Forensic Investigation of Hardened Concrete: Water-Cement Ratio

Transcription

1 Forensic Investigation of Hardened Concrete: Water-Cement Ratio Julius Bonini, PE - M+P Labs, Schenectady, NY boninij@mandplabs.com Andrew Smith, PhD - CERAM Research, Stoke-on- Trent, UK andrew.smith@ceram.com 1

2 Outline Introduction: Concrete Basics Constituents and Reactions Timeline Consequences of Improper Water-Cement Ratio Performance Impact Indicators: Pre-cure and Post-cure Methods to Estimate Water-Cement Ratio Capillary Porosity Optical Microscopy Paste Volume Analysis Other Forensic Methods Applied to Concrete 2

3 Water : Cement Ratio in Concrete Part 1 Introduction to Concrete 3

4 Concrete Basics Inert Reactive Coarse Aggregate + Fine Aggregate + Cement + Water Dolomite Sandstone Etc. (Regional) > 2mm Sand Additives Silica (Quartz) Silicates < 2mm Pozzolana OPC Flyash Slag Microsilica Other Components Admixtures: Water Reducers Retarders Accelerators Air Entrainment Agents Tricalcium Silicate (C 3 S) Dicalcium Silicate (C 2 S) Tricalcium Aluminate (C 3 A) Tetracalcium Aluminoferrite (C 4 AF) 4

5 Concrete Basics Concrete: Components and purpose (Theoretical) Cement binder Coarse aggregate inert bulking Fine aggregate inert bulking Water activator, catalyst, gives plasticity/fluidity to the solids Concrete in the Modern World : (Realistic) Cement binder Pozzolana cement replacement with binder properties when reacted with Ca(OH)2 (fly ash (pfa), slag (ggbfs), silica fume (microsilica), volcanic ash (pumice), glass (recycled), ceramic dust Coarse aggregate inert bulking Fine aggregate inert bulking Admixtures agents to address -air entrainment, water reduction (plasticisers& organic superplasticisers), waterproofing, low temperature working Water activator, catalyst, gives plasticity/fluidity to the solids 5

6 Concrete Basics Definitions: w-c = w-cm = Weight of water Weight of cement Weight of water Weight of cementitious material Cementitious Material = cement + flyash + slag + etc. (Reactive Pozzolana) 6

7 Cement Cement Nomenclature (for non-chemists) CaO C (calcium oxide) SiO 2 S (silicon dioxide) Al 2 O 3 A (aluminium oxide) Fe 2 O 3 F (iron oxide) H 2 O H (water) Cement Clinker Phases (Shorthand) C 3 S C 2 S C 3 A C 4 AF Tricalcium Silicate (Alite) Dicalcium Silicate (Belite) Tricalcium Aluminate Tetracalcium Aluminoferrite 7

8 The Basic Reaction Hydration of Silicate & Aluminate Compounds: 2( 2CaO SiO 2 ) + 4H 2 O 3CaO 2SiO 2 3H 2 O + Ca(OH) 2 + Heat 3CaO Al 2 O 3 +6H 2 O + Ca(OH) 2 4CaO Al 2 O 3 12H 2 O (Hexagonal) converts to 3CaO Al 2 O 3 6H 2 O (Cubic) over time Water Reaction is Critical Fundamentally converts slurry to a solid Water-Cement Ratio is critical, and difficult to control and verify Optimal theoretical value is 0.25 (0.40 practical) Water content can vary during batching, transport, setting 8

9 The Concrete Time Line Design Batching Placement Hardening Mix Based on Design Requirements Aggregates Cement Additives Pozzalana Water Transport To Site Poured Into Forms Setting Curing Mixing & Testing Batch Ticket Tested (Wet) -Slump -Compaction -Air Content -Unit Weight Finishing Design Batch Placement Initial Set Set Final Set Curing 90 Min. 45 Min. 2 8 Hours 28 + Days 9

