Earth Molded Concrete Sculpture:
By Bo Atkinson
My attraction to sand molding is the versatility and speed for casting concrete. The requirements can be as little as raw ingredients, minimal tools and rudimentary textbook information. Earthen molds can really be of any sand or loam, mine were usually sandy , usually unsifted and pebbly. This report describes a simplified method of concrete casting for sculptural purposes. I used simple mixes, seen here. I used My method was developed both by the need for making a living and by a love for the flowing cementitious processes. The following methods grew gradually over the years (since the 1980's). Technological text books were studied and applied. Primitive arts were also considered and tried. Experiments tested new ideas. What emerged was a preference for sand molding of relatively thin shelled concrete products.
The bridge above is real with kids running over it during a birthday party.( It is a 20ft bridge arch of semi laminated Ferro cement, wheeled into place, one end on an axel. The ferrocement was used as a mold to fill with readymix, after placement.) Sand molding enhances unique versus mass stylized. A timeless appeal emerges versus factory dated commercialism. My family often explored interesting places like the then (1954), overgrown park called Bomarzo, pictures below.
My appreciation for timeless mysteriousness came from growing up as an American child in the ruins of Rome (Italy). While archeology puzzled over datum and lineage, gardeners and kids enjoyed the found shards for decoration. We saw some large, dateless highway stones, smoothly worn by chariots, just getting land filled one day! We brought them home for garden seats!
More pics on google images. Slaves are said to have carved the Bomarzo statuary from stone but this timeless style today is economically feasible by use of earth molding or sand molding methods.
My article on sand casting concrete sculpture follows below. I am very glad to hear inquiries about my availability to work creatively. I'm also glad to receive comments on my web pages, pro and con. It all keeps me focused on writing more. : Newer email: boa1 at pivot dot net (Fix punctuation)- - - Thanks Tel : 207 342 5796 . . . (Maine, USA) To build a symbol hire a sculptor .
Earthen and sand molds
date to ancient times and were adapted for ceramic and metal casting from readily
available soil and mineral. With modern industrialized mass production, these
methods were reasonably forgotten as civilization hungrily needed mass production.
As surpluses from mass production become commonplace, the demand for unique
art forms again arises. The principles described in this article could be adapted
at a small or large scale with little investment required. I try to write this
article to fit many interests, from beginner to widely experienced persons.
Please bear with it.
Click here to see how I formed a 33ft/ 10M dome building with innertubes!
My approach to sculpture production and sales was very low budget and also a one man operation. During the 80's and 90's, a reliable income was made selling sculptural concrete birdbaths, fountains, furniture and small structures. This occupation allowed some experimentation. Concrete durability was proof of the methods described. I also built some ferrocement, FC additions to my own house in the late 1970s.
(Click here to see my photocopy catalog used for many years).
(Click here to see my FC house addition work in the 1970's).
My earliest cement work used laboriously tied mesh in ferrocement (FC) . I also did furnace and foundation work with wooden forms and other mold materials. Casting cement in reusable molds began to feel creatively restrictive to me. Too much bother in clean up, to store and to maintain. Rather than repeated productions from the same identical form, the idea of sculpting recyclable forms began to appeal to me. The shapes could vary in countless ways which also appealed to buyers. Also the earthy texture on product surfaces appealed to me, since it is simple to do and also gives an antique quality agreeable with stone. Late in the 1970s, I found that simple birdbaths, taking few minutes to produce, could easily sell with a good profit. The maintenance of a molding bed was so simple: organizationally, aesthetically and ecologically. More time was available for true needs and less was required for totalitarian commercialism.
Slurry And Sand Trick
Sand casting concrete
plays on a very advantageous trick. It is really juggling with the three states
of matter: solids, liquids and gas. In brief, water is used to bond the sand
when building the sand mold. Water distributes sand particles into compact
which hold for molding purposes. Instead of laboriously prodding stiff cement
into air tight cavity molds, a very soupy mix can be poured into a breathable,
thirsty earth mold. A wet, soupy mix, (a slurry), easily pours into cavities
or over surfaces like bird bath molds. My birdbath molds are simple hand
sand mounds. Cement simply pours over and readily "skins" the sand
surface. Where cement touches the sand, the excess water quickly seeps into
the pores of the mold, (cement particles do not seep) . This in turn stiffens
the cement which is draining the excess water. Furthermore the excess water
aids the temporary sand-mold constitution.
Vertical sand mold walls are more difficult, but I have readily applied soupy cement slurries by flinging with large paint brushes. Spraying is the more technological tool for this, where bigger scales of operation are needed and where labor cost is higher than equipment. But large brushes and careful timing allow considerable production. (Once a slurry of cement plus sand does skin over the considerably vertical sand mold surface, thicker cement or concrete can be carefully flung or placed as a second coat). Back to the "trick". Plenty of extra water is added during slurry mixing which, by the way, eases mixing generally. Excess water in a mix is normally very bad practice. But pouring this soupy mix on to a ready sand mold is an exception to the rule. While the cement in contact with the sand mold de-waters, the cement not closely contacting the sand remains soupy. This fluid concrete/ cement is easily evened out, while the drained portion holds reasonably well to the mold.
Low Slump Ideal
A closer consideration reveals that a thin coat of low slump cement forms at the sand mold boundary. Low slump, (fresh and stiff), of course is the ideal cement. At the same time, fresh mud initially tends to remain wet at a layer above the low slump cement, (above the drained or de-watered cement). The excess slurry- wet cement is easily swept along by a rubber gloved hand. The excess water eventually needs to be removed either by the mentioned de-watering or evaporation, or by removing overly thick sections of fresh wet mix. Herein is the means to control thickness of a thin shell product. Countless factors effect this rule, like ambient temperature, humidity, sun or wind exposure, etc.. Suffice it to say that smoothing should not be prolonged nor should mud quality be misdiagnosed.
