Hi Forum
I have been reading the page on Manufacturing Concrete Paving
www.pavingexpert.com/manuf01.htm
And was interested in what water/cement content/ratio "a semi-dry concrete mix" used in Hydraulic Press (HP) would be?
I believe to get cement to fully hydrate at least 25% water is required?
I have been using a superplasticiser and vibrating table and a 30% water to cement ratio. The 30% water content does not include any free water in the sand. The cement content included 5% silica fume.
On another subject I have found obtaining pozzolans in small quanties difficult, any pointers would be appreciated.
I have been able to obtain small quantities of un-densified silica fume/micro silica but not fly ash.
I believe sulfacrete by Blue circle/lafarge contains fly ash and cement and Hanson offer something similar.
Regards
Iain
Manufacturing concrete paving
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Brucieboy
- Posts: 162
- Joined: Sun Jul 17, 2011 1:21 am
- Location: Essex
Ian - yes, you're correct about the water/cement ratio. To fully hydrate cement it's about 0.25 to 0.27.
To produce consistent concrete, you really need to establish the moisture content of the sand so you can adjust the amount of added water to ensure it's the same w/c ratio for each batch.
When producing ready mixed concrete in smallish quantities, we normally use densified microsilica such as MS93 (grey in colour) from Carbon Enterprises at typically 10% by weight of cement - normally 40 kg/m3. It has a bulk density of about 600 kg/m3 and is a bit easier to use than the undensified lighter grey MS95 UD (bd 300 kg/m3) or white MS98 UD (bd 200 kg/3). The only benefit of using the white is that it's better if using pigments as it has less effect on the finished colour of the concrete.
I'm assuming the cement you're using is CEMI 52.5 and not the General Purpose stuff (32.5 or 42.5) which generally contains either a proportion of flyash or ggbs.
Why do you need flyash when you're already using microsilica? The latter should give you a much better finish - it's tends to produce concrete giving a mirror image of the mould.
To produce consistent concrete, you really need to establish the moisture content of the sand so you can adjust the amount of added water to ensure it's the same w/c ratio for each batch.
When producing ready mixed concrete in smallish quantities, we normally use densified microsilica such as MS93 (grey in colour) from Carbon Enterprises at typically 10% by weight of cement - normally 40 kg/m3. It has a bulk density of about 600 kg/m3 and is a bit easier to use than the undensified lighter grey MS95 UD (bd 300 kg/m3) or white MS98 UD (bd 200 kg/3). The only benefit of using the white is that it's better if using pigments as it has less effect on the finished colour of the concrete.
I'm assuming the cement you're using is CEMI 52.5 and not the General Purpose stuff (32.5 or 42.5) which generally contains either a proportion of flyash or ggbs.
Why do you need flyash when you're already using microsilica? The latter should give you a much better finish - it's tends to produce concrete giving a mirror image of the mould.
Retired DIY'er
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iain123
- Posts: 4
- Joined: Fri Oct 30, 2015 9:20 pm
- Location: Essex
Hi Bruice Boy
Thank you for your kind advice
Yes I have been using Snowcrete which is a CEMI 52.5
Maybe I don't need fly ash S as I new to the hobby of casting some decorative moulds, I been internet researching rather than a person with experience. I read opc+fly ash+silica fume makes makes a really strong mix.
I was advised to use the undesified silica fume as it needs less mixing than "professional" densified forms.
With regards to water content of the sand I would not known how to measure this as its just bagged sharp sand from b&q etc, can I do it with how the sand feels fairly accurately?
Not having an air entertaing plasticizer/admixture will this make the concrete more likely to crack with freeze thaw do you know?
I also have some colloidal silica, which I have been told is a pozzalan at about 2% of of the cement.
regards
Iain
Thank you for your kind advice
Yes I have been using Snowcrete which is a CEMI 52.5
Maybe I don't need fly ash S as I new to the hobby of casting some decorative moulds, I been internet researching rather than a person with experience. I read opc+fly ash+silica fume makes makes a really strong mix.
I was advised to use the undesified silica fume as it needs less mixing than "professional" densified forms.
With regards to water content of the sand I would not known how to measure this as its just bagged sharp sand from b&q etc, can I do it with how the sand feels fairly accurately?
Not having an air entertaing plasticizer/admixture will this make the concrete more likely to crack with freeze thaw do you know?
