FAQ Concrete casting/ready for comment
Battersby
--
T. M. Battersby, stuccoist.
http://www.battersbyornamental.com
tbatt...@satx.rr.com
Dan Spector <arch...@earthlink.net> wrote in message
news:_93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net...
> Because of its low cost and outdoor endurance, concrete is an important
casting
> medium to learn. Concrete can also be direct-sculpted, and we will touch
on that
> later, but first let's look at casting.
> Concrete is defined as a conglomerate stone of portland cement and various
> grades of rock and sand, and water. Compressive strengths of 5000 to 8000
psi
> are now common, and can go much higher. It is an exotherm, meaning as it
cures
> it produces heat, same as plaster and resin. Therefore adding heat
accelerates
> the cure, and vice versa.
>
> The size of the rock is crucial in concrete batch design. Consider a
bucket full
> of fist-sized rocks: the gaps between them are big. The points of contact,
where
> stress is transferred through the mass, are tiny. So let's add enough
gravel to
> fill the gap volume: now that stress can spread among hundreds of little
trails
> through the bucket. A bigger stone is less likely to shatter from
compression
> now that gravel surrounds it. Let's go further and add enough sand to fill
the
> voids between the gravels. Now the bucket feels like a solid mass. And we
> haven't added the cement yet!
>
> Portland cement was invented by the Romans and reinvented by the English
around
> 1850. It had a pleasant gray color which looked like the building stone
quarried
> at Portland, England, hence the name. Basically, it's a fine powder of
clays and
> limestone where all the molecular water has been driven out, same as the
> gypsum>plaster action. That cement powder seeks to combine with water. It
will
> take it from the air's humidity or your skin, so be careful. Never touch
cement.
> Do not let art teachers tell you to mix it by hand. It is a ferocious
alkali and
> will burn the skin without your being aware until later. Cement comes in
94 lb.
> bags because that much equals a cubic foot. It is produced in white,
grays, and
> buff.
>
> For construction purposes, the best concrete is the strongest for the
cheapest.
> Rock and sand are very cheap compared with cement. Concrete is
proportioned by
> the cubic yard (27 cu. ft.) We talk about the number of sacks of cement
per cu.
> yd.: 5 per is normal cheap concrete, and 6 per is good practice, what
you'd ask
> the guys to pour your driveway. Cast stone, high-grade concrete where
appearance
> is as important as strength, has around 8.
>
> You may be thinking, if more is better, then pure cement is best. Wrong.
Cement
> used "neat" has almost no strength, and shrinks a lot. Cast stone uses the
> cement's color as an element.
> Back to batch design: Suppose we start with 6 buckets of rock, 3 buckets
of
> gravel, and 2 buckets of sand (6:3:2) This is just a for-instance, not an
> approved formula. How much cement? 11 buckets of aggregate, divided by 27,
> equals .4, so if you want 6-bag concrete, multiply 6 by .4, equals 2.4
buckets
> of cement: 2 1/2. See? I don't care how big your bucket is, just keep
using the
> same size! Weigh that cement before you add it, you'll need its weight.
> Let's look at the size of the biggest rock you're using. Pouring a
footing? Or a
> ballerina? (You'll do both soon enough!) The rule is, the rock's diameter
=1/3
> the width of the void. A 6" wall gets 2" rock; a 3/4" ankle gets 1/4"
gravel.
> Using chunks of old concrete is approved.
> You can work with sand mix, no rock, but it's weaker. I have put nicely
colored
> sand mix on the surface of my mold, then filled it up with good ol'
Sakrete, and
> been very pleased. More on sand mixes later.
>
> Water content is super-important to your concrete. The less water used,
the
> stronger the concrete, but if it's too dry to flow, your casting will be
awful.
> Using too much water leaves tiny voids all through the concrete, which
later can
> absorb rain and shatter when it freezes. Basically, the water mixes with
the
> cement: the rock products are bystanders here. The way it looks when a
correct
> amount of water is used is like a thick soup, and the way waves go through
the
> fluid is distinctive. Since sand is often damp or wet, the water added
must be
> decreased to compensate.
> The correct ratio is 100 cement to 40 water by weight.
>
> The way concrete is mixed is important as well. You mix the rock products
dry
> with each other, then add the cement and mix, then the water. Mixing in a
bucket
> or a big pan with hand tools simply cannot equal what a mixer does. But
who
> would buy a mixer for infrequent use? You'll get decent quality if you'll
mix
> for a good long time. The hoe with a hole in the blade is meant for pan
mixing.
> Small bucket batches can be mixed with a square trowel, or a paddle mixer
on a
> half-inch drill.
>
> A huge advantage of concrete over real stone is your ability to add rebar
> (reinforcing bar.)
> It is considered fundamental to our modern world that steel and concrete
have
> the same coefficient of expansion. Sound pompous?
> Bridges-buildings-roads-tunnels-dams: we'd miss them.
> I'm saying that because steel and concrete expand and contract the same
amount
> when the temperature changes, they mate perfectly.
>
> The sculptor takes advantage of this by making rebar armatures and setting
them
> in the mold before he pours the concrete or wiggling them into it
afterward.
> (Remember the stone size?) Since you don't want the steel showing at the
> surface, you can get little nylon clip-on spacers called chairs at builder
> supply places. Heavy wire mesh is used for flatwork.
> I won't deal with bending, cutting or welding rebar here, but I should say
it's
> done every day, and rebar is commonly tied together with a soft iron wire
> (tiewire) you'll love anyway.
> In the most crucial usage, rebar is coated with epoxy; even the cut ends
are
> painted. That's the green color you've seen in bridge work.