The normal tendency of cement thixotropy or gelling must not be confused with the desirable low slump ideal. My method is to soon begin patting and smoothing the fresh surfaces, all over the fresh pour. Aggregate from 50% to 80% of the thickness of the desired shell thickness is a great help in sensing and controlling the finished thickness. (Relatively large aggregate also conserves cement for cost savings and reduces environmental impact). When wet, thickness sized aggregates "touch bottom" and can be sensed as a gauge at all points along a curved surface. A combination of patting and smoothing quickly establishes this. Patting is here the equivalent to screeding, tamping and floating used in regular concrete work. Where wet fines quickly work up to the surface, the patting is gaining.The better the compaction, the denser and stronger the result will be.
Well graded mixes seem to de-water even when larger aggregate is used. (Well graded mixes generally mean that aggregate particles of all sizes are included, from large to very fine). A simple slump test can be made by taking back some concrete freshly from the mold and packing it into a mud ball. Toss the ball up in the air, just slightly and catch again. Repeat a few times. If the ball can hold it's shape well, you have an acceptably moderate slump mix. If the ball too easily crumbles apart, the mix is too dry for sculpting. This is an old method for testing mixes generally. Eventually, conditions are visually apparent and easy to control. Ideal water content would tend to be near crumbly, but that condition is barely workable. This is where sand molds fluidly shape the finish while avoiding uncooperative crumbling.
Actually, the slurry application is not the only sand molding issue. It is the easiest for surface forming. I also apply all ranges from wet to near low slump mixes when in a hurry to use up surplus mix from more important work. Instead of wasting surplus, I quick mold birdbaths. The lower the slump, the greater the attention needed to avoid altering a sand mold surface. Spreading stiffer mix across the sand may drag sand and leave texture alterations in the resulting surface... Not to worry about bargain birdbaths.
Exception to exception..... If your mold is all pebble or with large stones intended to remain on the finished surface, then the lower slump, stiffer mix is essential. Slurries de-water less on pebbly sand, and not at all on stony molds. A quick correction for a pour not de-watering quickly enough is to cover with dry sand. This absorbs the excess water upwards. Stony molds make appealing bird baths, especially when you have nice stones.
Experimentation and practice with birdbaths and basins is a invaluable start. You may find that crudeness of your early work has an appeal for garden ornamentation. Eventually, a serious user should also consult standard concrete text books for a wider background in concrete practice.
For those tropical readers who may not know, ice can break enclosures like bottles or engine blocks. At a temperature just below freezing, Ice goes through a volume expansion. This expansion doesn't necessarily bother gently sloped basins. But extremely porous cement items, (soaking wet), can become pulverized, through a freezing winter exposure. The exception here has to do with the size and shape of the pores. When the pores are microscopically small enough, surprisingly enough, trapped freezing water cannot perform this expansion feat. This was interestingly discovered in the sidewalks of New York City, where animal fats were used to lubricate the concrete mixers, last century. The small amounts of fat mixed in, regulated the pore sizes and tended to reduce capillaries. This lead to development of air entrained cement modifiers to control pore qualities and to better resist freezing damage and also to restrict penetration of other aggressive chemical agents.
I kept some simple, curved shaped unreinforced birdbaths outdoors through many cold winters to observe for possible freeze damage. These were plain cement, not modified. None of these ice filled baths suffered frost damage and none of these had reinforcements. My rule of thumb is to reinforce basins over 2ft in diameter with one ring, near the lip. Someone pushed one of my 5 ft diameter basins which fell 4ft and it only chipped in one small place. The only reinforcement was the one or two wire rings. For more complex, and larger shapes, the use of additional rings appealed to me. >>>>Click here to see my current research with rings as a primary reinforcement element.
Low slump mixes with
the largest possible aggregate, well graded down to fine aggregates, shrink
the least during or after curing. If a relatively wet mix is left to cure without
de-watering, compacting or patting, then increased shrinkage of the wet areas
will occur during the cure, (hardening of cement). However, if a simple birdbath
is uniformly constituted, (same wetness throughout), and if the shrinkage therefore
occurs uniformly, then chances are, all is well. The main fault with this small
scale birdbath shrinkage is increased porosity in the product. Increased porosity
suggests a weaker product. But porosity in stone can optionally be admired from
non engineering angles, a broad ranging aesthetic subject in itself. Porous
stone has some practicality , as for bees and pollinators of the garden, (it
better suits their water drinking habits). Another caution is that reinforcements
would in effect be compressed or reverse tensioned, a worse than null condition.
Here cracking could be caused by hampered shrinkage.
An important note for larger or more complex work is that shrinkage usually manifests as cracks, along the weakest sections. One engineering ideal plans larger concrete pieces with "expansion joints". These joints control where the separation of the parts should occur due to minute shrinking. Shrinkage is an engineering issue even with low slump mixes, for large work. In a compact birdbath, this is of little concern. Chances are it could be of little concern in large structure if provision was made for slippage of the receding edges. In this instance, reinforcement would want to be of a post tensioning variety, (non interfering during curing and tightened after curing). In FC, shrinkage cracks can be so widely distributed as to be invisible. Highly distributed reinforcement can assist this distribution of shrinkage cracks converting them into distributed pores.
