Tips to Better Concrete

August 26, 2008 07:00 PM
Dan Anderson, Farm Journal Columnist


A long-handled bull float smooths and levels concrete immediately after it has been screeded. For a good finish, professionals warn not to overwork concrete.                     Photos: Portland Cement Association
Anybody can pour a concrete slab, but creating a smooth, durable concrete surface requires more than 2x4 forms and troweling with the backside of a #14 scoop shovel. Accurate planning, as simple as it sounds, may be the most important step. Consider using the following tips to save money and improve the quality of the concrete poured for farm shop floors and other agricultural purposes.
Don't add water. "Customers add water at the job site to make [concrete] easier to move and work, but extra water weakens the mix,” explains Alan Sparkman, executive director of the Tennessee Concrete Association. "You can reduce the final strength of a 4,000 psi mix down to 3,500 psi by adding just 2 gal. of water per [cubic] yard before it's placed in the forms.”
If a "looser” mix is desired to make placement easier, have water reducers added to the mix at the local ready-mix plant. Adding water reducers to the mix can typically cost $2 to $4 per cubic yard. But they can improve the workability of the mix without decreasing final strength.
Determine a project's correct load-carrying capacity. Sidewalk-grade concrete is usually rated at a crush strength of 3,000 psi. Custom mixes can achieve ratings in excess of 6,000 psi.
"Concrete in the 4,000 psi range works well for most flatwork on farms,” Sparkman says. "In most cases, the best way to increase load-carrying capacity is to increase slab thickness. Going from a thickness of 4" to 6" increases the load-bearing capacity by 50% using the same mix.”
Sparkman encourages customers to work with local ready-mix plant operators to formulate the appropriate concrete mix, including additives, for their particular project.
"If you're in an area that has freezing and thawing, and you're pouring an exterior slab, the American Concrete Institute (ACI) recommends using air-entrained concrete,” he says. "All concrete absorbs water to some degree. That water can freeze and create pressure within non-air-entrained concrete. Air-entraining additives create billions of microscopic bubbles, and those bubbles act like tiny cushions to minimize freeze and thaw cracking.”
Use proper reinforcement. Glen Davis, manager of American Concrete in Carroll, Iowa, says a farmer's strategy of using old livestock fence panels, steel posts or scrap woven wire fencing to reinforce concrete slabs is questionable.
"Nobody throws good metal into their concrete, it's always the bent and twisted stuff,” he chuckles. "It's tough to keep all of that bent and twisted material in the middle of the finished slab. Any reinforcing that ends up at the bottom of the slab, laying against the ground, doesn't do you any good,” he says.
Davis prefers fiberglass or other modern fiber-strand reinforcing that's blended in at the ready-mix plant. "Fiber is uniformly blended throughout the mix, so you get maximum reinforcing without the hassle of messing with metal reinforcing at the job site,” he says. "Fiber reinforcing adds a couple dollars per [cubic] yard, but it's money well spent.”
Sparkman notes reinforcing concrete slabs for most farm-duty flatwork isn't as much for load-carrying capacity as for crack control and thermal durability.
"All concrete expands and contracts with temperature changes,” he says. "For thermal movement, fiber reinforcing is as good as metal reinforcing, and you don't have to mess with keeping it in the middle of the slab while you pour concrete.”
Don't finish concrete too soon. "There's a tendency for amateurs to work concrete too soon and too much,” Sparkman says. "With any slab, you should pour it, use a screed to level the concrete, then bull float it once, maybe twice, to produce a smooth surface. After that, leave it alone. Depending on temperature and other factors, bleed water will come to the surface. After a while, that bleed water will disappear, and then you're ready to do your final finish.
"If you work it before the bleed water goes away on its own, you'll create a cap that traps the bleed water in the slab, and you'll probably have trouble with the finished surface scaling off sometime down the road.”
For exterior slabs, ready-mix plant manager Davis recommends broom-finishing the bull-floated surface after the bleed water disappears. At most, make one additional pass with a wood or magnesium float to create a non-slip surface prior to brooming.
"If you overwork an exterior slab, you can get too much cement and not enough aggregate in the top layer, and it can cause you problems in the future,” Davis says.
For interior slabs like shop floors where smooth finishes are desired, work the surface with a metal trowel only after the bleed water disappears.

Rent a concrete saw to make stress relief and crack-control joints in fresh concrete. Cut to half the depth of the concrete slab.

Locate joints and seams. All concrete eventually cracks to some degree to relieve internal and external stresses. Joints can control where those inevitable cracks occur.
"The rule of thumb used to be, take the thickness of the slab in inches and multiply it by three, and that was the distance in feet between cuts or joints,” Sparkman explains. "A 4"-thick slab would have been jointed every 12'. Lately, the trend has been to multiply thickness by 2.5, so a 4" slab would be jointed every 10'.”
For optimum crack management, put control joints or cuts in a square pattern. According to the joint calculation formula, a 4'-wide sidewalk would be jointed every 10'.
"There would be cracks within those 10'-long rectangles,” Davis says. "In that case, I'd joint it every 4' to keep the blocks as square as possible.”
ACI recommends that joints/cuts be half the depth of the slab. Concrete saws are the best way to achieve that depth on thick slabs used in farm shops or aprons outside their doors. Sparkman says that on large floors that require multiple pours, the seam between one pour and the next makes an adequate pressure-relief joint. "Plan ahead so that the seam falls where you need a relief joint,” he adds.
Many amateur concrete workers believe that an expansion joint (a thin strip of tar-impregnated fiber or other soft material) must be installed between multiple pours. Sparkman disagrees.
"There's usually no need to put an expansion joint between pours,” he says. "But you always need expansion joints if you're pouring a floor against a wall or in a situation where the two concrete surfaces will expand and contract in different directions.”
Allow time to cure. Curing is one of the most important steps in concrete placement—and one of the most neglected. Use a garden sprinkler to keep new concrete damp for at least a week to achieve maximum strength. Or, once the concrete has hardened so it won't absorb colors, cover the slab with a layer of burlap or straw and keep it moist without having to constantly run sprinklers.
Another option is to use spray-on curing compounds or one-step cure-and-seal products. Curing products work well when applied to new concrete at the recommended rates.
For smooth-finished shop floors, sealants will repel oil and chemical spills and make floors easier to keep clean. Sealants achieve best results when applied to new concrete before the surface is put to use.
Davis urges amateur concrete workers to work with ready-mix plant managers who are familiar with additives and mix strengths suited to agricultural situations.
"You're going to have to live with that slab for a long time, so take time to ask questions to get the best mix and get it finished right,” Davis says. 

A darby is a long, hand-held wooden float used to smooth and level smaller areas of freshly screeded concrete or in other areas where a large bull float is uncomfortable or hard to control.

You can e-mail Dan Anderson at
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