> Rebar is called by the number of eighths of an inch of its diameter.
Half-inch
> bar is #4.
>
> Concrete broken open after years have passed shows minuscule rust on the
steel.
> Good concrete breaks through, not around, its rock. If water does get to
the
> rebar, it rusts and slowly explodes the concrete around it. So inspection
and
> repair of your statuary is good practice.
>
> Pouring the statue: Prewet the rubber mold for low surface tension (fewer
> bubbles.) FRP and other hard molds are misted or wiped with form oil, a
variant
> of diesel oil. Bang and shake the mold as you pour it. Tip it around to
> dislodge bubbles. Concrete precasters use a vibrating steel table to
consolidate
> the concrete, but these are hard on molds. On-site casters use vibrators
run by
> air, dipped down into the forms. Without this action your concrete will
not
> attain full strength.
> Water will slowly rise to the top. Leave it alone.
> Concrete hardens in a few hours in the summer, and overnight otherwise.
Working
> in freezing weather is bad practice. Adding salts to your water to lower
the
> freezing point is questionable at best. Take off the mold only when the
casting
> is really hard. Wet the surface then and often, for several days. It will
keep
> strengthening. Its strength is commonly measured after 28 days. Pour a
cylinder
> when you pour a statue if testing will be required.
>
> If there were leaks in the mold, and colored cement was used, a strong
band of
> the color will show at the surface. White cement avoids this, but so does
> leak-proofing the mold with silicone caulk.
> The cement will cover the cast surface anyway. Many people dislike this
look.
> Remove the cement with a brush if very fresh, or with muriatic acid in
water any
> time later. This exposes the sand and some pebbles. The cement color is
still
> there, but muted by the other colors. Speaking of colors, concrete is
often
> tinted. Use only permanent colors made of crushed rock. The
> yellow-orange-red-brown-black pigments are cheap and readily available,
while
> green and blue are very costly. Choosing your sand and pebble colors for
> appearance graduates you into the 'cast stone' realm.
> If there are air holes, called 'bugholes,' you may fill them with a thick
mix of
> the cement and fine sand, rubbed in wearing gloves. To add new concrete to
old,
> paint the area with bonding agent, a white-glue relative, right before you
place
> the new concrete.
>
> Back to sand mix: Mixing sand mix with a small amount of water, just
enough so a
> fistful holds its shape, enables you to drypack the mold. This means
forcefully
> compacting the stuff against the mold wall, so rubber molds are not used:
too
> bouncy. Producers demold immediately, so molds can be recycled very fast.
Every
> cinder block is drypacked. You won't acidwash drypack, the texture is
always
> great. Keeping the cast items wet is still vital.
> Inclusions: Hey, we're artists, this is the fun part. Things can be stuck
into
> the concrete from above (pennies in the sidewalk) or glued to the mold
surface
> to be exposed later. Leaves and cloth can be trowelled into the surface.
Mosaic
> tiles, marbles, whatever.
> On the technical side, useful things like threaded anchors can be placed.
> Screw-eyes can later be screwed in, the piece can be erected, and the
screw-eyes
> can be removed. Loops are sometimes tied to the rebar, and cut off and
> patched-over later. Gluing EPS foam inside the mold creates perfect
channels and
> blockouts. Concrete castings are easily incorporated into brick walls if
you
> created mechanical keys for mortar.
> Direct sculpting of concrete: When a drypacked block is fresh, it's easy
to tear
> into with a chisel and a spoon. No money for stone? Cast it this way.
Using
> crushed limestone as your aggregate gives a wonderfully carvable mass.
Just keep
> some of the dry mix aside for repairs later.
> --
> Dan <arch...@earthlink.net>
> Be sure to see my website, http://www.archicast.com and follow the
Aintree
> thumbnail.
>
Dan Spector
Andrew Werby
Dan Spector wrote in message <_93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net>...
Because of its low cost and outdoor endurance, concrete is an important casting medium to learn. Concrete can also be direct-sculpted, and we will touch on that later,
[This can be the "Concrete-casting FAQ" and a subsequent one can cover direct sculpting.]
but first let's look at casting.
Concrete is defined as a conglomerate stone of portland cement and various grades of rock and sand, and water. Compressive strengths of 5000 to 8000 psi are now common, and can go much higher. It is an exotherm, meaning as it cures it produces heat, same as plaster and resin. Therefore adding heat accelerates the cure, and vice versa.
The size of the rock is crucial in concrete batch design. Consider a bucket full of fist-sized rocks: the gaps between them are big. The points of contact, where stress is transferred through the mass, are tiny. So let's add enough gravel to fill the gap volume: now that stress can spread among hundreds of little trails through the bucket. A bigger stone is less likely to shatter from compression now that gravel surrounds it. Let's go further and add enough sand to fill the voids between the gravels. Now the bucket feels like a solid mass. And we haven't added the cement yet!
Portland cement was invented by the Romans and reinvented by the English around 1850. It had a pleasant gray color which looked like the building stone quarried at Portland, England, hence the name. Basically, it's a fine powder of clays and limestone where all the molecular water has been driven out, same as the gypsum>plaster action. That cement powder seeks to combine with water. It will take it from the air's humidity or your skin, so be careful. Never touch cement. Do not let art teachers tell you to mix it by hand. It is a ferocious alkali and will burn the skin without your being aware until later. Cement comes in 94 lb. bags because that much equals a cubic foot. It is produced in white, grays, and buff.
[I've never seen buff cement. Does it have a special name? Who makes it? I have seen "Lumnite" (high alumina) cement, which is more refractory than Portland- I think they call it "ciment fondue" in Europe- do you know anything about that?]
