Forensics Investigations. Tom Scullion Texas A&M Transportation Institute Short Course 2016

Transcription

1 Forensics Investigations Tom Scullion Texas A&M Transportation Institute Short Course 2016

2 Objectives of Forensic Studies What is the cause of the problem What should the TxDOT District do now How can the problem be avoided in the future More research is needed New Specs New test procedures New Equipment

3 TxDOT s Forensics Process TxDOT District contacts DarHao Chen and head of Materials and Test Div. NTP written If approved; CST appoints a team of TxDOT and University personnel TTI has an IAC to conduct both Forensics Studies and Corridor Studies at minimal cost to District Study conducted (typically 1 week to 3 months duration) Short report written and presentation made to District requesting help Follow on studies identified

4 Forensics Studies and their Products Sulfate Heave Lab Test Procedures (145E + 3D Swell) Field Veris Equipment Thermal Segregation in Hot Mix Pave IR system Early Cracking in Overlays Balanced Mix Design High Performance Thin Overlays Project Acceptance testing Use of FWD

5 Case Study 1 Sulfate Heave (1997) US 67 South of Dallas Blow ups in stabilized subgrade layer during construction Pat Harris, Stephen Sebesta

6 Following is an abbreviated geochemical reaction model from Hunter (1989): Ca(OH) 2 Ca (OH) - (Ionization of lime; ph rises to 12.3) Al 4 Si 4 O 10 (OH) 8 + 4(OH) H 2 O 4Al(OH) H 4 SiO 4 (Dissolution of kaolinite at ph > 10.5) CaSO 4 2H 2 O Ca 2+ + SO H 2 O (Dissolution of gypsum) 6Ca Al(OH) (OH) - + 3(SO 4 ) H 2 O Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 26H 2 O (Formation of ettringite Expands approx 3 times original volume)

7 Spectrophotometer/Colorimeter Cost $400 Lab Test Tx Method 145E

8 Counties where high levels of sulfates (> 8000 ppm) have been detected on a TxDOT project

9 Veris 3150 Soil Mapping System (TxDOT Contacts Jimmy Si and Wade Blackmon) Cost of unit is approximately $30K Measures Electrical conductivity of top 2 to 4 feet

10 Search for Alternative Stabilizers using 3D Swell Molded in gyratory compactor 3 day dry back Wrapped in wet paper towel and rubber membrane Placed in chest on porous stone in water bath Monitor 3-D swell Target < 6% or less than control Tex Method 149 E

11 3D Swell Expansion * Double-mellowing : 3% lime or 3% Class F fly ash was added to 3% lime-treated mellowing materials at day 7, respectively, in order to achieve reach sulfate content 3,000 ppm

12 Case Study 2 Thermal Segregation problems 2002

13 Gen 1 Pave-IR Bar

14 Day 1 of Placement Operation many problem areas

15 Day 2 Operation Representative Results after Contractor Modified Operation

16 Carrot Stick

17 PAVE-IR PHOTOGRAPHS Second Generation Pave-IR Cooperation with Industry

18 Sebesta and Scullion 3 rd Generation

19 Case Study 3 Poor Performance of overlays over JCP (Houston IH ) 4 inch Item 341 Overlay 18 months old Replaced less than 3 years

20 Tx Method 248 TxDOT s Overlay Tester

21 Balancing Rutting and Cracking Properties HWTT.vs. Overlay tester

22 Conclusions The balanced mix design approach works The high performance mixes TxDOT is placing these days are working well RAP/RAS limits not needed if the BMD approach is enforced

23 Case 4 Frontage Road Failure Failure during construction 20 inches of Grade 1 base Underseal + 2 inches of Type D dense graded HMA Both base and HMA passed specs on density and thickness Is it HMA, base, subgrade or bonding problem??

24 Project Nuclear Density data Base Material SH6 Percent Compaction Frequency % 80.00% 60.00% 40.00% 20.00% More 0.00% Frequency Cumulative %

25 Cores from SH 6 Project No bonding issues

26 Properties Cracking (overlay tester cycles to failure) Rutting (Hamburg cycles to 0.5 inch rut) Lab Results on HMA Field Cores No problems in performance tests Site A (good) Site B (poor) Result Target 337 > K >10K Properties Result Cracking (overlay tester cycles to failure) Rutting (Hamburg cycles to 0.5 inch rut) Target 426 > K >10K

27 SH 6 FWD data max deflection vs distance A Test locations B

28 DCP data from low deflection area

29 DCP data from high deflection area

30 Conclusions and Actions Problem with compaction of base Density measurements did not catch problem One test per 3000 cu yd Base substantially less than OMC when test run Contractor compacted base on dry side HMA was good, bonding was good 1 mile of project rebuilt

31 Future Directions The FWD is the best tool for ensuring that the pavement is being constructed as designed Steps underway to incorporate both DCP and FWD testing in upcoming design build projects Some Districts conducting in-house testing to determine portable FWD to assist with project acceptance testing

32 Conclusions TxDOT has a very successful forensics program This program is a critical part of the research cycle Implementation includes new tools, test procedures and specifications We get better by learning from our mistakes not burying them