10/10/2007. Is it Dark at Night?

Transcription

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

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

3 Plastic Shrinkage Plastic Shrinkage Plastic Shrinkage Cracks Prevention Fog mist Evaporation retarder Man in a Can Buying Time Polypropylene Fiber Wind or sun screens Faster set 3

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

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

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

7 Cracking of Concrete Due to Shrinkage Original Length Unrestrained Shrinkage No Stress Developed Restraints Cause Tensile Stress When Tensile Stress is Greater Than Tensile Strength... Concrete Cracks. 200 to 600 psi Tensile Stress Developed 27 cubic feet 2 cubic feet are coming back out! Pounds of Water EXCESS WATER IN ONE CUBIC YARD OF CONCRETE WITH 500 POUNDS OF CEMENT Water/Cement Ratio 7

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

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

10 What happens? The paste shrinks more than the concrete, so when the concrete is over worked the aggregate gets pushed down leaving only paste near the surface. It shrinks more than the concrete below it, leaving a fine map pattern of cracking. Crazing Cracks Prevention Proper curing Moderate slump or use admixtures Don t finish bleed water into concrete Don t sprinkle or spray water on the slab Use surface retarders whenever it is hot, dry and windy. Especially cool dry and windy Windy means +5 MPH! Curling Concrete slabs curl due to differential moisture through the thickness, and the differing shrinkage response. Moisture Profile Differential Shrinkage Curling Response Cracking Upon Loading 10

11 Curling in Slabs Differential volume change from Differential moisture loss Temperature gradient Prevention Proper curing Proper jointing Low water mix- reasonable Cement content, max Agg. sizes No poly vapor barriers Prevent excessive bleeding Shrinkage reducing admixture Evaporation Control/retarder Jointing Those Planned Cracks Do not extend joint spacings over 1 1/2 times length versus the width. 2 to 3 times the depth ¼ the depth - minimum Example: A four foot wide slab the joints should not extend past 6 feet. 11

12 Compacting the Subgrade Base/subgrade 12

13 Base/subgrade Joint Planning Watch those corners Plan Joints - -prior to placement 13

14 Joint Minimization, Crack Elimination Shrinkage Reducing Admixes Warranty Slabs Cost savings on Joint stretching, maintenance. 40 X 40 Portland, 50 X 50 in Seattle Performance of SRAs Reduce shrinkage by as much as 80% at 28 days. Reduce ultimate shrinkage on the order of 25 to 50%. Reduce-not eliminate Allow up to 50 ft joint spacing with no cracking Glacier Northwest (6 thick, proper practices, 3 day wet cure, etc.) SRA job: Layout & Planning 14

15 Greg s baby No Restraint Details 15

16 EIC Winner SRA Impact on Curling Joint Minimization, Crack Elimination TYPE K Komponent Cement Shrinkage compensating Cement 15% Replacement of regular cement Big Box Floors Fred Meyer Old Navy Parr Lumber Additional Curing Requirements Operational Challenges- Silo or Bags 16

17 ORIGINAL LENGTH Potential Expansion Actual Expansion K-FIBER or REBAR CONCRETE FINAL LENGTH Figure 1 System-K with Fibers Type K with Rebar 24 ocew) Traditional Portland Concrete with Rebar 24 ocew) Red = Portland Cement & Rebars Blue = Type K & Rebars Green= Type K & K Fibers-no Rebars Test slabs were: 50 X 6 by 6 thick 50,000 square foot slabs Portland Concrete has 6270 lf of joints System-K Concrete has only 210 lf of joints OVER A MILE OF JOINTS ARE ELIMINATED Figure 3 17

18 How Type K Works Komponet System Conventional ODOT DECK w/c 0.45 Hydration Place w/c 0.40 Hydration w/c 0.25 Current DOT s Using Type K California DOT Michigan DOT Ohio DOT Mix Design Cement 15 Percent type K Standard Aggregates W/C ratio of 0.45 to 0.50 Slump of 5 to 6 inch Standard admixtures 18

19 Fred Meyer Distribution Center Fred Meyers Distribution Chehalis, Center WA Chehalis 60,000 square foot expansion x 55 joint spacing Costco Distribution Center Salt Lake City, UT 300,000 square feet 100 x 200 joint spacing 19

20 Old Navy Foam around Columns Bars at Corner 20

21 Parr Lumber Flatness Water Cure 21

22 Sofa Express 70,000 square feet Max Joint Spacing 130 No Rebar After 5 weeks FF = 98.6 FL = 61.0 No cracks Cracks Normally Occur at: 1. Mid-Panel 2. Columns in the Slab (notice no boxouts) 3. Perimeter Columns 4. Bollards 5. Re-entrant Corner 6. Plastic and Crazing Type K Advantages: No Drying Shrinkage Easily Placed Easily Finished Lower maintenance Costs No need for joints, dowels, etc. Up to 50,000 square feet without joints Length to width ratios increase No Curling Moisture is used - Vapor tests faster 22

23 Crack Control - Summary Prepare subgrade properly-watch restraints Specify and Order correct mix-don t add water! Plan and cut proper control joints Prevent excessive evaporation Start curing early Synthetic Fiber Reinforcement Shrinkage Reducing Admixtures Type K Cement Smokey the Bear Only YOU can prevent Cracks 23