For construction purposes, the best concrete is the strongest for the cheapest. Rock and sand are very cheap compared with cement. Concrete is proportioned by the cubic yard (27 cu. ft.) We talk about the number of sacks of cement per cu. yd.: 5 per is normal cheap concrete, and 6 per is good practice, what you'd ask the guys to pour your driveway. Cast stone, high-grade concrete where appearance is as important as strength, has around 8.
You may be thinking, if more is better, then pure cement is best. Wrong. Cement used "neat" has almost no strength, and shrinks a lot. Cast stone uses the cement's color as an element.
Back to batch design: Suppose we start with 6 buckets of rock, 3 buckets of gravel, and 2 buckets of sand (6:3:2) This is just a for-instance, not an approved formula. How much cement? 11 buckets of aggregate, divided by 27, equals .4, so if you want 6-bag concrete, multiply 6 by .4, equals 2.4 buckets of cement: 2 1/2. See? I don't care how big your bucket is, just keep using the same size! Weigh that cement before you add it, you'll need its weight.
Let's look at the size of the biggest rock you're using. Pouring a footing? Or a ballerina? (You'll do both soon enough!) The rule is, the rock's diameter =1/3 the width of the void. A 6" wall gets 2" rock; a 3/4" ankle gets 1/4" gravel. Using chunks of old concrete is approved.
[For casting sculpture, you usually want to use the aforementioned 8-bag mix- I generally mix 4 buckets of aggregate to 1 cement to get pretty close. ]
You can work with sand mix, no rock, but it's weaker. I have put nicely colored sand mix on the surface of my mold, then filled it up with good ol' Sakrete, and been very pleased.
[We should talk about this face-coating technique in greater depth- it is very useful for getting good detail, and works with smaller batches than are necessary to fill a whole large mold.]
More on sand mixes later.
Water content is super-important to your concrete. The less water used, the stronger the concrete, but if it's too dry to flow, your casting will be awful. Using too much water leaves tiny voids all through the concrete, which later can absorb rain and shatter when it freezes. Basically, the water mixes with the cement: the rock products are bystanders here. The way it looks when a correct amount of water is used is like a thick soup, and the way waves go through the fluid is distinctive. Since sand is often damp or wet, the water added must be decreased to compensate.
The correct ratio is 100 cement to 40 water by weight.
[Superplasticizers reduce the amount of water needed for the concrete to flow, eliminating hydraulic leakage at mold seams, and water trails. We should talk about some of the other admixes as well.]
The way concrete is mixed is important as well. You mix the rock products dry with each other, then add the cement and mix, then the water. Mixing in a bucket or a big pan with hand tools simply cannot equal what a mixer does. But who would buy a mixer for infrequent use? You'll get decent quality if you'll mix for a good long time. The hoe with a hole in the blade is meant for pan mixing. Small bucket batches can be mixed with a square trowel, or a paddle mixer on a half-inch drill.
A huge advantage of concrete over real stone is your ability to add rebar (reinforcing bar.)
It is considered fundamental to our modern world that steel and concrete have the same coefficient of expansion. Sound pompous? Bridges-buildings-roads-tunnels-dams: we'd miss them.
I'm saying that because steel and concrete expand and contract the same amount when the temperature changes, they mate perfectly.
The sculptor takes advantage of this by making rebar armatures and setting them in the mold before he pours the concrete or wiggling them into it afterward. (Remember the stone size?) Since you don't want the steel showing at the surface, you can get little nylon clip-on spacers called chairs at builder supply places. Heavy wire mesh is used for flatwork.
I won't deal with bending, cutting or welding rebar here, but I should say it's done every day, and rebar is commonly tied together with a soft iron wire (tiewire) you'll love anyway.
In the most crucial usage, rebar is coated with epoxy; even the cut ends are painted. That's the green color you've seen in bridge work.
Rebar is called by the number of eighths of an inch of its diameter. Half-inch bar is #4.
Concrete broken open after years have passed shows minuscule rust on the steel. Good concrete breaks through, not around, its rock. If water does get to the rebar, it rusts and slowly explodes the concrete around it. So inspection and repair of your statuary is good practice.
Pouring the statue: Prewet the rubber mold for low surface tension (fewer bubbles.) FRP
[Fiber-Reinforced Plaster?]
and other hard molds are misted or wiped with form oil, a variant of diesel oil. Bang and shake the mold as you pour it. Tip it around to dislodge bubbles. Concrete precasters use a vibrating steel table to consolidate the concrete, but these are hard on molds. On-site casters use vibrators run by air, dipped down into the forms. Without this action your concrete will not attain full strength.
Water will slowly rise to the top. Leave it alone.
Concrete hardens in a few hours in the summer, and overnight otherwise. Working in freezing weather is bad practice. Adding salts to your water to lower the freezing point is questionable at best. Take off the mold only when the casting is really hard. Wet the surface then and often, for several days. It will keep strengthening. Its strength is commonly measured after 28 days. Pour a cylinder when you pour a statue if testing will be required.
If there were leaks in the mold, and colored cement was used, a strong band of the color will show at the surface. White cement avoids this, but so does leak-proofing the mold with silicone caulk.
The cement will cover the cast surface anyway. Many people dislike this look. Remove the cement with a brush if very fresh, or with muriatic acid in water any time later. This exposes the sand and some pebbles. The cement color is still there, but muted by the other colors. Speaking of colors, concrete is often tinted. Use only permanent colors made of crushed rock. [Metallic oxides] the yellow-orange-red-brown-black pigments are cheap and readily available, while green and blue are very costly. Choosing your sand and pebble colors for appearance graduates you into the 'cast stone' realm.
