How to Work with Concrete

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Spiral stairs built of Concrete

Spiral stairs built of Concrete

Build Anything with Concrete

 

Concrete is one of the most versatile materials in modern use. Contemporary cities are essentially constructed of concrete; buildings, bridges, subway tunnels, water ducts, dams and sidewalks, in fact much of everyday life, including most home foundations,  is constructed of concrete. Concrete can also be used as mortar to lay brick,  forming large sail boats, concrete canoes, flower pots, sculpture, and casting other art forms; the potential and versatility of concrete is no less than amazing. You might not need a large-span bridge, but how about a foundation, a geodesic dome home, a pad for the barbecue, a garage floor, a set of stairs, or a beautiful, small statue? Working with concrete is not difficult. There are several basic ideas that if understood and applied, can make working with concrete easier and safer. Understanding concrete itself, how it sets, and observing a few other details will result in an improved outcome, whether creating a mundane utility slab, an elegant boat, or a piece of classic statuary.  

What is Concrete?

Concrete is a mixture of stone aggregate, sand, and cement. Other ingredients may be added for durability and working characteristics, such as ground pigments to alter the colour, various types of fibre or steel mesh and bars to provide strength. Air-entraining and other chemicals may be used to enhance characteristics, and accelerate or delay set times depending upon the application. Pouring, casting, hardness, time required to set and frost resistance may be altered by the addition of certain chemicals and additives.  In certain applications, Styrofoam beads, glass fibre, or even sawdust may be added to make concrete lighter or stronger. The term “cement” is erroneously but commonly used in place of the term “concrete” and ideally should be differentiated. ” Cement” is actually used to make concrete. Portland cement itself is an inorganic, manufactured product. It is made by the mixing of limestone, chalk, and specific types of shale, clays, and some types of sand. The mixture is ground up and roasted to about 2700 degrees F (1480 C) . The roasting process creates a complex “clinker” or lumpy agglomerated rock that consists of silicates and other compounds. . The clinker is subsequently cooled and ground into an extremely fine, dry powder. The powder is then blended with a fine gypsum powder to control the “setting rate” when it is eventually mixed with water. In use, the finely ground silicates “hydrate” or combine with the water, eventually “setting” or becoming hard. When cement is mixed with aggregates, sand, gravel and perhaps other additives, and mixed with water, it finally hardens to  becomes “concrete”.

Let us look at safety requirements of working with concrete!

Concrete is made of sand, rock, and Portland cement. Heavy to work with, concrete dry or wet must be handled carefully. Do not underestimate the weight of wet concrete; a small pail of wet concrete is heavy, so work sensibly. If you get cement on your skin, dry or wet, it is not necessary to panic , but DO wash it off with clean water as soon as reasonably possible. Dry cement has an affinity for water and will damage skin , as it derives moisture from it. Wet cement, by it’s very nature, contains calcium hydroxide, a high-pH, corrosive chemical. Because of the corrosive nature of concrete in wet OR dry form skin exposure must be minimized when handling any form of cement, including mortar. If concrete dust or wet concrete is accidentally splashed in the eyes, wash it out immediately, and seek medical help. Grit from concrete can damage the eyes; remember, the chemical nature of cement is aggressive and corrosive! DO use gloves and safety goggles when mixing, preparing, or working with wet concrete. Portland cement is a very fine powder and easily creates dust. Avoid breathing cement dust, which can be very damaging to lung tissue, so always wear face mask.