How much force should one use in this work. The proper degree of force used must be learned by experience. Never so much force during cementing as to deform the sand mold beneath, and always enough to even out fresh pours. The cement should not be applied in gelled- thixotropic state. This quality of cement can not be emphasized enough, and is the reason masons often agitate mixes during application. Most graphic is to watch how quickly a plain cement slurry can gel if left to stand. This has nothing to do with cement hardening or curing.... The proper force to apply during cement molding is learned by experience of sand mold building. Here again, mold building uses patting and smoothing. The larger the work piece, the heavier the work load will feel.
Methods will vary a great deal depending on project sizes. Bird baths, basins, stone furniture, garden bridges, etc., are all easily formed predominately in one sided molds with reasonable slopes for pouring concrete. Even house parts can be sand bed cast and later tilted or hoisted into position.
In larger sizes of work, it becomes helpful to use shovels and feet in stead of hands. The (shoed) foot is an excellent power tool. Through practice, the foot can improvise much of the hand technique described above. For reasonably sized basins and garden pools, the foot does excellent work. During hot weather, freshly poured concrete can begin setting, (curing), fairly soon. Foot and shovel can compact and smooth the pour quickly. By the time a person's limit is poured warm and smoothed, fresh concrete can become hard enough to stand on without deforming it. This refers to earth supported flat work. The texture may be rough and may be acceptable as is, however thin coats with finer mixes can also provide smoother finish. For cooler temperature pours, short boards or snow shoes can be used to move around slowly curing surfaces for smoothing work.
Joinery And Edges
Joining fresh cement with older cement deserves consideration, even on a sand mold. Poured slurry readily de-waters, as noted. On a large mold, one might not be able to keep all edges of the pour moving simultaneously. Keeping all edges of the pour moving nearly simultaneously would produce an excellent finished surface. Where an edge of the pour halts for just a while it de-waters and gels. Later advancing of that edge needs caution. The leading edge of a slurry application tends to bulge so that the actual contact between cement and sand is slightly cantilevered or hidden. If this is neglected, a slight crevice might appear in the finished work. But extra wet slurry applied along this edge can reach the true leading edge and fill the void. It is also helpful to stroke the edge area after this to leave no crevice in the finished work.
The bulging effect just mentioned can be profited from in at least two ways. The popular use is to produce the finished edge of your product quickly. With practice, the bulge of the leading pour edge can be pushed to the desired location of the finished product edge. This is a means to produce wavy, usually non-symmetrical, unique edges. The looks can be natural as in life forms or primitive as in rough hewn. The main point is that good control of the edge work can yield a smooth edge with little effort. The second way to profit from the bulging and crevice potential could be as follows. A very patient artist could develop a style to use those crevices like line drawings. The finished work would have the crevices all incorporate as a line drawn image. For that matter the piece need not even be solid like a basin. It could instead be like a fluid carving, essentially extruded like pastry toppings. This of course would require experimentation and personal styling.
Laminating Cement Layers
Laminated layers are sometimes needed. The need can be as odd as a change of mind as to thickness or as a well planned method for adding mesh layers. The need can also be for a finish coat. In all cases it is desirable to achieve a solid product with the least or with no voids trapped inside. Generally, it would seem desirable to apply additional layers the sooner the better. But mechanically, some form work is too fragile for immediate laminating. Among the fragile types are large vertical sand molds. Laminating layers suggests two approaches. One is by spraying the other by hand/ trowel. The spray offers an interesting principle of small blobs of cement, raining in, on to a surface with smaller air or voids between the applied blob and surface. By contrast, the hand or trowel size of cement blob can trap larger air bubbles or voids. These trapped voids are unintentional. The sprayer looses advantage on flat, horizontal work since there is always rebound, (courser particles bouncing off). This rebound falls back down without the velocity required for compaction. Whereas vertical spraying can reduce void forming, horizontal spraying canactually trap more voids. As with the hand method, this can be remedied. The comparative advantages are controversial, having to do with budgets for equipment and skills of operators. Most importantly, the scale or size of a job and the required speed of the work affects the choice of method used. Mechanisation of labor speeds it up and builds larger projects, but the smaller more gradual project has advantages of achieving the impossible or serendipity.
If smaller aggregate are used in two or more layers, then low slump, stiffer mixes are preferable to start with. Avoidance of trapping air or excess moisture between the two layers are both critical. Spraying or shooting cement avoids some fuss, but does leave rebound to waste or to deal with. Hand or trowel applications need to "scrub" or work the cement against the previous layer to remove these voids. Application is easiest to a fresh layer since the fines of both may mingle and the larger particles may penetrate the boundary. Application to a pre-hardened layer, will depend upon the successful placements of fines to completely cover the hard surface. This is difficult with stiff, harsh, low slump mixes, therefore it may be advisable to thinly coat the hardened surface with a slurry of cement and have it just barely moist when the stiffer mix is applied. Experience is the best understanding. To build confidence in your work, it is helpful to produce very thin wafers or cookies with your planned mix. The shape is not important, I just mold them by hand and plastic wrap or bury in earth to hold moisture. Curing can be speeded with gentle warmth, providing that drying is prevented. After curing the wafers are broken with your fingers or tool if needed. Then closely observe, with lens to search for voids along the fracture. Voids are either from trapped air or excess water. Likewise, try laminating cookies, using the exact methods you plan for your critical application, including the cure scheme. After curing and breaking, the laminated test cookies should reveal the quality of adhesion and voids between layers.
