Delivering Concrete to Survive the Environment. Dr. Peter Taylor

Transcription

1 Delivering Concrete to Survive the Environment Dr. Peter Taylor

2 Outline What can go wrong? How do we prevent it? 2

3 What Can Go Wrong? Internal Expansion External attack

4 Durability Ability of the concrete to survive the environment to which it is exposed Aggressive fluids Cold weather Unstable aggregates Slab deformation Overload

5 Aggressive fluids Sulfates External sulfates (eg soil) C 3 A in the cement Softening Complex chemistry

6 Aggressive fluids Salt crystallization Transported in solution Water evaporates leaving solid salt Not necessarily cold related 6

7 Aggressive fluids Soft water or acids Dissolves calcium compounds Problem in: Pure water reservoirs Some streets (Bourbon St., New Orleans) Industrial settings 7

8 Cold Weather Saturated Freezing and Thawing Thin flakes in paste Or deep cracks 8

9 Cold Weather De-icing salts Calcium oxychloride (MgCl 2 ) Friedel s Salt Calcium-chloro-aluminate Ettringite 9

10 Cold Weather Deicing salts Oxychloride Causes paste expansion at ~40 F Separates aggregate from paste 10

11 Cold Weather Deicing salts Ettringite Accelerates saturation Stark and Bollmann 2000 Moulzolf 11

12 Unstable Aggregates Alkali Aggregate Reaction Chemical reaction with some aggregates, paste alkali hydroxides and water Expansion

13 Unstable Aggregates D-Cracking Some limestone aggregates hold water Freezes Marks and Dubberke

14 Unstable Aggregates Popouts Porous aggregates Freezing Surface phenomenon May accelerate other distresses

15 Shrinkage Cracking Warping Expansion Slab deformation

16 Strength Support Overload

17 Slabs on Grade What's critical Name What is the cause D-Cracking Water freezing in coarse calcareous aggregate containing clays and or with a pore system that holds water What does it look like Cracks near joints, often an a curve at intersection of joints Where does it occur Full depth, cracks are parallel to free surface, mm part Where does it start How far does it go Prevention Repair Photo Deteriratio n Type Bottom of a joint Up to ~18" from joint Avoid As per ASR deleterious aggregates. Reducing maximum size Physical delays damage Joint Related Freeze thaw Water freezing in Thin flakes parallel saturated cement to free surfaces at paste with joints inadeqaute air void system Joints Where water is trapped A few mm Adequate air void system, low permeability Partial depth repair Physical Oxychloride MgCl2 reacts with some paste systems at about 40F to form expansive oxychloride compounds Cracks parallel to joint faces, sometimes up to 1" from previous cut or crack Joints Tops of saw cuts or in the kerf Up to ~9" from joint High SiO2 cementitious systems, low permeability, adequate air, limit use of MgCl2 Partial depth repair, Full depth repair Chemical Salt Scaling Salts or ice crystallizing below the surface, often related to poor surface finishing Flakes peeling off the surface of the slab Surface only Surface Can cover the whole slab Good finishing procedures, curing, adequate air void system, low alkali cement Grind Not sure Only Surface Popout Water freezing in low density aggregate Divot above aggregate particle At the surface Joints Whole slab surface Avoid deleterious aggregates None Physical Checial Reaction Alkali silica reaction Reaction between some silicates in aggregate, alkali hydroxides in pore solution and water Cracks mostly parallel to longitudinal joint Full depth Near edges Whole slab surface Avoid deleterious aggregates, low calcium SCM, Lithium comounds Remove and replace, rubblize and overlay, unbonded overlay Chemical

18 What do we measure now? Slump Strength Air Thickness

19 Performance Engineered Mixtures Seeking to: Understand what makes concrete good Specify the critical properties and test for them Prepare the mixtures to meet those specifications Delivering concrete to survive the environment

20 PEM A program to make everyone s life miserable

21 The Perfect Specification You get paid after [ ] years Or we do a test that predicts life Slump? Strength?

22 Why bother? Currently: Focused on strength Struggling to get durability Wrestling with unintended consequences

23 No. of ingredients Why bother? Cement, water, rock, sand, AEA Add SCMs, admixtures, int. aggregates, limestone Opening Weeks Days (or hours) Curing Weeks Days De-icing Sand, NaCl Other chlorides, formates, acetates Design life 20 years 100 years Knowledge base In house Contracted out

24 What Really Matters? Transport properties (everywhere) Resistivity / Formation Factor Aggregate stability (everywhere) ASR AASHTO R80 D-Cracking IPA Strength (everywhere) Flex or compression

25 What Really Matters? Cold weather resistance (cold locations) Air SAM LTDSC Shrinkage (dry locations) Prism Ring Workability (everywhere) VKelly Box

26 A Better Specification Measure the right things at the right time Prequalification This is what will be delivered Process control What was delivered will make it Acceptance Delivered as promised

27 A Better Specification A buffet of approaches (for the Agency) Prescriptive: w/cm, paste volume Performance: Formation factor Equivalency?

