Module 4:Troubleshooting Common Concrete Defects

1. Identifying Causes of Cracking, Blistering, and Scaling

Cracking

• Plastic Shrinkage Cracks:

• Cause: Rapid surface drying in hot/windy weather before concrete sets

• Appearance: Short, irregular cracks (often spiderweb pattern)

• Prevention: Use evaporation retardants, fog spraying, and windbreaks

• Drying Shrinkage Cracks:

• Cause: Excessive water in mix or improper joint spacing

• Appearance: Long, straight cracks between control joints

• Prevention: Proper water-cement ratio (0.40-0.50) and joint spacing (24-36x slab thickness)

• Structural Cracks:

• Cause: Insufficient reinforcement or subgrade settling

• Appearance: Wide, deep cracks that propagate through slab

Blistering

• Cause: Trapped air/water under sealed surface during finishing

• Contributing Factors:

• Over-troweling while bleed water is present

• High-slump concrete with excessive bleed water

• Rapid drying conditions

• Prevention: Delay finishing until bleed water evaporates

Scaling

• Cause: Freeze-thaw damage or deicer chemicals on non-air-entrained concrete

• Key Indicators:

• Flaking/spalling of surface (1/8"-1/2" deep)

• Exposed aggregates

• Prevention:

• Use 4-7% air entrainment in freeze-prone areas

• Proper curing (minimum 7 days)

• Avoid finishing while bleed water is present

2. Preventing Plastic Shrinkage and Crazing

Plastic Shrinkage Prevention

1. Environmental Controls:

• Erect windbreaks (6 ft height minimum)

• Use sunshades in hot weather (>85°F)

2. Surface Protection:

• Apply aliphatic alcohol evaporation retardant

• Begin curing immediately after finishing

3. Mix Design Adjustments:

• Use shrinkage-reducing admixtures

• Consider synthetic fibers (0.5-1.5 lb/yd³)

Crazing Prevention

• Cause: Overworking surface with steel trowels

• Solution:

• Final finish with magnesium floats instead of steel trowels

• Avoid adding water during finishing

• Maintain consistent curing conditions (no wet/dry cycles)

3. Repair Techniques for Surface Imperfections

Hairline Cracks (<1/8")

1. Routing and Sealing:

• V-cut crack with angle grinder (1/4" wide x 1/2" deep)

• Fill with polyurethane sealant

2. Epoxy Injection:

• For structural cracks - inject low-viscosity epoxy under pressure

Scaling/Spalling Repairs

1. Partial-Depth Repair:

• Saw-cut perimeter (minimum 1" beyond damage)

• Chisel out to sound concrete (1" minimum depth)

• Apply bonding agent + polymer-modified repair mortar

2. Topping Systems:

• For widespread damage - apply 2" bonded overlay

Blister Repair

1. Early-Stage:

• Pop blisters with roller when concrete is semi-hardened

• Re-trowel affected areas

2. Hardened Concrete:

• Grind down raised areas

• Apply micro-topping for seamless finish

Popouts

1. Remove offending aggregate

2. Fill with:

• Cementitious patching compound (for small voids)

• Sand-mix grout (for larger voids >1")

Key Quality Control Checks

Pro Tip: Always conduct moisture testing (ASTM F1869) before applying repairs to ensure proper bonding.