State DOTs want their bridges to have a service life of 75 years or longer, which means they need durable concrete. As a contractor, how can you be sure your concrete will meet their requirements?
“Water penetration is directly or indirectly the cause of the majority of disintegrations in concrete and the degree to which water penetration is permitted by the texture of any concrete is a direct measure of its strength and endurance.” — Concrete Engineers’ Handbook, 1918
This 100-year-old observation suggests that permeability is the best predictor of concrete durability. Yet the traditional approach to specifying durable concrete is prescriptive:
- Minimum cement content of 6 sacks (564 lb/yd3)
- Maximum water-cement ratio of 0.45
- Maximum slump (usually 4 to 6 in.)
- Aggregates sound, clean, well graded, and durable
- Adequate air-void system in freezing climate
- Proper batching, mixing, placement, finishing and curing.
What do we mean by “durable concrete”?
Concrete durability doesn’t mean anything unless we know the service conditions. Is it indoors or outdoors? Is it subject to freezing? Deicing salts? Industrial chemicals? Abrasion? Concrete exposure classes help us categorize the nature and severity of the exposure conditions. How long do we want it to last?
Something else we need to consider is what we mean by failure. Obviously, collapse is failure. But what about when corrosion has proceeded to the point of spalling of the cover? How about when rust stains appear on the concrete surface?
Naturally, we want to prevent catastrophic failure and at least postpone significant deterioration for a long time. The prescriptive specification listed above will get us most of the way there most of the time. But it isn’t necessarily the most effective specification, and it’s definitely not the most sustainable.
In the last 20 years, the concrete industry has been moving slowly toward performance specifications. That is, instead of specifying what the concrete must be, we specify what it must do.
A performance specification for durable concrete might include maximum permeability to water, maximum charge passed in the Rapid Chloride Permeability Test (ASTM C1202), cracking tendency (AASHTO T334), or maximum shrinkage (ASTM C157). Compressive strength may not even matter, or there could be a maximum limit on strength.
In the interests of both sustainability and better performance, more owners are accepting and even encouraging the use of supplementary cementitious materials such as fly ash and slag cement.
If you’re an engineer or owner, Beton can develop a concrete specification that meets your needs. Or if you’re a contractor who must meet someone else’s specification, we can design a durable concrete for you. We’ll conduct any testing you need to demonstrate that it works. And of course if you need us to, we’ll come to your job site to make sure everything goes smoothly.