MANAGING AIR IN CONCRETE

Transcription

1 LEARNING OBJECTIVES MANAGING AIR IN CONCRETE G. Terry Harris Sr. GCP Applied Technologies Why is air necessary in concrete How do we entrain air What are the factors that impact air in concrete Producing concrete with consistent air entrainment MYTHS Lochness Monster Flat Earth A Patriots win without cheating Perfect Air 3 4 AIR ENTRAINMENT FOR CONCRETE WHY?? 5 6 NPCA 1

2 Benefits of Air Entrainment Freeze/Thaw Durability Plastic Concrete Workability Reduced Segregation and Bleeding Improved Finishability in Lean Mixes Increased Yield Hardened Concrete Freeze Thaw Durability Reduced Permeability Resistance to Alkali and Sulfate Attack Reduce Density (Lightweight Concrete) Other?? 7 8 Concrete is porous -contains many capillaries and pores that hold water When water freezes it undergoes expansion Force of 10,000 psi Concrete has a max tensile strength of 1000 psi Tiny Bubbles Accommodate freezing (expanding) water inside pours of concrete Safety Relief Valves AIR ENTRAINMENT Fundamentals of air entrainment (9% in volume) GCP Applied Technologies Inc. Confidential 10 CHEMISTRY OF AIR ENTRAINING AGENTS Vinsol Resin: Industry Standard AEA Single Source; Limited Supply; Increasing price Quick air generation; Mid size bubble Tall Oil Proven Alternative to Vinsol Slower air generation; Smallest bubble Likes mixing Surfactants Proven Alternative to Vinsol Quick Air generation; Coarsest bubble Chemically Different/Functionally Identical 12 NPCA 2

3 13 14 AIR IN CONCRETE All concrete contains air to some degree Air content is measured by volume Without the use of an AEA, typical air contents are in a range of 1.5 3%. This is defined as entrapped air and these air voids are typically irregularly shaped and are 1 mm or greater in diameter. Entrained air is intentionally added through the use of AEA and the voids are spherical in shape and range from.01 to 1 mm in diameter. Air meters measure total air content, entrained + entrapped Entrapped Air vs. Entrained Air Air in Concrete: AIR IN CONCRETE Air Entraining Agents are not foaming agents Air is created by blending and folding action of a concrete mixer. AEA modifies the natural air into an entrained air void system Without an AEA the bubbles formed during mixing are not stable so continued mixing action breaks them down. AEA s react with calcium hydroxide released as the cement and water come in contact and forms a precipitate. This precipitate coats and stabilizes the bubbles as they form, allowing them to stand up to further mixing action NPCA 3

4 WHAT IS THE SECRET TO CONTROLLING AIR IN CONCRETE? CONSISTENCY WHAT IMPACTS AIR IN YOUR CONCRETE? Temperature Mixing Time Slump Mixer Capacity Batching Sequence Cement Content Cement Composition Cement Contaminants Sand / Aggregates Water Content Amount of Batch Water Water Reducers, Retarders Accelerators Superplasticizer Fly Ash, Slag Pigments Basically Everything Regardless of how many variables effect air, If you can measure it - you can begin to control it Material & Mix Proportion Variables Cement Pozzolanic Materials Coarse Aggregate Fine Aggregate Water Admixtures Cement Cement Increase = Increase in AEA demand Increased Fineness = Increased AEA demand Increased Alkali = Decreased AEA demand However, air may not be as stable, therefore air loss over time may be greater Other Cement Chemistry Silicates, aluminates, sulfate content -????? Pozzolanic Materials Fly Ash - typically increases AEA demand Increasing % = increase in AEA*** Fineness increase = increase in AEA L.O.I. increase (carbon) = increase in AEA Use of Type F ash typically requires more AEA than when Type C ash is used AEA demand with fly ash can be monitored through the use of the Foam Index Test Pozzolanic Materials Silica Fume Increase the AEA demand 5-10% typically requires % increase in AEA Higher than 10% will require even greater increase in AEA NPCA 4

