14 Jun 2012 ... Chair of ACI 238, Workability of Fresh Concrete, and is a member of ACI 211,
Proportioning Concrete Mixtures; ACI. 221, Aggregates; ACI 236, ...
June 14, 2012
Eric Koehler is Vice President of R&D for Verifi LLC, where he is responsible for the development of software and hardware for in-transit control of concrete properties. He is Chair of ACI 238, Workability of Fresh Concrete, and is a member of ACI 211, Proportioning Concrete Mixtures; ACI 221, Aggregates; ACI 236, Material Science of Concrete; ACI 237, Self-Consolidating Concrete; and ACI 522, Pervious Concrete. He is also a member of ASTM. He received his BS in civil engineering from Clemson University in Clemson, SC, in 2002 and his MS and PhD in civil engineering from the University of Texas at Austin in 2004 and 2007, respectively.
Total Water Control ACI Spring 2012 Convention March 18 – 21, Dallas, TX
Outline
Controlling Total Water Content During Transit Eric P. Koehler
01
Sources of Water in Concrete
02
Adding Water to Adjust Slump
03
Adding Admixture to Adjust Slump
04
Automated Slump Control Equipment
05
Mixing of Water and Admixture During Transit
06
Conclusions
Vice President of R&D Verifi LLC
[email protected]
American Concrete Institute
Spring 2012 Convention
March 21, 2012
Dallas, Texas
© 2012 Verifi LLC. All Rights Reserved.
Use of Slump to Control Water Content
Water is added to concrete from multiple sources
Deviations in slump may indicate a deviation in water content
Percent of Total Water
Accurately Measured?
Residual water from prior load
0 – 10%
rarely
Aggregate free water
10 – 40%
sometimes
• Water has a significant effect on slump. • Water is typically the largest variation during batching.
Does controlling to a target slump affect variation in water content? By controlling to constant slump, variation in total water was less than +/- 1.5% for 6 loads tested over 2 days
Batch water (metered)
60 – 90%
yes
Wash down (“slump rack”) water
0 – 10%
rarely
Driver added water
0 – 20%
sometimes
TOTAL WATER
100%
???
Metered batch water is measured most accurately. But, variability in other sources of water makes it difficult to determine how much batch water should be added.
• Wet batch (central mix): adjust to mixer power (amp meter) • Dry batch (truck mix): manual adjustment by driver to reach target slump
Once the truck leaves the plant, additional sources of variation are introduced.
Total Free Water (lb/yd 3)
300
Therefore, it is common to add water to reach a target slump during or immediately after batching.
water error -1.0% 1.4% 0.5% 0.0% 0.5% 1.1%
Design
250 200
Truck RS
150
Aggregate Batch
100 50
trimmed 17.3 pcy, 6.7%
Sources of Water in Concrete
0 D1M2 D1M3 D2M1 D2M2 D2M3 D2M4
To ensure accuracy of the slump vs. water content relationship: • Manage other likely causes of slump change, such as temperature • Recalibrate the slump vs. water content relationship periodically • Measure slump accurately
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The Challenge of Ready Mix Batching and Delivery Concrete must be batched in one condition to meet acceptance criteria later at the construction site.
Adding Water After Batching Adjusting to a target slump at the site by adding water increases variability. Scenario #2 requires more water.
Scenario #1 mix design A
Batching • raw material variation • aggregate moisture variation • residual material in drum
Transit • transit time variation • weather
Site • jobsite delays • mixing variation • jobsite-added water (if allowed) • sampling and testing variation • limited measurement and data
Adding Water After Batching When adjusting to a target slump on the jobsite by adding water, further variability is introduced due to the lack of accurate slump measurement
10 minute haul 50°F and cloudy 6 in. slump
Scenario #2 mix design A 60 minute haul 90°F and sunny 8 in. slump
Using Admixture to Achieve Target Slump plant
jobsite
• At the plant, water content is typically the largest source of variation in slump • Adjusting batch slump with water is effective for reducing variation in water content, provided other sources of variation in slump are properly managed • Initial admixtures, including optionally superplasticizer, still added at plant
• Transport to the jobsite introduces new sources of slump variation, for example: • transit time • concrete temperature • mixing • Adding superplasticizer instead of water at the jobsite ensures: • less water used • less variation in water content
• Adjustments to water are often made based only on a visual estimate of slump. • Contractors and other site personnel may request an increase in slump beyond what is ordered. • This introduces variability in water content, even if water is below the maximum w/cm in both cases.
3-inch (75 mm) slump
3-inch (75 mm) slump?
Recommendations to Control Total Water Content
Automated Slump Control Technology
Manage all sources of water “Back out” drum before loading to remove residual wash water Accurately measure aggregate moisture Limit wash down water Accurately measure water added on the truck
Accurately measure slump – don’t rely on visual assessments
Verifi® Concrete Control System Measures slump and temperature of concrete in mixer, drum speed, and water added
Control to the right slump – don’t allow unauthorized changes to the target slump at the site
Adds water and superplasticizer to slump target, automatically or manually
Limit the amount of water added on truck – don’t exceed the maximum water-to-cementitious materials ratio
Prompts driver to ensure concrete is fully mixed
Adjust slump with admixture once maximum water content is reached
Documents delivery process Enables change based on data
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Verifi Concrete Control System - Components
Experimental Testing: Slump Measurement Accurate slump measurement Testing conducted on 3 trucks at 2 plants
ASTM C 143 Slump (mm)
Verifi Ticket and Batch Feedback
0
50
100
150
200
250
300
12
300 2
Verifi Data Service Gateway
250
8
200
6
150
4
100
2
50
Verifi SMS Slump (mm) Verifi SMS Slump (mm)
radio
Verifi Verifi SMS Slump SMS Slump (in.)(in.)
