6 questions on fundamentals of designing continuously reinforced concrete pavements ... · 2020. 3....
TRANSCRIPT
6 QuestionsOn Fundamentals of Designing
Continuously Reinforced Concrete Pavements –How Well Do You Know
Concrete?
Elizabeth (Lisa) Lukefahr, P.E.
Texas Concrete Pavement Association
Early Texas Concrete Pavements
• Belknap Place San Antonio, 2-Lift Pavement built in 1914• 1880 Alamo Cement Kiln: first cement plant west of the Mississippi
River
Central Texas ACI Chapter – March 2020
Early Texas Concrete Pavements
Central Texas ACI Chapter – March 2020
What Design Methods Have Been Used in the Past?
Central Texas ACI Chapter – March 2020
A painting by Carl Rakeman of the construction of the first macadamized road in the United States (1823). In the foreground, workers are breaking stones "so as not to exceed 6 ounces [170 g] in weight or to pass a two-inch [5 cm] ring". Wikipedia
Then and NowConestoga Wagon
• Cargo Capacity: 6 tons
• Tare Weight: 0.7 tons
• Daily Vehicle Traffic ??
Semi-Tractor Trailer
• Cargo Capacity: 32.5 tons
• Tare Weight: 17.5 tons
• Downton IH-35 2018 AADT:218,960 9.3% Trucks
Central Texas ACI Chapter – March 2020
AASHO Road Test – The Origin of US Pavement Design
• No CRCP, JCP Only
• Failure based on roughness in wheel paths, cracking and surface patching
• 1972 AASHTO Interim Guide implied load transfer was equivalent for JCP and CRCP, resulting in equivalent thickness
• Simple nomograph
Central Texas ACI Chapter – March 2020
1984 – PCA Design Method
• Used JSLAB computer program to identify critical stresses and deflections at the pavement edge• Fatigue
• Erosion
• Also based on JCP
Central Texas ACI Chapter – March 2020
1993 AASHTO Guide for Design of Pavement Structures
• Provided recommendations for CRCP load transfer coefficients and for shoulder type
• However, distress mechanisms still based on JCP performance
• STILL AN EMPIRICAL DESIGN MODEL
Central Texas ACI Chapter – March 2020
Empirical vs. Mechanistic Design What’s the Difference??
• Empirical Model - based on direct observation, measurement and extensive data records
• Mechanistic Model - seeks to explain phenomena only by reference to physical causes
Example: Develop a Model to Help Lisa Lose (or at least maintain) Weight
Central Texas ACI Chapter – March 2020
Lisa’s Empirical ModelMeasured Observations
• When I eat only 5 cheesecakes per day for a month I gain 60 pounds
• When I eat nothing but celery every day for a month I lose 30 pounds
Model Results
• The rate of weight gain is 0.4 pounds per cheesecake
• The rate of weight loss with celery-only diet is -1 pound per day
Takeaway
• To maintain my current weight, I can eat 5 cheesecakes for every 2 days of celery-only eating.
Central Texas ACI Chapter – March 2020
Lisa’s Empirical ModelMeasured Observations
• When I eat only 5 cheesecakes per day for a month and I gain 60 pounds
• When I eat nothing but celery every day for a month I lose 30 pounds
Model Results
• The rate of weight gain is 0.4 pounds per cheesecake
• The rate of weight loss with celery-only diet is -1 pound per day
Takeaway
• To maintain my current weight, I can eat 5 cheesecakes for every 2 days of celery-only eating.
Central Texas ACI Chapter – March 2020
Any Limitations to the Above? Errors in Assumptions?
Lisa’s Mechanistic Model• Based on NASA theories of the energy consumption of astronauts in Zero-g
performing normal astronaut activities, the number of calories needed to maintain constant weight in a 30-day orbital trip is:
• Where b = the number of high protein freeze-dried packets consumed, p = the hours of stationary bike riding, and i = metabolic degeneration due to zero gravity
Practical Challenges
• It is known that protein, carbohydrate and fat calories are metabolized VERY differently
• I don’t ride bikes, but I do competitive Netflix Watching in 1.0 gravity environments
Central Texas ACI Chapter – March 2020
Lisa’s Mechanistic Model• Based on NASA theories of the energy consumption of astronauts in Zero-g
performing normal astronaut activities, the number of calories needed to maintain constant weight in a 30-day orbital trip is:
• Where b = the number of high protein freeze-dried packets consumed, p = the hours of stationary bike riding, and i = metabolic degeneration due to zero gravity
Takeaway
• To maintain my current weight….
