further development of post- tensioned prestressed concrete pavement in texas short course material...
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Further Development of Post-Further Development of Post-Tensioned Prestressed Concrete Tensioned Prestressed Concrete
Pavement in TexasPavement in Texas
Short Course Material
Stage I – Design of a PCP
TxDOT Research Project 0-4035TxDOT Research Project 0-4035
Developed by Center for Transportation ResearchThe University of Texas at Austin
This CD was developed in compliance with TxDOT Research Project 0-4035 Task 10.5. The material contained here intends to serve as a guide for pavement engineers to learn about the procedure followed in this project to design a post-tensioned prestressed concrete pavement (PCP).
This material only presents the design procedure; however, complementary information about Project 0-4035 might be found in the technical reports prepared for the project. This reports can be accessed through the Internet at the Center for Transportation Research’s Library Website.
http://www.utexas.edu/research/ctr/
IntroductionIntroduction
For any questions or comments related to this material or project, please contact the research agency:
Center for Transportation ResearchCenter for Transportation Research
At’n. Dr. Cesar I Medina-ChavezAt’n. Dr. Cesar I Medina-Chavez
3208 Red River Suite 1043208 Red River Suite 104
Austin, TX 78712Austin, TX 78712
Phone (512) 232-3100Phone (512) 232-3100
E-mail address: [email protected] address: [email protected]
ContactContact
Acronyms found in the literature that refer to Prestressed Concrete Pavement are: PSCP, PTCP, PCP
BackgroundBackground
In prestressed pavement the concrete slab is preloaded (before traffic loads)
Post-tensioned: it means preload is applied after concrete has gained sufficient strength
Why use Prestressing Techniques?
Concrete is essentially a compression material
Its strength in tension is low
Prestressing naturally involves compressive loading prior to application of service loads
Tensile stresses are reduced or eliminated
BackgroundBackground
Improve the design and construction
techniques for a cost effective state-
of-the-art pavement structure and
apply them in a new design
Project ObjectiveProject Objective
Reference MaterialReference Material
Literature Review(Chapter 2)
PCP use in Texas
(Chapter 3)
Evaluation of PCP in Texas(Chapter 4)
Design of PCP(Chapter 5)
Materials and Construction
Specifications(Chapter 6)
Discussion of Developments
(Chapter 9)
Monitoring Plan for New PCP
(Chapter 7)
PCP and CRCP Comparison(Chapter 8)
Conclusions and Recommendations
(Chapter 10)
I. Evaluation of Previous WorkI. Evaluation of Previous Work
II. New DevelopmentsII. New Developments
Click here to open the report (5.5 Mb PDF file)
Report 0-4035-1Report 0-4035-1
Prestressing principles date from 1888
Eugene Freyssinet started applications in 1910
PCP application goes back to the 1940s in England and France
Brief Prestressing HistoryBrief Prestressing History
APPLICATION OVERSEAS
• First application in England in 1943
• Then in Paris, France at Orly Airport
• Other projects in Austria, Belgium, Germany, The Netherlands, Switzerland, etc.
• Japan
EARLY DOMESTIC EXPERIENCE
• In 1953 in Maryland (Airfield - 7 in. PCP Patuxent River Naval Air Station)
• In 1959 in San Antonio, TX (Taxiway at Biggs Air Force Base)
• Other experimental projects:Pittsburgh, PA (1971)
Dulles Intl. Airport, VA (1971)
EARLY DOMESTIC EXPERIENCE
• After a feasibility study the FHWA decided to develop PCP projects in 3 States
• Characteristics of PCPs in PA, MS, AZ– Only longitudinal post-tension was applied– Post-tensioning applied in gap slabs– 24 ft-wide placements– 6 in.-thick slabs
Gap Slab
Active JointActive Joint
24 ft
Gap SlabGap Slab
Used in projects in PA, MS, and AZ
Disadvantages of Gap Slabs:Require 2 joints per slab
Additional construction operation
Delayed opening to traffic
Greater chance of failure
Higher costs
PCPPCP SlabSlab
400-600’
GapGap SlabSlab GapGap SlabSlab
3-8’ 3-8’
Gap SlabGap Slab
IMPROVED DOMESTIC EXPERIENCES
• IL – O’Hare Intl. Airport (1980)• TX – Highway IH-35 (1985)• PA - Highway US220 (1988)• IL – Greater Rockford Airport (1993)
• During the early 1980s CTR introduced innovations based upon the successes and failures of projects in PA, MS, an AZ– Used Central Stressing Pockets instead of Gap
Slabs– Applied Longitudinal and Transverse Prestress
• A 6 in-thick PCP overlay was constructed in 1985 near West, Texas in McLennan County
• After almost 20 years of service the PCP is in excellent condition under heavy truck traffic
