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• Concrete Strength Relies on the Right Mix and Reinforcement.

Reinforced concrete is barely a hundred years old, and engineers are still refining their assumptions of its properties. Yet some contractors (even large governmental agencies) have not changed their methods in the past 30 to 40 years.

Concrete alone lacks appreciable tensile strength. Steel reinforcing, or rebar, used in concrete cannot withstand compressive force by itself. Combining the strengths of concrete and steel produces the required structural properties.

Anyone who works with concrete knows that steel reinforcement provides tensile strength. But even experienced builders and designers commonly overlook an obvious consequence of this fact. Rebar must extend into concrete deep enough to develop that tensile strength. Instead of pulling out of the concrete, the bars will start to stretch.

Problems commonly arise at points where rebar changes direction and at intersections. Take, for example, a corner in a footing that has two horizontal rebars. Workers often place the outer bar wrapping around the outside of the corner, which is correct. But then they wrap the inside bar around the inside of the corner, losing a few essential inches of development length. Inside bars at corners should cross, run past each other, and extend toward the far side of the footing.

• Rebar Has to Go Deep Enough to Do the Job

Straight reinforcing bars can develop sufficient bond if they extend far enough into the concrete. When they don’t have thick enough concrete (such as at a wall corner or at a T-intersection), a hook at the bar’s end may substitute for the lack of available embedment. If a perpendicular bar is placed inside the hook, it spreads the force to a greater area of concrete. (Usually there is a whole row of hooked bars, and a single bar can run through all of the hooks.) We like to see a bar inside the bend at any change in bar direction.

Sometimes the tail of a hook may not fit where it’s shown on the plans. Usually you can rotate the tail of the hook to clear obstacles. As long as the hook extends into concrete to develop sufficient anchorage, it’s doing its job. You should check with the designer first, though. Sometimes hook tails need to lap with other bars.

• Less Water Mean Less Shrinkage and More Strength

Nuisance cracking in concrete has two major causes: shrinkage during the curing process and thermal expansion or contraction due to temperature swings. Generally, shrinkage is caused as the water in the concrete gradually dissipates. As much as one-third of the water, or 5% of the total volume of the concrete, can dissipate.

The strongest concrete uses only enough water to hydrate all of the cement in the mix. Excess water leaves space in the concrete when it evaporates, making the concrete less dense and therefore weaker. (Lost water affects the concrete’s structure on a molecular level, unlike the tiny bubbles that are left in air-entrained concrete.) But strong concrete is worthless if you can’t place it, and getting good workability requires more water.

Good workability means that you can consolidate the concrete around reinforcement and into corners by vibration. It does not mean that the concrete flows there by itself. If I overhear concrete workers complaining about how hard it is to work the mix, I know it’s good concrete.

Reducing the amount of water in the mix also makes the cured concrete less permeable to air, water, and salts. Although air seems harmless, the carbon dioxide it carries can react with water in a process called carbonation. In carbonation, water and carbon dioxide combine to form carbonic acid, a weak acid. Over time, enough acid lowers the concrete’s alkalinity to the point where steel corrodes much more readily. Using a drier mix and consolidating it well can forestall carbonation for a long time.

• Special Considerations for Slabs on Grade

Concrete slabs need to have a firm, even, well-compacted substrate, which begins with properly prepared original ground. Remove all sod, stumps, roots, other organic matter, large rocks, and wet, mushy soil. The soil must be compacted thoroughly and evenly. If you have unstable soil, plan on hiring an engineer or ending up with a cracked, buckled slab.

Placing gravel, crushed rock, or sand before pouring the slab makes an even surface to receive the concrete. As the concrete shrinks, the slab edges want to slide toward its center. A smooth surface of sand makes it easier for the concrete to slide, reducing its tendency to crack.

A layer of sand or gravel also creates a capillary break between the ground and the slab. Sand still wicks some water, so we usually recommend placing a vapor barrier on top of it. You should compact any sub base material well and dampen any exposed areas just before pouring the slab.

• Rebar Performs Better than Wire Mesh in Slabs

Steel rebar and wire mesh both serve the same function: They add structural reinforcement to the concrete. Rebar is only slightly more expensive than mesh but easier to keep centered in the slab. If mesh is properly placed, it should do just as good a job of strengthening the concrete as rebar.

