple of test mixes which were sent away for testing. This mixes failed, with one coming in at C22 and the other at C24.
The first test used a C30 mix 1:2:3 (Cement:Sand:Stone) and the second mix was C25 1:2:4. All were mixed in a good small concrete mixer. The slump test showed a slump of 3.5 cm. Clean water was used for both mixes and the materials had been washed and were dust free.

The stone used in the mixes varied in size up to 15 mm (max size of stone for the concrete pump that we are going to use). I thought that the sand was too course and more like gravel (approx 2mm).

Where did we go wrong?

 

[tony1851]

Is this for plain or reinforced concrete?
If its just plain and for foundation work, would that small drop in strength matter?

 

[Norcon]

Round stone or crushed?

If round then more cement is needed.

The test cube should sit for at least a month before testing and I've seen engineers suspend the cubes in water tanks for certain periods.

 

[Spanglish]

Hi all,

Thanks for your replies.

The concrete is for reinforced foundations and ring beams etc.
The stone is crushed.
What I can't understand is that we were aiming for C30 and din't even hit C25.

The testing really proves nothing because the sample is taken from the mixer and not the foundation. When the builder sees the inspector coming he puts another bag of cement in the mixer. But that's this country's moronic bureaucracy for you.

 

[Norcon]

Why don't you use readymix from a reputable company?
We pour about 70 cube of the stuff every week.

Your wasting the re-bar when you pour that weak stuff over it.

 

[^woody^]

Vibrate the cube

 

[Spanglish]

Hi Norcon,

Of course you are right, because of the location of the property we had discounted this idea.

Perhaps I could get your advice on this. We are 30-40 minutes from the nearest readymix supplier. The build is up in the mountains so once the readymix arrives it has to be then taken by dumper to a point where it can be pumped up to the build further up the hill. The road is a narrow single lane and services a number of other properties.

My question is how long can you ****** C25 without affecting the strength and workability?

 

[Norcon]

You would be better asking a concrete specialist who is involved with batching concrete.

We stick to one company as much as possible as we know the results are pretty good. From looking at their delivery note they add a WRA. Which has obvious benefits.
I've no idea what type.

One company a few weeks back added some other concoction and it held the air in the formwork resulting in a bad finish.

Also retarding concrete in the Irish climate is usually not needed.

I believe there are superplasticizers that can be used in conjuction with water reducing retarders to increase the strength.

We used 40 newton yesterday instead off 35 and the extra cost was only £2 per cubic/M.

it has to be then taken by dumper to a point where it can be pumped up

You can't even get the dumper to the site? Don't you have a digger to make access?
One digger we use can lift and carry 4 ton. So we use a cubic/M skip with that which is very useful.