28 A Better Specification AASHTO PP84 published in March 2017 Guide Specification / Standard Practice Deemed to satisfy Avoids bonus discussion that is local Provisional = meaning we can modify as we learn things

29 Construction QC should include Unit weight Calorimetry Maturity Strength development Air void stability And a response Risk management

30 Test Methods Tests for those critical properties VKelly / Box SAM Resistivity / Formation factor Sorptivity Ring / Dual ring 30

31 VKelly Measure initial slump (initial penetration) Start vibrator for 36 seconds at 8000 vpm Record depth every 6 seconds Repeat Plot on root time Calculate slope = VKelly Index 31

32 Box Test A test that examines: Response to vibration Filling ability of the grout (avoid internal voids) Ability of the concrete to hold an edge Ley

33 Box Test The edges of the box are then removed and inspected for honey combing and edge slump Ley

34 Super Air Meter Reports air content and SAM number SAM number correlates well with freeze thaw testing Ley

35 Formation Factor F = Resistivity (bulk) Resistivity (solution) Sample is fully saturated Solution resistivity = ~0.01 kω cm ASTM C1202 Classification Charge Passed (Coulombs) Resistivity (kω cm) a Formation Factor High >4,000 < Moderate 2,000 4, ,040 Low 1,000 2, ,040 2,080 Very low 100 1, ,080 20,700 Negligible <100 >207 20,700

36 Sorptivity Assess capillary absorption (ASTM C 1585) Condition sample for moisture content Seal the sides and top Measure mass increase over time with open face immersed in water

37 Ring Test Assesses cracking risk Starts immediately 37

38 How do we proportion to achieve design goals? Workability Transport Strength Cold weather Shrinkage Aggregate stability Aggregate System Type, gradation Paste quality Air, w/cm, SCM type and dose Paste quantity Vp/Vv

39 % Passing Workability Factor 2" 1 1/2" 1" 3/4" 1/2" 3/8" # 4 # 8 # 16 # 30 # 50 # 100 # 200 Percent Retained Percent Passing Doing the Sums Project Effect of gradation 9/29/2016 Aggregate System The wonders of a spreadsheet and a solver function Materials Blue= Input Data Red = Calculation Don t touch! Cementitious 472 Yellow = Output Don t touch! Coarse Agg 1" Black = Working Don t touch! Fine Agg Sand Intermediate 3/8" Sieve Analysis Data Max nominal aggregate size 1.00 inch (0.75, 1.0 or 1.5) Coarse 1" Fine Sand Intermediate 3/8" Combined Fineness Percent Cum. Sieve Modulus Percent mass Passing Retained Retained Sieve: % Pass % Mix % Pass % Mix % Pass % Mix % % % 2" /2" " /4" /2" /8" # # # # # # # Coarsness Factor Power 45 least difference 31.0 Tarantula error Workability Factor Power 45 error Adjustments 0.00 Adjusted Workability Factor Fine Coarse 31.5 >15 15 Measure V a Tarantula Greater than 15% on the sum of #8, #16 and # % of fine sand (#30-200) Individual and Combined Gradations C33 Sand /8" 1" 0 Sieve 0 # 200 # 100 # 50 # 30 # 16 # 8 # 4 Sieve 3/8" 1/2" 3/4" 1" Combined 1 1/2" 2" Power 45 Shilstone Chart IV Sandy II III Small Agg Mixture Max Density Limits I Gap V Coarse 0 #200 #50 #16 #4 1/2" 1" 2" Sieve (^0.45) Coarseness Factor 20 0

40 Does it Work? West Des Moines specification WI using the proportioning tool Other states looking at shadow evaluations in 2018

41 A framework is in place The details need work Ruggedness Limits Correlation with life Training Are we there yet?