5 Coarse Aggregate Crushed-stone entrains less air than rounded Gravel. Cleanliness Clay or silt (increased AEA demand) Crusher fines (increased AEA demand) Fine Aggregate Fine to coarse aggregate ratio increase = reduced AEA demand Sand traps bubbles in voids between grains 10% increase in sand content 1% - 2% increase in air content Sand gradation Consistency in gradation a must for consistency in AEA dosage requirements Middle fractions (#30 to #100 sieve) most effective in entraining air Increase in -#200 sieve material will increase AEA demand (crusher fines, clay, silt) Manufactured angular sand will Increase AEA demand SAND IMPACT ON AIR CONTENT Water Should be potable which means if you won t drink it you may not want to use it 29 Water Potable when possible - if not, then test. Hardness - increase in AEA demand as hardness increases if AEA is introduced directly to water. Minimal effect if AEA is introduced on to sand Use of reclaimed water may require a change in AEA addition rates Organic impurities may result in high air contents W/C ratio The higher the w/c ratio, the lower the AEA demand Water starved mixes can be air sensitive (moistures) Admixtures Air Entraining Admixtures Water Reducers/Retarders High Range Water reducers Accelerators Corrosion Inhibitor VMA Viscosity Modifying Admixture NPCA 5

6 Admixtures Water Reducers/Retarders Typically will lower AEA demand. Lignosulfonate based products may require reducing the AEA by 30-60%. Others - (HC and carbohydrate based products may require a 20-40% reduction. Zylas usually require a 5-20% increase versus lignin based WRA Accelerators CaCl 2 based - Slight increase in air content - typically 0.5-1% Non-chloride accelerators - minimal effect- test to verify. Admixtures High Range Water Reducers Naphthalene / Melamine HRWRs Minimal effect on AEA addition rate (Test to verify) air content may increase along with slump increase Polycarboxylate based HRWR s Typically a 20-75% reduction in AEA is required. Controlled with built-in defoamer package - may respond differently to different materials - TEST Admixtures Corrosion Inhibitor Calcium nitrite - slight to moderate increase in AEA demand - depending upon addition rate Shrinkage Reducing Admixture Require an increase in AEA addition rate - Test VMA Viscosity Modifying Admixture These may reduce AEA demand with harsh materials Other Materials Pigments May require an increase in AEA addition rate - Test Carbon black - May require tenfold increase in AEA, iron oxide based black coloring recommended Other Materials????? Test Test Test EFFECT OF SOME LIQUID PIGMENTS ON AIR CONTENTS WHAT IMPACTS AIR AND HOW TO CONTROL IT? Production and Construction Variables Sampling & Testing Mixing Time and Speed Batch Size Temperature Moisture & Slump Variation Plant & Equipment maintenance Admix Discharge location and Batch Sequencing Erratic Air Can often be Linked to these Variables 37 NPCA 6

7 Sampling and Testing Sampling Properly mixed Representative??? Sampling and Testing Sampling Properly mixed Representative??? Test Equipment/Calibration How often do you calibrate your air meter? ASTM C231: must be made as frequently as necessary and at intervals not to exceed three months Sampling and Testing Sampling Properly mixed Test Equipment/Calibration Test Procedures Unit Weight for Verification GCP Applied Technologies Inc. Confidential 41 WHAT TALE WILL THE UNIT WEIGH TELL?? Sampling and Testing Unit Weight for Verification If your unit weight goes down Only 2 things will make concrete lighter Air Water Air check will point to the correct answer 2015 GCP Applied Technologies Inc. Confidential NPCA 7

8 Unit Weight UNIT WEIGHT VS AIR CONTENT VS STRENGTH Strength Air Content Unit Weight Strength Sampling at plant Wash down rear fins and chute Insure that concrete is thoroughly mixed Run mix back and forth in drum several times prior to obtaining sample this coats drum to avoid loss of mortar in sample Discard first material down chute (1/2 wheel barrow) Remix sample in wheel barrow prior to testing Sampling at site Sampling from truck Re-mix revolutions prior to discharging Discharge 1/2-1 yd 3 before sampling If load has to be tested before discharging, follow procedure for sampling at plant after remixing revolutions. Recombine sample prior to testing. WITH DIFFERENT MIXER TYPES MIXING TIMES WILL VARY. PLANT TO PLANT AND MIXER TO MIXER GCP Applied Technologies Inc. Confidential 48 NPCA 8