[Can you recommend any particular types of aggregate, or caution us against any? Marble chips, for instance, substituted for sand, permit sanding and even polishing of the surface, for a terrazzo effect.]
If there are air holes, called 'bugholes,' you may fill them with a thick mix of the cement and fine sand, rubbed in wearing gloves. To add new concrete to old, paint the area with bonding agent, a white-glue relative, right before you place the new concrete.
Back to sand mix: Mixing sand mix with a small amount of water, just enough so a fistful holds its shape, enables you to drypack the mold. This means forcefully compacting the stuff against the mold wall, so rubber molds are not used: too bouncy. Producers demold immediately, so molds can be recycled very fast. Every cinder block is drypacked. You won't acidwash drypack, the texture is always great. Keeping the cast items wet is still vital.
Inclusions: Hey, we're artists, this is the fun part. Things can be stuck into the concrete from above (pennies in the sidewalk) or glued to the mold surface to be exposed later. Leaves and cloth can be trowelled into the surface. Mosaic tiles, marbles, whatever.
On the technical side, useful things like threaded anchors can be placed. Screw-eyes can later be screwed in, the piece can be erected, and the screw-eyes can be removed. Loops are sometimes tied to the rebar, and cut off and patched-over later. Gluing EPS foam inside the mold creates perfect channels and blockouts. Concrete castings are easily incorporated into brick walls if you created mechanical keys for mortar.
[Mortar is a mixture of cement and sand, with some clay added for plasticity. There are also proprietary "thinset" mortars which are extra sticky, used for setting tile, etc.]
Direct sculpting of concrete: When a drypacked block is fresh, it's easy to tear into with a chisel and a spoon. No money for stone? Cast it this way. Using crushed limestone as your aggregate gives a wonderfully carvable mass. Just keep some of the dry mix aside for repairs later.
--
Dan <arch...@earthlink.net>
Be sure to see my website, http://www.archicast.com and follow the Aintree thumbnail.
[How about some sources of supplies?]
Andrew Werby
Dan Spector
--
Dan <arch...@earthlink.net>
http://www.archicast.com
----------
In article <MQgS4.48418$k5.14...@news1.frmt1.sfba.home.com>, "Andrew Werby" <and...@computersculpture.com> wrote:
Dan Spector wrote in message <_93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net <mailto:93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net> >...
Because of its low cost and outdoor endurance, concrete is an important casting medium to learn. Concrete can also be direct-sculpted, and we will touch on that later, but first let's look at casting.
[This can be the "Concrete-casting FAQ" and a subsequent one can cover direct sculpting.]
>whatever.
Concrete is defined as a conglomerate stone of portland cement and various grades of rock and sand, and water. Compressive strengths of 5000 to 8000 psi are now common, and can go much higher. It is an exotherm, meaning as it cures it produces heat, same as plaster and resin. Therefore adding heat accelerates the cure, and vice versa.
The size of the rock is crucial in concrete batch design. Consider a bucket full of fist-sized rocks: the gaps between them are big. The points of contact, where stress is transferred through the mass, are tiny. So let's add enough gravel to fill the gap volume: now the stress can spread among hundreds of little trails through the bucket. A bigger stone is less likely to shatter from compression now that gravel surrounds it. Let's go further and add enough sand to fill the voids between the gravels. Now the bucket feels like a solid mass. And we haven't added the cement yet!
>Seems like this compacted mass shd be the basis for cement content.
Portland cement was invented by the Romans and reinvented by the English around 1850. It had a pleasant gray color which looked like the building stone quarried at Portland, England, hence the name. Basically, it's a fine powder of clays and limestone where all the molecular water has been driven out, same as the gypsum>plaster action. That cement powder seeks to combine with water. It will take it from the air's humidity or your skin, so be careful. Never touch cement. Do not let art teachers tell you to mix it by hand. It is a ferocious alkali and will burn the skin without your being aware until later. Cement comes in 94 lb. bags because that much equals a cubic foot. It is produced in white, grays, and buff.
[I've never seen buff cement. Does it have a special name? Who makes it? I have seen "Lumnite" (high alumina) cement, which is more refractory than Portland- I think they call it "ciment fondue" in Europe- do you know anything about that?]
>We used to could get buff, now it's special order here. This has nothing to do with alumina cement, which is available from refractory supply places.
For construction purposes, the best concrete is the strongest for the cheapest. Rock and sand are very cheap compared with cement. Concrete is proportioned by the cubic yard (27 cu. ft.) We talk about the number of sacks of cement per cu. yd.: 5 per is normal cheap concrete, and 6 per is good practice, what you'd ask the guys to pour your driveway. Cast stone, high-grade concrete where appearance is as important as strength, has around 8.
You may be thinking, if more is better, then pure cement is best. Wrong. Cement used "neat" has almost no strength, and shrinks a lot. Cast stone uses the cement's color as an element.
Back to batch design: Suppose we start with 6 buckets of rock, 3 buckets of gravel, and 2 buckets of sand (6:3:2) This is just a for-instance, not an approved formula. How much cement? 11 buckets of aggregate, divided by 27, equals .4, so if you want 6-bag concrete, multiply 6 by .4, equals 2.4 buckets of cement: 2 1/2. See? I don't care how big your bucket is, just keep using the same size! Weigh that cement before you add it, you'll need its weight.
> I am not satisfied with this although it's the way I was taught.