Reinforcement should stay centered in the slab and not trip concrete workers as they move about. However, if the reinforcement is wire mesh, there’s no way to avoid trampling it while placing the concrete. So the mesh gets embedded in the sand beneath the slab. Although it’s not impossible to keep the mesh centered uniformly in the slab, it is difficult.

Placing a grid of #3 rebar at 18 in. centers provides adequate slab reinforcement and places for nimble feet to step. Supporting the bars on metal “high chairs” or precast concrete cubes (“dobies” where I live) keeps them in proper position as concrete is poured over them. The cost difference between mesh and rebar is almost negligible, and the reinforcement ends up where it belongs.

Reinforcement falls into two main categories: structural and shrinkage/temperature. Structural reinforcement provides strength to resist bending, compression, or tensile loads. Shrinkage/temperature reinforcement reduces the concrete’s tendency to crack as it dries or as it contracts or expands due to temperature.

For the latter, chopped fibers of polypropylene, nylon, steel, glass, palm fronds, or the like may be added to the concrete mixture. Because the fibers are automatically distributed throughout the concrete during mixing, there are no concerns about proper placement. For most slabs on grade that require no structural reinforcement, fiber reinforcing can be placed in the mix at the batch plant.

• Keep the Concrete Wet and Warm

Builders always want to hurry to strip the forms, but leaving them in place a few days holds the moisture in the concrete. A week of wet curing would make any engineer happy. Keep the concrete wet by covering it with plastic or wet burlap, or with spray from a fog nozzle. Slabs can also be flooded. Curing compounds that seal in moisture when sprayed on concrete are available.

Rapid drying or freezing severely reduces concrete’s strength and results in weak slab surfaces. A little time invested in proper curing protects the finished product you worked hard for.

For further information, the American Concrete Institute publishes several references and the model code, ACI 318-89. The Aberdeen Group offers several publications and references intended for contractors and builders. “Design and Control of Concrete Mixtures” by the Portland Cement Association offers 200 pages of information including mixing, placing, finishing, and testing.

• Size and Grades of Rebar

Rebar comes in many sizes and grades. In residential work, we mostly use bar sizes #3, #4, or #5. These sizes translate to the diameter of the stock, measured in 1/8-in. increments; #3 bars are 3/8-in. in dia, #4 bars are 4/8-in. (1/2), and #5 bars are 5/8-in. The grades 40 and 60 refer to the yield (tensile) strength (40,000 psi and 60,000 psi, respectively). Grade 60 is harder to cut and bend. Both grades are priced the same. The designer usually specifies which one to use for a particular purpose. If the grade is not specified, I buy the softer grade 40 for short lengths and bends, and grade 60 for long straight runs with few or no bends.

• Wiring the Rebar Together until the Pour

To lap or cross rebar, we used prelooped wire ties. Commonly available in lengths of 6 in. and 8 in., the latter are handy for tying together pairs of #4 or #5 bars. A bag of 5,000 8 in. ties costs about $45 and will last for several residential-size foundations. Wire is also available on spools, but we find that less convenient.

The simplest tie requires merely bending the tie diagonally over the bars and, using a tool called a twister, hooking the loops and spinning the wire. Over twisting the wire wills imply break it. (The wire adds no strength or integrity once the concrete has been placed.) The twisted wire is then wrapped around the bar so that it doesn’t extend toward the exterior surface of the concrete. The wire might rust if it remains exposed to the elements or could lead water into the embedded rebar if exposed below grade. Incidentally, spools of wire are also handy for hanging the rebar at a consistent height in the footings. Because the footing is covered later by the waterproofed foundation wall, the wire is never exposed to the elements.

• Rebar Maintains Control of Concrete Shrinkage

In addition to solving problems related to shear or tension, rebar is also specified for shrinkage control of concrete. Because the water in poured concrete is lost by evaporation as it cures, concrete shrinks in volume. Rebar doesn’t prevent shrinkage but binds both sides of the eventual cracks into a single wall plane.

