Concrete: driveway, walk and steps.

Why concrete fails here

River Forest sits on clay. Clay holds water, expands when it freezes, and shrinks when it dries. A slab poured on clay heaves in January and settles in August, and after a few cycles it cracks along whatever line the stress finds. Add the freeze-thaw cycles of a Chicago winter, road salt tracked in from Lake Street, and the roots of hundred-year oaks, and a driveway poured wrong lasts eight years instead of thirty.

Every part of a good concrete estimate is a defense against one of those four things.

What is under the slab

The single most important line on a concrete estimate is the base. Good practice on clay is to excavate the soil, lay four to six inches of crushed stone, and compact it mechanically in lifts. The stone drains water away from the underside of the slab so it cannot freeze there, and it spreads the load so the slab does not bridge soft spots and crack.

Pouring directly on clay, or on two inches of uncompacted gravel raked out by hand, is where cheap driveways come from. You cannot see it after the pour. Ask for the base depth, the material, and how it is compacted, and be there the morning before the trucks come.

Thickness, mix and steel

Thickness. Four inches is the standard for walks and patios. Driveways carry cars and the occasional delivery truck and want five inches, six at the apron and anywhere a garbage truck turns. A four-inch driveway is a common shortcut.

Mix. Exterior concrete in this climate should be at least 4,000 psi, air-entrained, with a low water-cement ratio. Air entrainment puts microscopic bubbles in the mix that give freezing water room to expand; without it, the surface scales off in the first winter. It should be on the estimate by name.

Steel. Concrete cracks. Steel does not stop that; it holds the crack tight so the pieces do not separate and heave. Welded wire mesh is the minimum; rebar on a grid, set on chairs so it sits in the middle of the slab and not on the gravel, is better for driveways. Fiber reinforcement in the mix helps with surface cracking and does not replace steel.

Reading the estimate, row by row

Line these rows up across every estimate you collect. Where two of them differ, that is where the price difference usually lives.

Control joints and where cracks go

Since concrete will crack as it cures and moves, control joints are cut or tooled into the surface to tell it where. A joint every eight to twelve feet on a four-inch slab, in a grid, with the panels roughly square, and the joint at least a quarter of the slab depth. Expansion joints, with a compressible filler, go where the new slab meets the house, the garage, the public walk and any other fixed structure, so the slab can move without pushing.

A driveway with joints twenty feet apart, or none, will crack randomly. A driveway poured tight to the garage slab with no expansion joint will heave the garage floor.

Pitch, drainage and the basement

Every slab near the house should pitch away from it, about two percent, a quarter inch per foot. A driveway pitched toward the garage sends water under the door and into the basement stairwell; a patio pitched toward the house is a basement waterproofing problem waiting for the next storm. Where the driveway slopes toward the garage because the lot does, a trench drain in front of the door with a pipe to daylight is the answer, and it belongs on the estimate.

See the waterproofing guide for why this matters more here than the concrete contractor may mention.

The apron and the public sidewalk

The strip of driveway between the sidewalk and the curb is the apron, and with the public sidewalk and the curb it belongs to the Village. Work on any of it needs a Village permit and must follow the Village construction details, which specify thickness, reinforcement and the profile at the curb, with an inspection before the pour.

The Village rates public sidewalk squares during its annual inspections. It replaces those rated Red at full Village cost, and for those rated Yellow it offers a 50/50 cost share on request. Before you pay a contractor to replace the walk in front of your house, ask Public Works whether it is on the list. Your contractor can pour the private walk and the Village's crew the public one, in the same season, and the public walk costs you half or nothing.

Finishes

A broom finish, dragged across the surface for traction, is the standard for driveways and walks, and the right one in a climate with ice. Smooth trowel finishes are slippery outdoors. Exposed aggregate and stamped or colored concrete cost more, look good on patios, and need sealing on a schedule. Whatever the finish, the edges should be tooled and the joints straight; those are the parts you see from the porch every day.

Curing, sealing and the first winter

Concrete reaches most of its strength in a week and keeps curing for a month. Keep it damp for the first few days, stay off it for two, keep cars off a driveway for a week, and never let a truck on it for a month. A good contractor will tell you this and may apply a curing compound.

The first winter is the hard one. Do not use salt or ice melt on new concrete; use sand. A penetrating silane or siloxane sealer applied after the first month protects the surface from salt and water for a few years and is worth the modest cost on a driveway.

What drives the price

Square footage sets the base. Then demolition and hauling, if there is old concrete to remove. Then the spec: base depth, thickness, air-entrained mix and rebar together add 20 to 40 percent over the shortcut version and roughly double the life. Then finish, drainage and the apron. Access matters too; a back patio with no side-yard access for a truck means pumping or wheelbarrows, and both cost time.

Red flags

• No base depth, or "gravel base" with no compaction stated.
• Four-inch driveway.
• No psi or air entrainment on the page.
• Steel not mentioned, or "fiber" offered as a substitute.
• No pitch or drainage line.
• No permit line. Everything on this page needs one.

Questions to ask

• How deep is the base, what is it made of, and how do you compact it?
• What thickness, what psi, and is the mix air-entrained?
• Rebar or mesh, and how do you keep it in the middle of the slab?
• Where are the control joints and expansion joints?
• Which way does it pitch, and where does the water go?
• Who pulls the Village permit for the apron, and is the public walk on the Village program?
• When can I park on it?

The short list

Compacted stone base, depth stated. Five inches on a driveway. Air-entrained 4,000 psi. Rebar on chairs. Joints on a grid, expansion at the house. Pitched away, drain where needed. Permit and apron detail. Seven lines, and the driveway outlasts the car, the next car, and most of the oaks’ remaining branches.

Permits, programs and the Village