[For casting sculpture, you usually want to use the aforementioned 8-bag mix- I generally mix 4 buckets of aggregate to 1 cement to get pretty close. ]
Let's look at the size of the biggest rock you're using. Pouring a ballerina? Or a pedestal for her? (You'll do both soon enough!) The rule is, the rock's diameter =1/3 the width of the void. A 6" wall gets 2" rock; a 3/4" ankle gets 1/4" gravel. Using chunks of old concrete is approved.
You can work with sand mix, no rock, but it's weaker. I have put nicely colored sand mix on the surface of my mold, then filled it up with good ol' Sakrete, and been very pleased.
[We should talk about this face-coating technique in greater depth- it is very useful for getting good detail, and works with smaller batches than are necessary to fill a whole large mold.]
More on sand mixes later.
Water content is super-important to your concrete. The less water used, the stronger the concrete, but if it's too dry to flow, your casting will be awful. Using too much water leaves tiny voids all through the concrete, which later can absorb rain and shatter when it freezes. Basically, the water mixes with the cement: the rock products are bystanders here. The way it looks when a correct amount of water is used is like a thick soup, and the way waves go through the fluid is distinctive. Since sand is often damp or wet, the water added must be decreased to compensate.
The correct ratio is 100 cement to 40 water by weight.
[Superplasticizers reduce the amount of water needed for the concrete to flow, eliminating hydraulic leakage at mold seams, and water trails. We should talk about some of the other admixes as well.]
>My buddy Bob Ferguson used to be enthusiastic re superplaz but not any more.
I'll find out why not. And I'm not getting these people into fly ash.
The way concrete is mixed is important as well. You mix the rock products dry with each other, then add the cement and mix, then the water. Mixing in a bucket or a big pan with hand tools simply cannot equal what a mixer does. But who would buy a mixer for infrequent use? You'll get decent quality if you'll mix for a good long time. The hoe with a hole in the blade is meant for pan mixing. Small bucket batches can be mixed with a square trowel, or a paddle mixer on a half-inch drill.
A huge advantage of concrete over real stone is your ability to add rebar (reinforcing bar.)
It is considered fundamental to our modern world that steel and concrete have the same coefficient of expansion. Sound pompous? Bridges-buildings-roads-tunnels-dams: we'd miss them.
I'm saying that because steel and concrete expand and contract the same amount when the temperature changes, they mate perfectly.
The sculptor takes advantage of this by making rebar armatures and setting them in the mold before he pours the concrete or wiggling them into it afterward. (Remember the stone size?) Since you don't want the steel showing at the surface, you can get little nylon clip-on spacers called chairs at builder supply places. Heavy wire mesh is used for flatwork.
I won't deal with bending, cutting or welding rebar here, but I should say it's done every day, and rebar is commonly tied together with a soft iron wire (tiewire) you'll love anyway.
In the most crucial usage, rebar is coated with epoxy; even the cut ends are painted. That's the green color you've seen in bridge work.
Rebar is called by the number of eighths of an inch of its diameter. Half-inch bar is #4.
Concrete broken open after years have passed shows minuscule rust on the steel. Good concrete breaks through, not around, its rock. If water does get to the rebar, it rusts and slowly explodes the concrete around it. So inspection and repair of your statuary is good practice.
Pouring the statue: Prewet the rubber mold for low surface tension (fewer bubbles.) FRP
[Fiber-Reinforced_ Plaster?
>Polyester]
and other hard molds, including plaster, are misted or wiped with form oil, a variant of diesel oil. Bang and shake the mold as you pour it. Tip it around to dislodge bubbles. Concrete precasters use a vibrating steel table to consolidate the concrete, but these are hard on molds. On-site casters use vibrators run by air, dipped down into the forms. Without this action your concrete will not attain full strength.
Water will slowly rise to the top. Leave it alone.
Concrete hardens in a few hours in the summer, and overnight otherwise. Working in freezing weather is bad practice. Adding salts to your water to lower the freezing point is questionable at best. Take off the mold only when the casting is really hard. Wet the surface then and often, for several days. It will keep strengthening. Its strength is commonly measured after 28 days. Pour a cylinder (this is Official stuff) when you pour a statue or footing if testing will be required.
If there were leaks in the mold, and colored cement was used, a strong band of the color will show at the surface. White cement avoids this, but so does leak-proofing the mold with silicone caulk.
The cement will cover the cast surface anyway. Many people dislike this look. Remove the cement with a brush if very fresh, or with muriatic acid in water any time later. This exposes the sand and some pebbles. The cement color is still there, but muted by the other colors. Speaking of colors, concrete is often tinted. Use only permanent colors made of crushed rock. [Metallic oxides] the yellow-orange-red-brown-black pigments are cheap and readily available, while green and blue are very costly. Choosing your sand and pebble colors for appearance graduates you into the 'cast stone' realm.
>Never, ever use salty beach sand or pebbles. Marble sand and dust, used by cultured marble producers, give beautiful results.
[Can you recommend any particular types of aggregate, or caution us against any? Marble chips, for instance, substituted for sand, permit sanding and even polishing of the surface, for a terrazzo effect.]
>I'll never agree with omitting the sand or other "fines" in concrete.
If there are air holes, called 'bugholes,' you may fill them with a thick mix of the cement and fine sand, rubbed in wearing gloves. To add new concrete to old, paint the area with bonding agent, a white-glue relative, right before you place the new concrete.
Back to sand mix: Mixing sand mix with a small amount of water, just enough so a fistful holds its shape, enables you to drypack the mold. This means forcefully compacting the stuff against the mold wall, so rubber molds are not used: too bouncy. Producers demold immediately, so molds can be recycled very fast. Every cinder block is drypacked. You won't acidwash drypack, the texture is always great. Keeping the cast items wet is still vital.