According to Val Presto, a structural engineer from Harvard, Massachusetts, “This shrinkage will result in 1/16-in. to 1/8-in. cracks at about every 20 ft. at the top of an unreinforced wall. Add more water to the concrete, and you get more shrinkage – cracks that are perhaps every 10 ft. to 15 ft. – and the concrete is weaker. Horizontal bars will minimize the cracking by causing multiple fine line cracks instead.”

Prest also says, “Temperature and shrinkage-designed steel for the average 10-in. thick residential foundation is commonly spaced on 12-in. centers, or every 10-in. horizontally for #4 bars and every 15 in. for #5 bars.” For horizontal placement in wall forms, we almost always tie rebar to the tie rods (or form ties) that hold the outer and inner concrete formwork together. After the concrete subcontractor sets up the outer wall and pushes the tie rods through, we place the steel on these tie rods and wire the rebar to them. The verticals are tied to the horizontal bar and to U-shaped dowels in the footing.

Although steel is specified for its temperature and shrinkage control, Prest says, “ninety-nine percent of the time, steel is designed for shrinkage, not temperature.” Some exceptions would be bridges, concrete roadbeds, and large retaining walls built on highways exposed to direct sun. 

• A Driveway Is Only as Good as the Soils Beneath It

A crucial part of proper driveway design is making sure the materials below the concrete are adequate. The first 6 in. to 8 in. of material directly below the concrete is the base. The sub base is the soil 8 in. to 12 in. below the base, and the sub grade is usually the native or naturally occurring soil below the sub base. The design thickness of each layer    depends on the soil being built on. Acceptable natural soils such as sand and gravel let moisture drain. If the sub grade consists of this type of soil, then it can be compacted to serve as the base and sub base, and more excavating and filling are unnecessary.

However, if the sub grade is clay, peat or fine-grained silty soil that holds moisture and drains poorly, removal of up to 20 in. of sub grade solid might be necessary, depending on the support value of the soil. If you have doubts about the soil characteristics in your case, it’s worth hiring a soils engineer to do an evaluation.

• Establish Driveway Elevations Early for Proper Drainage

Prior to excavating and backfilling, the exact elevation of the top of the drive should be established. Then, as earthwork is being done, base grades can be brought up wit equipment usually to within an inch of their required height, which saves on hand grading later. For the best drainage, we try to slope the driveway at least ¼ in. per running foot away from the house.

Some situations prevent proper drainage, such as an area of concrete that is locked between a house and a garage. In these cases a catch basin may have to be installed as part of the driveway’s drainage system.

The best way to remove water from a catch basin is to use a drainpipe at least 4 in. in dia. That returns to daylight or to a storm sewer that is located safely away from the house. A second method is connecting the catch basin to a dry well. In the most extreme cases, a sump pump is installed to pump collected water to a safe place. The last solution is the most costly and probably should be used only with the recommendation of an engineer.

• Forms Determine the Final Grade of the Driveway

After the soils have been layered and compacted satisfactorily and the proper grades established, the driveway forms can be laid out and installed. We usually make our forms out of 2x4s if the driveway is to get a 4-in. slab or 2x6s for a 6-in. slab. The forms need to be staked strongly enough to hold the concrete and to withstand screeding without movement, so we drive our stakes every few feet. Wooden stakes are okay, but they don’t hold up well to being driven and generally can be used only a few times. On most jobs we use commercially available round metal stakes with predrilled holes for nailing the stake to the form.

The driveway featured here was placed on a nice, gently sloping lot. We began by setting the forms to the natural grade along one side. On the opposite side we ran a string at the exact width of the finished slab. We staked the forms for this side along the string at roughly the correct height. Next, we leveled from one side to the other set the exact height of the opposite forms, driving the stakes deeper or pulling them up to adjust the height. We’ve found that a laser level is the quickest and easiest tool for setting the height of our forms, although a transit or even a water level can be used.

Our forms usually receive a coating of release agent (a special oil available from concrete-supply houses) to provide better consolidation of the adjacent concrete and to make removal easier. After the forms are set, we grade the base layer of soil to its exact elevation and run the compactor over it one last time. Then we dig the edges of the driveway down a couple of inches. The thicker concrete along the perimeter provides additional support as well as protects against erosion of the soils next to the drive. We incorporate a single run of rebar along the edges for additional support, as well as an 8-in.-thick by 12-in.-wide thickened area at the road.