Texas CaseTexas Case
Concept Developed by CTR researchers
Requires 1 joint per slab
Advantages are:No delayed opening to traffic
Decreased construction time
Less chance of failure
Less maintenance
Central Stressing PocketCentral Stressing Pocket
Existing PCP in TexasExisting PCP in Texas
Existing PCP in TexasExisting PCP in Texas
Total of 32 Prestressed Pavement SlabsTotal of 32 Prestressed Pavement Slabs
7 – 440 ft x [ 17 ft + 21 ft ]7 – 440 ft x [ 17 ft + 21 ft ] 9 – 240 ft x [ 17 ft + 21 ft ]9 – 240 ft x [ 17 ft + 21 ft ]
7 Slabs @ 440 ft = 3080 ft7 Slabs @ 440 ft = 3080 ft9 Slabs @ 240 ft = 2160 ft9 Slabs @ 240 ft = 2160 ft
Total = 5240 ftTotal = 5240 ft
IH - 35IH - 35 NN
Exit 351Exit 351Wiggins RoadWiggins Road
FM 1858FM 1858
FrontageFrontage
Waco 15 milesWaco 15 miles
Existing PCP in Texas on IH-35Existing PCP in Texas on IH-35
Cross section construction history, according to TxDOT’s files:
1952
JCP
Granular base
Natural soil
Lime stabilized sub-base
12"
5"
6"
1975
ACP4"
1985 and current
PCP6"
Existing PCP in TexasExisting PCP in Texas
• Outline a procedure that can be used as a guide for designing any PCP
• Use past experiences and apply them in the new design• Foreign, PA, MS, AR, IL• TX – Old PCP and Precast Slab Concrete
Pavement in Georgetown, TX• Apply process to design a PCP on IH-35 near
Hillsboro, TX in Hill County
Design of New PCPDesign of New PCP
Methodology
• Literature review– Documentation of previous work (domestic and
abroad)
• Selection of construction site and evaluation of existing pavement conditions
• Site visits• Development of work plan
Design of New PCPDesign of New PCP
Approach
• Factors affecting design– Traffic Loads– Temperature effects– Moisture effects– Slab friction resistance– Prestress losses– Transverse prestress– Joint movement
Design of New PCPDesign of New PCP
Design Considerations
• Design variables– Foundation strength and embankment
properties– Pavement thickness– Magnitude of prestress– Slab length– Slab width
Design of New PCPDesign of New PCP
Design Considerations
• Design Steps for New PCP in Hillsboro, Texas– Embankment issues– Condition survey– Deflection measurement– Existing pavement back calculation– Traffic data analysis– Thickness design– Slab length
Design of New PCPDesign of New PCP
Implementation of Design Considerations
Location
Between Mileposts 365 and 368 both directions
Design of New PCPDesign of New PCP
Condition Survey- Evaluate existing pavement condition
Design of New PCPDesign of New PCP
Condition Survey- Evaluate existing pavement condition
Design of New PCPDesign of New PCP
– Pavement in Fair Condition
– Transverse Cracks
– Longitudinal Cracks
– Rutting
– Patches
– Alligator Cracking
Condition Survey Summary
Design of New PCPDesign of New PCP
Condition Survey Quantitative Analysis
•Use Pavement Distress Index (PDI)•Mathematical combination of distresses
•Assign weight factor (WF)•Assign severity factor (SF)
•14 different distresses•Severity: Low-Moderate-High•To calculate PDI, each distress is assigned WF and SF
Design of New PCPDesign of New PCP
Where:
Di = deducted points of the ith type distress,
Sij = weight of the jth severity class of the ith type of distress,
Eij = extent of the jth severity class of the ith type distress,
n = number of distress types,
m = number of severity classes
Pavement Distress Index (PDI)
n
i
m
jijij ESDiPDI
1 1
100PDI Chia-Pei-Chou
TRB Record 1592
Design of New PCPDesign of New PCP
n
i
m
jijij ESDiPDI
1 1
100PDI Chia-Pei-Chou
TRB Record 1592
Distress weight factor (49-100)
Severity factor (based on distress 0.24-1.00)
Extent (percent of area/occurrence frequency)Extent (percent of area/occurrence frequency)
Design of New PCPDesign of New PCP
Pavement Distress Index (PDI)
Direction No. of subsections Group
Number
Scores*
NB 21 1,2 77 - 97
SB 22 1,2,33,4 91 - 86 - 6363 - 99
Maximum Score: 100.00
*PDI Chia-Pei-Chou TRB Record 1592/CTR Report
87.0
84.7
SummarySummary
Pavement Distress Index (PDI)
Design of New PCPDesign of New PCP
Structural Evaluation of Existing Pavement
Design of New PCPDesign of New PCP
Deflection Data Collection Using RDD
Design of New PCPDesign of New PCP
Deflection Data Collection Using FWD
Design of New PCPDesign of New PCP
Back Calculation of Pavement Structure
1962 1977 1990 Current
Flexible base CRCP
Compacted foundation course
Natural soil
Lime stabilized subgrade
ACP
Flexible base
ACP
ACP
8"
4"
6"
6"
1"
3-5"
Design of New PCPDesign of New PCP
Back calculation of layer properties
1010
109134
194268
43513075
681760
Natural Soil
SG
SB
CRCP
ACP
Computed Modulus (ksi)
SB
Computed Modulus (ksi)
NBLayer No.