Inclusions: Hey, we're artists, this is the fun part. Things can be stuck into the concrete from above (pennies in the sidewalk) or glued to the mold surface to be exposed later. Leaves and cloth can be trowelled into the surface. Mosaic tiles, marbles, whatever.
On the technical side, useful things like threaded anchors can be placed. Screw-eyes can later be screwed in, the piece can be erected, and the screw-eyes can be removed. Loops are sometimes tied to the rebar, and cut off and patched-over later. Gluing EPS foam inside the mold creates perfect channels and blockouts. Concrete castings are easily incorporated into brick walls if you created mechanical keys for mortar.
[Mortar is a mixture of cement and sand, with some clay added for plasticity. There are also proprietary "thinset" mortars which are extra sticky, used for setting tile, etc.]
>Brick mortar is what I mean here and it has lime as well. I don't want to get into that here though. Mortar colors are not necessarily cement colors.
Direct sculpting of concrete: When a drypacked block is fresh, it's easy to tear into with a chisel and a spoon. No money for stone? Make it this way. Using crushed limestone as your aggregate gives a wonderfully carvable mass. Just keep some of the dry mix aside for repairs later.
--
Dan <arch...@earthlink.net>
Be sure to see my website, http://www.archicast.com and follow the Aintree thumbnail.
[How about some sources of supplies?]
Nope. Everything should be findable locally--except colors, which come from whatsername.
Andrew Werby
Dan Spector
More notes
notes added
--
Dan <arch...@earthlink.net>
http://www.archicast.com
----------
In article <MQgS4.48418$k5.14...@news1.frmt1.sfba.home.com>, "Andrew Werby" <and...@computersculpture.com> wrote:
Dan Spector wrote in message <_93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net <mailto:93S4.45840$g4.12...@newsread2.prod.itd.earthlink.net> >...
Because of its low cost and outdoor endurance, concrete is an important casting medium to learn. Concrete can also be direct-sculpted, and we will touch on that later, but first let's look at casting.
[This can be the "Concrete-casting FAQ" and a subsequent one can cover direct sculpting.]
>whatever.
Concrete is defined as a conglomerate stone of portland cement and various grades of rock and sand, and water. Compressive strengths of 5000 to 8000 psi are now common, and can go much higher. It is an exotherm, meaning as it cures it produces heat, same as plaster and resin. Therefore adding heat accelerates the cure, and vice versa.
The size of the aggregate (rock products) is crucial in concrete batch design. Consider a bucket full of fist-sized rocks: the gaps between them are big. The points of contact, where stress is transferred through the mass, are tiny. So let's add enough gravel to fill the gap volume: now the stress can spread among hundreds of little trails through the bucket. A bigger stone is less likely to shatter from compression now that gravel surrounds it. Let's go further and add enough sand to fill the voids between the gravels. Now the bucket feels like a solid mass. And we haven't added the cement yet!
>Seems like this compacted mass shd be the basis for cement content.
Portland cement was invented by the Romans and reinvented by the English around 1850. It had a pleasant gray color which looked like the building stone quarried at Portland, England, hence the name. Basically, it's a fine powder of clays and limestone where all the molecular water has been driven out, same as the gypsum>plaster action. That cement powder seeks to combine with water. It will take it from the air's humidity or your skin, so be careful. Never touch cement. Do not let art teachers tell you to mix it by hand. It is a ferocious alkali and will burn the skin without your being aware until later. Cement comes in 94 lb. bags because that much equals a cubic foot. It is produced in white, grays, and buff.
[I've never seen buff cement. Does it have a special name? Who makes it? I have seen "Lumnite" (high alumina) cement, which is more refractory than Portland- I think they call it "ciment fondue" in Europe- do you know anything about that?]
>We used to could get buff, now it's special order here. This has nothing to do with alumina cement, which is available from refractory supply places.
For construction purposes, the best concrete is the strongest for the cheapest. Rock and sand are very cheap compared with cement. Concrete is proportioned by the cubic yard (27 cu. ft.) We talk about the number of sacks of cement per cu. yd.: 5 per is normal cheap concrete, and 6 per is good practice, what you'd ask the guys to pour your driveway. Cast stone, high-grade concrete where appearance is as important as strength, has around 8.
You may be thinking, if more is better, then pure cement is best. Wrong. Cement used "neat" has almost no strength, and shrinks a lot. Cast stone uses the cement's color as an element.
Back to batch design: Suppose we start with 6 buckets of rock, 3 buckets of gravel, and 2 buckets of sand (6:3:2) This is just a for-instance, not an approved formula. How much cement? 11 buckets of aggregate, divided by 27, equals .4, so if you want 6-bag concrete, multiply 6 by .4, equals 2.4 buckets of cement: 2 1/2. See? I don't care how big your bucket is, just keep using the same size! Weigh that cement before you add it, you'll need its weight.
> I am not satisfied with this although it's the way I was taught.
[For casting sculpture, you usually want to use the aforementioned 8-bag mix- I generally mix 4 buckets of aggregate to 1 cement to get pretty close. ]
Let's look at the size of the biggest rock you're using. Pouring a ballerina? Or a pedestal for her? (You'll do both soon enough!) The rule is, the rock's diameter =1/3 the width of the void. A 6" wall gets 2" rock; a 3/4" ankle gets 1/4" gravel. Using chunks of old concrete is approved.
You can work with sand mix, no rock, but it's weaker. I have put nicely colored sand mix on the surface of my mold, then filled it up with good ol' Sakrete, and been very pleased.
