Coefficient of Thermal Expansion of Concrete Pavements

Transcription

1 Coefficient of Thermal Expansion of Concrete Pavements Erwin Kohler Ramon Alvarado David Jones University of California Pavement Research Center TRB Annual Meeting, Washington D.C. January 24 th, 2007

2 Importance of CTE for pavements Joint opening LTE Thermal curling cracking Joint sealant perf. spalling Even potential for catastrophic failures such as blow ups

3 FHWA CTE Testing FHWA has CTE from over 1,800 samples Lab cast and drilled cores from LTPP sections Result from 670 tests is that CTE ranges between 5.0 and 7.0 microstrain/ F. Objective: validating data for the Mechanistic-Empirical Pavement Design Guide (ME-PDG)

4 TxDOT CTE Testing Concretes made with coarse aggregates from > 30 sources in Texas Large variance in CTE with concrete containing river gravels, More consistent CTE with crushed limestone aggregates Objective: improve reinforcement design and construction specs for CRCP

5 CTE in ME-PDG Transverse crack predictions highly dependent on the assumed CTE value

6 Experiment to Study Effect of CTE Variable Factor Levels 1 COTE (2) /ºF 7 10e -6 /ºF 2 Axle Load Spectra (2) Urban Rural 3 Traffic Volume (1) TI: 16 4 PCC Thickness (2) 9 in. 12 in. 5 Base Type (1) Cement Treated Base 6 Dowels (2) Dowels No Dowels 7 Shoulder Type (3) Asphalt Shoulders Tied Shoulders Widened Truck Lane 8 Joint Spacing (2) 15 ft. 19 ft. 9 Climate Regions (3) Mountain Valley South Coast 10 Subgrade Type (1) SP 11 Strength (1) 626 psi Total Number of Cases: 288

7 Effect on cracking and faulting % Slabs Cracked Faulting (in.) CTE (x10-6/ºf) 4 7 CTE (x10-6/ºf) /ºF corresponds to limestone or granite aggregate; /ºF corresponds to quartzite, cherts

8 Testing Procedure AASHTO TP60 + recommendations by Texas DOT

9 Steps to test CTE 1. Specimen preparation : - 100mm (4 inches) diameter cores - Cut flat surfaces top and bottom - Length from 165 to 210mm (6 ½ to 8 ¼ inches) 2. Submerge specimen in limewater for at least 2 days. 3. Measure exact length of the specimen 4. Specimen is placed in the testing frame which is submerged in water 5. Test

10 Steps to test CTE (cont d)

11 Steps to test CTE (cont d)

12 Temperature sequence Step Temperature Duration 1 10 C 30min 2 Change from 10 C to 50 C 2hr 15min 3 50 C 30min 4 Change from 50 C to 10 C 2hr 15min 5 10 C 30min

13 Software s screen capture

14 Thermal cycling 1. The thermal cycling is automatically repeated three times to obtain more stable readings, as explained later 2. Each cycle takes ~6 hours entire test is ~18 hours.

15 Frame correction Correction to account for thermal deformation on the frame that supports the LVDT. Correction obtained using cylinders of known CTE: 3 stainless steel aluminum 6061 T-6

16 Regression to determine CTE 0.2 Relative displacement (mm) Test 2, falling: CTE=6.60 y = 0.002x R 2 = Test 1, falling: CTE=6.10 y = x R2 = Test 2, rising: CTE=6.63 y = 0.002x R2 = Test 1, rising: CTE=6.40 y = 0.002x R 2 = Temperature (C)

17 Effect of consecutive thermal cycles Better regressions are obtained with consecutive thermal cycles Reduction in the difference between the rising and falling CTE Lower CTE

18 Effect of consecutive thermal cycles R R 2 CTE (microstrain/f) CTE Rising (heating) Falling (cooling) Thermal Cycle Thermal Cycle CTE decreases with additional cycles: 3rd CTE is lower than 1st CTE in 76% of cases 3rd CTE is lower than 1st CTE by 0.1 / F in 48% of cases 3rd CTE was on average 0.15 / F lower than 1st CTE

19 3 rd CTE vs 1 st CTE rd CTE (10-6 / F ) st CTE (10-6/ F )

20 Effect of concrete saturation 1 oven-dried core, saturated for 4 days 2 oven-dried cores, immediate test

21 CTE at high saturation levels 105% Saturation (%). 100% 95% 90% 85% Time (hours) 9 CTE (microstrain/ F) % 98% 99% 100% 101% Saturation (%)

22 Oven-dried cores, immediate test CTE(microstrain/ F) Rising Falling CTE (microstrain/ F) Time (hours)

23 Comparison With Results From Other Laboratories CTE (microstrain/f) UCPRC TX or FHWA TX-1 FHWA-1 FHWA-2 FHWA-3 FHWA-4 TX-2

24 50 rigid pavement sites, 56 composite pavement sites (ac overlay)

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27 Mechanistic inputs being collected thickness, joint spacing, accumulated traffic, subgrade type, solar reflectivity, etc. Data will be used to verify the effect of CTE and other factors on concrete pavements.

28 Number of cores All data Histograms of CTE values Geographical Variability Number of cores District 2 n= District 4 n=42 across the state: 4.5 to 6.7 microstrain/ F. Number of cores District 10 n= District 11 n=4 District 2 : 6.3 District 4 : 5.2 District 10: 6.4 District 11: CTE (microstrain/f) CTE (microstrain/f)

29 Aggregate types in CA District 2: alluvial or glacial deposits. A mix of sedimentary (sandstone) and volcanic (basalt) rocks Districts 4 and 10: sedimentary (predominantly sandstone), more angular and probably quarried. District 11: predominantly granitic and probably quarried

30 CTE spatial variability CTE (microstrain/ F) Site 4-SCL-85 Northbound Southbound CTE (microstrain/ F) Site 10-SJ-580 Eastbound Westbound Postmile Postmile

31 CTE spatial variability. CTE (microstrain/ F) 7.0 Site 4-SOL Eastbound CTE (microstrain/ F) Site 2-SHA Northbound 4.0 Southbound Postmile Postmile CTE (microstrain/ F) Site 4-SON-101 Northbound Southbound CTE (microstrain/ F) Site 11-IMP-86 Southbound

32 Summary and Conclusions CTE being evaluated from in-service pavements in California Involves thermal cycles in a waterbath Range is 4.5 to 6.7 microstrain/ F 3-cycle testing is good practice: Better regressions Reduction in difference between ramps (Lower CTE) Continue work

33 Thanks Erwin Kohler University of California Pavement Research Center Project Scientist, PhD Civil and Environmental Engineering, UC-Davis