10 Where Problems Occur: Batching to Placement Rinse Water Plant Water Sand & Aggregate Moisture Transit Water Slump Adjustment Atmospheric Water Design Batch Placement Initial Set Set Final Set Curing 90 Min. 45 Min. 2 8 Hours 28 + Days 10

11 The Results Coarse Aggregate Fine Aggregate Paste Volume Fly Ash & Slag Content Air Voids: entrained & entrapped Reinforcement Location 11

12 SECTIONS EVALUATED IN A SCANNING ELECTRON MICROSCOPE SEM 12

13 The Results SEM Imaging Assess Micro-cracking Fine aggregate distribution Identify Other Constituents Fly Ash Slag Other Constituents (Exotics) 13

14 IMAGE ANALYSIS SYSTEM 14

15 The Results Imaging Analysis Results % Relative Area of Constituents Coarse & Fine Aggregate Paste Volume Air Voids Depth Profile 15

16 Water-Cement Ratio in Concrete Part 2 Adding water to cement 16

17 Cement Hydration C = CaO S= SiO 2 A= Al 2 O 3 F= Fe 2 O 3 H= H 2 O 2C 3 S + 6H C 3 S 2 H 3 + 3Ca(OH) 2 2C 2 S + 4H C 3 S 2 H 3 + Ca(OH) 2 3CaO Al 2 O 3 + 6H 2 O + Ca(OH) 2 4CaO Al 2 O 3 12H 2 O(Hexagonal) converts to 3CaO Al 2 O 3 6H 2 O(Cubic) over time C 4 AF + 2Ca(OH) H C 3 AH 6 + C 3 FH

18 Hydration Rates 18

19 Hydration Strength Development 19

20 Strength Development 20

21 How Much Water is Enough? Theoretically, for an OPC cement and a given mix design with no admixtures: however w-c = 0.25 w-c = 0.15 is required to hydrate all the cement. is physically absorbed by cement paste and thus not available for hydration. therefore w-c = 0.40 w-c = > 0.40 represents a minimum water content to achieve full hydration. excess water remains as free water in the cementitious structure and forms capillary pores, BLEEDS to the surface, or lost by evaporation. w-c = < 0.40 restricted hydration of cement particles. 21

22 Water Problems During Curing Early stage problems with concrete associated with water Bleeding migration and accumulation of water at the surface of concrete (horizontal pour) - loss in volume of concrete (densification) - segregation of aggregate (settlement) - slower hydration = retardation of strength gain Drying Shrinkage excessive loss of water from the surface of concrete due to evaporation. Rate of Evaporation > Bleed - plastic shrinkage crack development - desiccation of surface concrete Design Batch Placement Initial Set Set Final Set Curing 90 Min. 45 Min. 2 8 Hours 28 + Days 22

23 Consequences of High w-c Ratio Decrease in Strength Concrete is usually mixed with more water than required for the hydration reactions improves workability Excess water remains in microstructure pore space Pores weaken the concrete 23

24 Consequences of High w-c Ratio Decrease in Durability Shrinkage occurs as water not consumed by hydration leaves the system The higher the additional water, the higher the shrinkage potential Saw-cut contraction joints absorb normal shrinkage Excess water can result in cracking beyond the joints, reducing durability: Internal cracks: weaken the structure Surface cracks: decrease freeze-thaw resistance, allow ingress of chlorides 24

25 Indicators of High w-cratio Pre-cure Slump Slump is a measure of consistency and workability of wet concrete Standardized test.. should yield consistent results for a given mix design Compare as-mixed to as-placed slump values: Excessive slump could be an indicator of added water However, could also be caused by other factors (eg, overdose of air-entrainment) 25

26 Indicators of High w-cratio Pre-cure Unit Weight Nominal unit weight (density) can be calculated from the mix design Measured at placement to confirm proper mix A decrease in unit weight could be an indicator of added water Design Batch Placement Initial Set Set Final Set Curing 90 Min. 45 Min. 2 8 Hours 28 + Days 26