Use these important rules for working with cement:

  Always mix the ingredients together DRY and mix uniformly PRIOR to adding water. Doing so coats the aggregates with cement and the mixing process is more uniform and efficient, resulting in a stronger bond. Dry additives such as oxide pigments and concrete-enhancers are also added, mixing uniformly prior to adding any water. If making concrete for general utility casting, mix concrete by using a uniform ratio of cement, sand, and aggregate. Remember the magic ratio 3:2:1 which represents three parts gravel (aggregate) , two parts sand, and one part cement by VOLUME. The ratio can be changed marginally, for example to 5:3:1 or 5:3:2 to meet the requirements of specific applications. Concrete used to assemble brick and block structures is called mortar. For the making of mortar, where handling characteristics such as stickiness and plasticity are essential use a ratio of 3:2:1 , but in this application it represents 3 sand, 2 cement, and 1 lime.  Lime is included in mortar mix to allow the mortar to stick easily to brick or concrete block when applied, and flow or squeeze out when bedding the block, brick or stone in the mortar. Keep in mind that commercially available “mortar cement” is cement that has had lime added in advance. Mix that specific cement with screened, sharp, clean sand for the best mortar. Common “stucco”, an “applied concrete siding” for buildings is fine, screened concrete, again made sticky by the addition of lime, and also includes various fibres for strength to alleviate cracking on the wall. Apply stucco uniformly over a steel screen mesh that has been fastened to the wall. Spread it evenly, allow it to set, and re- coat with a second smoothing coat if desired, and finally a more fluid “colour coat”.

Work with Concrete: Mixing

The addition of water to the dry ingredients for concrete whether it is for common construction work, mortar, stucco, or fine art casting is critical. Too much water will result in weak concrete. Adding water prior to mixing of ingredients can result in excessive slump, poor casting, poor bonding and weaker concrete. Remember, when you work with concrete, remember that  concrete is stronger with less water, so add only what you need to get the mixing, pouring, casting or plasticity characteristics that you require. To start with, add about the same volume of water, as the volume of the cement put into the mix. When adding the water, add more as necessary, but add the last of it judiciously and mix well, for concrete changes flow characteristics surprisingly fast. Simply adding another pint of water into a small concrete mixer will make a substantial difference in fluidity of the mixing concrete.. Remember, less water  results in stronger cement.

 

Reinforcement of Concrete.

Installing steel reinforcing is a standard practice to increase the tensile strength of concrete. Although concrete has very high compressive strength, it has little tensile strength. Steel “rebar” and steel mesh are installed in concrete slabs and structures to ensure adequate tensile strength. Place steel mesh close to the bottom one-third of footings or slabs for maximum weight-bearing capability, and  ensure it is buried within the concrete itself, and not merely covered up by it. Support steel or mesh with commercially available stand-offs, or use appropriate small pieces of block or cement rubble, remembering to check the position of the mesh as you pour the concrete. “Lift up” the mesh into the cement if necessary. *As an aside, do NOT leave large rocks in concrete slab castings.  They can, and will eventually cause stress cracking.

Slabs

  For large slabs, install 7/16″ or larger steel “rebar” every 12″, 16″ or 24″ both ways for maximum strength, wire tying the steel together at all intersecting points. For very large slab structures, install expansion joints or cut narrow stress grooves about an inch deep in the finished surface every 10 or 12 feet . Cut them with a rotary concrete or diamond saw and fill them with flexible caulking. If a slab is an unbalanced design, expansion and contracting forces will crack concrete; providing stress grooves will encourage cracking handsomely along the straight, uniform stress grooves rather than random, unattractive cracking. Alternatively large slabs can be poured in individual castings progressively, using a temporary wooden “sleeper” form to provide a straight edge for one slab, which, on the subsequent casting, will become the “form” for the following adjacent slab. When cured, such joints will also provide stress relief, but should also be grooved and caulked to maintain a superior appearance.