• Expansion Joints Allow the Concrete Slab to Move with Changes in Weather

Another crucial part of the driveway layout is planning for contraction and expansion joints. Contraction joints are added during placement, so I’ll discuss them later. Expansion joints, installed before concrete placement, allow the driveway to move both horizontally and vertically. Most people think concrete is solid and unmoving. However, concrete not only moves in relation to other solid structures, such as foundations and roadways but also expands and contracts with temperature changes and moves as soil conditions beneath the slab change.

Expansion joints provide a full division between different sections of concrete placement. For the driveway featured in this article, expansion joints were placed between the driveway and the sidewalk to the front door, between the driveway and the garage apron and between the two main driveway slabs that were placed or poured separately. We didn’t need an expansion joint between the driveway and the asphalt roadway, but a joint is required if the roadway is concrete.

An expansion joint consists of a thin layer of energy-absorbing material such as asphalt-impregnated fiberboard, plastic foam, wood, cork, or rubber. For most driveways, we use ½-in.-thick fiberboard installed with a plastic cap strip on top, flush with the finished height of the slab. After the concrete has set up, the cap strip is removed, and we fill the top of the joint with a joint sealant, SL-1 from Sonneborn, Chem-Rex Inc., which protects the joint from moisture penetration and UV-degradation. The joint sealant also matches the color of the concrete to add a more pleasing look to the joint.

• Order the Right Concrete Mix

Concrete mixes vary depending on the application. But for most driveways, we use a six-bag limestone m ix (4,000 psi) with approximately 6% air entrainment. Air entrainment is the incorporation of microscopic air bubbles throughout concrete to prevent scaling, the flaking or peeling that occurs on cured-concrete surfaces.

The mix order should also include slump requirement, or the wetness of the concrete when it’s delivered. Slump is measured on a scale of 1 to 12 with 1 being the driest mix. For most driveways, we request a slump of 4 to 5, which is easy to spread but can be worked shortly after it is poured. After a mix has been prepared to specifications, adding water can weaken it. The concrete supplier is responsible for the slump as well as the strength of the mix, and the concrete should arrive as ordered. If concrete arrives too wet, it can be sent back.

• Screeding Creates the Level of the Slab

Trucks in our area are able to distribute concrete pretty evenly by controlling the flow of material and the direction of the chute. We work the concrete along the edges to consolidate it and to remove any voids. Then the concrete is raked to a rough elevation just slightly higher then the forms and expansion joints. We make sure we never get too far ahead of the screeding process to that any excess can be easily raked down to areas waiting for concrete.

Screeding cuts in the grade of the slab and consolidates the concrete before bull floating. It’s usually done with a long, straight x4 (we sometimes use an aluminum box beam screed rail), slightly longer than the width of the driveway. The 2x4 or screed rail that rides on the forms is pulled across the wet concrete with a side-to-side reciprocating motion.

After two or three yards have been placed, the edges should be hand-floated and cut in with an edging tool. The screeded concrete can now be bull-floated. A bull float is a wide, flat metal float mounted on the end of a long handle. As the float is pushed and pulled over the screeded concrete, the leading edge must be elevated to keep from digging in. If the float is mounted on the handle at a fixed angle, bull floating can be a real workout. The best bull floats have a blade that rotates back and forth by simply twisting the handle; this design allows the operator to keep the handle at a constant, comfortable angle. As the bull float rides over the concrete, it creates surface tension that brings water to the top, which smoothes the slab and fills in minor voids at the same time.

After bull floating, the concrete should be left alone until all bleed water on top of the wet concrete has evaporated. At this point the concrete should be strong enough to support a crewmember on kneeboards, which I’ll describe later, and is ready for finishing. Finishing the concrete too early can trap water and create a weak surface with a high water/cement ratio.

• Contraction Joints Give the Slab a Place to Crack

The first part of the finishing process is lying out and cutting the contraction or control joints. Contraction joints act as score marks in the concrete; they create weak points and encourage any cracks that might develop to occur at the joints. To understand how contraction joints work, I need to explain why concrete cracks.