>I mixed a sandmix from yellow sand and white cement for the tone needed. This was patted in place in the mold and allowed to sit an hour. Sakrete was mixed normally and poured over it, and heavy mesh was embedded in it.
[We should talk about this face-coating technique in greater depth- it is very useful for getting good detail, and works with smaller batches than are necessary to fill a whole large mold.]
More on sand mixes later.
Water content is super-important to your concrete. The less water used, the stronger the concrete, but if it's too dry to flow, your casting will be awful. Using too much water leaves tiny voids all through the concrete, which later can absorb rain and shatter when it freezes. Basically, the water mixes with the cement: the rock products are bystanders here. The way it looks when a correct amount of water is used is like a thick soup, and the way waves go through the fluid is distinctive. Since sand is often damp or wet, the water added must be decreased to compensate.
The correct ratio is 100 cement to 40 water by weight.
[Superplasticizers reduce the amount of water needed for the concrete to flow, eliminating hydraulic leakage at mold seams, and water trails. We should talk about some of the other admixes as well.]
>My buddy Bob Ferguson used to be enthusiastic re superplaz but not any more.
I'll find out why not. And I'm not getting these people into fly ash or plastic fibers.
The way concrete is mixed is important as well. You mix the rock products dry with each other, then add the cement and mix, then the water. Mixing in a bucket or a big pan with hand tools simply cannot equal what a mixer does. But who would buy a mixer for infrequent use? You could rent a little one, but you'll get decent quality if you'll mix for a good long time. The hoe with a hole in the blade is meant for pan mixing. Small bucket batches can be mixed with a square trowel, or a paddle mixer on a half-inch drill.
A huge advantage of concrete over real stone is your ability to add rebar (reinforcing bar.)
It is considered fundamental to our modern world that steel and concrete have the same coefficient of expansion. Sound pompous? Bridges-buildings-roads-tunnels-dams: we'd miss them.
I'm saying that because steel and concrete expand and contract the same amount when the temperature changes, they mate perfectly.
The sculptor takes advantage of this by making rebar armatures and setting them in the mold before he pours the concrete or wiggling them into it afterward. (Remember the stone size?) Since you don't want the steel showing at the surface, you can get little nylon clip-on spacers called chairs at builder supply places. Heavy wire mesh is used for flatwork.
I won't deal with bending, cutting or welding rebar here, but I should say it's done every day, and rebar is commonly tied together with a soft iron wire (tiewire) you'll love anyway.
In the most crucial usage, rebar is coated with epoxy; even the cut ends are painted. That's the green color you've seen in bridge work.
Rebar is called by the number of eighths of an inch of its diameter; half-inch bar is #4.
Pouring the statue: Prewet the rubber mold for low surface tension (fewer bubbles.) FRP
[Fiber-Reinforced_ Plaster?
>Polyester]
and other hard molds, including plaster, are misted or wiped with form oil, a variant of diesel oil (Construction products dealer again.) Bang and shake the mold as you pour it. Tip it around to dislodge bubbles. Concrete precasters use a vibrating steel table to consolidate the concrete, but these are hard on molds. On-site casters use vibrators run by air, dipped down into the forms. Without this action your concrete will not attain full strength.
Water will slowly rise to the top. Leave it alone.
Concrete hardens in a few hours in the summer, and overnight otherwise. Working in freezing weather is bad practice. Adding salts to your water to lower the freezing point is questionable at best. Take off the mold only when the casting is really hard. Wet the surface then and often, for several days. It will keep strengthening. Its strength is commonly measured after 28 days. Pour a cylinder (this is Official stuff) when you pour a statue or footing if testing will be required.
Concrete broken open after years have passed shows minuscule rust on the steel. Good concrete breaks through, not around, its rock. If water does get to the rebar, it rusts and slowly explodes the concrete around it. So inspection and repair of your statuary is good practice.
If there were leaks in the mold, and colored cement was used, a strong band of the color will show at that place on the surface. White cement avoids this, but so does leak-proofing the mold with silicone caulk.
The cement will cover the whole cast surface. Many people dislike this look. Remove the cement with a brush if very fresh, or with muriatic acid in water any time later. This exposes the sand and some pebbles. The cement color is still there, but muted by the other colors. Speaking of colors, concrete is often tinted. Use only permanent colors made of crushed rock. [Metallic oxides] the yellow-orange-red-brown-black pigments are cheap and readily available, while green and blue are very costly. Choosing your sand and pebble colors for appearance graduates you into the 'cast stone' realm.
>Never, ever use salty beach sand or pebbles. Marble sand and dust, used by cultured marble producers, give beautiful results. White and black together -use Black Beauty sand, a sandblasting product- is very attractive.
[Can you recommend any particular types of aggregate, or caution us against any? Marble chips, for instance, substituted for sand, permit sanding and even polishing of the surface, for a terrazzo effect.]
>I'll never agree with omitting the sand or other "fines" in concrete.
If there are air holes, called 'bugholes,' you may fill them with a thick mix of the cement and fine sand, rubbed in wearing gloves. To add new concrete to old, paint the area with bonding agent, a white-glue relative, right before you place the new concrete.
Back to sand mix: Mixing sand mix with a small amount of water, just enough so a fistful holds its shape, enables you to drypack the mold. This means forcefully compacting the stuff against the mold wall, so rubber molds are not used: too bouncy. Producers demold immediately, so molds can be recycled very fast. Every cinder block is drypacked. You won't acidwash drypack, the texture is always great. Keeping the cast items wet is still vital.