27 Indicators of High w-cratio Effect of added water on unit weight clues from the batch tickets: Example Batch Calculations: Design vs. Actual Pre-cure As Mixed As Placed Material Unit of Specific Mix Actual Act Water Actual Act Water Moisture Moisture Measure Gravity Design Batch (gal) Batch (gal) #2 Stone lbs Sand lbs % % 5.0 Cement lbs Fly Ash lbs Water lbs Air Entrain. oz per 100 wt Reducer oz per 100 wt Total Water - Batch: 32.0 Add'l Water (truck/site): 7.0 Total Water - Actual: 39.0 w-cm Specific Gravity Unit Weight lbs/ft Added water reduces unit weight 27

28 Indicators of High w-c Ratio Post-cure Segregation of Aggregate Large amount of excess water reduces viscosity of cement paste Coarse aggregate segregates, with larger pieces settling toward bottom Can be exacerbated with vibratory compaction 28

29 Indicators of High w-c Ratio Shrinkage Cracks Caused when evaporative losses exceed bleed rate Surface cracks or deeper Deep cracks evidenced as cracks in paste only, around the aggregate Correct depth of reinforcement steel important to keep cracks tight at surface Post-cure 29

30 Water : Cement Ratio in Concrete Part 3 Test methods 30

31 Measurement of Water in Hardened Concrete Where water is or has been in the concrete: Capillary Porosity Aggregate Porosity Liquid water found within the cement paste as capillary voids, typically >50µm Water can be found within the pore structure of the aggregates used in the concrete Combined Water For concrete w:c typically between 0.20 and 0.25 (of hydration) often taken as 0.23 in the absence of any additional information Lost Water Bleed water that is subsequently evaporated from the surface of the concrete. 31

32 Complicating factors: Measurement of Water in Hardened Concrete Bleed Water Damaged Concrete Poorly Compacted Concrete Carbonated Concrete Water lost from the surface of the concrete, by evaporation, during early stage curing. Cracked concrete due to mechanical damage, frost or chemical attack Large irregular voids result in inaccurate measurements of porosity Carbonation process releases combined water Cement ContentMeasurement Inaccurate cement content measurement can result in incorrect w:c calculations 32

33 Measurement of Water in Hardened Concrete Complicating factors (cont d): Aggregate Porosity and Combined Water Special Aggregates Air Entrainment Admixtures The aggregate itself may have a porosity and or combined (mineral) water in the structure Same issues as with aggregate porosity and or combined water where the values are likely to be high Creates additional voids that are in addition to the capillary porosity of the concrete Water-proofers and chemical water repellents can result in poor measurement of the capillary pore structure 33

34 Measurement of Water in Hardened Concrete Methods: Capillary Porosity Chemical analysis to measure free water Included in British Standards Optical Microscopy Impregnate sample with fluorescent epoxy Developed in Finland Paste Volume by Image Analysis Estimation based on measured paste volume % Not covered by ASTM standards 34

35 Capillary Porosity Method British Standard: BS :1988 Testing concrete -Part 124 Methods for analysis of hardened concrete Clause 7. Original water content Calculation of : Total water : cement ratio (including water absorbed into the aggregates) or Free water : cement ratio (excludes water absorbed into the aggregates) 35

36 Capillary Porosity Method Testing concrete - Part 124 Methods for analysis of hardened concrete Calculation of : Total water : cement ratio (including water absorbed into the aggregates) or Free water : cement ratio (excludes water absorbed into the aggregates) By measuring: Capillary porosity of the concrete (using 1,1,1-trichlorethane not water) Combined water of the concrete Cement content of the concrete Aggregate porosity (water absorption value or 1,1,1-trichlorethane method) Aggregate combined water

