I'm planning to design and make some decorative square concrete blocks with
holes in.

Well-known patterns include a square suspended inside the outer square; and
four quarter circles, one centred on each corner, with radii equal to the
block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry

Harry Davis wrote:
I'm planning to design and make some decorative square concrete
blocks with holes in.

Well-known patterns include a square suspended inside the outer
square; and four quarter circles, one centred on each corner, with
radii equal to the block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry

You mean screen blocks?

I have known quite a few knowledgable people try to make these over the
decades and none of them ever managed to make more than a few that didn't
fall apart.

You've got to try different mixes, strengths and moisture levels.

Don't use concrete, use grit sand, mixed at about 3:1 with cement and with
only a drop of water - it needs to be of a consistency that you can squeeze
it together into a ball and it hold it's shape, akin to slightly damp sugar.

after you've made one, it needs to cure for quite a while before you can do
anything with it, maybe up to a week, and even then it may be another month
before it's cured completely, all in all, a pain in the arse and it'd
probably be cheaper and easier to go and buy some

"Phil L" wrote in
:

Harry Davis wrote:
I'm planning to design and make some decorative square concrete
blocks with holes in.

Well-known patterns include a square suspended inside the outer
square; and four quarter circles, one centred on each corner, with
radii equal to the block's side-length.

What mix of concrete or screed should I use?

You mean screen blocks?

Yep.

I have known quite a few knowledgable people try to make these over
the decades and none of them ever managed to make more than a few that
didn't fall apart.

You've got to try different mixes, strengths and moisture levels.

Don't use concrete, use grit sand, mixed at about 3:1 with cement and
with only a drop of water - it needs to be of a consistency that you
can squeeze it together into a ball and it hold it's shape, akin to
slightly damp sugar.

after you've made one, it needs to cure for quite a while before you
can do anything with it, maybe up to a week, and even then it may be
another month before it's cured completely,

I'm grateful for this advice. Sounds like a bit of a challenge, then!
Might some plasticiser in the mix be useful? Or something other than a
standard cement? I'm not sure whether Mastercrete already contains some.

all in all, a pain in the
arse and it'd probably be cheaper and easier to go and buy some

Ah but I'll be designing my own! :-)

Harry

Harry Davis wrote:

"Phil L" wrote in
:

I have known quite a few knowledgable people try to make these over
the decades and none of them ever managed to make more than a few that
didn't fall apart.

You've got to try different mixes, strengths and moisture levels.

Don't use concrete, use grit sand, mixed at about 3:1 with cement and
with only a drop of water - it needs to be of a consistency that you
can squeeze it together into a ball and it hold it's shape, akin to
slightly damp sugar.

after you've made one, it needs to cure for quite a while before you
can do anything with it, maybe up to a week, and even then it may be
another month before it's cured completely,

I'm grateful for this advice. Sounds like a bit of a challenge, then!
Might some plasticiser in the mix be useful? Or something other than a
standard cement? I'm not sure whether Mastercrete already contains some.

all in all, a pain in the
arse and it'd probably be cheaper and easier to go and buy some

Ah but I'll be designing my own! :-)

I spotted this comment elsewhe

http://www.pavingexpert.com/cgi-bin/...ST;f=2;t=11044

"In the70s, it was screen block walling. You could buy moulds for
screen blocks in Woolies (TruFact!) and every wannabe
builder-gardener was churning out up to two-dozen blocks a week
in their back yard, soon to be assembled into what could only be
described as a "wall with character".

Rarely did any two blocks have the same colour or texture due to
the vagaries of mixing concrete in a 2-gallon bucket (litres
hadn't been invented back then), and even more rare was a block
with actual 90° corners. Many had bulging sides due to the moulds
deforming when filled with the mix, and on completion the whole
thing usually resembled something dreamed up by Dali on one of
his bad nights.

