September 14, Structural Engineers Assn. of Ohio Cold & Hot Weather & Freezing. Ken Hover, P.E., Cornell University

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1 Cold Dry Hot Dry Hot and Cold Weather Concrete and Freeze-Thaw Resistance Evaporativity Concrete Temperature Ken Hover, P.E. Cornell University Cool Wet Hot Wet Evaporativity Temperature Effects Moisture Effects Concrete Temperature The Weather Challenge in Ohio 1

2 Columbus, Ohio Monthly Average Temperatures Why Concrete Temperature Matters 1.) Reaction only proceeds in a water-filled space 2.) Rate of reaction exponentially dependent on temperature 3.) Volume depends on extent of reaction and loss of water Hydration 4 Concrete Temperature (C) C 23C 36C 4 Concrete Temperature (C) ydration Rate Relative H Compared to Lab Cure % 1 % 5 % 52 F 73 F 96 F Concrete Temperature (F) ydration Rate Relative H Compared to Lab Cure Risk of Freezing Concrete Temperature (F) kch7@cornell.edu 2

3 16 Concrete Temperature (C) Net loss due to freezing Setting Time Relative Compared to Lab Cure Concrete Temperature (F) Temperature and Stages in the Life of Concrete Columbus, Ohio Monthly Average Temperatures 52F = 5% Hydration rate compared to 73F Lab Temp 52FC = 2% Set Time compared to 73F Lab Temp Temperature and Water Demand for Workability Increased Water Demand with er Concrete Temperature About 1 gallon / yard per 1 degrees F kch7@cornell.edu 3

4 6 Curing Temperature ( C) Temperature and Compressive Strength ve Strength (psi) Compressiv day strength ve Strength (MPa) Compressiv Curing Temperature ( F) 6 Curing Temperature ( C) ve Strength (psi) Compressiv day strength 1 day strength ve Strength (MPa) Compressiv Constructability bl Problems! Curing Temperature ( F) Columbus, Ohio Monthly Average Temperatures 52F = 5% Hydration rate compared to 73F Lab Temp 52FC = 2% Set Time compared to 73F Lab Temp These Observations Lead to Concrete Temperature Specifications kch7@cornell.edu 4

5 F Specified Min. 28-day Optimum Std. Lab Cure Specified Max C Concrete Temperature Concrete Temp perature (F) National way Institute Concrete Materials Module University of Nevada at Reno, Minutes after Batching 15 Heat of Hydration Heat of Hydration Heat of Hydration 4 15 W lamps 6SackMix= 564 lb x 19 BTU = 17, BTU / CY 564 lb Cement / CY 64 Watts / CY kch7@cornell.edu 5

6 Concrete Temperature (C) Cold Weather Concreting ydration Rate Relative H Compared to Lab Cure of Freezing Risk Concrete Temperature (F) Hot Weather Concreting kch7@cornell.edu 6

7 Moisture Matters 1.) Reaction only proceeds in a water-filled space 2.) Rate of reaction exponentially dependent on temperature 3.) Product depends on extent of reaction and loss of water Surface Drying and Shrinkage Hydration How soon until this concrete starts to dry? Concrete Slab on Ground in dry air 1.) Bleed water appears on concrete surface 2.) Bleed water evaporates 3.) Drying of concrete surface kch7@cornell.edu 7

8 Humidity Air Texas Evaporation Pressure Water Vapor Ev vaporation WIND Water Temperature ativity /hr Evapora lb/ft 2.5 lb/ft 2 /hr, Va. SF Overlay Days From Jan 1 Freeze Thaw Damage and Deicer Scaling Resistance kch7@cornell.edu 8

9 Freeze Thaw Damage and Deicer Scaling Resistance Control porosity by controlling water Control Finishing Curing & Shrinkage control Temperature Control Frost-Resistant Aggregate Air-Entrained Paste Air Entrained Concrete Concrete to which a detergent has been intentionally ti added d for the purpose of stabilizing air bubbles formed during the concrete-mixing process. Air Void Disadvantages Strength Reduction Porous Paste Aggregate Interface Required Compensations er water content Admixtures er cement content Cost Heat Shrinkage kch7@cornell.edu 9