R = 0.95 10
Initial calibration of each truck conducted on limited set of mixtures Slump measurement was accurate across much wider range of mixtures and test conditions
Verifi Reporting
Third Party Batch Software
0
0 0
2
4
6
8
10
12
ASTM C 143 Slump (in.)
• Materials (cement, SCMs, aggregates, HRWR, VMA, AEA) • Mixture Proportions • Unit Weight (110 – 150 lb/ft3) • Multiple Locations • Load Size • Transit vs. Stationary
Typical Delivery: Water and Superplasticizer Adjustment
Automated Admixture Addition on Truck
Successful adjustment of water and superplasticizer to target slumps Admixture use pre-programmed centrally in web-application 250
Slump, mm
Admixture injected under pressure to reach concrete in drum
Limits applied to amount of admixture added
at site, slump on target
200 150
batch slump target achieved, add superplasticizer
slump
100 50 0
slump loss, admixture additions
delivery target
batch target
mixing complete, slump too low, add water
100
admixture 50
water 0:00
Managing Water with Automated Admixture Addition Water After Batching
Batch below maximum water content to allow for water addition in transit.
Water At Delivery
0:30
1:00
1:30
0
2:00
Reduction in Slump Variability with Auto Slump Control Slump at Discharge vs. Target Before Auto Slump Control
max water (design)
Water (l) Admixture (l*10)
User selects slump, system automatically determines correct dose
With Auto Slump Control (Water Only)
Average: 2.86 StDev: 2.50
Average: 0.69 StDev: 1.78
Automated admixture addition in transit can results in less total water use and less variation in total water. batch water
Opportunity to reduce cement content.
adjust to delivery slump with water adjust to batch slump with water
without auto slump control
adjust with water to batch slump, adjust to delivery slump with admixture
Less variability in slump at discharge Lower slump at discharge = less water added Improved customer satisfaction due to a more consistent concrete: finishability, air content, setting time, strength development
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Data Available to Control Total Water Real Time Feedback
Mixing of Superplasticizer in Transit
Post Delivery Adjustments
Batch operator sees slump after batching and water added to reach target slump. This can be used to adjust subsequent batches.
Operations and QC management receive reports on slumps and water and admixture added to every load. This can be used to ensure correct water content and redesign mixtures.
Superplasticizer additions can be fully mixed at agitating speed
Polycarboxylate-based superplasticizer added at agitating speed (2.6 rpm) or mixing speed (12.5 rpm), dose of 0.06%solids/cement Stable slump reached at equal number of revolutions 250
250
200
Drivers see accurate slump reading and an estimate of water need to reach target slump. They can add a precise amount of water through in-cab interface.
200
Test A: 2.6 rpm
150
Test C: 12.5 rpm
Slump, mm
Slump, mm
Test C: 12.5 rpm
Test B: 12.3 rpm
150
Test B: 12.3 rpm
100
Test A: 2.6 rpm
100
50
50
0
0
0
5
10
15
20
25
30
35
0
10
20
Time, minutes
Mixing of Superplasticizer in Transit
30
40
50
60
70
Benefits of Adding Admixture at the Jobsite
Two different polycarboxylate based superplasticizers added at agitating speed
• Compensate for variations in haul time, weather, and other factors
To reach stable slump, PC-05 required 30 revolutions and PC-04 required 60 revolutions
• Less amount and variation in total water – possible reduction in cement
Test D: PC-05 at 0.10%s/c, 3.2 rpm
• Consistent final slump and other concrete properties
• Transport large loads – use low slump in transit to avoid spillage, then higher slump at jobsite • For long hauls, ability to wait to bring slump to target at jobsite
250 Test D: PC-05 at 0.10%s/c, 3.2 rpm
200
90
Adding Admixture at Jobsite
Mixing requirements primarily a function of admixture and revolutions
250
80
Revolutions
200 Test E: PC-04 at 0.09%s/c, 3.1 rpm
Automated addition can ensure:
Slump, mm
Slump, mm
Test E: PC-04 at 0.09%s/c, 3.1 rpm 150
150
100
• Correct dose is added
100
• Concrete is fully mixed prior to discharge
50
50
0
0 0
5
10
15
20
25
30
35
40
45
50
• Slump is correct prior to discharge • No manual dosing required 0
20
40
Time, minutes
60
80
100
120
140
160
Revolutions
Conclusions Water is added to concrete from numerous sources, leading to variations in total water content New technology is available to measure and control concrete in the truck • Accurate slump measurement • Accurate and automated water addition and measurement
Automated admixture addition enables improved concrete quality • Provide desired slump and avoid exceeding maximum w/cm • Reduce total amount of and variation in amount of water added
Thank You.
Superplasticizer can be successfully mixed in transit, even at agitating speed
4