Design of New PCPDesign of New PCP
Design of New PCPDesign of New PCP
The existing asphalt pavement was visually inspected and a pavement distress index (PDI) was calculated for each highway direction.Next, pavement deflections were measured using the rolling dynamic deflectometer (RDD) and the falling weight deflectometer (FWD). Those deflections were used to estimate or backcalculate the elastic properties of the pavement.The next step involved the estimation of the projected traffic for the design lane. In this case, the calculation of the projected equivalent single axle loads (ESALs) was performed by TxDOT and the information was provided to the researchers to conduct a pavement fatigue analysis.
• Layer elastic properties were backcalculated using deflection information
• ESALs information was provided by TxDOT
• ESAL projections were done for 30 years
Design of New PCPDesign of New PCP
Design Process Recap
• Design of equivalent pavement30 year life
114 million ESAL applications
Concrete flexural strength: 700 psi
Concrete modulus of elasticity: 4,000 ksi
• Two different conditions were analyzedOverlay of existing pavement
Pavement on median
Design of New PCPDesign of New PCPElastic Design for Fatigue Loading
Design of New PCPDesign of New PCPElastic Design for Fatigue Loading
The design was performed for two different pavement cross sections, for an overlay of the existing asphalt pavement and a new pavement to be constructed on the existing median.In both cases, an equivalent continuously reinforced concrete pavement (CRCP) was designed using AASHTO’s design procedure.The thicknesses obtained were as follows:
Equivalent CRCP for overlay: 14 in.Equivalent CRCP for pavement on median: 15 in.
Equivalent CRCP for Overlay
15.0”
10”
8”
4”
6”
2
2
5,000 lb5,000 lb
12”
125 psi125 psi
CRCP: E = 4,000,000 psi = 0.15
AC: E = 760,000 psi = 0.35
CRCP: E = 3,075,000 psi = 0.15
Sub-base: E = 268,000 psi = 0.35
Subgrade: E = 134,000 psi = 0.35
Natural Soil: E = 10,000 psi = 0.40
TT
Equivalent CRCP for Pavement on Median
14”
7”
2
2
5,000 lb5,000 lb
12”
125 psi125 psi
CRCP: E = 4,000,000 psi = 0.15
AC: E = 700,000 psi = 0.35
Natural Soil: E = 10,000 psi = 0.40
TT
Design of New PCPDesign of New PCPElastic Design for Fatigue Loading
The next step was to calculate the minimum required prestress for various PCP thicknesses.The analysis included estimation of the prestress for various thicknesses from 6 in. to 15 in.The required prestress increased as the thickness decreased. This means they are inversely proportional. For instance, a 6 in.-thick PCP for the overlay design requires 49.1 psi of prestress to compensate for thickness. Likewise, a 14.0 in.-thick PCP will not require any additional prestress. That would be the equivalent CRCP entire thickness.
Thickness vs. Required Prestress
0
20
40
60
80
100
120
PCP Thickness (in)
Req
uir
ed
Pre
stre
ss (
psi
)
Existing Pavement Overlay 49.1 36.6 26.8 17.3 14.2 9.5 5.4 1.7 0
New Pavement on Median 107.6 82.6 62.6 46.8 34 23.2 14.1 6.5 0
6 7 8 9 10 11 12 13 1414 / 15
The next step in the design is to consider the effect of the combination of environmental stresses and wheel load stresses. The following conditions should be met:
• Critical stresses at PCP must not cause fatigue failure of the prestressed slab
• Combination of wheel loads, slab DT, and moisture stresses should not exceed flexural strength of concrete
Design of New PCPDesign of New PCP
Elastic Design for Environmental
Stresses and Wheel Loads
f = allowable flexural stress in the concrete
sp = effective prestress
st = stress caused by applied wheel load
sc = curling stress due to slab temperature differentials
sF = friction loss between slab and supporting layer
Design of New PCPDesign of New PCP
f + sp st + sc + sF
Elastic Design for Environmental
Stresses and Wheel Loads
The basic equation to be used is:
Design of New PCPDesign of New PCP
Using the basic equation previously shown, the PCP is designed as follows:
• Prestress at end of slab is computed• Prestress losses are estimated and discounted• Strand spacing is calculated
• Iterative process• Vary length and thickness
• Other properties considered are– Aggregate type, Poisson’s ratio, flexural strength,
modulus of elasticity– Climatic variables
Design of New PCPDesign of New PCP
It is recommended to design PCP using a spreadsheet and vary slab length and thickness. Final design characteristics are based on engineering judgment.