Concrete begins to crack before receiving any loads whatsoever. As concrete curs and drives, water is absorbed into base materials and evaporates through the surface, which causes the concrete to shrink or contract. Cracks form in the concrete as a result. Contraction joints provide the relief needed so that these cracks form along a joint instead of randomly in the surface of the slab.

Maximum spacing of contraction joints should follow this rule of thumb. Multiply the thickness of the slab by 2 ½, and that number represents the maximum distance in feet between joints in any direction. The slab for this driveway was 4 in. thick, so the maximum distance between the joints is 10 ft. (4 x 2.5 = 10).

The depth of the joint should be no less than one-quarter of the thickness of the slab and should be cut in either during the finishing process or immediately afterward For this driveway we cut the joints with special tools called groovers. We begin by stretching a string between our layout lines and snapping it to leave an impression in the wet concrete. We work the groovers along straightedges to cut in the joints. Thicker slabs require deeper joints that are cut with saws equipped with blades designed to handle fresh, or green, concrete.

Temperamental is a literal description of concrete. Temperature, along with humidity, influences the pour more than any other factor.

• Hot Weather Pours

When it’s hot, and the humidity is low, every minute is important. If you spend time fussing around, when the last wheelbarrow of concrete is finally off the truck, the first section of floor you placed will probably be hard enough to walk on.

Here are some strategies that help in hot weather:

• Even if a polyethylene vapor barrier is not required, use one. It blocks the moisture from dropping through the sub gravel.
• Have lots of help available. The sooner you get the truck unloaded and the concrete leveled, the better your chances will be of getting a good finish.
• Have two finishers working the slab: one with a magnesium float, and another following behind with a steel trowel.
• Although it compromises compressive strength, consider using a wetter mix to buy a little more working time.
• If more than one truckload is needed, coordinate the arrival times carefully. If a fresh truckload of concrete has to sit and wait an hour while you finish unloading the first truck, you may find that concrete from the second truckload will set up before you are ready for it.
• Areas that receive direct sunlight set up much quicker than shaded areas.
• Start wetting down the slab as soon as the final finish has set. Few things will weaken concrete as much as “flash” set, where the concrete dries to quickly.

• Cool-Weather Pours

When the temperature is cool, concrete initially reacts in slow motion. After the slab is placed and the bleed water slowly evaporates, you’ll wait hours for the slab to tighten up enough to start hand troweling. When it’s finally ready to be troweled, you’d better be there because that window of opportunity for finishing doesn’t stay open much longer on a cool day than it does on a warm day.

Here are a few cool-weather tips:

• Don’t wet the mix any more than necessary.
• If a polyethylene vapor barrier isn’t required, don’t use one. Any moisture that drains out of the slab will speed the set.
• Pour as early as possible to avoid finishing the slab after dark.

• Cold-Weather Pours

When the temperature is cold, a whole new set of rules comes into play. Concrete cannot be allowed to freeze. That tender, finely finished surface you just troweled on the slab will turn to mush if it’s allowed to freeze. Fortunately, the chemical reaction that takes place when concrete hardens generates heat.

Here are some strategies that help in a cold-weather pour:

• Ask your concrete supplier about using warm mixing water to prevent problems during transit on days when the temperature is well below freezing.
• Having the supplier add calcium to the mix accelerates the initial set of the concrete, and the concrete achieves the strength to resist freeze/thaw stress faster. The amount of calcium is measure as a percentage of the cement content and ranges from ½% to 2%. Talk to a veteran concrete finisher before deciding when and how much calcium to add to the mix. Too much calcium produces the same problems as hot, dry weather. It’s important to note that calcium is corrosive to steel and should never be used in steel-reinforced concrete.
• Always be sure that all components of the sub base are frost-free.
• Provide supplemental heat to keep the building above freezing.
• Cover the slab with polyethylene and then spread an insulating layer of straw or hay at least 4 in. thick on top, or use an insulating tarp.
• The best strategy: Pour when cold temperatures are not an issue.

Foundations and Concrete Work. Taunton Press, Inc. 2003, Pg. 4-33, Pg. 43.

Foundations and Concrete Wrk. Taunton Press, Inc. 2003. Pgs. 4-33. Pg. 43. Foundations and Concrete Work. Taunton Press, Inc. 2003. Pgs. 4-33. Pg. 43.