How do I pour a small concrete slab?

A small slab, such as a patio, a pad for a grill or trash cans, or a short walk, is a job many homeowners can do with bagged concrete, some lumber and a helper. Whether it lasts comes down to the work before and after the pour: a utility locate, a firm base, forms set to the right slope, joints in the right places, and a week of curing. This guide covers slabs on the ground that carry people and light loads. A slab that will hold up a shed, a garage or a room is a foundation, and a different job.

Before you dig: utility lines and permits

Virginia law says: "No person shall make or begin any excavation or demolition without first submitting a locate request to the notification center."^[1] Virginia 811 asks homeowners to contact it at least 3 working days before excavation, and to outline the area where you plan to dig in white paint or white flags.^[2] Once the lines are marked, it explains that state law sets a two-foot tolerance zone on either side of the marks, where only certain digging, such as with hand tools, is allowed.^[2]

Utilities mark only their own lines. Virginia 811 lists lines running from your home to a detached garage or shed, invisible pet fences, and septic and propane lines among the private lines that will not be marked.^[3] If any of those could be near the slab, find them before you dig.

Whether a slab needs a building permit is a local call. Prince William County lists "Concrete (patio) slabs on grade that are not designed to support a future building addition" among work that needs no building permit, and notes that zoning or watershed approval will be required.^[4] Pittsylvania County lists "On-grade patios" under work that needs no building permit, and adds that setbacks and other zoning requirements may still apply.^[5] Other localities set their own rules, so ask your county or town building office, and check your HOA rules, before you buy materials.

Plan the slab

Thickness. Quikrete's instructions say sidewalks should be at least 4 inches thick, and simple slabs for small bases are normally 4 to 6 inches thick, depending on the load.^[6]

Slope. Water has to run off. A slope of about 1/4 inch per foot away from the house is generally recommended, but check local codes and conditions.^[6]

Base. A slab is only as good as what is under it. VDOT's sidewalk specification has its contractor excavate, shape and compact the earthen foundation to a firm, even surface, with unsuitable material removed and replaced.^[7] For a home slab that means digging out topsoil and soft spots and tamping what is left. Quikrete adds that where drainage is poor, dig deeper and add several inches of gravel or crushed stone to the subbase.^[6] A hand tamper is usually enough for a small pad; a rented plate compactor helps on anything larger.

How much concrete. Multiply length by width by thickness, all in feet, to get cubic feet. An 80 lb bag of Quikrete Concrete Mix yields about 0.60 cubic feet,^[8] so a 10 by 10 foot slab 4 inches thick (about 33 cubic feet) takes about 56 bags. If you order ready-mixed concrete, add a margin on top of the volume you worked out, since forms and ground are never perfectly true. If you go that route, check access first: a loaded truck is very heavy and may not be able to cross soft ground, loose dirt or residential curbs.

Protect your skin, eyes and lungs

Wet concrete is caustic. Quikrete's safety data sheet for its concrete mixes says the product "Causes severe skin burns and serious eye damage".^[10] It warns that burns from Portland cement may not cause immediate pain or discomfort, and they can become worse even after contact has ended.^[10] The dry mix is a hazard as well. The mix contains crystalline silica, and its hazard statements include "May cause cancer through chronic inhalation".^[10] Quikrete warns: "Prolonged or repeated inhalation of dust may cause lung damage or cancer."^[8] Quikrete calls for impervious gloves such as nitrile, a mask and eye protection,^[8] and for a NIOSH-approved respirator such as an N95 in poorly ventilated areas, when used for extended periods, when use is frequent, or when permissible exposure limits may be exceeded.^[10] Add long sleeves and rubber boots, mix outdoors, and open bags gently to keep the dust down. If concrete gets on your skin, Quikrete says to remove all product from the body and rinse thoroughly with water, and to see a health care professional right away if you suspect a cement burn.^[10] Do not kneel in wet concrete without waterproof pads.

Pour day

Have every tool, every bag and a helper ready before you mix the first batch; concrete does not wait. Quikrete's data sheet calls for about 6 pints of water per 80 lb bag, with a final water content of about 6 to 9 pints per bag.^[8] Resist the urge to make it soupy; extra water weakens the mix and makes the slab more likely to crack.

Watch the forecast. In cold weather, do not pour on a frozen subgrade; VDOT's specification has its contractor prevent the temperature at the surface of the concrete from falling below 40 degrees F during the first 72 hours immediately following concrete placement.^[7] In hot, dry or windy weather, the same specification says care shall be taken to protect the concrete from shrinkage cracking by applying the curing medium at the earliest possible time after finishing operations and after the sheen has disappeared from the surface.^[7]

Joints and finishing

Concrete shrinks, so it will crack somewhere. Joints decide where. Quikrete's instructions cut 1-inch control joints every 6 to 8 feet in a 4-inch slab.^[6] VDOT's sidewalk specification divides the slab into sections approximately 5 feet long with control joints that extend into the concrete at least 1/4 of the depth, and where a slab is more than 7 feet wide it adds joints lengthwise to obtain uniform blocks that are approximately square.^[7] Aim for square or nearly square panels. Where the slab meets the house, steps or another slab, put in a strip of premolded joint filler so each part can move on its own; VDOT requires preformed joint filler between its sidewalk and any adjacent fixed structure.^[7]

Timing matters more than technique. Quikrete says troweling takes place after the surface moisture has evaporated from the surface and the concrete has lost its sheen,^[6] and that when a coarse finish is desired for nonslip footing, floating, edging and grooving may be the only steps performed.^[6] Wait for the sheen to go, then trowel or broom. Overworking the surface usually does more harm than good.

Curing

Curing is keeping the slab wet and warm enough while it gains strength. Quikrete says to cure for 5 days in warm weather at 70 F or higher, or 7 days in colder weather at 50 to 70 F, using wet burlap, plastic sheeting, a sprinkler or a curing compound.^[8] If you cure with water, Quikrete says water curing with lawn sprinklers, nozzles or soaking hoses must be continuous to prevent interruption of the curing process,^[8] so a hose run once in the evening does not count. Quikrete also says to protect concrete from freezing during the first 48 hours,^[8] and to carefully remove the forms after the concrete has hardened, 48 hours or so.^[6] For vehicles, read how long new concrete should cure before you drive on it.

Stop and call a pro if

• You have no locate ticket or the marks are missing. Do not dig. If you hit a line, Virginia 811 says to stop digging, call 911 if a hit line releases flammable, toxic or corrosive gas or liquid, then notify the utility and Virginia 811.^[2]
• The slab will carry a structure. A shed, garage, room or anything a building official would call a foundation needs to be designed for its load; ask the building office before you plan it.
• The slab is big, or a truck cannot reach it. Past a few dozen bags, a crew with ready-mix, a pump or a buggy is usually faster and more even.
• It meets a street, curb or public sidewalk. Quikrete notes that the part of a driveway between the street and the sidewalk is usually controlled by the local municipality, and permission must usually be obtained before cutting a curb.^[9]
• The ground is soft, wet or recently filled. A base that will not compact needs more than a hand tamper.
• You find cracks you cannot explain. See why new concrete cracks.

Getting it done in Virginia

If you would rather hire it out, read what to ask a concrete contractor before hiring them and what a concrete slab costs in Virginia. Loclis lists DPOR-licensed businesses with their licence number and class; look up concrete contractors in Blacksburg, browse concrete and flatwork, or see every concrete and hardscaping answer.

When you are ready, describe the slab once and licensed local businesses quote it.