To Crack or Not to Crack? To Crack or Not to Crack? That is the - PDF document

10/10/2007 To Crack or Not to Crack? To Crack or Not to Crack? That is the Question That is the Question Causes and Prevention Causes and Prevention Presented by: Dave Frentress Presented by: Dave Frentress Does Concrete Crack? Does Concrete Crack? Is it Dark at Night? Cracking Cracking � A recent NRMCA survey identified A recent NRMCA survey identified cracking as the number one cracking as the number one complaint in the concrete industry. complaint in the concrete industry. � How many types of cracks are there? How many types of cracks are there? 1

10/10/2007 20 Causes of Cracks 20 Causes of Cracks Types of Cracks After Hardening Before Hardening Physical Chemical Thermal Structural Plastic Construction Thermal Shrinkable Rebar Freeze Thaw Loads Plastic Formwork Early Frost Aggregates Corrosion Shrinkage Movement Damage Drying Alkali Seasonal Creep Plastic Sub grade Shrinkage Aggregate Variations Settlement Movement Crazing Carbonation Early Thermal Settlement Traffic Contraction Movement Vibration D Cracking Sulfate Identifying Causes of Cracks Identifying Causes of Cracks � Identify crack type Identify crack type • When did they show up? When did they show up? • What type of pattern? What type of pattern? � Look at L L Look at k t k t • Mix design Mix design • Ambient conditions Ambient conditions • Structure design & dimensions Structure design & dimensions • Placing, finishing, curing Placing, finishing, curing Plastic Shrinkage Cracks Plastic Shrinkage Cracks � Show up before concrete is set Show up before concrete is set � Typically disconnected Typically disconnected - - not through full not through full depth or thickness of structure depth or thickness of structure � Rapid evaporation Rapid evaporation - - slow bleeding slow bleeding � Weather Weather • Hot Dry Windy Hot Dry Windy • Cold Cold - - slow set Temp Drop slow set Temp Drop � Low bleed concrete Low bleed concrete • Low w/ c Low w/ c HRWR HRWR • High air High air Silica fume Silica fume 2

10/10/2007 Plastic Shrinkage Plastic Shrinkage Plastic Shrinkage Plastic Shrinkage Cracks Plastic Shrinkage Cracks Prevention Prevention � Fog mist Fog mist � Evaporation retarder Evaporation retarder • “Man in a Can” “Man in a Can” • Buying Time B B Buying Time i i Ti Ti � Polypropylene Fiber Polypropylene Fiber � Wind or sun screens Wind or sun screens � Faster set Faster set 3

10/10/2007 Man in a can… Man in a can… Plastic Shrinkage Cracks Plastic Shrinkage Cracks Prevention Prevention � Faster set Faster set � Higher early strength Higher early strength � Cover/ protect concrete Cover/ protect concrete Drying Shrinkage Cracks Drying Shrinkage Cracks � Show up 4 days to 8 weeks Show up 4 days to 8 weeks � Through entire structure at weak Through entire structure at weak point point � Normal shrinkage N N Normal shrinkage - 1/ 2” per 100’ l h i k l h i k 1/ 2” 1/ 2” per 100’ 1/ 2” 100’ 100’ � Aggravated by Aggravated by • High water content High water content • Insufficient control joints Insufficient control joints • Inadequate curing Inadequate curing 4

10/10/2007 Factors Which Affect Factors Which Affect Shrinkage Cracking Shrinkage Cracking � Paste to aggregate ratio Paste to aggregate ratio � W/ C ratio W/ C ratio � Aggregate type and � Aggregate type and Aggregate type and Aggregate type and gradation gradation � Cement Cement � Weather Weather � Finishing practices Finishing practices Drying Shrinkage Cracks Drying Shrinkage Cracks Prevention Prevention � Lower shrinkage mix Lower shrinkage mix � Larger Coarse Agg. size Larger Coarse Agg. size � � Lower water content Lower water content � Max .45 w/ c Max .45 w/ c � � Proper control joints Proper control joints Psst Psst - - they they actually are cracks actually are cracks - - planned ones planned ones � Per ACI Per ACI � � On the Plans! On the Plans! � � Proper curing Proper curing � If you want Curing, you MUST specify it. If you want Curing, you MUST specify it. � � Bid Item Bid Item � Batched into the load? Batched into the load? At the Plant… At the Plant… 5

