Proper attention to reliable practices and procedures for constructing exterior flatwork can yield long-lasting outdoor surfaces for foot and vehicle traffic.
Concrete is the material of choice for driveways, walks, stoops, porches, and patios, which are collectively called exterior flatwork. Concrete flatwork, when properly constructed, is extremely durable, can carry heavy loads, conforms well to many designs and finishes, and is easy to maintain.
Because exterior flatwork is outdoors, it has to contend with rain, lawn sprinklers, and fertilizers in warm conditions and with snow, ice, and road salt in cold conditions. While the environment surrounding the flatwork plays a role in its long-term durability, professional placement and finishing techniques can usually protect flatwork from deteriorating, even in the worst environments. It's very important to prepare a suitable base for the flatwork slab, order the right concrete for the job, place the concrete correctly, and finish the slab properly. Proper attention to reliable practices and procedures for constructing exterior flatwork can yield long-lasting outdoor surfaces for foot and vehicle traffic.
Soil and gravel base
Prepare the bearing soil before setting up forms for flatwork. Level the soil, and compact it to 95% of its original capacity with a vibrating plate compactor. After leveling and compacting the bearing soil, distribute a layer of gravel that's 2" to 4" thick. Gravel keeps water from reaching the underside of the slab because the spaces between granules of gravel are too large for capillary action to take place. Use single grade gravel with no fines. Thoroughly tamp the gravel with a plate compactor to create a level surface for the concrete. Crushed concrete may be used instead of gravel, if it's the material that's more readily available in the market.
Forms
Forms establish the slope, thickness, and elevation of the flatwork. To establish the proper thickness, set driveway, walk, stoop, porch, and patio forms for a 4" slab and apron forms for a 6" slab. Construct the forms at the slope and elevation noted on the construction documents to ensure proper drainage. Form and slope stairs according to local codes. Lay rebar or wire mesh on chairs, so the concrete will completely surround it. The rebar or wire mesh should rest approximately in the center of the slab thickness. Use rebar to pin the driveway slab section to the garage slab.
ProTip!If pouring a walkway to the home's entrance at the same time as the driveway, install a sleeved dowel to prevent stresses from transferring between the two applications.
Expansion joints
Expansion joints are installed during form construction to give individual sections of flatwork room to expand during changes in temperature and humidity. Install 1/2" expansion joints between different applications, such as between the driveway and sidewalk, the sidewalk and apron, and the driveway and garage. Pressure-treated wood strips can be used as expansion joints.
ProTip!For large flatwork slabs, such as driveways, consider segmenting the job into smaller concrete pours with 2x4s or expansion joints. A crew of a certain size can manage only so much concrete at a time.
Concrete order
Concrete consists of aggregate, sand, cement, and water. The types and quantities of these materials in a concrete mix greatly affect its strength and durability. Order concrete with the correct aggregates, air entrainment, psi, water-to-cement ratio, and slump. Avoid soft and crystalline silica aggregates, and order enough aggregate to allow good distribution. Order air entrainment of 6% to 8% when pouring concrete in freezing markets, so the cured concrete will have an air entrainment of 4% to 6%. The psi should be at least 4,200 for driveways and aprons and at least 3,000 for walks, stoops, porches, and patios, unless otherwise specified in the construction documents. The water-to-cement ratio should be no more than 0.45, with a 3.5" to 4" slump.
Concrete delivery
Concrete delivery is the process of moving concrete from a ready-mix batching plant to a jobsite. During delivery, the concrete is slowly mixed in the delivery truck to prevent it from stiffening. When concrete arrives onsite, check the delivery ticket to make sure the mix hasn't been in transport more than 30 minutes in hot weather or 45 minutes in cool weather. If the concrete has been in transport for too long, reject it, and request a new batch of concrete, rather than adding water to the mix. Adding water to the mix will produce weak flatwork that will be prone to cracking, spalling, and other problems.
Water-to-cement ratio
In concrete mix, the water-to-cement ratio is the weight of the water divided by the weight of the cement. It's important to control the water-to-cement ratio to ensure the concrete's strength and durability. Use only the amount of water needed to ensure proper curing. The water-to-cement ratio should be in the range of 0.35 to 0.45. To check the water-to-cement ratio, perform a slump test on the mix; it should have a slump of 3.5" to 4". If the slump is greater than 4", the water-to-cement ratio is too high. A trained professional should perform the slump test, following the ASTM C143 Standard Test Method for Slump of Hydraulic Cement Concrete guidelines. Pumped mixes are more liquid than mixes delivered by chute and consequently will have a slump higher than 4". However, the water-to-cement ratio should still be in the range of 0.35 to 0.45.