Wet Mix Considerations
For lack of good metering equipment, I generally mixed by feel, not by weight or exactitude. My disadvantages should encourage a person to go ahead and start sand casting without too much worry about exact mixes or excessive details. The hands on experience is the most important thing. For sandy mixes- 2 to 3 sand to 1 cement by volume is a reasonable mix to start learning with. Precise mixes are intended for larger scale efficiency, not a hindrance to beginning the art. (Use rubber gloves. I found all types useful, depending on circumstances. For general Sand casting the thin latex type allow the sense of touch which is important. I like to keep old, worn latex gloves over a solid inner glove. This conserves on glove consumption.) 2008 page on Bo's cement mixes.
While I have tried and enjoyed many kinds of cement, I have always returned to the most commonly available and economical cement: Portland Type I . This or a near equivalent is sold almost everywhere. All the standard information can be reviewed in text books. I 'll add only this.... There are numerous aggregate variations, additives and treatments for many diverse effects. To seriously explore this is fun in an alchemical sense. However for sand casting, (as described here), chemical additives can leach into the sand and waste. For other modes of cement sculpture, additives certainly make good sense. In most cases, I would prefer mixing in the largest aggregate which can be accommodated, with all gradations of size down to the very smallest. One of the admixes of interest is in fact fine particles smaller than cement particles, as small as a 100 X smaller. These add density and true bonding, but might not de-water as fast in the sand and slurry method. Some of theses are byproducts of metal industry, some are naturally volcanic and some are specifically manufactured. I found them a bit pricey due to shipping. Some areas may have surpluses at low cost. They make plenty of good sense for longevity and improved strength of your work. Key words to search for these: microsilica, silica fume, pozzolans and the newest similar material: metakaolin. These super fines should not be breathed, although the microsilica I bought was stabilized in soluble granules and had no dust at all. Note precautions.
Since I often used glacial till sand and gravel, (dug from a pit), I generally chose the best gradations immediately available. These have not been ideal gradations by any means and some soft stone was in the till. Improved proportioning reduces the cement paste required. Searching for better aggregate qualities is worthwhile. I have always purchased truck loads and used the pile as extra molding space. One thing to consider with sand piles....Very near trees might have roots underneath, which will try to surface into your pile. Apparently tree roots need well aerated soil. Your sand pile tends to smother them. So if your sand pile lasts more than a year or so, you may find there roots in the sand. Also outside sand piles tend to collect wind blown leaves, blossoms, etc. I needed to keep sand piles swept smooth to avoid embedding debris.
Storage And Lumpy Cement
The beginner should be cautioned that cement needs very dry storage conditions. There often is a discount in bulk buying, challenged by potential lumping of cement over time. In moderate humidity stored cement can take on moisture and start to get slightly lumpy, even during one month. My bet is that the ultimate compressive strength lost in lumpy bag cement is equal to the force needed to re-pulverize it. (In other words very soft lumps equal virtually no loss of cement strength). Also, bags at the bottom of a stack can feel very hard due only to storage not necessarily moisture. When lumps are getting a little harder to pulverize but still dissolve quickly in water, I am inclined to make the cement slightly richer, almost always by "feel" of all the issues at the moment. Bagged cement usually begins to develop lumps near the bag surface. In some cases, hard lumps can be removed and still plenty of good soft cement remains. For the seriously continuing cement user, I recommend keeping bags in a shed or well drained sunny area. Generally I like keeping one old bag around as descant for the new bags, especially when waves of high humidity happen by. My old bag is usually on top of two pallets with plastic sheet below the bag. An older plastic sheet below the pallet will keep weeds from growing up into the stack. The newer bags can pile on top of the descant bag(s). Next a much larger plastic sheet should cover bags nearly to ground. For outdoors a tarp as well is best, weighted or tied down against wind. I would prefer bulk cement if a steady flow of work seemed possible. I hate wasting the paper bags, which are plastic laminated and can't be easily recycled. Possibly water soaking the bags to extract pulp would interest a determined recycler? I've used some old, harder lumpy cement over the years, generally for less critical masonry work in the garden. Even bags that were years old, and very granular, not easily dissolving, did eventually harden, (taking much longer even for initial hardening). Lumps bigger than eggs I generally throw in to my fish pond to compensate for acid rain. Sometimes old lumps are great for stabilizing muddy work areas or driveways. Here is a 2007 page devoted to this subject.
Larger shell structures may call for tensile reinforcement like Ferrocement. FC has long been an experimenter's art form which only very recently is becoming code regulated. The essential idea is to finely distribute steel into a thin shell of concrete. FC has only rarely been used in sand molds, having most often been adapted to other mold systems and non- mold methods as well. While I explored many variations of these I presently focus on minimal steel use in contoured thin shells. I tend to prefer wire and smooth bar over other modes of tensile reinforcement. Overlapping meshes or overlapping hooked wires or hooked bars is often entirely adequate for tensile connecting, (in my mind at least). I never sew together meshes any more. To hold meshes down, I use weights like a hand fulls of cement or concrete. Stones serve well on flat work and sticks can hold vertical mesh. Often enough, once the cement has been applied over the whole mesh, it imparts sufficient stickiness to hold the mesh or small bars. The property of thixotropy or tendency of cement to gel soon after placement also increases the tack. An occasional stiff wire-- pin, clamp, clip, crimp, tie or such is also handy. Welding can also be useful with critical bar reinforcement but so much can be done without welding. Generally, time is teaching me that excess steel is not really effective where much less steel can often serve equally well. I build bear sized sculpture by first hand forming a sand plus water core. Then cement is applied to the sand core as a sculpted shell. Steel reinforcement is embedded between according to strength requirements. Short chain pieces can be anchored in for later hoisting. The point here is that endless improvisation is possible with rough and ready technique .Countless suppliers offer a variety of fibres for mixing with cement. I've lost interest in these, primarily due to clean up prolongation and potential interferences with quicker finishing. Also, random fibres are less efficient than anchored bars and wires, my favorite reinforcement. Where mesh is desired, this can be applied just after the first mold skinning described before. My objections to mesh use include..... The difficulty of fitting curves, since meshes can really bend only in one direction. Deforming mesh spherically looses intended, manufactured mesh integrity and is exceedingly labor intensive. Cut mesh punctures gloves easily, wasting gloves prematurely. I prefer using hand woven wire for unique art. Smooth steel is available in hard or soft and in bar stock or ordinary wire. The cold rolled bar from steel yards is good stuff, a little more expensive than soft wire, but rated stronger. I really like the straight long, 1/8 inch, (3 mm) cold rolled bar. These can bend and weave easily by hand, (with a "J" or "U" shaped hook, at ends). You can space them apart and run them parallel or singly. In some cases, weaving makes sense, just a little like an under woven baskets. It's not so hard to learn intuitively. You can poke straight ends in the sand mold for later bending and anchoring in other work.
