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Uniform Service Life Design
Guide
by Mike Bartholomew, P.E. / CH2M HILL
2015 AASHTO SCOBS MEETING
TECHNICAL COMMITTEE T-9 – BRIDGE PRESERVATION
APRIL 21, 2015
SARATOGA SPRINGS, NY
Presentation Overview
Service Life Design – What is it?
Historical Background – What’s been done?
Current Status / Gaps – What’s being done?
NCHRP Research Problem Statement for Service Life Design – What’s next?
Service Life Background
Bridge Design has historically focused solely on structural engineering aspects
Selecting materials by their strength properties (f’c, fy) and sizing components to resist loads
Extremely important, but does little to ensure that a structure will remain in use for a given period of time
Service Life Background
When a structure reaches the end of its life
The cause is primarily because the material components have begun to deteriorate
Not from unanticipated loads
But by loss of strength from steel corrosion and concrete cracking/spalling, as a result of the environmental exposure conditions
Service Life Background
Significant research has been
completed over the past 25 years
on how materials deteriorate with
time (particularly reinforced
concrete)
Mathematical models have been
developed to model deterioration
Service Life Design
Principles
All Materials Deteriorate with Time
Every Material Deteriorates at a Unique Rate
Deterioration Rate is Dependent on
The Environmental Exposure Conditions
The Material’s Protective Systems
Service Life Design (SLD)
Design approach to resist Deterioration
caused by Environmental Actions
Also called Durability Design & often Design for
100-year Service Life
Similar to design against Structural Failure
caused by External Loads
What we know as Strength Design
Service Life Design
Types of Deterioration
Reinforcing Steel Corrosion
Concrete Cracking, Spalling,
Delamination
Structural Steel Corrosion
following breakdown of
Protective Coating Systems
Service Life Design
Environmental Actions
Exposure to Chlorides from Sea
Water or De-Icing Chemicals
Exposure to CO2 from many Wet/Dry
Cycles
Exposure to Freeze/Thaw Cycles
Alkali-Silica Reaction (ASR)
Abrasion (ice action on piers,
studded tires on decks)
Service Life Design
Mathematical Modeling of Deterioration vs Time
Based on Magnitude of Environmental Actions
Resistance Properties of the Materials
Deterioration – Fick’s 2nd Law
Models Time to Initiate Corrosion from Chloride Ingress in Uncracked Concrete (Cracks < 0.3 mm or 0.012”)
C(x,t) Chloride concentration at depth & time kg/m3
x, t Depth from surface / time mm, yr
erf Mathematical error function -
Ccrit Critical chloride content (to initiate corrosion) kg/m3
Co Initial chloride content of the concrete kg/m3
Cs Chloride concentration at surface kg/m3
Dapp,C Apparent coefficient of chloride diffusion in concrete
mm2/yr
C x t( ) Co Cs Co 1 erfx
2 Dapp c t
Ccrit
Chloride Profiles vs. Age constant Dapp,c = 15.1 mm2/yr
20 40 60 80 100
Depth, mm
Ch
lorid
e C
on
ten
t, k
g/m
3
5
10
15
Ccrit =1.59
Cs =17.7
10 yr
50 yr
100 yr 120 yr
Service Life Design History
Duracrete: EU Brite/EuRam
Probabilistic Performance-based Durability Design of Concrete Structures, 1996-2000 (started in 80’s)
LIFECON: European Community Competitive and Sustainable Growth Program
Life Cycle Management of Concrete Infrastructures for Improved Sustainability, 2001-2003
International Federation for Structural Concrete (fib)
Model Code for Service Life Design, 2006
Service Life Design History
First introduced to US Bridge Community at the
2005 ASBI Convention in Washington, DC
“Design and Construction of Segmental Concrete
Bridges for Service Life of 100 to 150 Years”, by Steen
Rostam / COWI, Denmark
Strategic Highway Research Program (SHRP2)
Project R19A, 2007-2013
“Bridges for Service Life beyond 100 Years: Innovative Systems, Subsystems & Components”, by Atorod
Azizinamini / Florida International University
Current Specifications for
Service Life Design
fib Bulletin 34 – Model Code for
Service Life Design (2006)
fib Model Code for Concrete
Structures 2010
ISO 16204 – Durability – Service
Life Design of Concrete
Structures (2012)
Service Life Design
Strategies
• Avoidance of deterioration – Strategy A
• Design Based on Deterioration from the Environment – Strategy B
Deemed to satisfy provisions
Full probabilistic design
Semi-probabilistic or deterministic design
Avoidance of
Deterioration
Also called the “Design-Out” approach
Achieved by either:
Eliminating the environmental exposure actions
(e.g., interior of buildings with controlled temperature & humidity)
Providing materials with resistance well beyond the requirements needed
(e.g., stainless steel reinforcement)
Deemed to Satisfy
Method
Prescriptive approach used in most major design codes
e.g., In severe environment, use concrete with w/c ratio < 0.40, 2½” cover
Based on some level of past performance
No mathematical deterioration modeling
Simplistic and not quantifiable
Lowest level of reliability
Full Probabilistic Design
Uses mathematical models to describe observed physical deterioration behavior
Model variables are:
Environmental exposure actions (demands)
Material resistances (capacities)
Variables represented by mean values and distribution functions (std. deviations, etc.)