Concrete and Pavement Properties:
28-day Flexural Strength f = 700 f = 700 f = 700Safety Factor (ACI=2) SF = 2 SF = 2 SF = 2
Allowable Flexural Stress f design = 350 f design = 350 f design = 350
Concrete Modulus of Elasticity (psi) E = 4500000 E = 4500000 E = 4500000Concrete Poisson's Ration 0.15 0.15 0.15Coefficient of Thermal Expansion (/°F) = 6.00E-06 = 6.00E-06 = 6.00E-06
Concrete Unit Weight (pcf) 144 144 144Slab Length (ft) L = 250 L = 300 L = 350Temperature Differential (°F/in) T = 3 T = 3 T = 3Coefficient of Friction (slab-support)
max= 0.92 max= 0.92
max= 0.92
Thickness (in) D1 D2 D3 D1 D2 D3 D1 D2 D3
D = 8 9 10 D = 8 9 10 D = 8 9 10Tensile Stress at Bottom of Slab (psi)
t = 63.9 54.4 51.3 t = 63.9 54.4 51.3
t = 63.9 54.4 51.3Critical Stress Factors (edge condition) CSF = 1.3Required Fatigue Prestress (psi)
PR = 27 17 14 PR = 27 17 14
PR = 27 17 14
Wheel Load Stress (corrected, psi) t design = 83.1 70.7 66.7
t design = 83.1 70.7 66.7 t design = 83.1 70.7 66.7
Curling Stress at Slab Center (psi) c = 381 429 476
c = 381 429 476 c = 381 429 476
Friction Stress (psi) F = 115 115 115
F = 138 138 138 F = 161 161 161
Concrete and Pavement Properties
Prestressing Tendon Properties:
Tendon Ultimate Strength (psi) Ss = 270000 Tendon Yield Strength (psi) Sy = 229500Tendon Elastic Modulus (psi) Es = 28000000 Tendon Yield Force (lbs) Fy = 49572
Tendon Area, 0.6 in. Diameter (in2) a= 0.216 Tendon Prestressing Force (lbs) F = 34700Tendon Wooble Coefficient K= 0.001 Time After Stressing (design hr) t hr = 262800
Design Tendon Strenght (decimals) DSs= 0.8 Fjack= 46656
Concrete Shrinkage Strain (in/in) s = 0.00015
Concrete Creep Strain (in/in) k = 0.000075
Concrete Strain (Shrinkage and Creep) cu = 2.25E-04Time After Stressing (design yrs) t = 30
Design of PCP Using Spreadsheets
Prestressing Tendon Properties
Climatic Factors: Max - MinSeasonal Temperature Change from Summer to Winter (T Season) = 86 45 41Temperature Change During Summer Day (T Summer) = 110 86 24Temperature Change During Winter Day (T Winter) = 48 10 38
Climatic Factors
Design of New PCPDesign of New PCP
Design of New PCPDesign of New PCP
Summary of PCP Characteristics
Design Feature Recommendation
Design life 30 years
Projected traffic for design life 114 million ESALs
Concrete design flexural strength 700 psi
Concrete modulus of elasticity 4,000 ksi
Thickness of PCP slab 9 in.
Length of PCP slab 300 ft
– A similar expansion joint as in previous PCP in McLennan County will be used
– Joint has proven to be durable and adequate– A better neoprene seal will be used
Asphalt Concrete Layer
Existing JCP Pavement
Weld
Neoprene Seal
½” ØNelsonDeformed Bars
1 ¼” ØStainlessSteel Dowel
DowelExpansionSleeve
*L4 x 3 x ¼
*
Transverse Expansion Joint
Design Practical ConsiderationsDesign Practical Considerations
Transverse
Expansion
Joint
• The new design is based on improvements of previous experiences (PCP near West, TX)
• New PCP is an interesting project that will promote research and innovative construction practices
• Two designs have been conducted– PCP overlay on existing asphalt pavement– PCP on median
• Joint spacing will be limited to 300 ft• Thickness will be 9 in.
Discussion of DevelopmentsDiscussion of Developments
• Construction of PCP allows a more efficient use of construction materials• Less concrete
• Application of prestressing forces makes use of the compressive strength of concrete and reduces tensile stresses
• A well constructed PCP highly reduces maintenance tasks costs
Concluding RemarksConcluding Remarks
Dr. B. Frank McCulloughProject Supervisor, CTR
Dr. Moon C. WonProject Director, TxDOT
AcknowledgementsAcknowledgements