Concrete Shrinkage Testing and Challenges

Transcription

1 Concrete Shrinkage Testing and Challenges Presented by Jafar Allahham MS student University of Utah NC2 Meeting April 26 th, 2017 Little America Hotel

2 Acknowledgements Others at Univ. of Utah Assisting on Project: Amanda Bordelon, Ph.D., P.E. (PI on project) Catalina Arboleda (PhD student) Min Ook Kim (PhD student) Lingkun Li (MS student) Siddartha Rayaprolu (MS student) Set-up of Restrained Ring Apparatus Funded by: Utah Department of Transportation (Project Grant No ) Thomas Hales Bryan Lee Scott Strader Scott Nussbaum Nicholas Clark Vincent Liu Department of Civil and Environmental Engineering at University of Utah

3 Concrete Shrinkage 4 types of shrinkage Autogenous Plastic Carbonation Drying Other volumetric changes Creep Thermal Expansion and Contraction

4 Why Concrete Shrinkage is a Problem

5 Common Factors Affecting Shrinkage in Concrete Temperature of the environment. Relative humidity and curing method. Water-to-Binder ratio and paste content. Aggregate size and air content. Specimen size.

6 PCA Manual

7 Free shrinkage (microstrain) all 28% cement volume content stored in humidity chamber environment Age of concrete (days) 0.54 w/c 0.49 w/c 0.43 w/c 0.38 w/c 0.33 w/c 0.28 w/c

8 Aggregate Size and Air Content Aggregates volume remains constant as the concrete dries restrains paste less net shrinkage. Air entrained mixtures exhibit higher shrinkage due to higher overall paste content.

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10 Specimen Size Concrete thickness Thinner earlier cracking 100 Age of cracking (days) Concrete thickness (inches) Delatte et al. 07 Weiss et al. 02 Allahham et al. 16

11 Concrete Shrinkage Testing

12 Shrinkage Testing ASTM C157 Un-restrained (free) Shrinkage Test 1 to 4 rectangular prism sizes possible (depends on max aggregate size) 1 prism most common for mortar 3 prism for 1 coarse aggregate 4 prism for 2 coarse aggregate Standard recommends moist curing for 28 days to minimize variation. After initial curing, samples stored in controlled environment (same as all shrinkage test methods)

13 Additional Factors Studied Free Shrinkage Shrinkage (10-6 in/in) Time (Days) Refrigerator Humidity Chamber Storage University Room of Utah 130C UDOT Lab Lab Fog Room RH (%) Temperature ( C) Location Min Max Average Min Max Average Refrigerator Humidity Chamber University of Utah UDOT Fog Room

14 Specimen Size Effect Free Shrinkage Even with same aggregate size (here sand only) Smaller samples shrink more (more surface area per volume leads to more drying) Similar trend as restrained (smaller concrete sample = more shrinkage) Shrinkage (10-6 in/in) x1x10" prism 2x2x10" prism 3x3x10" prism Time (Days)

15 Concrete 1.5 thick Ring Test Methods Outer diameter of concrete 16 Outer diameter of steel 13 Height 6 ASTM C1581 Concrete 3 thick Outer diameter of concrete 18 Outer diameter of steel 12 Steel inside ring 0.5 thick Height 6 AASHTO T334

16 Pro and Cons of the Test Methods Free Shrinkage Pros + Relatively simple setup. + Can be used as a mix design quality control. Cons Not correlated to actual field performance Concrete is usually restrained. Requires control of temperature and humidity in sample storage room. Restrained Shrinkage + Simulates restrained conditions. + Mixtures can be evaluated in 28 day timeframe. + Can study concrete pavement mixtures or bridge deck mortar. Steel must be custom made. Requires control of temperature and humidity in sample storage room. High testing setup cost. Concrete may never crack.

17 Ring Shrinkage Test Set-up Requirements

18 Data Fluctuation Average Strain after Temperature Compensation (10^-6 in/in) Trial 2 both 2 rings failed at 8 days Trial 1 both 2 rings failed between 3 and 7 days Age (days)

19 Environmental Controls Temperature 73 +/- 3 F and relative humidity 50% +/- 4%. We found room humidifier not producing 50% nor maintaining consistent RH levels Temperature fluctuations reduced with humidifier, but still too high each day Temperature (F) Fahrenheit( ー F) Humidity(%rh) /13 4/13 4/14 4/14 4/15 4/15 4/ Relative Humidity (%)

20 Room Environment Fluctuation Temperature (F) Relative Humidity (%) 10/ Before humidifier 10/21 11/4 11/18 12/2 12/16 12/30 1/13 10/7 10/21 11/4 11/18 12/2 12/16 12/30 1/ After humidifier

21 Summary of Ring Test Challenges High dependability on the surrounding environment All shrinkage methods require tight climate controls. At minimum, need to visually monitor the restrained rings Poor quality of the strain gauge software can cause problems. Strain gauges can be temperature-compensating. AASHTO T334 ring test method is not a favorable method to compare mix designs May never crack ASTM C157 free shrinkage method preferred Can be used to compare different mixtures and environmental conditions.

22 Questions? Report of this work can be found at: Jafar Allahham, Amanda Bordelon, Lingkun Li, and Siddartha Rayaprolu. Review and Specification for Shrinkage Cracks of Bridge Decks, Utah Department of Transportation Report No. UT-16.17, December 2016.