• I need to develop a way to figure out how to adapt what seemed like a perfectly brilliant concept in my mind to some sort of practical application
Central Texas ACI Chapter – March 2020
Mechanistic-Empirical Models• The best of both worlds
• Great for applications where it is difficult to fully describe phenomena with currently known scientific principles
• like pavement design
Central Texas ACI Chapter – March 2020
Central Texas ACI Chapter – March 2020
Mechanistic-Empirical Pavement Design Guide (MEPDG) - NCHRP 1-37(A),
circa 2004
• Mechanistic – Empirical Concept (AASHTO July 2008 MEPDG Manual of Practice)
• The design and analysis procedure calculates pavement responses (stresses, strains, and deflections) and uses those responses to compute incremental damage over time.
• The procedure empirically relates the cumulative damage to observed pavement distresses (i.e., calibration with actual field data).
Central Texas ACI Chapter – March 2020
Central Texas ACI Chapter – March 2020
• Select pavement type and design strategy
• Enter climate, material and traffic loading data
• Enter trial structure
• The software analyzes the pavement’s performance for the given design life.
• The structural response model operates with mechanistic models that calculate the pavement responses to given traffic loading and climatic environment.
• The pavement responses, such as pavement and base material degradation, are then converted using the
embedded empirical transfer functions to pavement distresses that are accumulated over time.
MEPDG Process
Central Texas ACI Chapter – March 2020
Central Texas ACI Chapter – March 2020
Transfer Function
6 Questions Impacting Response, Transfer Function and Damage Accumulation of ME-
Continuously Reinforced Concrete Pavement1. Concrete temperature is uniform throughout depth; concrete
stresses from temperature variations are uniform through the cross-section; slab will contract and expand in one dimension; there is full (100%) and uniform contact between the concrete slab and the subbase
2. The more lanes tied together (wider), the more transverse steel is needed to prevent longitudinal cracking
3. Transverse Crack Spacing and Crack Width are related
4. Short spacing between transverse cracks increases punchoutpotential, as LTE will decrease
5. LTE will be poorer in winter (concrete contracts and cracks widen) than in summer
6. Steel mat higher than center will better control cracks and reduce punchouts
Central Texas ACI Chapter – March 2020
Question 1 (combo)
True or False• Concrete temperature is uniform throughout depth;
• Thus, concrete stresses from temperature variations are uniform through the cross-section
• Slab will contract and expand in one dimension
• There is full (100%) and uniform contact between the concrete slab and the subbase
Central Texas ACI Chapter – March 2020
Question 2True or False
The more lanes tied together (wider), the more transverse steel is needed to prevent longitudinal cracking• The subgrade drag equation adapted to pavements, including
CRCP (no transverse joints)
Central Texas ACI Chapter – March 2020
Question 2 – Empirical MeasurementsThe more lanes tied together (wider), the more transverse steel is needed to prevent longitudinal cracking
Central Texas ACI Chapter – March 20200-5444-2 Megan Stringer, et. al, Oct 2008
Question 2 – Empirical MeasurementsThe more lanes tied together (wider), the more transverse steel is needed to prevent longitudinal cracking
Central Texas ACI Chapter – March 20200-5444-2 Megan Stringer, et. al, Oct 2008
SGDT does not predict actual behavior for CRCP; stress is local
RESULT: Transverse steel spacing set at 4’, for constructability reasons,
regardless of overall highway width
BUT…. Empirical Measurements DID show that stresses intie bars at longitudinal joints was much higher than previously theorized.
Central Texas ACI Chapter – March 20200-5444-2 Megan Stringer, et. al, Oct 2008
Question 2 – Empirical Measurements
BUT…. Empirical Measurements DID show that stresses intie bars at longitudinal joints was much higher than previously theorized.