On my travels, I sometimes see surviving relics of these
constructions, defying all laws of nature, and I really ought to
photo-document them before they finall succumb to weathering
and/or gravity. They are a thing of genuine amazement and awe!
"

Chris
--
Chris J Dixon Nottingham UK


Plant amazing Acers.

Chris J Dixon wrote:
I spotted this comment elsewhe

http://www.pavingexpert.com/cgi-bin/...ST;f=2;t=11044

"In the70s, it was screen block walling. You could buy moulds for
screen blocks in Woolies (TruFact!) and every wannabe
builder-gardener was churning out up to two-dozen blocks a week
in their back yard, soon to be assembled into what could only be
described as a "wall with character".

Rarely did any two blocks have the same colour or texture due to
the vagaries of mixing concrete in a 2-gallon bucket (litres
hadn't been invented back then), and even more rare was a block
with actual 90° corners. Many had bulging sides due to the moulds
deforming when filled with the mix, and on completion the whole
thing usually resembled something dreamed up by Dali on one of
his bad nights.

On my travels, I sometimes see surviving relics of these
constructions, defying all laws of nature, and I really ought to
photo-document them before they finall succumb to weathering
and/or gravity. They are a thing of genuine amazement and awe!

Yep, that's about the size of it, in short, they're ugly, deformed and just
plain crap, even the shop bought ones look so 1970's even when new.

To the op, they really need to be pressed - no air pockets at all, which is
nigh on impossible, unless the mix is extremely wet, then vibrated, but then
you get a shiny surface all over and they take forever to cure.
Comercially made ones are made using a dry mix which is mechanically
compressed, then pushed out of the mould and cured that way, yours will have
to sit in the mould for at least a week otherwise it will crumble when you
try to get it out

On 06/09/2013 17:06, Phil L wrote:
Chris J Dixon wrote:
I spotted this comment elsewhe

http://www.pavingexpert.com/cgi-bin/...ST;f=2;t=11044

"In the70s, it was screen block walling. You could buy moulds for
screen blocks in Woolies (TruFact!) and every wannabe
builder-gardener was churning out up to two-dozen blocks a week
in their back yard, soon to be assembled into what could only be
described as a "wall with character".

Rarely did any two blocks have the same colour or texture due to
the vagaries of mixing concrete in a 2-gallon bucket (litres
hadn't been invented back then), and even more rare was a block
with actual 90° corners. Many had bulging sides due to the moulds
deforming when filled with the mix, and on completion the whole
thing usually resembled something dreamed up by Dali on one of
his bad nights.

On my travels, I sometimes see surviving relics of these
constructions, defying all laws of nature, and I really ought to
photo-document them before they finall succumb to weathering
and/or gravity. They are a thing of genuine amazement and awe!

Yep, that's about the size of it, in short, they're ugly, deformed and just
plain crap, even the shop bought ones look so 1970's even when new.

To the op, they really need to be pressed - no air pockets at all, which is
nigh on impossible, unless the mix is extremely wet, then vibrated, but then
you get a shiny surface all over and they take forever to cure.
Comercially made ones are made using a dry mix which is mechanically
compressed, then pushed out of the mould and cured that way, yours will have
to sit in the mould for at least a week otherwise it will crumble when you
try to get it out


I saw an episode of The Savager where he made a concrete table top. He
used an old orbital sander to vibrate the mould.

--
Dave - The Medway Handyman www.medwayhandyman.co.uk

"Harry Davis" wrote in message
...
I'm planning to design and make some decorative square concrete blocks
with
holes in.

Well-known patterns include a square suspended inside the outer square;
and
four quarter circles, one centred on each corner, with radii equal to the
block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry

I wouldn't bother. The bought ones are pressed into the mould using a fairly
dry mix.
Anything you could make would be crap (ie fall to bits pretty toot sweet.)

On 06/09/13 20:01, harryagain wrote:
"Harry Davis" wrote in message
...
I'm planning to design and make some decorative square concrete blocks
with
holes in.

