Uniform Service Life Design Guide - Transportation

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

mbarthol@ch2m.com