ballast to concrete ratio please?

[muckington]

After 2 days you could probably still scratch into it with a stick.

but what temprature

 

[Ivor Windybottom]

Dunno. Off t'internet somewhere.

If there's no urgency then give it a couple of weeks. There are tower blocks that have fallen down where the builders have removed formwork and supports too soon.

I don't think there's too much difference with modern cement. I concreted a patch this year, I could rub the surface off with my boot a few days later.

 

[stevie888]

Dunno. Off t'internet somewhere.

If there's no urgency then give it a couple of weeks. There are tower blocks that have fallen down where the builders have removed formwork and supports too soon.

I don't think there's too much difference with modern cement. I concreted a patch this year, I could rub the surface off with my boot a few days later.

Modern cements are finer ground than in times of yore. This means that in a given volume of concrete there are a greater number of particles of cement which are therefore closer together. The setting process is the growth of dendrites (finger like structures) which interlock binding the mass together so the smaller gaps mean that the dendrrites interlock sooner.

We had a major fright in the late 70s when we tested a load of cubes at 7 days from our first major pour. It was a 60odd metre heavily reinforced pour and the concrete was to be 40N.

Average strength 27N measured. No worries: this is 2/3 of 28 day so we're looking at 3/2 x 27 so over 40. Come 28 days they only test at about 37N so the 7 day was about 75% and this was the norm for the whole job.

 

[muckington]

Modern cements are finer ground than in times of yore. This means that in a given volume of concrete there are a greater number of particles of cement which are therefore closer together. The setting process is the growth of dendrites (finger like structures) which interlock binding the mass together so the smaller gaps mean that the dendrrites interlock sooner.

Do you think the amount of water will make any differance?

 

[Notch7]

Do you think the amount of water will make any differance?

Hard to tell because ballast can be bone dry or soaking wet.

Interestingly in winter you tend to get less in your bulk bag than in the summer…..you are paying for water in the winter months

 

[stevie888]

Do you think the amount of water will make any differance?

Yes. Simplictically more water = less strength.

The hydration process needs about 20% of the weight of cement any water above this makes the mix more fluid but has to go somewhere. Some of it will rise to the surface dragging cement particles with it leaving a soft dust surface (bleeding) and some will just form water pockets weakening the finished product.

 

[foxhole]

Less water = stronger mix.

 

[robinbanks]

Less water = stronger mix.

Before I tell you that's nonsense, I'd like you to define "stronger"

 

[noseall]

Do you think the amount of water will make any differance?

Workability - yes. As said, too much water encourages 'dusting', when it lies on the surface. Unless you are building the Hoover dam, don't worry too much about strength, as long as you mix the product well and are no weaker than 6:1.

 

[muckington]

Hard to tell because ballast can be bone dry or soaking wet.

Interestingly in winter you tend to get less in your bulk bag than in the summer…..you are paying for water in the winter months

are they doing it on weight or volume?

 

[muckington]

Yes. Simplictically more water = less strength.

The hydration process needs about 20% of the weight of cement any water above this makes the mix more fluid but has to go somewhere. Some of it will rise to the surface dragging cement particles with it leaving a soft dust surface (bleeding) and some will just form water pockets weakening the finished product.

I know some one that made the mix dry and relied on rain water
I have done one drum the other day, came back after the heavey rain and there was no rain water sittig on the top so must of gone straight though

 

[stevie888]

I know some one that made the mix dry and relied on rain water
I have done one drum the other day, came back after the heavey rain and there was no rain water sittig on the top so must of gone straight though

Or run off the top if you'd finished it properly.

 

[robinbanks]

I know some one that made the mix dry and relied on rain water

I wouldn't

 

[Atgni]

I don't know about strength and ratio but I have laied some concrete where I have ran out of cement for the volume so I have added more dry gravel and then come to remove it years later and it's a nightmare to chisel away

 

[Diver Fred]

I was taught that the correct mix is 1 cement, 2 sand, 3 (20mm) stone. If you use ready mixed ballast then for best you need to mix large quantities. If wish a weaker mix (i.e. a little cement then an extra 1/2 measure of both sand and stone but NO weaker)
As for the right amount of water I go by sound - I aim to hear a 'flumf' sound. Start with half a measure of water in the mixer before adding the dry materials, add a little more as the mix progresses to ensure the mix is workable.