6 Questions On Fundamentals of Designing Continuously Reinforced Concrete Pavements ... · 2020. 3....

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


What Design Methods Have Been Used in the Past?


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

https://en.wikipedia.org/wiki/Macadamized_road

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


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


1984 – PCA Design Method

• Used JSLAB computer program to identify critical stresses and deflections at the pavement edge• Fatigue

• Erosion

• Also based on JCP


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


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


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.


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.


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


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


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


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).



• 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


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


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


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)


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


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.


Question 2 – Empirical Measurements

BUT…. Empirical Measurements DID show that stresses intie bars at longitudinal joints was much higher than previously theorized.



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


Question 3

Transverse Crack Spacing and Crack Width are related

Central Texas ACI Chapter – March 2020Suh et al., 2001


True or False

Short spacing between transverse cracks increases punchout potential, as load transfer efficiency (LTE) will decrease


Question 4

Short spacing between transverse cracks increases punchout potential, as load transfer efficiency (LTE) will decrease


Question 4


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


Question 5


LTE will be poorer in winter (concrete contracts and cracks widen) than in summer


Ha, et. al., 0-5832-P1 2012

Question 6

True or False

Steel mat higher than center will better control cracks and reduce punchouts





Predicted Punchout rate for steel placed higher than mid-depth and mid-depth




• 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


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?


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


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


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"


Thanks for your time and attention!


6 Questions On Fundamentals of Designing Continuously Reinforced Concrete Pavements ... · 2020. 3....

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