Well-known patterns include a square suspended inside the outer square;
and
four quarter circles, one centred on each corner, with radii equal to the
block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry
I wouldn't bother. The bought ones are pressed into the mould using a fairly
dry mix.
Anything you could make would be crap (ie fall to bits pretty toot sweet.)


Not everybody is incapable of DIY harry.

Even if you are.

I've successfully cast concrete many times.

--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

"The Natural Philosopher" wrote in message
...
On 06/09/13 20:01, harryagain wrote:
"Harry Davis" wrote in message
...
I'm planning to design and make some decorative square concrete blocks
with
holes in.

Well-known patterns include a square suspended inside the outer square;
and
four quarter circles, one centred on each corner, with radii equal to
the
block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry
I wouldn't bother. The bought ones are pressed into the mould using a
fairly
dry mix.
Anything you could make would be crap (ie fall to bits pretty toot
sweet.)


Not everybody is incapable of DIY harry.

Even if you are.

I've successfully cast concrete many times.


TurNiP, my last DIY concrete job was nearly two hundred tons for my earth
shielded house.

Its a long time since I did this stuff, so memory might be a bit off on some details...

I'm planning to design and make some decorative square concrete blocks with
holes in.

What mix of concrete or screed should I use?

Think we used a wet 3:1 mix. You need strong & dense.


I have known quite a few knowledgable people try to make these over
the decades and none of them ever managed to make more than a few that
didn't fall apart.

that surprises me, we did ok, and had no fancy equipment to work with.

You've got to try different mixes, strengths and moisture levels.

why?

Don't use concrete, use grit sand, mixed at about 3:1 with cement and
with only a drop of water - it needs to be of a consistency that you
can squeeze it together into a ball and it hold it's shape, akin to
slightly damp sugar.

There are 2 approaches, a wet mix as we used, that has to stay in the mould till set and cured for days. Factories use a dryish mix and eject from the mould immediately. Dryish mixes need strongly vibrating or pressing. Wet wants vibration to remove air pockets, but dont overdo it or the mix separates somewhat.


Cure is slow. You can speed cure a bit by covering with polythene. More heat & more damp.


all in all, a pain in the arse

not IME. More intricate moulds for screen blocks do take time, so to get any throughput you need to go with dryish mix and shaker table - not hard to set up but needs doing.

It's far cheaper than buying blocks, but it is work.


You can buy mixes that give a much finer finish, most slabs, blocks etc dont use them.

IIRC we did at least a dozen per day per person. With wet mix the number of moulds is the big limit. Pink ones sold for more back then - bet there aren't many of those still in use!

Rarely did any two blocks have the same colour or texture due to
the vagaries of mixing concrete in a 2-gallon bucket (litres
hadn't been invented back then), and even more rare was a block
with actual 90� corners. Many had bulging sides due to the moulds
deforming when filled with the mix, and on completion the whole
thing usually resembled something dreamed up by Dali on one of
his bad nights.

Any job can be screwed up by incompetence. There's no reason to though. Don't underestimate the mould accuracy required, 1mm out does show in a final wall, 2mm looks lousy.

in short, they're ugly, deformed and just
plain crap, even the shop bought ones look so 1970's even when new.

They look however you choose them to. You can make crap if you like, or make something special. I'd sugest taking time over choosing a block design, and making the mould with precision & style. The blocks are, if things go ok, just copies of the mould cavity.


yours will have
to sit in the mould for at least a week otherwise it will crumble when you
try to get it out

with wet mix, yes. With dry you need to make a tool that presses it out without putting the slightest amount of sideways force on it, as factories do it.

An old orbital sander to vibrate the mould is ok for a wet mix

Polythene is the best release agent, it gives a smoooth finish

One decision to make early on is whether you need 1 or 2 sided blocks.

These days I'd include some chopped mixed synthetic fibre to reduce crack risk. Shred some carpet or scrap clothing.