Let's work with concrete: Pouring and Casting

  For pouring slabs such as sidewalk, foundation or other flat structures, forms must be set up. Use appropriate dimensioned lumber set up to equate the OUTSIDE dimensions required. Level, plumb, and fasten all forms securely in place. Concrete is heavy and if poured carelessly, will easily displace or break flimsy, poorly built forms! Pile dirt against the side of forms and stake adequately with heavy dimensioned lumber such as 2×6′s driven into the ground. Brace parallel sides and reinforce corners by adding a cross-brace at the top, securely nailed to both sides to ensure they are not forced apart. Installing polyethylene beneath poured slabs is also highly recommended to avoid the movement of moisture up into, and through the concrete. Install and position reinforcing steel or mesh appropriately. When pouring the concrete, tamp, tap the forms, and otherwise vibrate the concrete adjacent to the forms to prevent “honeycomb” or holes in the cast surfaces. Remember, the more concrete is jiggled or vibrated, the more it will flow, often bringing excessive water to the surface. Strip, or remove wooden forms from cement after 3 days. They may be removed earlier, however, do exercise caution, as the edges and corners of “green cement” castings may be easily broken off. Wooden forms usually come off easily, because as concrete cures, the wooden forms dry and  usually crack away from the concrete to some degree.

Hardening, Forming and Finishing

  Ordinary cast concrete will harden generally within three to four hours, depending upon the temperature and the mix that was used, so ensure you have it cast in place and surface-finished prior to that time.  When you work with concrete, observe  that when concrete is beginning to set, the wetness, or “shine ” begins to disappear. Pay attention and plan to have the final troweling close to completion at that time. If the temperature is high and it is difficult to work the surface, spray a small amount of water on top of the casting and work it to perfection. In casting slabs, if wet spots on concrete are setting too slowly in cool weather, a small amount of cement is often sprinkled on the wet surface and troweled, repeating as necessary. Certain concrete surface-hardening chemicals may also be sprinkled upon the wet surface. A motorized trowel may be used on large, flat surfaces to trowel and polish a hardening concrete slab to perfection. Keep new concrete moistened and covered with polyethylene for at least five days after completion for the optimum development of strength and to reduce cracking. Remember also that a richer concrete mix (more cement) will harden more rapidly and will ultimately be stronger. Although it may harden in 4 hours, concrete does not develop it’s full strength for about 5 days and it’s maximum strength until about 28 days later, but it progressively continues to get harder for many years.

Concrete Objects Large and small  that Look Nice,  Hold Water, and Float

DIY'er projects and work with concrete do not  have to be mundane, functional pads, blocks,  or foundations.  Set up a suitable form around supporting mesh, cast the cement, allow it to set partially, and before it sets too hard, “design” or carve the surface, much as you would a clay sculptureCreate any form of artwork you wish.  Additional layers or detail may be added within a day or two by keeping the concrete moist and plastering additional finishing material on the surface. Cut and form wire mesh into any desired, hollow shape. Fill the hollow with supporting material such as a sand-filled bag, styrofoam or crumpled paper, or temporarily fasten a suitably-shaped backing such as canvas or burlap on the inside. Proceed with the application of concrete on the surface of the mesh. The concrete used in this specialized construction is usually made using finer aggregate. For artwork and sculpture use iron-free crushed marble or limestone, and for very fine work, stone dust or metakaolin may be used as the aggregate component. Use a drier mix concrete mix so that it remains as placed and does not slump, or collapse. Finish as required, and allow it to set for a couple of days prior  to moving the object to avoid damage. Larger objects including tanks, sailboat hulls, geodesic domes and “free-form” domestic homes are constructed using ferro cement.   Steel mesh, similar to galvanized chicken wire is set up, formed, and thin layers of mortar are subsequently applied repeatedly, or sprayed with suitably prepared, fluidized, special cement, until the desired thickness is achieved usually 10-30 mm thick, depending upon projected use. Ferro cement construction  is used for expensive yachts, canoes, water tanks, and other creative endeavors. Similar to methods used for stucco, ferro cement structure is light, very strong and durable. Using these methods, one can build almost anything using concrete. With ease of construction and substantial benefit, concrete has been in use since the 1800′s.

There must be something  modern and dedicated Jack-of-all-trades can do with it too. Think about it. Think up some concrete ideas.

Is that Incoming I hear?

Photo credit:  By Mathias Bigge  Wikimedia CommonsFacebooktwittergoogle_plusredditpinterestlinkedinmailFacebooktwittergoogle_plusredditpinterestlinkedinmail

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