28 day Concrete Strength after 10+ years 1

[CaliEng]

Is the 28-day concrete strength exceeded after long periods of time? I know concrete gets stronger over time, but unlikely to stop after 28 days. So, for instance, if a concrete is 3500 psi at 28 days, can one assume it is actually stronger than this after 10 or 20 years?
Thanks.

 

[LittleInch]

You appear to have answered your own question...

The question is by how much does it get stronger, now that's interesting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Rabbit12]

Do some cores and find out.

 

[KootK]

Google "concrete maturity curves" and you'll quickly get a sense for what is realistic. You'll get continuing gains beyond 28d but it's definitely a diminishing returns thing unless something about the environmental conditions changes significantly and you only had partial curing to begin with. I routinely specify 56 day strengths for certain elements where I deem that appropriate.

 

[STrctPono]

When we seismically retrofit bridges, we do use a higher compression strength to account for the increasing strength of the concrete as well as the fact that most ready-mix suppliers provide higher strength concrete than is required anyway. In the end, standard practice for us is a 30% increase in strength. As Rabbit12 said, coring the concrete will be the only true way to check this.

Apply the following information as you see fit. Below excerpts from M.J.N Priestly Seismic Design and Retrofit of Bridges

 

[MIStructE_IRE]

It keeps getting stronger. But the only real way to determine it is core & crush.

There are plenty of non destructive strength tests, but i take most of them with a pinch of salt..

 

[BigH]

No mention is made of concrete using fly ash. Fly ash retards the earlier strength gain as the cementitious fly ash needs the portland cement hydration to start "hydrating".

On a job in Indonesia, the Contracdtor used 50% fly ash in a low cementitious concrete (160 kg/m3 at 50:50). The 90 day strength ratio to 28 days (average) was 1.8 (vibrated) to 2 (rodded). The 80 day strength ratio to 28 days (average) was 2.2 (vibrated) to 2.5 (rodded). The fine aggregate was not washed to maintain a low fines content. In testing the fine aggregate also was somewhat pozzalonic. The Contactor also used 50:50 cement/fly ash for structural concrete. The fine aggregate was washed. The 90 day to 28 day ratio was found to average 1.5.

Many specs identify 90 day characteristic strength when using fly ash although I have also seen references that make no such distiction.

 

[dik]

What strength do you need? likely the 'scatter' from any testing will negate any gain... Is it for flexure or bearing? Unless you have high amounts of reinforcing, concrete strength has little effect on flexural strength. Shear strength is improved a tad... based on sqrt().

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[LittleInch]

I was involved once in trying to remove some concrete installed in the 1930's. Contractor reckoned it was tougher than granite and I think they tested it at 80N.

Took an absolute age to chip this stuff out little chunk by little chunk....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[STrctPono]

concrete strength has little effect on flexural strength

What!? I'm curious on why you say that. For prestressing, concrete tension stress limits are in direct correlation to compressive strength.

Isn't un-reinforced concrete design in ACI also related to compressive strength?

 

[JedClampett]

STrctPono, in classical reinforced flexural analysis, the concrete strength only affects the size of the compression block. So instead of a=1.7" it might equal 1.5". The distance from the CG of the reinforcing to the centerline of the compression block is likely in the order of 18 to 20 inches for a 24 inch beam. So you're only increasing the moment arm by 1% or 2%. And the moment arm times the steel strength is the flexural capacity.
As far as unreinforced concrete design, I try to stay away from it.
Concrete strength, however, has a huge (proportional) effect on column capacities.

 

[dik]

Sorry... I was thinking that it was normally reinforced concrete. For normal RC, the statement is still correct. For plain concrete, again, the gain is small. Based on the SQRT(f'c).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Thanks Jed...and that's why I noted it was for small amounts of reinforcing...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bridgebuster]

[link the only real way to determine it is core & crush.]Link[/url]

The proof is in the pudding. In the mid-90's I was on a bridge rehab project; 14 bridges built in the mid to late 50's. Non Air entrained concrete, f'_c =3,000. We did a lot of coring and nothing came up short. I found some of the test results (6 of the bridges). We had anywhere from 4,300 to 10,200 psi.

 

[BridgeSmith]

When we seismically retrofit bridges, we do use a higher compression strength to account for the increasing strength of the concrete as well as the fact that most ready-mix suppliers provide higher strength concrete than is required anyway. In the end, standard practice for us is a 30% increase in strength

Just to clarify, that's the maximum strength, in order to make the retrofit adequate for the worst case plastic hinging moment, correct? You're not suggesting arbitrarily using a concrete strength 30% greater than design for checking shear capacity, etc., right?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

Dik,

I was under the initial impression that you were mentioning pure concrete material properties (think flexure beam test). I see now that I hastily misinterpreted your initial comment and that you were referring to flexure in conjunction with limit state design.

BridgeSmith,

For seismic retrofit, in most cases you don't consider the contribution of concrete shear strength in plastic hinge zones anyway. The capacity is derived from the reinforcing steel alone. I am not suggesting to arbitrarily bump the concrete strength by 30% if pushover analysis is OP's end goal (which I doubt that is what they were getting at). But if OP is checking shear of an old structure, then it is a real possibility that a 10, 20, or even 30 year old structure will be seeing a 30% increase in material strength. I understand that that's not standard practice for bridge load ratings but is nonetheless a possible approach for OP's situation. However, now that we are on the topic, if justifying higher shear capacity was the end goal, there are much more code-approved ways of doing so... Using the modified compression field theory approach for starters, which for an old structure was probably never considered.

 

[dik]

Is it for flexure, bearing or shear?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

...if OP is checking shear of an old structure, then it is a real possibility that a 10, 20, or even 30 year old structure will be seeing a 30% increase in material strength.

My comment was to convey a caution that, while that 30% increase is possible, maybe even likely, I don't think it would be prudent assume that increase, without verification, for calculating shear capacity.

...if justifying higher shear capacity was the end goal, there are much more code-approved ways of doing so... Using the modified compression field theory approach for starters,

That I'll agree with.

Rod Smith, P.E., The artist formerly known as HotRod10