The wiki article covers some of this. Look at industrial block/paviour making on youtube, it'll explain much.


NT

wrote in
:

Many thanks for your very helpful post.

A few follow-up questions if I may!

Would that be sharp sand and mastercrete? (I've been doing a lot of
concreting recently, and the main reasons I bought mastercrete were
because it comes in plastic bags rather than paper ones, making lifting a
bit less hassle, and because I thought that since I'm a bit of a newbie,
buying something a bit away from the bottom of the range would compensate
for my lack of experience. So far, all has gone well, and I was
pleasantly surprised by the whiteness of the final colour, but I
digress...)

Don't use concrete, use grit sand, mixed at about 3:1 with cement and
with only a drop of water - it needs to be of a consistency that you
can squeeze it together into a ball and it hold it's shape, akin to
slightly damp sugar.

There are 2 approaches, a wet mix as we used, that has to stay in the
mould till set and cured for days. Factories use a dryish mix and
eject from the mould immediately. Dryish mixes need strongly vibrating
or pressing. Wet wants vibration to remove air pockets, but dont
overdo it or the mix separates somewhat.

Did you make your own moulds or buy them?

Cure is slow. You can speed cure a bit by covering with polythene.
More heat & more damp.

all in all, a pain in the arse

not IME. More intricate moulds for screen blocks do take time, so to
get any throughput you need to go with dryish mix and shaker table -
not hard to set up but needs doing.

I might leave the dry mix until later, because at the moment a big
throughput isn't required; but for future reference, how do you set up a
shaker table?

You can buy mixes that give a much finer finish, most slabs, blocks
etc dont use them.

Buying mixes seems like cheating! What is the difference in the
components? White cement? Some kind of different sand?

They look however you choose them to. You can make crap if you like,
or make something special. I'd sugest taking time over choosing a
block design, and making the mould with precision & style.

Is timber the material to use? I've been wondering whether something
might be doable with silicone, but it would need not to fall apart after
the first few blocks.

yours will have to sit in the mould for at least a week otherwise it
will crumble when you try to get it out

with wet mix, yes. With dry you need to make a tool that presses it
out without putting the slightest amount of sideways force on it, as
factories do it.

Got it; strong in compression, weak in tension.

An old orbital sander to vibrate the mould is ok for a wet mix

Thanks for this. Roughly how long do you need to do it for?

Polythene is the best release agent, it gives a smoooth finish

Thanks for this too!

One decision to make early on is whether you need 1 or 2 sided blocks.

These days I'd include some chopped mixed synthetic fibre to reduce
crack risk. Shred some carpet or scrap clothing.

The wiki article covers some of this. Look at industrial block/paviour
making on youtube, it'll explain much.

And all of this! :-)

Harry

On 07/09/2013 00:54, Harry Davis wrote:
Is timber the material to use? I've been wondering whether something
might be doable with silicone, but it would need not to fall apart after
the first few blocks.

Rather depends on which silicone! Standard bathroom sealer is unlikely
to be up to it. This is a link to various silicone rubbers used by,
among others, sculptors. The same company also offers other mouldmaking
materials.

http://www.tiranti.co.uk/EdgeImpactS...cone+Ru bber+

One of the standard casting techniques used with rubber moulds is to
make an original item. Then cover the item with the rubber material to a
usable thickness. Then cover that with a substantial layer of plaster -
using all sorts of reinforcement such as metal bars and scrim. You
therefore end up with a mould that has the fine details, easy release
and flexibility of rubber on the inside, but the strength and rigidity
of the plaster on the outside.

Provided the design is simple enough you would only need a simple
one-part pour-in design - but the same basic technique can be used to
make fully 3D objects by designing a multi-part mould - suitably trapped.

--
Rod

On Saturday, September 7, 2013 12:54:59 AM UTC+1, Harry Davis wrote:
wrote in
:

Many thanks for your very helpful post.
A few follow-up questions if I may!
Would that be sharp sand and mastercrete?

just building sand & cement.