I also have some colloidal silica, which I have been told is a pozzalan at about 2% of of the cement.
regards
Iain
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Brucieboy
- Posts: 162
- Joined: Sun Jul 17, 2011 1:21 am
- Location: Essex
As you're using a semi-dry mix, are you using a forced-action, not drum, type mixer? A forced-action mixer is essential to obtain a uniform distribution of all ingredients.
An air entraining agent is not usually used in a semi-dry mix as it's difficult to generate the right size and distribution of air bubbles necessary to provide resistance to the effects of frost.
The problem with bagged sand from diy stores such as b&q is that not only can the moisture content vary from bag to bag, but it could also come from different sources depending on which one they decide to stock at any particular time. It's much better to obtain it direct from a quarry / pit so at least you know the source is consistent. It depends how fussy you want to be with the finished product.
If you're experienced, you can roughly assess the moisture content by "feel". However, if you do decide to measure it, it's determined by accurately weighing a representative quantity of damp sand (say 500g) then drying it in an oven until it's weight is stable. The moisture content is the difference between the damp weight and dry weight divided by the dry weight x 100 to express it in percentage terms. Do not include the weight of the pan or tray, a common error causing incorrect results. Be careful if using a microwave as too high a setting can cause the particles to burst. Always use appropriate ppe (personal, protective equipment), particularly heat-proof gloves, as you're dealing with hot materials.
With regard to colloidal silica, personally it's a bit of an overkill if you are already using undensified microsilica. It does improve durability but if you're using good quality aggregates, CEMI 52.5 at the right proportion, microsilica, superplasticiser and low w/c ratio, these should be sufficient to produce good quality concrete for paving slabs if well compacted with a hydraulic press.
On the basis you're using a hydraulic press (or going to in the future), I assume you're making the slabs as a business for supply to the open market?
An air entraining agent is not usually used in a semi-dry mix as it's difficult to generate the right size and distribution of air bubbles necessary to provide resistance to the effects of frost.
The problem with bagged sand from diy stores such as b&q is that not only can the moisture content vary from bag to bag, but it could also come from different sources depending on which one they decide to stock at any particular time. It's much better to obtain it direct from a quarry / pit so at least you know the source is consistent. It depends how fussy you want to be with the finished product.
If you're experienced, you can roughly assess the moisture content by "feel". However, if you do decide to measure it, it's determined by accurately weighing a representative quantity of damp sand (say 500g) then drying it in an oven until it's weight is stable. The moisture content is the difference between the damp weight and dry weight divided by the dry weight x 100 to express it in percentage terms. Do not include the weight of the pan or tray, a common error causing incorrect results. Be careful if using a microwave as too high a setting can cause the particles to burst. Always use appropriate ppe (personal, protective equipment), particularly heat-proof gloves, as you're dealing with hot materials.
With regard to colloidal silica, personally it's a bit of an overkill if you are already using undensified microsilica. It does improve durability but if you're using good quality aggregates, CEMI 52.5 at the right proportion, microsilica, superplasticiser and low w/c ratio, these should be sufficient to produce good quality concrete for paving slabs if well compacted with a hydraulic press.
On the basis you're using a hydraulic press (or going to in the future), I assume you're making the slabs as a business for supply to the open market?
Retired DIY'er
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iain123
- Posts: 4
- Joined: Fri Oct 30, 2015 9:20 pm
- Location: Essex
Hi Bruice boy
Thanks again for your advice, my interest is a hobby not business.
No i am not using a hydraulic press, in my original question i wanted to known what water cement ratio would be classified as semi dry to compare with my low water "wet mix".
With my vibrating table i find i get bubbles on the surface of the concrete as it spreads out in the mould.
"As you're using a semi-dry mix, are you using a forced-action, not drum, type mixer? A forced-action mixer is essential to obtain a uniform distribution of all ingredients". I will definatelly have to give this more thought.
regards
Iain
Thanks again for your advice, my interest is a hobby not business.
No i am not using a hydraulic press, in my original question i wanted to known what water cement ratio would be classified as semi dry to compare with my low water "wet mix".
With my vibrating table i find i get bubbles on the surface of the concrete as it spreads out in the mould.
"As you're using a semi-dry mix, are you using a forced-action, not drum, type mixer? A forced-action mixer is essential to obtain a uniform distribution of all ingredients". I will definatelly have to give this more thought.
regards
Iain