BASICS OF CONCRETE. Durability

Transcription

1 BASICS OF CONCRETE Durability

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7 What Deterioration Mechanisms Are We Trying To Prevent? Corrosion Alkali-Silica Reaction (ASR) Alkali-Carbonate Reaction (ACR) Delay Ettringite Formation (DEF) Freeze-Thaw Scaling External Sulfate Attack 7

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9 What Deterioration Mechanisms Are We Trying To Prevent? Corrosion Alkali-Silica Reaction (ASR) Alkali-Carbonate Reaction (ACR) Delay Ettringite Formation (DEF) Freeze-Thaw Scaling External Sulfate Attack All of these mechanisms involve water... and the rate at which they proceed is somewhat dependent on the ease with which water (and any salts dissolved therein) can move through the concrete pore structure 9

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11 Control the water ingress, and we have solved the majority of our durability problems. How? 11

12 Holistic Approach to Durability Environment Design Materials Proportions Properties Concrete Production Placing & Finishing Curing Workmanship QC & Testing Cracking Maintenance Influenced by concrete producer & contractor Durability 12

13 Material Considerations for Durable Concrete Cement and Supplementary Cementing Materials Water to Cementitious ratio (Permeability) Use of materials with the correct chemistry and proportions to mitigate 13

14 Some of the following slides contain hardcore chemistry, and you will be require to commit at least one of these to memory! You will be tested at the end of this presentation. 14

15 Water to Cementitious Ratio (Permeability) Outflow = Q Hydrostatic pressure, h l X-section area = A Coefficient of permeability, k Q A l h 15

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17 High Permeability Increases in the water content or w/c ratio results in higher porosity cement becomes more dilute Porosity is directly related to strength and durability 17

18 Low Permeability Lower w/c results in smaller pores and less connectivity of the pores 18

19 Permeability (m/s) Water to Cementitious Ratio (Permeability) large spread of data indicates sensitivity to other mix parameters, maturity, curing and methods of measurement Water/Cement (Adapted from Hearn et al, 1996) 19

20 Permeability (m/s) Water to Cementitious Ratio (Permeability) Reduce k by 10X! Reduce w/cm Increase strength by ~ 50 to 60% Water/Cement (Adapted from Hearn et al, 1996) 20

21 Cement Chemistry Chemical Name Compound Percentage Tricalcium Silicate Alite (C 3 S) Dicalcium Silicate Belite (C 2 S) Tricalcium Aluminate Tetracalcium Aluminoferrite C 3 A 1-12 C 4 AF 5-15 Calcium Sulfate CŜH 2 <5 Sodium Oxide Na 2 O <1 Potassium Oxide K 2 O <2 21

22 Cement Hydration Goal: Increase C-S-H Reduce CH Reduce Porosity How do we achieve this goal? Age: Minutes Hours Days 22

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24 Use of SCM s (Fly Ash) 24

25 Use of SCM s (Fly Ash) Calcium Silicate Hydration of Portland Cement C 3 S + Water CSH + CH C 2 S + Water CSH + CH Hydration with Fly Ash CH + Silica + Water CSH CH + Aluminate +Water CAH 25

26 Pozzolanic Reaction Uses up CH which is weak and brittle Improves Transition Zone Porosity Aggregate Calcium Hydroxide CSH Ettringite 26

27 Pozzolanic Reaction Uses up CH which is weak and brittle Improves Transition Zone Porosity Aggregate Calcium Hydroxide CSH Ettringite

28 Permeability (m 2 ) Effect on Permeability Fly Ash OPC Age (days) 20X reduction With fly ash! 29

29 Not all Fly Ash is Created Equal CaO Class F MgO Class C MgO Fe 2 O 3 SiO 2 CaO Al 2 O 3 SiO 2 Fe 2 O 3 Al 2 O 3 30

30 Sulfate Attack 1.0 Control % C 20% C % F Time (months) M. Thomas 31

31 Sulfate Attack Testing Using Class C Fly Ash 32

32 Delayed Ettingite Formation (DEF) Related to Early Age Temperature History and Cement Chemistry Water C-S-H Water A S A S S A A S S Cement Grain A S A S A S = Ettringite Paste Water 33

33 DEF in TxDOT Structure 34

34 DEF in Prestressed Concrete Bridge Girder 35

35 Material Considerations for Durable Concrete Aggregates Reactivity Gradation D-Cracking Susceptibility 36

36 Reactivity of Aggregates (ASR) Water Water Paste K + Na + OH - Na + Alkali Silica Gel OH - SiO 2 K + Aggregate Na + OH - Water OH - K + Na + OH - K + Water Alkali 37

