Why specify 3,000 lb concrete but only require 2,500 lb
Why specify 3,000 lb concrete but only require 2,500 lb
(OP)
I am a Civil PE in Northern California and we have always specified on our plans the following; Use 3,000lb concrete, f'c = 2,500 used for design therefore no special inspection required.
A friend recently asked me if we are only designing using a compression strength of 2,500 lbs., then why bother with the note to use 3,000 lb concrete? All I could say was "thats what we have always done". I don't think this is a very good answer for a design engineer.
So could someone explain the reason or history of this spec?
Thanks, Eric
A friend recently asked me if we are only designing using a compression strength of 2,500 lbs., then why bother with the note to use 3,000 lb concrete? All I could say was "thats what we have always done". I don't think this is a very good answer for a design engineer.
So could someone explain the reason or history of this spec?
Thanks, Eric
RE: Why specify 3,000 lb concrete but only require 2,500 lb
...then came the day when we (the Contractor) had "problems" with concrete placement of a bridge deck. The DOT inspector closely followed the test cylinder breaks for that day. They did not reach 3000 psi at 28 days, but at 56 days they just "squeaked" over the requirement.
During that 56 day "waiting period", we could not, and did not wait - construction of the bridge continued on schedule. I still shutter to think what would have happened is we had to tear out 56 days of work because the concrete design had been worked out to "a nat's eyelash". The differential cost of stronger concrete is cheap insurance on some (but not all) projects.
www.SlideRuleEra.net
RE: Why specify 3,000 lb concrete but only require 2,500 lb
To avoid special inspection during placement of concrete, one may design based on 2500 psi but specify a higher strength concrete. If special inspection is to be provided, there is no need to assume a weak concrete for design.
I think it is similar to allowing only half strength for expansion anchors or masonry shear when special inspection is not provided.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
Let us suppose that for footings a design strength of 2500 psi is sufficient for strength considerations but for durability f'c of 4,000 psi is required. Under these circumstances, will special inspection of the concrete and rebar for footings be required?
RE: Why specify 3,000 lb concrete but only require 2,500 lb
henri2 makes a good point in that strength of specified concrete is not always chosen on the basis of required structural strength but insofar as strength is a good measure of so many other concrete properties, strengths might be chosen to match these other properties. Instance - for a concrete lined side road ditch in a mountainous area, the strength of the concrete from a structural viewpoint needn't be more than, say 1500 psi if that - no loads other than a foot of water or so. But, given the velocity of the water flow in high gradiant drains, the water would eat away such low strength concrete and you would specify a higher strength, as henri2 points, out for the durability - erosion aspects.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
RE: Why specify 3,000 lb concrete but only require 2,500 lb
A situation very similar to the scenario mention by SlideRuleEra occurred at a very large pour for a hospital here. The pour continued, even after some initial tests raised questions about the mix, and nothing really happened until after the 28d tests. This caused every core drill in this part of country to on the job site, still no good. The project was bit by the 1 in 20 rule mention by BigH as all the 56d test samples and additional core samples failed using 3 different labs. Every air compressor and jack hammer you find ended up on this job.
According to the Geotechnical people the second time around the mix exceeded the specified requirements by at least 1000 psi. There were two GT inspectors on site at all times during the second pour. Sure glad this is not for profit hospital.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
The other arguement about inspection requirements is interesting but I think we cannot assume that any concrete would have to be broken out due to low strength. It is a requirement that all steps are taken during mix design, concrete production, placement and sampling that the correct grade of concrete is used in the permanent works - this is not really a design issue.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
RE: Why specify 3,000 lb concrete but only require 2,500 lb
It is my opinion that mix designs should be carefully tailored to the application...if you need strength and durability for an application, then consider them both. If strength is the only consideration, then design for that, but the whole mix design is important, from cement content, to w-c ratio, to aggregate size, to aggregate ratios....they all have an effect on the performance.
Remember, good concrete is made with water, cement, and aggregates.....bad concrete is made with the same ingredients.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
RE: Why specify 3,000 lb concrete but only require 2,500 lb
RE: Why specify 3,000 lb concrete but only require 2,500 lb
It is relatively common that the concrete strength is reduced due to segregation or mixing of soil with the concrete.
RE: Why specify 3,000 lb concrete but only require 2,500 lb
RE: Why specify 3,000 lb concrete but only require 2,500 lb
Many, if not most concrete repairs are due to either poor onsite workmanship such as insufficient cover to reinforcement or poor quality concrete from a durability standpoint.
in relative terms, an increase in concrete grade often does cause great financial pains but gives benefits in reduced covers (or greater net durability), slightly better young's modulus (thus deflection), flexibility during construction (ie striking slabs or loading slabs early) and possibly lower amounts of reinforcement (particularly if shear is a governing criteria).
Such are the benefits of increase concrete grade during construction that i find many contractors propose it themselves to allow for greater onsite flexibility and thus programme advantages!
all in all, i wouldnt regularly recommend on skimping on concrete grade!
blihp
RE: Why specify 3,000 lb concrete but only require 2,500 lb