Air entrainment
Air entrainment is achieved by adding a surfactant to the concrete mix; the surfactant causes extremely small air bubbles to form throughout the mix. As the concrete cures, these air bubbles leave tiny air pockets throughout the concrete slab. When water in the concrete freezes and expands, the air pockets absorb the stresses and prevent cracks from forming. Order air-entrained concrete in markets with climates that are subject to freeze-thaw conditions. When concrete arrives onsite, test for air entrainment. A trained professional should perform the air entrainment test, following the ASTM C231 Test for Air Content of Freshly Mixed Concrete by the Pressure Method guidelines.
Placement
Concrete placement is the process of pouring concrete from a pump or chute and spreading it to its final location. Always keep the concrete pump or chute within 3' of the ground during placement to prevent the aggregate from separating from the cement paste. Also, place the concrete in small loads, and spread it no more than 3' in any direction from the point where it was placed.
ProTip!Dampen the gravel base just before pouring concrete to help prevent the concrete from stiffening too early.
Screeding
Screeding the concrete after placement fills low spots, evens out high spots, and distributes the concrete uniformly, creating a level surface. Screed the concrete immediately after placement. Work the concrete from the center of the slab to the edges. Don't overwork the concrete; lightly saw the screed back and forth to level the concrete without pushing the aggregate to the bottom.
ProTip!Place and screed the concrete in rapid succession before the bleed water rises to the surface.
Floating
Exterior flatwork should have a surface that's more porous than interior flatwork, so water vapor can pass into and out of the flatwork. To achieve the appropriate surface, finish exterior flatwork with a bull float, making no more than two passes with the float to submerge any aggregate remaining above the surface and to create a uniform finish. Allow bleed water to evaporate from the surface prior to bull floating. After bull floating, continue finishing the surface with an aluminum or magnesium hand float to smooth tool marks and create a uniform texture on the surface. Don't use a steel trowel, because it can seal the surface, trapping water underneath. Don't finish the surface aggressively; this causes the sand and water to rise toward the top portion of the slab and the aggregate to settle toward the bottom portion. Minimize finishing work to keep from weakening the surface.
ProTip!If plastic cracks form on the surface while the concrete is still workable, float over the cracks, and re-smooth the surface.
Control joints
Control joints prevent shrinkage cracks from forming in visible areas and reducing the strength and durability of finished flatwork. Control joints may be tooled in or sawed in. If tooling in the control joints, tool them in while the concrete is still workable to ensure clean joints. Knock down the edges of the joints with a magnesium or aluminum hand float. If sawing in control joints, saw them in as soon as the concrete can support the saw to prevent early shrinkage cracks. In driveway slabs, space control joints at intervals of 24 to 30 times the slab thickness. For example, in a 4" slab, space control joints no more than 10' apart with a layout of 10' x 8'. In porch and patio slabs, run control joints from inside corners, and use control joints to separate major areas of different proportions. Also, use control joints to isolate cast-in penetrations.
Broom finishing
Broom finishing produces a uniform raised grain to prevent slip hazards on the flatwork. Broom finish after the concrete is stiff enough that sand particles don't move when the broom is applied. Typically, the surface can be broom finished immediately after floating. To achieve a broom finish, pass a soft, nylon-bristle broom over the concrete surface.
Curing
Concrete needs a certain amount of moisture to cure properly. In hot and dry climates, keep the surface moist using one of four methods: 1) Mist water on the flatwork for a minimum of three days. 2) Apply curing blankets to the flatwork for a minimum of three days. Periodically wet the blankets down. Don’t apply curing blankets until the concrete has cured enough to resist surface damage. 3) Cover the surface with polyethylene sheeting for a minimum of three days. Don’t apply polyethylene sheeting until the concrete has cured enough to resist surface damage. Secure all of the edges to prevent wind from drying the surface. 4) Apply spray-on curing compounds.
Concrete also needs a certain amount of heat to cure properly. When the temperature is below 40°F, keep the surface warm during the curing process by applying curing blankets to the flatwork for a minimum of three days.