10 Clustering of Air Bubbles Disadvantages Stickiness Finishing Problems Bleeding, Plastic Cracking, Delaminations Variable Volume Yield problems Interaction with other admixtures ACI 32 - Construction of Floors Entrained air not recommended when smooth, dense, hard troweled finish required Porous Paste Air Voids ACI 31 - Specification Intentionally entrained air should not be incorporated in normalweight concrete slabs to be given a smooth, dense, hard troweled finish. Random point: freezing begins Air Voids Pressure Ice Expands Water and Ice move in direction of least resistance Pressure Pressure The longer the flow path, The higher the pressure The Longer the line, the higher the pressure for a given pumping rate. Pressure Pressure kch7@cornell.edu 1

11 The LESS permeable the concrete, The higher the pressure Pressure Pressure Pressure The smaller the hose diameter, the higher the pressure for a given flow rate. How far can water and ice travel in the capillary pores until the pressure cracks the paste?.1 INCHES OR THEREABOUTS 1 mil poly slab underlayment ~ 1 mils ~ 1 mils ~ 1 mils ~ 1 mils Double thickness = 2 mils.2 INCHES OR THEREABOUTS kch7@cornell.edu 11

12 Does a Large Air Void protect more paste than a Small Air Void? YES! Protected Paste: Asset This is why we want the air voids! Air Void Volume: Liability Costs 2-25 psi Per percent Air Volume And leads to other problems like delayed bleeding and blisters Does a Small Air Void protect more paste PER PERCENT Air Volume than a Large Air Void? Does a Large Air Void protect more paste than a Small Air Void? Does a Small Air Void protect more paste PER PERCENT Air Volume than a Large Air Void? Does a Large Air Void protect more paste than a Small Air Void? YES! YES! Absolutely! Absolutely! The smaller the air void, the higher the ratio of Asset/Liability! The smaller the air void, the higher the ratio of Protected Paste Volume/Air Content Wanted: Large number of small voids Well distributed in paste Prefer bubbles around 5 mils < About 1 mils to nearest bubble Air vol about 18% of paste vol kch7@cornell.edu 12

13 Characteristics of AE Concrete 4-8% air bubbles by volume of concrete 1 1 to 2 ft 3 Air bubbles / CY billion air bubbles / CY 2 2 million in 2 of air bubble surface area / CY 15 SY of bubble surface area / CY Bubble Size Matters! How much Air do you need? How much paste do you have to protect? How severe is the exposure? Who are you asking? w/c =.45 Required Air Conte ent in Concrete (%) Water Content kg/m Average Bubble Radius = 7 mils Average Bubble Radius = 6 mils Average Bubble Radius = 5 mils Average Bubble Radius = 4 mils 43 in 2 / in 3 5 in 2 /in 3 6 in 2 /in 3 75 in 2 /in 3.7 in Average Bubble Radius = 3 mils 1 in 2 /in Water Content lb/cy kch7@cornell.edu 13

14 Durability Factor Durability Vs. Spacing Factor Computed Spacing Factor Materials Issues Frost-Resistant Aggregate w/c or w/cm.45 Controlled water content Air content 18-22% of paste content Periodic check of air bubble size Periodic check of air bubble stability Control the water SCM s and Scaling Slag/FA/OPC concrete can be scaleresistant. Air quality key. Charcoal is the problem. Cure for at least 7 days. Finish on time. (Delayed Bleeding) Slag & Fly Ash may be more sensitive to poor construction practice than OPC. Late season placement needs protection Construction Issues Control water Periodic check of air stability Timing of finishing operations Timing of finishing operations No surface water addition Wet curing kch7@cornell.edu 14

15 Standard Specs won t produce scaling resistance! Specifying Freeze-Thaw Resistance W/C max =.45 (in-place) Air content = 18% paste vol In place spacing factor max.11 Check ability of materials to stabilize a satisfactory air void system Specifying Scale Resistant Concrete W/C max =.45 (in-place) Wet cure 7 days (truly wet) Air vol = 2% of paste vol Spacing factor < =.8 inches Check ability of materials to stabilize a satisfactory air void system Timing of Finishing is critical Not too soon / not too late kch7@cornell.edu 15