Did you make your own moulds or buy them?

made from 2x4


I might leave the dry mix until later, because at the moment a big
throughput isn't required; but for future reference, how do you set up a
shaker table?

motor, offset weight on shaft, bolt to underside of table. Table mounted on rubber lumps. It will get wet, so pick an LV motor.

You can buy mixes that give a much finer finish, most slabs, blocks
etc dont use them.
Buying mixes seems like cheating! What is the difference in the
components? White cement? Some kind of different sand?

Memory not good here, but iirc both sand & cement are different


Is timber the material to use?

its very available, cheap & workable. Tendency to warp is a problem. Do use heavy enough timber to eliminate even tiny tendency to bulge.

I've been wondering whether something
might be doable with silicone, but it would need not to fall apart after
the first few blocks.

I've no experience with rubber moulds. I don't see how one would get sub 1mm accuracy with them


yours will have to sit in the mould for at least a week otherwise it
will crumble when you try to get it out
with wet mix, yes. With dry you need to make a tool that presses it
out without putting the slightest amount of sideways force on it, as
factories do it.
Got it; strong in compression, weak in tension.

Theres no tensile or compressive strength when its cast. Its simply a case of not moving the sand granules, even slightly.


An old orbital sander to vibrate the mould is ok for a wet mix
Thanks for this. Roughly how long do you need to do it for?

Pass. We didn't use any type of vibrator machinery.


NT

wrote in
:

On Saturday, September 7, 2013 12:54:59 AM UTC+1, Harry Davis wrote:
wrote in
:

(...)

Did you make your own moulds or buy them?

made from 2x4

(...)

Is timber the material to use?

its very available, cheap & workable. Tendency to warp is a problem.
Do use heavy enough timber to eliminate even tiny tendency to bulge.

(...)

yours will have to sit in the mould for at least a week otherwise
it will crumble when you try to get it out
with wet mix, yes. With dry you need to make a tool that presses
it
out without putting the slightest amount of sideways force on it,
as factories do it.
Got it; strong in compression, weak in tension.

Theres no tensile or compressive strength when its cast. Its simply a
case of not moving the sand granules, even slightly.

Many thanks for all of this very useful advice. I'm almost ready to start
now!

One last question: am I right that the polythene used as a release agent
should be cut into pieces of exactly the same shape as the surfaces
inside the mould, with vertical pieces glued, otherwise the block will
show folds from the polythene?

Thanks again!

Harry

On Saturday, September 14, 2013 11:18:51 AM UTC+1, Harry Davis wrote:
wrote in
:
On Saturday, September 7, 2013 12:54:59 AM UTC+1, Harry Davis wrote:
wrote in
:

(...)
Did you make your own moulds or buy them?
made from 2x4
(...)
Is timber the material to use?
its very available, cheap & workable. Tendency to warp is a problem.
Do use heavy enough timber to eliminate even tiny tendency to bulge.
(...)
yours will have to sit in the mould for at least a week otherwise
it will crumble when you try to get it out
with wet mix, yes. With dry you need to make a tool that presses
it
out without putting the slightest amount of sideways force on it,
as factories do it.
Got it; strong in compression, weak in tension.
Theres no tensile or compressive strength when its cast. Its simply a
case of not moving the sand granules, even slightly.

Many thanks for all of this very useful advice. I'm almost ready to start
now!
One last question: am I right that the polythene used as a release agent
should be cut into pieces of exactly the same shape as the surfaces
inside the mould, with vertical pieces glued, otherwise the block will
show folds from the polythene?
Thanks again!
Harry

We didnt. Its a long time since I did this, I'm pretty sure we just arranged the folding to happen somewhere that would be out of sight. Expectations have increased since then. Ideally wrap the poly round each wood member before screwing the mould together. Pull it tight a little, but avoid creasing.


NT