37 Capillary Porosity Method Calculation of Original w:c Ratio Aggregate Control Sample (direct measurements on representative sample of aggregate used) Measure the capillary porosity of the aggregate (q) as % Measure the combined water content of the aggregate (Y) as % Measure the capillary porosity of the concrete (Q) as % Measure the combined water content of the concrete (X) as % Determine cement content of concrete (C) as % Determine the aggregate content of the concrete (F) as a % Such that: W total = Q + X (YF/100) therefore Total w-c ratio = W total /C W free = Q + X F/100 (q+y) therefore Free w-c ratio = W free /C 37

38 Calculation of Original w:c Ratio Without Aggregate Control Sample (direct measurements on representative sample of aggregate not available) Measure the capillary porosity of the aggregate (q) as % or assume equal to water absorption value. (If not available only Total w:c rationcan be calculated) Measure the capillary porosity of the concrete (Q) as % Determine cement content of concrete (C) as % Determine the aggregate content of the concrete (F) as a % Assuming combined water of hydration in concrete = 0.23 x Cementcontent of the concrete Such that: Capillary Porosity Method Total w-c ratio = Q/C Free w-c ratio = (Q/C qf/100)

39 Optical Microscopy Method NORDTEST Method NT Build 361 November 1999 Impregnation of concrete sample using fluorescent epoxy under vacuum Or Fluorescent Liquid Replacement (FLR) -not applicable above w-c

40 Optical Microscopy Method w:c= 0.35 w:c= 0.50 w:c = 0.60 w:c = 0.70 From Concrete Petrography February 2011 (Concrete Society 2010) 40

41 Optical Microscopy Method From Concrete Petrography February (Concrete Society 2010)

42 Optical Microscopy Method 4mm 4mm Estimated w-c = > 0.70 Known w-c = 0.55 From Concrete Petrography February 2011 (Concrete Society 2010) 42

43 Paste Volume by Image Analysis Inputs: By Estimating Nomograph: Measured paste volume % in hardened concrete sample Relative area by image analysis techniques Point-count method Weight of cement per unit volume Based on batch ticket From: Concrete Petrography, St. February 2011 John D.A. et al. Elsevier,

44 Paste Volume by Image Analysis By calculation: Pure OPC Binder Blended w/ Cementitious Material W C W/C C CM = VD C C D W CM VD CM CM DM Estimated water-cement ratio Weight of cement (in kg) per cubic meter of concrete Weight of cementitious material (in kg) per cubic meter of concrete V Volume of paste (%) -excluding the air voids > 0.15 mm - including aggregate < 0.15 mm D Density of cement (kg/m 3 ) DM Weighted average density of cementitious material (kg/m 3 ) = From: French, WJ Concrete Petrography, Quarterly Journal of Engineering Geology, 24, 17-48,

45 Water : Cement Ratio in Concrete Part 4 Other issues

46 Other Forensic Methods for Hardened Concrete Unit Weight and Absorption Aggregate Volume and Distribution Air Void Content and Distribution Cement Content Presence of Cement Replacement Materials: flyash, slag, microsilica Placement/Depth of Reinforcement Mineralogy Assessment of Aggregate ASR Assessment (Alkali-Silica Reaction)

47 ASR 47

48 Element Mapping of ASR Gel From Concrete Petrography (Concrete Society 2010) February

49 HIGH FLY ASH CONTENT SEM Image & EDS Spectrum Powder Sample of Poor Concrete Very High Fly Ash Content SEM Image & EDS Spectrum Powder Sample of Good Concrete Normal Paste

50 Reactive Recycled Aggregate 50

51 Questions 1) What are the four main components of Portland cement? C 2 S, C 3 S, C 3 A & C 4 AF 2) What is the maximum time generally allowed between batching and placement of concrete? 90 Minutes 3) What is the ideal w-cratio? 0.25 theorectical 0.40 full hydration 4) As w-cratio increases, what is the expected impact on strength and durability of the concrete? Strength and durability both decrease 5) What are three methods to determine w-c ratio of hardened concrete? Capillary Porosity, Optical Microscopy and Paste Volume by Image Analysis 51