Inclusions: Hey, we're artists, this is the fun part. Things can be stuck into the concrete from above (pennies in the sidewalk) or glued to the mold surface to be exposed later. Leaves and cloth can be trowelled into the surface. Mosaic tiles, marbles, whatever.
On the technical side, useful things like threaded anchors can be placed. Screw-eyes can later be screwed in, the piece can be erected, and the screw-eyes can be removed. Loops are sometimes tied to the rebar, and cut off and patched-over later. Gluing EPS foam inside the mold creates perfect channels and blockouts. Concrete castings are easily incorporated into brick walls if you create mechanical keys for mortar.
[Mortar is a mixture of cement and sand, with some clay added for plasticity. There are also proprietary "thinset" mortars which are extra sticky, used for setting tile, etc.]
>Brick mortar is what I mean here and it has lime as well. I don't want to get into that here though. btw: Mortar colors are not necessarily cement colors.
Direct sculpting of concrete: When a drypacked block is fresh, it's easy to tear into with a chisel and a spoon. No money for stone? Make it this way. Using crushed limestone as your aggregate gives a wonderfully carvable mass. Just keep some of the dry mix aside for repairs later.
Direct sculpting also includes the world of ferrocement, where a dense wire armature is packed with concrete, or sprayed with Gunite and floated and trowelled.
Battersby
> For construction purposes, the best concrete is the strongest for the
cheapest.
I cast without sand most of the time, and have for years. I'm not building
bridges. Direct modeling, and work produced with a profile blade, is
composed of cement, lime, sand. Working with cement and working with
"concrete" are two different animules. I agree with the majority of what you
have put in your "FAQ". But cement has a much broader spectrum of use than
just as an ingredient in concrete. Modeled in place, turned, cast using
various formulas, and combinations of these techniques. I've put some images
on a webpage at my site to illustrate (links below, four pages).
> Nope. Everything should be findable locally--except colors, which come
from
> whatsername
Blue Mountain Pigment 800- 581-1924. Ask for John These are high quality,
and I can have them shipped from Pennsylvania to my door cheaper than buying
locally.
http://www.plastermaster.com/pigment/index.htm
There are also bagged cement specialty products available that are super
strong (18,000 psi) and demold in less than twenty minutes. I really like
these, just add water. There is an example at the second link below.
If anyone has time, please go to these pages and give me some feedback on
loading time, image quality, fit of page on your monitor (hopefully no horiz
scrollbar when viewing at 800x600), etc. And/or the work itself.
http://www.battersbyornamental.com/pyramid.htm
http://www.battersbyornamental.com/battresidence.htm
http://www.battersbyornamental.com/battresidence2.htm
http://www.battersbyornamental.com/battresidence3.htm
Thanks,
Battersby.
Andrew Werby
Battersby wrote in message ...
>Dan Spector <arch...@earthlink.net> wrote
>> For construction purposes, the best concrete is the strongest for the
>cheapest.
[It is possible to get crushed marble or limestone with a range of particle
sizes, like 1/2" to fine, which makes a nice casting aggregate that can be
eroded smoothly. If you use sand, then the hard quartz grains pull out and
roll around, so you can't get it really smooth. The marble and limestone, on
the other hand, is about the same hardness as the cement, being more or less
the same thing, chemically. I knew a cast-stone sculptor who built his own
crushing machine- basically two steel plates at the bottom of a hopper, one
of which reciprocated like a book cover- he'd load it with marble or
soapstone scraps and the crushed rock came out the bottom. Good to have if
you're into this and don't have a chain-gang handy...]
>
>I cast without sand most of the time, and have for years. I'm not building
>bridges. Direct modeling, and work produced with a profile blade, is
>composed of cement, lime, sand. Working with cement and working with
>"concrete" are two different animules. I agree with the majority of what
you
>have put in your "FAQ". But cement has a much broader spectrum of use than
>just as an ingredient in concrete. Modeled in place, turned, cast using
>various formulas, and combinations of these techniques.
[For this FAQ, some of those various formulas would be great. Perhaps you'd
like to elaborate on the direct modeling and profile pulling for a direct
concrete FAQ?]
I've put some images
>on a webpage at my site to illustrate (links below, four pages).
>
>> Nope. Everything should be findable locally--except colors, which come
>from
>> whatsername
>
>Blue Mountain Pigment 800- 581-1924. Ask for John These are high quality,
>and I can have them shipped from Pennsylvania to my door cheaper than
buying
>locally.
>
>http://www.plastermaster.com/pigment/index.htm
>
>There are also bagged cement specialty products available that are super
>strong (18,000 psi) and demold in less than twenty minutes. I really like
>these, just add water. There is an example at the second link below.
[If you've got a supplier for this (preferably with a website, but not
neccessarily) it would be good to put in. I've used something called
"Quik-crete" with similar properties, but I'm not sure if it's universally
available. I think these are basically concrete with an accellerator- does
anybody know what it is?]
>
>If anyone has time, please go to these pages and give me some feedback on
>loading time, image quality, fit of page on your monitor (hopefully no
horiz
>scrollbar when viewing at 800x600), etc. And/or the work itself.
>
>http://www.battersbyornamental.com/pyramid.htm
>
>http://www.battersbyornamental.com/battresidence.htm
>
>http://www.battersbyornamental.com/battresidence2.htm
>
>http://www.battersbyornamental.com/battresidence3.htm
>
>Thanks,
>Battersby.
>--
>T. M. Battersby, stuccoist.
>http://www.battersbyornamental.com
>tbatt...@satx.rr.com
[The images look great, load fast (using a cable modem, anyway) - but I
couldn't see those pin-holes you were complaining about- maybe a close-up is
called for?]
Andrew Werby
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