1. Designing Resilient Bridges:
Life Cycle Corrosion Modeling Applied to Current Practice
Alex Brent Rollins
Director, Civil Engineering Materials Research Laboratory
The University of Tennessee at Chattanooga
2016 CIS: Tech Session 1A
Charleston, SC
April 4, 2016

2. What is the Biggest Threat to our Nation’s Bridges?
The United States depends on its transportation infrastructure for the efficient movement of goods and services throughout the nation.
Bridges are critical transportation assets; the loss of a bridge can have tremendous economic impact in a state or region.
Securing our nation’s bridges has been seen as important by DHS, FHWA, and others from an All-Hazards Risk perspective.
However, perhaps the greatest threat to our nation’s bridges is time.

3. What is the biggest threat to our Nation’s Bridges?
One in six bridges needs to be replaced (ASCE)
Corrosion of reinforcing steel is the primary reason for bridge failures.
This research involved corrosion life cycle modeling of bridges based on current concrete mix designs, rebar cover, deck thickness, and geographic location.
At current state budget levels and life expectancy, replacing all of these bridges will take 200+ years if NO maintenance to other bridges is performed.
Changes in how we design bridges are needed now to offset maintenance and replacement costs for future generations.

4. Corrosion Modeling Software Used
The project utilized Life 365, a probabilistic corrosion model that calculates chloride diffusion rates based on concrete mix design. It was developed jointly by NIST, ASTM, and ACI with significant industry support.
All fifty states and 177 urban areas were studied, with data from State Departments of Transportation regarding mix designs, rebar cover, and deck thicknesses.
Best-case scenarios were employed to approach the problem from a conservative standpoint. For example, if a state DOT allowed 25% supplementary cementitious material, but didn’t require it, modeling was done at the 25% rate. This returns the greatest life cycle expectancy.
Life 365 also includes a life cycle cost analysis module that allows comparison of concrete mix design performance up to 500 years.

5. Example Results

6. Example Results

7. Results, Current Practice
Life Expectancy, Years
Number of States
Percent of States
0.0 – 10.0
0 %
10.1 – 20.0 8
16 %
20.1 – 30.0 12
24 %
30.1 – 40.0 2
4 %
40.1 – 50.0 6
12 %
50.1 – 60.0 5
10 %
60.1 – 70.0 3
6 %
70.1 – 80.0
80.1 – 90.0 1
2 %
90.1 – 100.0 7
14 %

8. Results, Current Practice

9. Results, Current Practice
Life Expectancy, Years
Number of Bridges
Approx. Percent of Bridges
0.0 – 10.0
0 %
10.1 – 20.0
87595
15 %
20.1 – 30.0
149312
25 %
30.1 – 40.0
24177
3.5 %
40.1 – 50.0
66999
11 %
50.1 – 60.0
50152
9 %
60.1 – 70.0
35702
6 %
70.1 – 80.0
28696
5 %
80.1 – 90.0
4518
1 %
90.1 – 100.0
33272
0.5 %
109441
19 %

10. Results, Current Practice

11. Results, Current Practice
86 % of states have current standard specifications for bridge construction that result in lower corrosion-based life cycle than the widely-recommended 100 year design life.
56 % of states have current standard specifications for bridge construction that result in lower corrosion-based life cycle than the widely-accepted bare minimum of 50 year design life.
This methodology is unsustainable, so a solution is suggested.

12. Proposed Mix Design Changes
Three primary contributors to low (<50 years) corrosion life expectancy were observed:
1) Lower than industry-acceptable limitations on maximum supplementary cementitious materials
2) Relatively high w/cm ratio maximums
3) Insufficient rebar cover
As seen previously, most corrosion life expectancy issues can be solved by:
1) Increasing the amount of allowable supplementary cementitious materials
2) Reducing maximum allowable w/cm ratios

13. Economic Impact of Proposed Mix Changes
Supplementary cementitious materials are recycled products such as fly ash, ground granulated blast furnace slag, and silica fume.
With the exception of silica fume, all are typically significantly less expensive than Portland cement.
Lower w/cm ratios will require greater use of high range water reducers.
Overall, 100+ year design life concrete is possible utilizing concrete mixtures that are very near or even less costly to produce than current practice.

14. Other Durability-Enhancing Technologies
In addition, the following durability-enhancing technologies are available for consideration:
Epoxy coated or stainless reinforcing steel
Waterproofing membranes
Penetrating sealers such as colloidal nano-silica or other pore-blocking technologies

15. Conclusions
SCMs have historically been seen as “fillers” in concrete. Resistance to SCM use is declining, allowing for increased use in several states.
We must get away from a strength-only design philosophy and seriously consider life cycle cost and durability.
The mix design changes presented represent an initial cost neutral or even savings over current mix design practice.
Long-lasting concrete mix designs for bridges are not only possible, but arguably imperative for national security.
The funding stream for transportation infrastructure must be increased to address current repair problems.