How concrete works: mix, strength, cracking and curing

Cement is not concrete

People use the words interchangeably, but cement is one ingredient. NRMCA describes concrete as a paste of cement and water that coats sand and stone and binds them into a rock-like mass^[1]. By volume, a typical mix is about^[1]:

Ingredient	Share of the mix
Aggregate (sand and stone)	60% to 75%
Water	15% to 20%
Cement	10% to 15%
Entrained air (in many mixes)	Another 5% to 8%

The hardening is a chemical reaction called hydration, not drying^[1]. That is why curing keeps concrete moist instead of letting it dry out. See concrete vs cement.

What psi means

Concrete strength is rated by compressive strength in pounds per square inch (psi), usually tested at 28 days. The model residential code (IRC Table R402.2) sets minimums by use and climate^[5]:

Use	Negligible weathering	Moderate	Severe
Basement and interior slabs (not garages)	2,500 psi	2,500 psi	2,500 psi
Porches, carport slabs, exterior steps, garage floors	2,500 psi	3,000 psi	3,500 psi

In moderate and severe climates the code also calls for entrained air, 5% to 7% by volume, on those exposed slabs^[5]. Air bubbles give freezing water room to expand. Higher psi alone does not make a slab freeze-proof. See concrete mix and PSI explained.

Why all concrete cracks

Concrete shrinks as it sets and dries, and it is weak in tension. NRMCA explains that when shrinkage is restrained, tensile stress builds until the concrete cracks^[2]. Cracks cannot be prevented entirely, only controlled^[2]. NRMCA's list of causes of excessive cracking includes too much water in the mix, poor subgrade preparation, missing joints, fast drying and poor curing^[4].

Two myths worth dropping:

• "Mesh or rebar prevents cracks." NRMCA states that wire mesh will not prevent cracking; it tends to keep cracks and joints from opening up^[2].
• "A crack means the slab failed." Most early shrinkage cracks are cosmetic. Offset, widening and sinking are the warning signs.

See why concrete cracks and our concrete repair section.

Joints: planned cracks

Joints give shrinkage a place to go. NRMCA's rules^[2]:

• Space joints at no more than 24 to 36 times the slab thickness. A 4-inch slab gets joints about every 10 feet, never more than 15.
• Keep panels square or nearly so, no longer than 1.5 times their width.
• Cut joints at least a quarter of the slab depth, and not less than 1 inch.
• Saw cuts go in within 4 to 12 hours after finishing. Wait too long and random cracks can form first.

See control joints and spacing.

Curing, weather and timing

Curing keeps new concrete moist and at a workable temperature so it reaches its designed strength. NRMCA's guidance^[3]:

• Cure for at least 3 to 7 days, or as the specification requires.
• Keep the concrete above 50 F.
• In cold weather, do not let it cool faster than 5 F per hour for the first 24 hours, and protect it from freezing until it reaches at least 500 psi.
• Concrete that dries out can lose as much as 50% of its potential strength.

HomeGuide's practical timeline for a driveway: walk on it after 1 to 2 days, drive on it after 7 to 10 days, and keep heavy trucks and RVs off for 30 days^[7]. See curing new concrete, pouring in cold or hot weather and when can you drive on new concrete.

Ready-mix, bags and cost

For anything bigger than a post hole or a small pad, concrete arrives by truck. HomeGuide puts ready-mix at $119 to $172 per cubic yard delivered and poured, and installed flatwork at $5 to $10 per square foot^[6]. Bagged mix makes sense for small repairs and setting posts. Mixing many bags by hand makes it hard to get consistent strength. See ready-mix vs bagged concrete.

How long concrete lasts

HomeGuide lists concrete driveways at 30 to 40 years^[8] and patios at 30 to 50 years^[9]. Life depends on the base, the climate, deicing salt, drainage and how well the slab was cured. See how long concrete lasts and the concrete thickness guide.

Reading a delivery ticket

Every ready-mix truck carries a ticket. It lists the mix ordered, the volume, the time the concrete was batched and any water added on site. Ask your contractor to keep it. If a slab scales or cracks badly in its first winter, the ticket shows whether the mix matched the order and whether extra water went in at the job.

Every concrete basics guide

• Concrete vs cement
• Concrete mix and PSI explained
• Why concrete cracks
• Control joints and spacing
• Curing new concrete
• Pouring in cold or hot weather
• Ready-mix vs bagged concrete
• How long concrete lasts
• When can you drive on new concrete
• Concrete thickness guide

When to call a pro

• Any pour bigger than a small pad, or anything that carries vehicles, a structure or a hot tub.
• Pours planned in freezing weather or extreme heat, which need protection plans.
• Cracks that are offset, widening, or paired with sinking or heaving.
• When a contractor proposes adding water on site or skipping curing: ask why first.