37 ASR in Concrete 38

38 ASR in Prestressed Concrete Bridge Girder 39

39 ASTM C 1260 for Fine Aggregates Limestone & Dolomite ASTM C 1260 Expansion 0.6 Consider ALL Aggregates Reactive % of sands tested thus far exceed 0.1% expansion 79% of sands tested thus far exceed 0.2% expansion Evens the playing field for all aggregate suppliers No premium for nonreactive aggregate Requires all concrete mixtures to incorporate a mitigation option No aggregate reactivity testing eliminates the discussion of whether ASTM C 1260 or ASTM C 1293 is the proper test to conduct. Natural Sands

40 How to Prevent ASR Use non-reactive aggregates Previous slide showed this is probably not realistic Eliminate moisture Large elements have enough internal moisture to drive reaction Limit Alkalis 3.5 lb/cy specfication Use Fly Ash or other SCM combinations 41

41 Need to have enough Class F fly ash Day Mortar Bar Expansion Results for Texas Class F Ash % Expansion % 15% 20% 25% 27.5% 30% 35% 40% 45% 50% % Replacement 42

42 Gradation of Aggregates Well-graded aggregate Low permeability Gap-graded aggregate Same w/c High permeability 43

43 Internal Curing w/lightweight Aggregates 44

44 IC Improves Concrete Properties 45

45 Corrosion High ph in concrete protects steel from corrosion forever unless Chlorides from deicing salts or seawater penetrate through the concrete cover and reach the steel or CO 2 CO 2 from the atmosphere penetrates through the concrete cover and reaches the steel In which case the steel will corrode!! 46

46 Corrosion Protection Strategies Non-chloride de-icers Cl Cl Membranes or sealers Low-permeability concrete Coatings on steel Non-corrosive materials Chemical corrosion inhibitors 47

47 Carbonation 48

48 Carbonation Cure, Cure, Cure Properly 49

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50 Freeze-Thaw and Surface Scaling 51

51 Oops! 52

52 Freezing Water in Concrete 53

53 Air Entrained Concrete 54

54 Grimes Jasper Newton Franklin Morris Do We need Air Entrainment? Effective Freeze-Thaw Cycles Below 17 F for more then 5 hours with moisture present Unburnt/Activated Carbon Primarily an issue for North Texas Fly Ash or Air? 1 El Pasd EL PASO 0 Hudspeth Culberson JeffDavis Presidio 10 5 Reeves Loving Brewster Winkler Ward Dallam Sherman Hansford Ochiltree Lipscomb Hartley Moore Roberts Hemphill Oldham Potter Carson Gray Wheeler Deaf Smith Randall Armstrong Donley Parmer Castro Swisher Briscoe Hall Bailey Lamb Hale Floyd Yoakum Gaines Andrews Pecos Ector Hockley Terry ODESSA Crane Lubbock Dawson Martin Midland Upton Terrell AMARILLO LUBBOCK Lynn Crosby Garza Motley Dickens King Knox Kent Stonewall Haskell Borden Scurry Fisher Jones Howard Mitchell Nolan Glasscock Reagan Crockett ValVarde Sterling Irion Coke SAN ANGELO Schleicher Sutton Kinney CHILDRESS Cottle Tom Green Edwards Foard ABILENE Taylor Runnels Concho Menard Kimble Real Uvalde Zavala 10 Coleman Wilbarger Baylor Collingswoth Throckmorton Callahan Mc Culloch Mason Kerr Bandera Brown Wichita WICHITA FALLS Archer Young Eastland BROWNWOOD Medina Frio San Saba Gillespie 5 Llano Palo Pinto Comanche Kendall Mills Bexar Atascosa Clay Jack Erath Blanco SAN ANTONIO Hamilton Lampasas Burner Comal Parker Cooke Grayson Fannin Wise Denton Collin FORT WORTH Hood Hays Coryell Guadalupe Wilson Karnes Bosque Trayis Tarrant Johnson Bell Williamson AUSTIN Caldwell WACO Gonzales Hill Mc Lennan Bastrop Dewitt Goliad DALLAS Dallas Falls Ellis Milam Lee Fayette Lavaca Rockwall BRYAN YOAKUM Victoria Navarro Robertson Burleson Hunt Lamar Delta Rains Van Zandt Henderson Freestone Brazos Washington Colorado Austin Jackson Calhoun Leon Wharton Madison PARIS Hopkins Anderson Fort Bend Matagorda Wood Smith Houston Walker Red River TYLER Titus Camp Cherokee Trinity Montgomery Harris HOUSTON Brazoria Upshur San Jacinto Rusk LUFKIN Polk Liberty Bowie ATLANTA Cass Harrison Angelina Marion Panola Tyler Hardin Shelby BEAUMONT Chambers Jefferson Sabine Orange 1 0 Dimmit La Salle Mc Mullen Live Oak Bee Refugio Webb Jim Wells San Patricio Nuceces Duval CORPUS CHRISTI LAREDO Kleberg Zapata Jim Hogg Brooks Kenedy Starr Hidalgo Willacy PHARR 55

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