Central Texas ACI Chapter – March 20200-5444-2 Megan Stringer, et. al, Oct 2008
Question 2 – Empirical Measurements
RESULT: Tie Bar Spacing changed from 3’ to 2’
True or False
Transverse Crack Spacing and Crack Width are related
• That is, there is a certain amount of stress that needs to be relieved, and since stress and strain are proportional, widely spaced cracks should be wider
Central Texas ACI Chapter – March 2020
Question 3
Transverse Crack Spacing and Crack Width are related
Central Texas ACI Chapter – March 2020Suh et al., 2001
Question 3 – Empirical Measurements
True or False
Short spacing between transverse cracks increases punchout potential, as load transfer efficiency (LTE) will decrease
Central Texas ACI Chapter – March 2020
Question 4
Short spacing between transverse cracks increases punchout potential, as load transfer efficiency (LTE) will decrease
Central Texas ACI Chapter – March 2020
Question 4
Question 4 – Empirical Measurements
Short spacing between transverse cracks increases punchout potential, as LTE will decrease
Central Texas ACI Chapter – March 2020Ha, et. al., 0-5832-P1 2012
True or False
Load Transfer Efficiency (LTE) will be poorer in winter than in summer
• concrete contracts in cold weather which cause the cracks to open
• Open cracks lose rock-to-rock contact
Central Texas ACI Chapter – March 2020
Question 5
Question 5 – Empirical Measurements
LTE will be poorer in winter (concrete contracts and cracks widen) than in summer
Central Texas ACI Chapter – March 2020
Ha, et. al., 0-5832-P1 2012
Question 6
True or False
Steel mat higher than center will better control cracks and reduce punchouts
Central Texas ACI Chapter – March 2020
Question 6 – Empirical Measurements
Steel mat higher than center will better control cracks and reduce punchouts
Central Texas ACI Chapter – March 2020
Predicted Punchout rate for steel placed higher than mid-depth and mid-depth
Question 6 – Empirical Measurements
Steel mat higher than center will better control cracks and reduce punchouts
Central Texas ACI Chapter – March 2020
• Test section constructed (~12 yrs )
• Monitoring to see if (and how much) steel mat placement affects punchouts
• Research is probably indicating that for thicker, single-mat CRCP, it will be advantageous to raise the reinforcing steel location
6 QuestionsTrue or False for Texas CRCP?
Q1 Concrete Behavior Items FALSE
Q2 SGDT FALSE and Poor Tie Bar Stress Prediction
Q3 Crack spacing and width arerelated
FALSE
Q4 Crack spacing and LTE related NOT COMPLETELY TRUE
Q5 LTE related to temperature Too Small to be Significant
Q6 steel mat higher than neutralreduces punchouts
TBD
Central Texas ACI Chapter – March 2020
Would You Use a Model That Incorporates Empirically Dubious
Concepts?
• Can a transfer function make an incorrect theory of behavior perform correctly in a model?
• Can you calibrate your way past a bad theory or a poortransfer function?
Central Texas ACI Chapter – March 2020
CRCP ME Design(TxDOT/Texas Tech Project 0-5832)
• Use 60 Years of CRCP experience to develop ME Design that more closely matches Texas experiences
• Use Volumes of Prior Research to Develop FEM Response Model
• Use Volume of Existing field measurements to develop transfer functions
Central Texas ACI Chapter – March 2020
All Models Have Inherent Limitations
• Know the limitations
• Don’t expand the inference space• Normalization of Deviance
• Adjust Model based on actual results
• Document Assumptions
• Continuing Education
• Involvement with Technical Organizations
Central Texas ACI Chapter – March 2020
Concrete Cracks
• I never wanted to believe my Dad was stealing from his job as a roadway maintenance worker. But when I got home all the signs were there.
• Two fish swim into a concrete wall. One turns to the other and says: "Dam"
Central Texas ACI Chapter – March 2020
Thanks for your time and attention!
Central Texas ACI Chapter – March 2020