9 NOTE THE SPEED OF THE AIR GROWTH WITH A TWIN SHAFT MIXER NOTE THE SPEED OF THE AIR GROWTH WITH A PAN MIXER Mixing Time and Speed Inconsistent mixing will cause Air to vary Worn Blades will decrease entrained Air Mixing Time and Speed Inconsistent mixing will cause Air to vary Worn Blades will decrease entrained Air Batch size Expect Longer mix time for larger loads Very small loads in a drum mixer could make air difficult to entrain. AIR PROFILE OBSERVATIONS Air Profile 54 NPCA 9

10 Mixing Time and Speed Scenario #1 Delivered % 5% revs 4% revs revs 4.66% - 4: % - 5:03 3% revs PRODUCER WITH VARIABILITY LOOKING AT WHAT WAS GOING ON IN THE BATCH ROOM ON THIS PARTICULAR DAY. STARTED OUT PRETTY NORMAL revs revs 57 Scenario #2 Scenario #3 6.5% 6.5% 5% 5% 4% revs 4% revs 3% Driver arrives at bed, adjusts load - now 4.7% instead of 6.5% 3% Driver starts mixing and has a conversation with the placing crew Production told driver to hurry, he leaves Plant at 30 revs revs Starts to unload and now has 3.8% instead of 6.5% revs Mixing / Time Mixing / Time EFFECT OF MIXING TIME ON AIR CONTENT Controlling the Mixing Time and Action is an important Tool to Control the AIR WHAT IS THE RIGHT AMOUNT OF WET - MIXING TIME? IS IT 30 SECONDS? When you come to a fork in the road take it Yogi Berra NPCA 10

11 60 SECONDS? 90 SECONDS? IS IT 170 SECONDS? MIXER ON OFF ON OFF ON DYNAMIC VS STATIC AIR CONTENTS Temperature An increase in concrete temperature will decrease the air content. Increases of 70 F to 100 F may reduce air content by - 25% Concrete uses more air Reductions of 70 F to 40 F may increase air content by +40%. Concrete uses less air Moisture & Slump Variation Variations in Moisture can effect AEA dosage efficiency and slump An increase in Slump will increase Air Contents from 2-7 a slump increase of 1 will increase air content approx. 0.5% Very high slump concrete may have lower air 7 + very low slump concrete (< 1 ), may require extremely high AEA addition rate to achieve desired air contents As time elapses slump loss occurs. Air loss is then recovered by adjusting slump.. (be careful of retempering with water) NPCA 11

12 Retempering With water Will raise air content somewhat - dependent upon slump increase/mix temperature/age of concrete Minimal effect on air void system With AEA Allowed under ASTM C-94 Can adjust to targeted air content - restores air void system May require relatively high addition rate Mix minimum of 30 revolutions after adding Re-test Plant & Equipment maintenance Maintain good lubricating practices at the cement plant, concrete batch plant, or concrete delivery vehicle Oil & Grease will either increase or decrease the amount of entrained air, depending on their composition. Material Transport Contamination Any other materials hauled besides main material? other Admix Discharge and Batch Sequencing Inspect Batch Sequencing Discharge AEA separate from other admixtures Avoid contact of AEA directly with hot water Inspect Admix Dispenser system for Proper Function Is it consistently dispensing the desired dosage Inspect Admix Discharge Location Preferred Discharge Location : Sand Charging Belt or Weigh Hopper Plant Water-Discharge Pipe Is all the admix dispensing to the desired location at the desired Time Be sure the AEA is not combining with other Admixtures before being introduced to the mix EFFECT OF SEQUENCING ON AIR CONTENT Adjust the Sequencing Is an important Tool to Control the AIR 2015 GCP Applied Technologies Inc. Confidential 70 SEQUENCING AND CONSISTENCY: SAND / STONE / CEMENT / - DRY BLEND - WATER / ADMIX SEQUENCING AND CONSISTENCY: SAND / STONE / CEMENT / - DRY BLEND - WATER / ADMIX NPCA 12