Preference For Smooth Bars Or Wires
My argument against rebar is that I prefer putting all the steel mass into tensile function, none into intermediate anchoring. Also you can't find rebar in wire sizes. In my mind a rough coat of galvanizing or even surface rust offer plenty of traction. Rebar is essentially steel bar with stamped surface ridges or raised treads. The standard idea has been that these offer better traction between steel and stone. Bent "J" hooks at a bar end seem adequate for anchoring the bar at both ends in my thinking. Additional treads offer little additional function. One effect seems to me equivalent to wedges. Think of the quarry method for splitting stone... You drive a bunch of tiny wedges in a line, that's it. You crack large stone with aligned wedges. Rebar tread stampings seemly introduce a comparable defect into the cement, more so than the smooth rod or wire would. Why add potential defects at all? Smooth rod or wire presents less wedging factor to cement, when it endures heavy dynamic stress. Why not present gentler contact between stone and steel? Why add the complication? Why not put your steel money where the tension is? Why invest steel money in compressive tasks which raised tread stampings suggest? Slipping of steel is not a real issue in thin shell cement. There is no way you can pull well anchored steel out of stone, without breaking something first.
Cell Like Reinforcement
Reinforcement for thin
shell concrete products is not necessarily needed. Curving structure better
distributes stress whereas thinly flat structure tends to concentrate stress.
Flattened structure calls for more reinforcement to handle stress. As the curvature
gets larger in ratio to the shell thickness, a thin shell surface becomes comparatively
flatter. Therefore less stress is distributed. Therefore deformation is less
resisted. Stress in this general sense implies resistance to deformation. Our
thin shells of concrete have compressive strength dependent on solidity of stone.
Stone has very limited tensile strength, or resistance to stretching or flexing.
Stone depends upon a buttress effect of neighboring stone particles. When stone
particles and molecules are packed more tightly, the compressive strength is
higher. Increasing voids between particles or molecules reduces compressive
strength. Nevertheless, maximum strength is not always needed.
Like the buttress effect, voids enclosed by cellular walls can effectively extend useful strength. Less material can fill more space. Foams are an example. Foamed stone like pumice is an example. The principle applies to all sorts and sizes of voids. Extruded, hollow bricks or blocks have long employed the essential idea. I would point to truss structures as a similar principal. Three dimensional trusses, like the octet truss, compare a little closer to the structure of pumice. Life forms employ cellular structure most elegantly.
Efficient building takes advantage of cellular concepts. With sand molds and artfulness, larger cellular structure can easily be made. The distribution of stress can be gainfully planned. The natural curves which life forms model for us can be copied to advantage. For example, the tapering curves where tree roots attach to trunk. These are essentially tapered curves which are strong models for floor joists and roof rafters. All applications of ribbing in a built structure can benefit from that tapering and curved joinery. Another useful model from nature is honey comb to strengthen near flat surfaces . Ribbing or studding integrally with wall or roof can be easy and efficient through sand molding. Here is a link to some further developments with cell like reinforcement.
Sand as the main mold
material was described because it is generally available and easily gives the
results described. Variations with other earth types and materials will vary
the experience. Spaying water on a sand mold tends to wash the fines behind
the larger surface particles. These larger sand particles will stick to the
slurry and give a sandy texture to the finish. This tends to hide unintended
irregularities as sand textured paint does. Instead of sand usage, finer soils
can be used as mold surfaces, to yield smoother finish. Clay can be sprayed
onto a sand mold and the clay can be elaborately smoothed before pouring or
spraying the cement. I rarely spent that much time with molds, wanting to make
a good wage with simpler production. At the other extreme, gravel offers pebble
coated finishes. This is easily done by having a preferably graded gravel pile.
Molds are simply hollowed out and poured. If pebbles are too unstable, a few
hand fulls of sand can stabilize a small mold. The the sand will wash behind
the pebbles easily. The cement can then properly adhere directly to pebbles
without more voids. Still another approach would coat a sand mold with selective
pebbles and pour.