10/10/2007 Added to the load! Added to the load! On the jobsite… What happens? What happens? Moisture loss creates volume Moisture loss creates volume i i l l l l reduction and tensile forces, these reduction and tensile forces, these exceed the concretes ability to exceed the concretes ability to resist shrinkage and the concrete resist shrinkage and the concrete cracks. cracks. Strength Results using W/C Values………… Strength Results using W/C Values………… 6

10/10/2007 Cracking of Concrete Due to Shrinkage Cracking of Concrete Due to Shrinkage Original Length Original Length Unrestrained Shrinkage Unrestrained Shrinkage No Stress Developed No Stress Developed Restraints Cause Restraints Cause Tensile Stress Tensile Stress When Tensile Stress When Tensile Stress is Greater Than is Greater Than Concrete Cracks. Tensile Strength... Tensile Strength... 200 to 600 psi Tensile Stress Developed 200 to 600 psi Tensile Stress Developed 27 cubic feet 2 cubic feet are coming back out! EXCESS WATER IN ONE CUBIC YARD OF CONCRETE WITH 500 POUNDS OF CEMENT 400 4 350 3 of Water 300 2 250 1 1 Pounds o 200 200 150 100 50 0 .25 .30 .35 .40 .45 .50 .55 .60 .65 .70 .75 Water/Cement Ratio 7

10/10/2007 Understanding Shrinkage Understanding Shrinkage Even after hydration, concrete is a porous material Why does concrete shrink Why does concrete shrink as it dries? as it dries? � Pores lose water due to Pores lose water due to hydration and evaporation hydration and evaporation � As pores become less than As pores become less than fully saturated, meniscus fully saturated, meniscus forms at the air forms at the air- -water water interface due to surface interface due to surface tension tension � The surface tension of pore The surface tension of pore solution which forms solution which forms meniscus also exerts inward meniscus also exerts inward pulling force on the side walls pulling force on the side walls of the pore of the pore � These forces in all pores in These forces in all pores in range of 2.5 to 50 nm is range of 2.5 to 50 nm is primary cause of shrinkage primary cause of shrinkage � C����������� P������� C����������� P������� Thermal Shrinkage Cracks Thermal Shrinkage Cracks � Show up 1st or 2nd night Show up 1st or 2nd night � Seasonal temperature variations Seasonal temperature variations � High cement mass concrete High cement mass concrete � May or may not go through entire May or may not go through entire May or may not go through entire May or may not go through entire structure structure � Shatter or three point are frequent Shatter or three point are frequent � Caused by Caused by • Rapid cooling of entire structure Rapid cooling of entire structure • Differential cooling Differential cooling 8

10/10/2007 Thermal Shrinkage Cracks Thermal Shrinkage Cracks Prevention Prevention � Reduce temperature rise Reduce temperature rise • Less cement Less cement - - more pozzolan more pozzolan � Increase early age strength Increase early age strength • Accelerators A A Accelerators l l t t � Reduce temperature loss or gradient Reduce temperature loss or gradient • Cover or insulate Cover or insulate • Beware Thermal shock! Beware Thermal shock! Crazing Crazing Crazing Cracks Crazing Cracks � Map cracking 1 to 7 days of age Map cracking 1 to 7 days of age � Shrinkage of surface paste Shrinkage of surface paste � Very fine Very fine - - depth of < 1/ 8” depth of < 1/ 8” � No impact on durability No impact on durability p p y y � Rapid loss of surface moisture Rapid loss of surface moisture • Lack of curing Lack of curing • High W/ C ratio (over a .45) High W/ C ratio (over a .45) • Finishing bleed water into surface Finishing bleed water into surface • “Blessing” the slab surface with water “Blessing” the slab surface with water • “Jitterbugging” “Jitterbugging” 9