13 TROUBLESHOOTING TAIL END DOSING THE AEA. NOTE THE RAPID AIR BUILD AND STEADY RESULTS Who, what, when, where? times of day cements aggregates temperatures admixtures? - order of addition plants etc., etc When does it not occur? 75 TROUBLESHOOTING EXAMPLE OF AIR ISSUE 1 Review information check mix design and yield Issue: infrequent but severe swings in air contents causing lost loads comparative performance data - history Vs test lab Wild swings in air content on the same mix for a particular project 10%+ Eliminate variables Determine causes Multiple loads batched back-to-back with moderate AEA dosages (20 30 ozs/load) would give us consistent air. Make adjustments (1 at a time) EXAMPLE OF AIR ISSUE EXAMPLE OF AIR ISSUE Further Investigation Reviewed testing procedures and mixing times Reviewed printouts from batches comparing air content results of problem mixes and loads just prior to good and problem loads? Or After Discovered when low slump concrete with high AEA dosage (~200 ozs/load) was batched just prior to problem project, air contents were high (10% +) Looked further into the plant setup, sequencing, and the discharge of our products into the mix Spent time up in the plant, looking at multiple loads with different quantities of AEA (and other admixes) to look for inconsistencies Problem Discovered Weigh hopper configuration created very steep angle on natural sand AEA was running down slope and building up at end of weigh hopper With large AEA dosages, probably % of the AEA was not getting in the load AEA would run down face of weigh hopper and get picked up in the next load NPCA 13

14 INSIDE WEIGH HOPPER AEA REMAINING AFTER BATCH EXAMPLE OF AIR ISSUE 1 EXAMPLE OF AIR ISSUE 2 Problem Solved We changed the timing on the AEA until after aggregate weigh hopper started to empty After about 20 seconds, the top of the aggregate pile started to cave in, forming a pocket for AT 30 We adjusted the discharge point to feed into this pocket, and all of the AEA, even large quantities, get contained in this area and feed into the load Issue: Low Strengths Strip strength Okay - just slightly lower 7 day strength low but close and no gain afterwards Oh and by the way - We weighed a cylinder and it was 2lbs light EXAMPLE OF AIR ISSUE EXAMPLE OF AIR ISSUE Further Investigation Further Investigation Tested okay at Lab Unit weight taken at pour beds light on certain mixes Sent cylinder Samples to Lab for Petrography Started reviewing materials and Processes at the plant NPCA 14

15 EXAMPLE OF AIR ISSUE Further Investigation EXAMPLE OF AIR ISSUE Further Investigation EXAMPLE OF AIR ISSUE 2 Problem Solved??? Learned that aggregate was dry some times Learned that the Aggregate absorption was now close to 2% - double what it was thought to be Tried to Increase initial water Changed the timing on the AEA until after Cement and additional water PLANT INVESTIGATION HAVING ERRATIC AIR CONTROL Sand / Aggregates AEA Cement Pre Water 87% Superplasticizer Retarder Final water 100% Accelerator (if needed) Wet Mix time 35 seconds seconds 15s 40s 4 yard Pan Mixer PLANT INVESTIGATION HAVING ERRATIC AIR CONTROL Sand / Aggregates AEA Cement * Pre Water 87% PLANT INVESTIGATION HAVING ERRATIC AIR CONTROL Sand / Aggregates AEA Cement Pre Water 90% Superplasticizer Superplasticizer Retarder Retarder Final water 100% Final water 100% Accelerator (if needed) Accelerator (if needed) Wet Mix time 35 seconds Wet Mix time 65 seconds * 6 out of 10 batches had air in spec Discharge 30 seconds 20 out of 20 batches had air in spec Discharge 30 seconds 30 seconds 45s 85s 120 seconds 150 seconds 30 seconds 45s 85s 120 seconds 170 seconds NPCA 15

16 ADJUSTING THE SEQUENCING AND MIX TIME MADE ALL THE DIFFERENCE IN AIR CONSISTENCY Before After 93 Thank you for your attention MANAGING AIR IN CONCRETE G. Terry Harris Sr. GCP Applied Technologies Questions and Comments 2015 GCP Applied Technologies Inc. Confidential 94 NPCA 16