After the cement / concrete has been formed, compacted and dewatered in the mold, the exposed surface can be finished. Often that condition has been adequate, in my mind, with no further treatment. One very nice surface treatment is to simply spray lightly, misting is better yet. The fines will quickly wash away from the surface only. Now larger aggregate is exposed. Curved work will always drain these fines . The resulting pool of fines and water can then be vacuumed out, or perchance they will drain to mold edge. One can leave them or scrape them away, (the water having neatly drained into sand). Another popular wall finish half mists the fresh cement surface to leave fine patterns of exposed aggregate outlined by wavelets of cement fines. In this case, barely enough mist is applied to wash a small area, and the run off is insufficient to stream. Still another very popular finish is fossilized leaves. Beautiful patterns are obtained from leaves of almost any plant or weed. Leaves can be carefully placed on sand molds and poured with stiffer but not too course a mix, (and preferably no excess water in the mix, except to compensate for evaporation during pour). Concrete with larger aggregate can be applied after coating the leaf surface. The leaves produce fossil effects which are prized by customers and clients.I also sold many basins which utilized pretty patterns found inside and outside select tires. Additionally, tire inner tubes offered a completely new dimension for other molding effects. For non-earth, nonabsorbent molding it is very important to use low water mixes since excess water will tend to group and become trapped at the nonabsorbent surface... This will appear as voids on your molded surface. Visible voids like this suggest that your thicker sections might also have voids and this affects the overall strength. I combined sand molding with inner tube and tire molding, so I applied wet mix to sand and drier mix to the rubber, within the same product. The rubber needs mold release. Economical releases include diluted soap, wax or grease. Solution was painted on and allowed to dry. Application of cement could not allow much disturbance of the rubber surface to avoid allowing these releases to absorb into cement. Spraying on a gel coat of cement before cement application, gives the smoothest result, but I rarely fussed that much.
Lastly I mention a form of surface smoothing which can be carried to the extent of a polished effect. It relates directly to steel troweling to a smooth finish, (check regular concrete texts). Curved sculptural surfaces can likewise be progressively smoothed as the cement proceeds through stiffening. This pre-cured smoothing can be done in phases. Small foam rubber rollers work reasonably well on exposed frershly placed, wet surfaces. Rubbing with gloved hand is also effective even through stiffening phases. Rotary polishing tools with wooly pads are good for stiffer phases. More cement paste can be added if needed later during the pre-cured period. Still another tool for this polishing might be "fiber blasting", a tech I have not explored much. Material such as fine saw dust or wood flower can be blown through sand blasting equipment to polish tenderly firm cement surfaces which are intricate and otherwise difficult to polish. Timing and temperature are very critical with any finishing. Smoothing and polishing is much more labor and equipment intensive but worth it for some people. Sanding with wood finishing tools is also a workable method, for example when the hardening gets ahead of us. I tended to focus on the simpler methods. I prefer an aged styling which can not readily be placed. It therefore mystifies and suggestively points to imagination, an area I enjoy.Color Pigments are easy enhancements which can be used mixed as slurry coatings, or call it "cement painting" . These tend to produce earth tone colors. These are sometimes embedded throughout the entire cement body. I have preferred pure, chemically stable oxide, powder pigments over other preparations since this gives me the alchemical advantage. Pigments should be stable, minerals with no adverse chemical or fading reaction. There are countless premixed products which include cement enhancements such as water reducers, super-plasticisers, self leveling agents and, extra hard- solid fines. One should always test new concoctions to access performance. Color matching is notoriously tricky from batch to batch, so be forewarned. I haven't minded color variations which for some of us, adds to the timeless quality of stone work. My pure pigment source was the only pure pigment source for my region, Davis Colors, 1-800-3469433. They might eventually find a distributor to mailorder small orders, if enough small buyers encourage them. They will try to help you locate suppliers, but you need to be very persistent if you want to regularly buy small quantities of a wide array of color. In the mean time they have $300 minimum orders. You can try a nice range of generally earth tone colors. Your other handy source is your local masonry/ building suppliers who most often carry only red and black pigments. I have mixed various pure pigments with no problem.
Stains and glazes of countless concoctions also add possibility. This is more of an alchemical area I am reluctant to recommend. The very easiest thing to try for brighter than earth tone colors, is to use what might be termed high solids paints. A rough and ready formula could simply be white portland cement, (more expensive), combined with colorants, and perhaps a super high range water reducer to help achieve a better surface. Selective pigments are very durable, from companies like Davis. Additions like very fine but hard sands will reduce shrinkage and boost strength. Greater care need be taken for moist cures, not too hot early in the cure. Companies will send sometimes samples, if you sound serious enough. Still another alternative admix, (don't combine with other chemical additives), is latex or acrylic. These can be mixed in a slurry with color and fine sand, then painted over surfaces. These plastic admixes want only one day near moist cure and then should dry for a week to cure properly.
A wild card colorant whose chemical composition is not always certain is your local paint store paint colorants. Usually an attendant won't mind giving you a little in a small container, dispensed from their machine. You should provide a suitable small liquid container. An exciting range of colors can be easily had this way, perhaps at modest cost. However, even with white Portland cement, the color will fade considerably, since too much colorant will weaken the result. I would be sure to ask for colorants used in water base exterior application paints, (though one colorant may be used for both). While the chemistry may be unpredictable, I think this approach is better than using acid etching stains. Test with small wafers before doing much more. For a glossier effect, you can buy the paint store stain base which is clear resin like liquid and add the colorants yourself. Painting with "high solids concrete sealers" is a quick way to add gloss and perhaps even to seal moisture in for better cures. There is a very wide variety available. I have tried just plain linseed oil with pigment which proved durable for years outside, on a protected wall surface. (One coat of linseed is less glossy). An interesting, truer stone glaze idea for that translucent, glossy, porcelain effect might be Porcelite TM. (http://www.porcelite.com). It is sold as a castable but I bet you can paint it. It is said to work with glass beads and colored sands and sounds like a nearly translucent, refined stone effect. It's expensive and I 'm not likely to use it unless that penthouse job comes in. ^_^ Back to sand casting....
