Concrete Strength and Durability

[RFreund]

It is my understanding that higher strength concrete is more durable because it has more cement which means less pores / reduced permeability which slows down the rate at which water/chemicals can penetrate the concrete. My question is - is there anyway to correlate the two. Meaning if 2,500psi is used verse 4,000 psi, is there some way to say that 'all else equal' the 2,500 psi concrete has a design life that is 15 years shorter than the 4000psi concrete?

In my specific case this is for a topping slab which is a wearing surface over a waterproofing system. The topping slab has no real structural purpose. However, is there a way to predict a relative life between a higher or lower concrete strength?

Thanks!

EIT

http://www.HowToEngineer.com

 

[JAE]

1. Not sure there is a direct correlation. Too many other variables perhaps?
2. Sometimes too much cement creates too much shrinkage and more cracks...i.e. perhaps less life.



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[oldestguy]

I'd look at the w/c ratio also. Also, bonding that topping to the lower slab. That takes care.

 

[BridgeSmith]

As JAE stated, more cement generally means more shrinkage cracking, so higher strength may not mean less permeability overall. Shrinkage reducing admixtures or modified concrete, such as latex, polyester or silica fume modified can decrease permeability. It depends on whether the high strength is needed or not. Until recently, silica fume modified concrete was our go-to for bridge deck overlays (partially because of the 7ksi strength and partially that it doesn't require prep beyond removing the weak concrete and cleaning the surface), but lately it has gotten so expensive we are experimenting with those other options.

 

[epoxybot]

Since you are asking about 2500psi concrete and speaking of a wear surface, does that mean foot traffic or something heavier. 2500psi concrete is ~sidewalk concrete~ and is quite durable for that purpose. Since it is over a waterproofing system, is it safe to assume it is not bonded? If it isn't bonded, then I think your durability issue is somewhat more related to thickness vs slab size and any aesthetic that might apply to joint placement. Just make sure that the drains are able to drain at the waterproofing level or you will have water ponding between the topping slab & the waterproofing and it will surely in time leak somewhere.

 

[RFreund]

Thanks for all the good information.
You are correct that it is not bonded. It will see car / light truck traffic (parking garage).
Some further specifics about my situation: 4,000psi was spec'd, but the test cylinders came back less. Because the slab really isn't "structural", the owner might except the lower strength concrete. However, they are wanting to know if the slab will be less durable and thus needing to be replaced sooner. Essentially trying to establish a 'credit value'.

Thanks again!


EIT

http://www.HowToEngineer.com

 

[Agent666]

Vehicle wheel traffic will tend to wear the surface more of a lower strength concrete as well, so don't forget about that in terms of durability. If it is carrying wheeled traffic over a drainage membrane then it has some structural function. Its deisgn often relies on the flexural strength of the concrete, so having a lower strength does potentially open it up to cracking over time and breaking up under repetitive wheel loads, and/or accommodating the ongoing long term deflection effects from creep/shrinkage of the structure underneath.

For rubber tyred traffic my local code suggests a minimum of 30MPa (~4300Psi) concrete for 50 year design life. So you might want to look for local literature on the wear characteristics as this is another factor in terms of long term durability. The fact that you have 2500Psi (17.5MPa) concrete would raise some eyebrows with respect to it being fit for purpose, both from a strength perspective in transferring the wheel loads and/or longer term wear characteristics. I guess they have minimums for a reason for this situation.

 

[BridgeSmith]

We have bridges in service where the deck concrete was specified as 3,000psi (typical strength after 40 or 50 years has been tested as around 4,000psi). Most of them needed overlays after about 20 to 30 years due to the surface being polished to the point where there was not sufficient roughness for vehicle traction.