Probabilistic, Monte-Carlo type analysis to compute level of reliability
Full Probabilistic Design
Reliability based like that used to develop
AASHTO LRFD code for structural design
Sophisticated analysis beyond typical
experience level for most practicing
bridge engineers
Work effort may be regarded as too time
consuming for standard structures
Usually reserved for use on large projects
Semi-Probabilistic Design
Uses same mathematical model as Full Probabilistic Design
Load Factors on Environmental Demands
Resistance Factors on Material Properties
Direct solution to model equations
Not enough data to properly determine appropriate factors and reliability level
Method expected to be adopted by Codes in the future
Current US Status of
Service Life Design
SHRP2 Project R19A Implementation Assistance Program (IAP) sponsored by AASHTO & FHWA
Subject Matter Expert (SME) Team
CH2M HILL
Buckland & Taylor
Participating Agencies
Iowa DOT
Oregon DOT
Pennsylvania DOT
Virginia DOT
FHWA Central Federal Lands
Service Life Design Gaps
Current International SLD guides
Not specific to bridges
Address concrete structures only
Lack of Deterioration Modeling for Steel Structures
AASHTO LRFD lacks SLD provisions on deterioration modeling
US Bridge community unfamiliar with SLD
Proposed Uniform Guide
for Service Life Design
AASHTO T-9 has developed NCHRP
Problem Statement
Title: Development of Guidelines for Uniform
Service Life Design for Bridges
Recently approved by AASHTO Standing
Committee on Research (SCOR) for Fiscal
Year 2016
Goals of the NCHRP
Research Project
Extend the Research from SHRP2 R19A to develop:
Standardized environmental loading parameters by geography and climate
Nationally recognized life cycle cost/benefit parameters
Example designs for various SLD strategies
Procedures for assessing remaining life of existing structures
Life extension technologies for existing post-tensioned concrete structures
Tasks of the NCHRP
Research Project
Develop annotated outline for a Service Life
Design procedure for review
Develop a final recommended AASHTO
Uniform Service Life Design Guide (Manual)
Create Demonstration Design Guides for
training
Develop standardized templates for Bridge
Life Cycle Cost Analysis
Prepare an Implementation Action Program
Uniform Guide for SLD
– Suggested Outline
Introduction
Service Life Design Strategies
Structure Life Duration
Environmental Exposures Classes
Deterioration Mechanisms & Models
Durability Limit States
Construction Requirements (Contract Documents)
In-Service Requirements
Assessment of Existing Structures
SLD Guide – Introduction
Basic Description of the Overall Concept
of Service Life (or Durability) Design
Terms & Definitions
Reliability Based Approach
Identify the Key Drivers to Design
Environmental Exposure
Deterioration Mechanisms
Material Resistance
How to Apply
Service Life Design
Strategies
Avoidance of Deterioration
Use of Materials With Resistance Well Beyond Required (e.g., Stainless Steel)
Full Probabilistic
Semi-Probabilistic (Needs more data to establish Environmental Load Factors and Durability Resistance Factors)
Deemed to Satisfy (Need to develop reliable prescriptive requirements)
Target Reliabilities
Structure Life Duration
Main Structural Elements (75-100 Years or more)
Conventional System Structures (75 Years?)
May be based on planned life of facility and potential obsolescence
Monumental Structures (100 Years or more)
Replaceable Components (25-30 Years)
Wearing Surfaces
Joints
Bearings
Temporary Structures (< 10 Years)
Set Based on Owner’s Requirements
Environmental Exposure
Classes Chloride Ingress in Sea Water (XS1-XS3 in EN-206)
Define the Chloride Concentrations (Salinity will vary by Geography and distance from the Ocean) for the Following Severities:
Atmospheric
Submerged
Tidal/Splash/Spray Zones
Chloride Ingress from Application of De-Icing Materials
Similar Severities as above (De-Icing application rate varies by climatic conditions)
Carbonation
Define Surface Concentrations of CO2 (varies by humidity, wet-dry cycles, and proximity to industrial areas)
Deterioration Mechanisms
& Models
Reinforcing Steel Corrosion (All of these are for initiation phase, need propagation models)
Chloride Ingress from Sea Water
Chloride Ingress from De-Icing Salts
Carbonation
Concrete Deterioration (No mathematical models currently accepted)
Freeze Thaw Attack
Chemical Attack
Abrasion
Durability Limit States
Steel Reinforcing in Concrete
Depassivation or end of Initiation Phase (also onset of Corrosion)
Cracking
Spalling
Loss of Section
Collapse
Uncoated Structural Steel
% Loss of Section ?
Preservation Issues After Initiation
Contract Document
Requirements Concrete
Trial Mix Designs (Performed during design or construction startup)
Chloride Diffusion Coefficient (Sampling requirements)
Inverse Carbonation Coefficient ( “ )
Reinforcing Steel
Obtained Cover Dimensions (Frequency of cover meter measurements)
Critical Chloride Content (May require research)
Coatings
Material Resistance Parameters (Chloride Diffusion Coefficient?)
Obtained Film Thickness
Re-Coating Requirements
In-Service Requirements
Maintenance and Inspection Plan
“Birth Certificate”
Like an Owner’s Manual
Developed
Monitoring
Sampling during service to compare actual behavior
to predicted
Assessment of Existing
Structures
Establish Guidelines for Condition
Assessment and Prediction of Remaining
Service Life
Follows Same Principles of New Structure
Service Life Design
Summary
Service Life Design is a design approach
to limit bridge deterioration due to
environmental exposures
SLD Principles have been used
successfully in Europe since the 1990’s
SLD is being implemented in the US now
Additional research is needed to define
environmental parameters in the US
Questions
Thank you for your attention
Contact:
Mike Bartholomew, P.E. / CH2M HILL