Curing Cement Sculpture
For specialized cement materials I usually review instructions for each new cement product, at least when things
are critical. From there I plan my timing. Portland Type I is said to cure for
30 days, but can be quickened with application of heat. Too often in the contracting
world I have observed that concrete floors are cured susceptible to premature
drying outside. Dry, hot and windy weather can weaken strength results considerably.
So coating fresh cement with sprayed on sealers is sometimes critical to getting
good results. Thin shell concrete is more needy of all the strength available
from it's cement, which can only be maximized with moisture conserving cure
conditions. Time and temperature are the primary factors in how long it takes
cement to cure. The more of either, the more the cure, which theoretically tapers
off with infinitesimally additional strength gain for 50 or more years.
Sand or loam can also advantageously cover the cement sculpture to conserve moisture during cure. After pulling small sculpture out of the sand mold, I like to nest my smaller products in a stack. I do not brush the surfaces yet (and also leave any leaves for fossilization intact). These provide extra holding power for moisture during the cure. I fill any freshly molded basins with a little water. Then I wrap the stack with plastic sheet. I simply overlap the plastic and clamp it shut, in close pin fashion. When needed I can add water through the top from a garden hose spray. This keeps humidity high inside. The sun will then heat up the products for an accelerated finish cure. Where the entire mass can heat up almost too hot to touch, I consider a day or two sufficient cure finish time. (More days if lower temperature is reached). Larger pieces are best left in the mold and covered. Where plastic sheet is not desired, a layer of earth is a great substitute . More watering and time will be needed to compensate for evaporation. Combining damp, sandy, papery or cloth like material with plastic vapor barrier is even better.Cure Testing
Testing is usually not necessary especially with ones accumulated experience. Beginners would do well to experiment with tests, just to gain experience. Experienced builders benefit from casual testing just to monitor curing status of their work. For rushed jobs, new materials, or for critical work I adopted a simple and reliable test method. I use small leftovers of mix which cured along side with my work for testing the cure status. My primitive style of testing is to break and observe which aggregate have split along with the cement. If hard stones have split, the sample passes. If they don't split, the sample proves uncured or weak. (Where they don't split, you see aggregates have pulled out of one side). My glacial till for aggregate has many stone types, some stones are stronger than others. Glacial till can include some very weak stone. My main use for testing is usually just to assure myself that the piece has cured and that moist curing time has been sufficient. When strong aggregate split in your test piece, then you know your concrete has reached that level of strength already.
Fireplace And Oven
Refractory materials are preferred for high temperature products because they withstand the entire range of temperature up to candescent heat or near their fusing point. Portland cement withstands some heat, but use of silica based sands and metal reinforcement introduces incompatible expansion rates. The steel expands more than the cement at temperatures just below candescent heat and tends to expand. This can stress the cement. Using fire brick sand or compatible aggregate all improve heat resistance somewhat. Near 50/50 metal alloys of and nickle and iron , some with doses of other metals reduce the thermal expansion remarkably. These alloys can be suited for stone or glass, to avoid disproportionate expansion, where it matters, (as supporting light filaments). But Portland cement does depend on it's hydraulic bond (or water) to remain solid. Loosing water at high temperatures and over time will weaken hydraulic cements. Refractory bonded cements are available at higher cost. Adaptable to the sand and slurry trick are fireclay mixes and hydraulic, calcium aluminate cement. Hydraulic cement brands I am familiar with are Fondu and Secar for refractory temperature service . When mixed with suitable aggregate like firebrick sand or corundum sand, these works as a hydraulic cement. If later, slowly fired to respective fusion points these will fuse as a ceramic (above candescent heat, check directions). The main trouble with cements like these is that you cannot fire short of the fusion temperature without loosing your hydraulic bond so you almost need a ceramic kiln for elevated firing to regain good strength. This proved too costly for my budget. For low cost casting, suitable fireclay mixes (clay plus fire brick-sand or grog), without hydraulic cements, are probably the best solution. Applying fireclay mixes to sand molds is somewhat a hybrid version of ramming. As the mix dewaters on the sand, the patting acts much like ramming, which itself is a wide www searchable building method. (Earth ramming for houses and refractory ramming mixes for high temperature work). The sand mold allows easy construction of curved ovens. Commercial brands of refractory clay mixes may give great results. Years ago I used products from: A P Green Refractories Inc , Green Blvd, Mexico, MO 65265 ... They produced some especially low cost mixes which seemed to be fire clays mixed with firebrick aggregate. One of these in particular seemed not to suffer at all from incomplete ceramic firing. Today with all the corporate take-overs, it might be difficult to find these older mixes. Fireplaces and cooking ovens rarely if ever reach candescent, industrial levels of heat, so low cost mixes are perhaps worthy to explore. I explored a number of higher costing industrial mixes and failed to invest in proper firing to attain good fusion. Portland cement makes good chimneys, although ceramic liners are important for heavily used chimneys, (cold climate, wood fire use). For light barbecue use, liners may not be wanted, although inordinate grease buildup could cause a damaging chimney fire.
The simple earth molding method described here provides a healthy mix of physical exercise and probably just the right amount of employment if you need income. Sculptural work can exercise the human body in a balanced way, so that we don't over repeat the same identical actions too often. I love opportunity to work with flowing and rounded shapes. The least lovable parts for me were mixing cement and selling products. Nevertheless, considering that I work and live far from good sales locations, the business was still very favorable. Mixing dusty cement outdoors or with exhaust fan allows some avoidance of the dust since the dust cloud can be seen and avoided. But someday, getting a bin system or half ton bag system with mechanized metering of mixes, would sure be nice. I especially dislike the cement dust. It should not be so hard for a manufacturer to supply low cost, reliable metering equipment, just right for the small user. In time I might improvise my own. This would boost my production way up, and my spirit also ;)I love simple sand molds because little time and material is wasted while the environment and worker are less taxed. The sand holds moisture for desirable cement curing. Mold and product are separated easily and brushed off later on. Sand is recycled anew, just add water. This system adapts both indoors and outside, in a tent, or in the sun. All ambient conditions have advantages and cautions to learn about.
