7.4 Concrete Defects, Joints, and Cold-Weather Practices
Key Takeaways
- Match defect to cause: honeycomb=poor consolidation, scaling/dusting=finishing over bleed water, plastic shrinkage/crazing=rapid surface drying, spalling=corroding low-cover rebar.
- Control (contraction) joints locate shrinkage cracks (saw-cut 1/4 the slab depth); construction joints are stop/start points; isolation joints allow independent movement.
- Control-joint spacing ~2-3 times slab thickness in inches converted to feet (a 4-in slab = ~10 ft); saw-cut early before stress builds.
- Cold weather (ACI 306, below 40 deg F): keep concrete above 50 deg F and protect from freezing until ~500 psi or it loses ~50% strength; never place on frozen ground.
- Markup adds a percent to cost; margin is a percent of price (20% margin on $50,000 cost = $62,500 price, higher than a 20% markup price of $60,000).
The final concrete topic covers defects, joints, and weather practices — heavily scenario-based on the NASCLA exam. References include ACI 224 (cracking), ACI 306 (cold weather), and ACI 305 (hot weather). Examiners test whether you can match a symptom to its cause and select the correct joint type.
Common defects and causes:
| Defect | Cause |
|---|---|
| Honeycomb | Poor consolidation / oversize aggregate / segregation |
| Scaling | Finishing over bleed water; no air entrainment + freeze-thaw |
| Dusting | Finishing with bleed water; low cement at surface |
| Plastic shrinkage cracks | Rapid surface drying (hot, windy) before set |
| Crazing | Surface map-cracking from rapid surface drying |
| Spalling | Corroding rebar (low cover) expanding |
| Efflorescence | Soluble salts carried to surface by moisture |
Trap: scaling and dusting both trace back to finishing over bleed water.
Joints control where concrete cracks and how it moves:
- Control (contraction) joints: tooled or saw-cut grooves that create a weakened plane so shrinkage cracking occurs at the joint, not randomly. Cut to a depth of 1/4 the slab thickness.
- Construction joints: where one placement stops and the next begins (use keyways or dowels).
- Isolation (expansion) joints: full-depth separation from walls, columns, footings to allow independent movement.
Trap: a control joint does not prevent cracks — it locates them.
Worked joint numeric: Control joint spacing rule of thumb is about 24 to 36 times the slab thickness (in feet), commonly 2-3 times the slab thickness in inches = feet.
For a 4-inch slab: spacing ~ 2.5 x 4 = 10 ft maximum. Depth of saw cut: 1/4 x 4 in = 1 in.
Trap: oversized panels or skipped joints lead to random shrinkage cracking. Saw-cut early (within hours, before shrinkage stress builds).
Cold-weather concreting (ACI 306) applies when the average daily temperature is below 40 deg F for 3+ days. Goals: keep concrete above 50 deg F during placement/curing and protect from freezing until it reaches 500 psi.
Practices:
- Heat mixing water and/or aggregate.
- Use Type III cement or an accelerator (non-chloride near steel).
- Insulating blankets/heated enclosures.
- Never place on frozen ground.
Trap: fresh concrete that freezes before ~500 psi suffers permanent strength loss (~50%).
Hot-weather concreting (ACI 305) risks rapid slump loss, plastic shrinkage cracking, and flash set above ~90 deg F.
Practices: use a retarder, cool the mix water (chipped ice), dampen subgrade and forms, place during cooler hours, fog/cure immediately.
Markup vs. margin trap (business crossover): On a $50,000 concrete job, a 20% markup = 50,000 x 0.20 = $10,000 added (price $60,000). A 20% margin means cost is 80% of price: price = 50,000 / 0.80 = $62,500. Margin always yields a higher price than the same-percentage markup.
A 5-inch-thick interior slab develops random cracking across the floor. Which joint type, properly spaced, would have controlled where the cracks formed?
Under ACI 306 cold-weather practice, what is the main risk if fresh concrete freezes before reaching about 500 psi?
Cracking and Common Defects
Know the defect-to-cause map. Plastic shrinkage cracks (early, hot/windy, rapid surface drying) — mitigate with fogging/windbreaks. Drying shrinkage cracks (long-term moisture loss) — control with joints. Scaling/spalling (freeze-thaw on non-air-entrained concrete or finishing bleed water). Honeycombing (voids from poor consolidation or form leakage). Crazing (fine surface map cracks). Dusting (weak surface from finishing bleed water or low cement).
Joints — Three Types
- Control (contraction) joints: deliberate weakened planes (saw-cut 1/4 of slab depth, spaced ~24–36× slab thickness in feet) so the slab cracks in a straight, hidden line. A 4-in slab → joints ~8–12 ft apart.
- Construction joints: where one pour stops and the next begins (keyed or doweled to transfer load).
- Isolation (expansion) joints: full-depth, separate the slab from columns/walls so each moves independently. Saw control joints early (within ~6–12 hours) to beat random cracking.
Cold- and Hot-Weather Concreting
Cold weather (ACI 306, below ~40°F): use heated water/aggregate, accelerators (non-chloride), insulating blankets/enclosures; protect from freezing until it reaches ~500 psi. Hot weather (ACI 305, above ~90°F or high evaporation): use chilled water/ice, retarders, place early morning, fog and cure immediately to prevent plastic shrinkage. Never place concrete on frozen ground.
Common Exam Traps
- Trap: Control joints prevent all cracking. They direct cracking to a planned line.
- Trap: Saw control joints days later. Cut early (6–12 hrs) before random cracks form.
- Trap: Honeycombing comes from too much water. It comes from poor consolidation/form leaks.
- Trap: Placing on frozen ground — never do it.
A control (contraction) joint is being saw-cut in a 4-inch slab. What is the appropriate cut depth?
Worked Joint-Spacing Example and Repair Basics
Example: A 6-in slab using a 30× thickness rule spaces control joints at 30 × 6 in = 180 in = 15 ft maximum. Lay out joints to form roughly square panels (avoid long, thin panels that crack across the middle). For repairs, match the method to the defect: epoxy-inject structural cracks, patch spalls with a bonding agent and repair mortar, and grind/overlay scaled surfaces. The exam rewards diagnosing the cause first — repairing a moving crack without addressing the joint just cracks again.
Load Transfer at Joints and Curling
At construction and control joints in slabs that carry wheel loads, dowels or a keyway transfer shear so the two sides deflect together — without load transfer the edges spall under traffic. Slabs also curl when the top dries and shrinks faster than the moist bottom, lifting the edges and corners; proper curing and joint spacing limit it. The exam connects these back to causes: a curling, spalling slab usually traces to inadequate curing, missing load transfer, or joints spaced too far apart.