11.6 Concrete Curing, Joints, Testing & Defects

Hydration Does Not Stop at the Pour

Quick Answer: Curing is maintaining adequate moisture and temperature so cement hydration continues after placement. Concrete that dries out early never reaches its design strength no matter how good the mix was. Minimum curing is commonly 7 days for normal cement and about 3 days where Type III high-early-strength cement is used.

Concrete does not "dry" — it hydrates. Water is a reactant. Concrete kept continuously moist keeps gaining strength for months; concrete allowed to dry at day two stops permanently at whatever strength it had reached. In Arizona's low humidity, that difference can be 30% or more of design strength, and it is invisible until the cylinders come back.

Curing Methods and Duration

Warning: a curing compound sprayed too thin, or applied hours late after the surface has already dried, does not cure — it just makes the slab look wet. Coverage rate and timing are both specification items.

Joints: Three Types, Three Jobs

Concrete shrinks as it dries and moves as it changes temperature. Jointing controls where the resulting cracks go.

Spacing rules of thumb. Control joint spacing in feet is commonly two to three times the slab thickness in inches: a 4-inch slab gets joints roughly every 8 to 12 feet, a 6-inch slab roughly every 12 to 18 feet. Panels should be kept close to square — an aspect ratio beyond about 1.5 to 1 invites a crack across the middle of the panel regardless of the joints at its edges.

Sawcut timing is the other half. Cut too early and the saw ravels the edge; cut too late and the slab has already cracked on its own. The window for conventional wet sawing is roughly 4 to 12 hours after placement depending on temperature and mix, and early-entry dry saws move that window earlier. In July in Phoenix, the window closes fast — crews that plan to cut "first thing tomorrow" find random cracks waiting for them.

Testing: What Each Test Actually Proves

Slump is the one candidates over-read. A 4-inch slump load and a 4-inch slump load can have entirely different strengths, because a superplasticized 0.42 mix and a watered-up 0.58 mix can arrive at the same consistency. Slump verifies that the load matches what was ordered; it is not a strength test.

Cylinders are the acceptance test. Standard practice is a set cast per specified volume or per day's placement, with breaks typically at 7 and 28 days. Two curing regimes answer two different questions:

• Standard-cured (lab-cured) cylinders are stored under controlled moisture and temperature. They test the concrete as delivered and are the basis for accepting or rejecting the mix.
• Field-cured cylinders sit at the jobsite under the same conditions as the structure. They test the adequacy of curing and protection in place, and are used to decide when forms may be stripped or a slab may be loaded.

That distinction is a favorite exam item. If the field-cured cylinders break low but the standard-cured cylinders pass, the mix was fine and the curing failed — which is a contractor problem, not a supplier problem.

Worked example. A specification calls for 4,000 psi at 28 days. Cylinders break at 2,900 psi at 7 days and 4,150 psi at 28 days. The concrete complies: 7-day strength typically runs roughly 65–70% of 28-day strength, so 2,900 psi at 7 days was on track, and the 28-day result is the acceptance criterion.

Reading Defects Backwards to Their Cause

Notice how many of these trace to finishing too early. Bleed water must rise and evaporate before final floating and troweling; a crew that closes the surface while water is still underneath produces dusting or delamination almost every time. This is also why exterior air-entrained flatwork gets a broom or float finish rather than a hard steel trowel.

Exam tip: the exam presents a defect and asks for the cause. Sort by timing. Cracks in the first hours point to evaporation. Surface texture failures point to finishing. Flaking in service points to air entrainment. Cracks ignoring the joint pattern point to jointing depth, spacing, or timing.