Bo's 2008 page on "sitting planters". Suggested as a way to garden and of special interest to retirees and sustainable lifestyle enhancement.
Some specific information on my sand casting technique was published in a 2007 book. This 2007 book documents some sand casting and ring reinforcement methods. Many high resolution color photos show multiple steps of forming 5 easy to make pieces-- Easy examples, which call for a reader's personal, imaginative input. It is close up views of the process. The publisher did not hire me to develop imaginative input. Rather what could be rushed together over 2 days, without prior development. The rough texture product fits naturalized, somewhat wild, gardening styles, not the highly trimmed, polished-manufactured stylization.
Pages 8 & 9 The mix formulae terms are technically jumbled, but the context is forgiving. (The mix recipe given is not what i used that say. It might have been confused with a discussion about "permable concrete" or "porous concrete" or "pervious concrete", where andy aggregates are omitted). I am not a purest with general mixes and most often use approximate measures like handfuls, when mixing small batches for sculpture. The publisher hired me to demonstrate sand casting and i thankfully accepted the job which lasted several days. I perhaps should have methodically presented an official mix recipe list but there are variables and specific contexts worthy of another chapter. Cement-rich mixes are appropriate as long as the size of the casting is not too large and as long as the shrinkage is proportionate in all parts .Shrinkage due to rich mixes is almost imperceptible in small pieces of work. Heavy steel reinforcement in larger pieces can unhappily resist cement shrinkage, causing cracks. My wire reinforcement rings can tolerate shrinkage, much better than linear reinforcement geometries. Large, contiguous, concrete floor shrinkage is very noticeable. Therefore large scale concrete practice emphasizes just enough cement paste to "scientifically" fill between aggregates and in this manner avoid shrinkage, (while also conserving on resources).
Page 10 The copper coating on MIG welding wire is probably just atoms thick and therefore does not prevent ordinary corrosion. Yet it does resist humidity for a short period of time. This is perhaps a minor issue, except that if one stores the wire after the plastic hermetic seal is broken, it will corrode, almost on any shelf, except perhaps in an arid desert climate. Copper is actually fairly reactive in terms of bonding with building materials. Yet when thoroughly encapsulated in cement any such reaction is insignificant and likely stable in the sense of durability, as long as the piece is not broken (to expose wires through damage or cracks). Normal concrete porosity is generally too small in pore size to permit ordinary corrosion, except where unusual extremes are encountered or where, for example, the whole structure acts like an antenna and electrically exchanges regular charges between ground and atmosphere. (See here and here for further corrosion study). Even a lawn sculpture can conduct minute electrical charges, just as trees or mountains or lightning arrestors do. Corrosion may manifest over many decades or much sooner in extreme conditions. Steel rust can accumulate to a point of rupturing thin cement, but on the other hand ferrocement boats using simple cement, have lasted very long periods of time.
As I am a partial author of this book, it is possible the following was avoided, but it is an extremely important point in my opinion:
Page 12 Another point i tried to stress was only partly covered on page 12. I attempted to explain that Portland cement manufactures ensure that the bagged Portland cement absorbs atmospheric humidity which considerably reduces cement quality. Hence, more cement is needed to compensate for lost purity. Note the lumpy nature of the cement (powder) product in the picture on page 12. Look closely at the paper bag with plastic lining. The picture clearly reveals countless small holes punched in the bag by the manufacture. These holes eventually admit humidity and spoil the product. I have been told by a chief engineer at a cement fabrication plant that the holes were desirable to prevent caking and he rejected my point that humidity enters through the holes. Look at the lumps in the cement product, which should be powder instead of lumps.
Click here for a short article on Ferrocement Faux Bois, (a page with some photos of a restoration job i got in the year 2000).
To build a symbol hire a sculptor!
Bo's Competitive Concepts For Building "Green" (Globally Efficient).
2006 Structural Coffer Study
Other Concrete Sculpture Methods
Besides sand molding, I also love forming concrete in in free air like a mason. I usually use welded steel skeletons, as seen in my building shop picture. Another sculptor once had me clad her styrofoam and paper- mache sculptural skeleton with concrete. For this we used latex modifier to give the cement some integral reinforcement. The latex modifier for cement increases the cost of materials, but saves labor of reinforcing and speeds curing. The styrofoam remained inside and we built the large statue in place to avoid moving it. This worked beautifully. Using styrofoam as a light weight aggregate is gaining popularity. Recycled foam packaging material can be chopped up or used directly as "styro peanuts". Commercial styro aggregate is also available, also web searchable and now called eps. These make cement much less droopy for free air sculpting. The result is weaker than solid stone. Recycling styrofoam as an environmental solution appeals to me but the chemical instability of this and some other plastics does not appeal to me. So long as the placement is not within an enclosed living space, I suppose there is no harm.
Bo's catalog, showing sand cast products sold in the 80's and 90's. Customized production of anything, available by request.
Below: Artichoke leaves in sand cast lantern stand, for the Flyingconcrete 06 Workshop.
Note: These pages are placed in the public domain and are furnished "as is". The author assumes no responsibility for the use or misuse of the concepts in this series. All authorities should be satisfied first, as might be required, by relevant laws, before any building proceeds.
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