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Concrete is a time-proven building product, however it is the inherent nature of the product to crack. Design and construction professionals make allowances for this normal occurrence by using crack control measures prior to and during the construction process.
Cracks that occur during the finishing process are a result of:
- Surface Shrinkage : Caused by rapid water loss, or by
- Settlement while the concrete is still in plastic or has yet to gain strength.
Cracks, which occur after hardening, are the result of drying-shrinkage, curling, and/or other thermal expansion/contraction. Using well-established concrete construction practices will minimize and control cracking at the job site. Quality workmanship and "know how" can prevent normal cracking from becoming a cracking "problem".
QUALITY CONCRETE STARTS WITH A SOLID SUB-BASE Sources:American Concrete Institute (ACI) Publications: ACI 332Guide for Residential Construction ACI 302Guide for Concrete Floors and Slabs ACI CCS-1Concrete Craftsman Slab on Grade Additional information is available. Click on a topic below to download information, courtesy of the National Ready Mixed Concrete Association in Silver Spring, Maryland. - Dusting Concrete Surfaces- Sealing Concrete Surfaces- Crazing Concrete Surfaces- Cracking Concrete Surfaces- Plastic Shrinkage Cracking- Joints in Concrete Slabs on Grade- Cracks in Concrete Basement Walls- Discrepancies in Yield- Low Concrete Cylinder Strength- Concrete Slab Moisture- Ordering Ready-Mixed Concrete- Making Concrete Cylinders in the Field- Self-Consolidating Concrete- Strength of In-Place Concrete- Curing In-Place Concrete- Hot Weather Concreting- Concrete Blisters- Finishing Concrete Flatwork- Chemical Admixtures for Concrete- Flexural Strength of Concrete- Flowable Fill Materials- Radon Resistant Buildings- Vapor Retarders Under Slabs on Grade- Concrete Pre-Construction Conference- Testing Comprehensive Strength- Pervious Concrete- Curling of Concrete Slabs- Delamination of Troweled Surfaces- Loss of Air Content in Pumped Concrete- Grout- Discoloration- Synthetic Fibers for Concrete- Corrosion of Steel in Concrete- Job site Addition of Water- Cold Weather Concreting- Supplementary Cementitious Materials- High-Strength Concrete- Structural Lightweight Concrete- Maturity Methods to Estimate Strength - A compacted, uniform sub-base is essential to avoid settlement cracking. - Concrete thickness must be equal throughout the placed section for control joins to function properly. - Exterior concrete should have grades established to promote good drainage away from dwellings. ESTABLISH A JOINT PLAN PRIOR TO THE POUR - Tooled or sawed control joints should be at least 1/4 the depth of the slab. - Joint spacing should be no more than ten feet apart for 4-inch thick concrete (Spacing = 30 times thickness in inches to a maximum of 15 feet, regardless of thickness.) - Joint placement should be completed within 24 hours after placement and finishing of the concrete surface. - Isolation joins are used to separate dissimilar construction and mist be full depth. PLACE CONCRETE AT A REASONABLE SLUMP - Use pumps, chutes, and wheelbarrows to avoid segregation. - The correct slump for residential requirements is 3 to 5 inches. - Excessive water will delay set time and reduce strength. - Excessive water also contributes to cracking, dusting, and scaling. - Use producer approved admixtures, like super plasticizers, to improve concrete flow. SHRINKAGE CRACKING CAN BE MINIMIZED OR AVOIDED - Direct sun, wind and low humidity cause rapid moisture loss from the surface, resulting in shrinkage cracks occurring before concrete hardens. - Freshly placed concrete must be protected from the environment. Use windbreaks, sunscreens, and fogging when necessary. - Dampening sub-grade prior to placement and immediate use of curing compounds minimize rapid water loss. - Environmental conditions cannot be ignored. CURING IS MANDATORY TO MINIMIZE CRACKING - Controlling the rate of water loss from the concrete after placement reduces the potential for cracking and surface defects. - Constant water curing is preferred. The early application of a curing compound is a practical alternative to water curing. - Pigmented curing compounds aid in even application and results. AVOID RE-ENTRANT CORNERS WITH PROPER PREPARATION - Cracks running out from the corners of columns or other rigid fixtures, which extend through the floor slab, are re-entrant corners. - Use expansion joint material to create an isolation joint around these fixtures or block them out. - Incorporate rigid structures into your joint plan to avoid cracking between slab corners and the fixture. AVOID CRAZING BY PROPER CURING - Shallow hairline cracks, commonly called "map cracking" are unsightly but do not affect the slabs' durability. - Troweling bleedwater into the surface will promote excessive surface shrinkage (crazing). - Starting the the curing process as early as possible can help minimize the occurrence of crazing.