Concrete Techno ology and Codes Non-structural Cracks in Concrete Letter Type of Cracking A B Plastic settlement C D E Plastic shrinkage F G Early thermal contraction H I J Subdivision Most Common Location Over reinforcement Deep sections K Thin slabs (and walls) Against formwork Natural Corrosion of reinforcement M I Calcium chloride Alkali-aggregate reaction Time of Appearance Rapid early drying Ten minutes to Excess bleeding Top of columns conditions three hours Trough and waffle Change of depth slab Diagonal Roads and slabs Reinforced concrete Rapid early drying Thirty minutes to six Random Low rate of bleeding slabs hours Reinforced concrete Ditto plus steel near Over reinforcement slabs surface Excess heat External restraint Thick walls generation One day or two or Rapid cooling three weeks Excess temperature Internal restraint Thick slabs gradients Floated concrete L Secondary Causes/Factors Arching Long-term drying shrinkage Crazing Primary Cause (excluding restraint) "Fair faced" concrete Slabs Columns and beams Inefficient joints Impermeable formwork Over troweling Lack of cover Precast concrete Excess calcium chloride Damp locations Reactive aggregate plus high-alkali cement Excessive shrinkage inefficient curing Rich mixes Poor curing Poor quality concrete Several weeks or months One to seven days, sometimes much later More than two years More than five years Why does concrete shrink? Non-structural Cracks z Fresh Settlement Plastic shrinkage z Hardened Drying shrinkage Thermal dilation Before Hardening Early frost damage z Plastic z Shrinkage Settlement z Construction movement Formwork Sub-grade PLASTIC SHRINKAGE CRACKING Typical Plastic Shrinkage Cracking Evaporation of Surface Moisture from Concrete Concrete Techno ology and Codes Plastic Settlement Subsidence Cracking Resistance to subsidence by top reinforcement Cracks usually appear along reinforcement bars… ..but can also form from differential settlement Resistance to subsidence by void tubes in hollow core After Hardening z Physical Shrinkable aggregates Drying shrinkage Crazing z Chemical Corrosion of rebar Alkali aggregate reaction Carbonation After Hardening z Thermal Freeze – thaw cycles External seasonal temperature variations Early thermal contraction External restraint Internal temperature gradients z Structural Accidental overload Creep Design loads Concrete Techno ology and Codes Drying Shrinkage Cracking Why cracks form Shrinkage and Cracking Drying Shrinkage Cracks Occur: In thin sections (low V/S ratio) z When: Several weeks after casting z Causes: z Excess water in mix High paste content Inefficient joints Poor curing z Remedies: Reduce water content of mix Improve curing Cracking tendency depends on many factors Shrinkage is Size Dependent Theoretical Shrinkage Stresses Mitigation of Drying Shrinkage Cracks z Aggregate z Content Size z z Workability z Ease of placement Consolidation Admixtures Chemical Mineral z Curing Eliminate external restraints by allowing joint movement P id crackk controll Provide steel distribution The role of fibers… z Fibers will: Reduce plastic shrinkage cracking Reduce bleeding z What about other types of cracking? Fibers can: Bridge cracks Distribute stresses and limit crack widths Volume is critical! Tensile Deformation Plain Concrete Tensile Deformation Steel Reinforced Concrete Tensile Deformation Fiber Reinforced Concrete Concrete Techno ology and Codes Thermal Cracking Internal Thermal Restraint HOT CENTER Temperature Rise and Cement Type ACI 224 Tolerable Crack Widths Exposure condition Dry air or protective membrane Humidity, moist air, soil Deicing chemicals Seawater and seawater spray; wetting and drying Water-retaining structures Tolerable crack width, in. 0.016 0.012 0.007 0.006 0.004 Crack Control Control of Cracking z Joints Isolation Contraction Construction z Reinforcement Cover Size of crack Frequency (numerous tight cracks) What’s allowable (crack width) Take an example… Sidewalk set above pavement z 500 ft long pavement strips p p z α = 6x10-6 in/in/°F z Approximately 0.7”/100’/100°F z Result… Omission of full depth isolation /expansion joint z Expansion p of adjacent pavement results in cracking and buckling of concrete sidewalk z Mid-Panel Cracking Crack Repair Fix large cracks prior to exposure Use: z MMA Epoxy injection Sealers Routing and sealing Effect of cover z Importance of curing on cracking z Summary Many forms of cracking z Concrete is weak in tension z Shrinkage! z Jointing z Concrete Techno ology and Codes Questions?
