huangyhg

strength of lightweight cmu
i am working on a project in which our specs call for minimum compressive strength of 2500psi for lightmweight cmu blocks (85-105pcf). i got an rfi from the contractor saying that high strength blocks (i.e. 2500psi) are medium weight. i was under the impression that we sould specify both lightweight and higher strength, is this correct? what problems could result from our specs?
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here are a couple of thoughts:
1. when specifing any product, why not call a few manufacturers in your area to find out what local practice is? they are the ones that will be bidding on your work and would have the best handle on what is available, doable and cost effective for your area. generic specs. have their place, but rarely have correct information regarding local practice.
2. i seem to recall seeing test reports on concrete block that show compressive strength of normal weight and light weight block about equal. your spec. should ask for submittal of test data on the block the contractor will use. you can actually get this from the block supplier at the time you write your spec. to verify it ahead of time.
3. make sure you know why you are asking for 2,500 psi lightweight block (or whatever) in your specification. if it turns out that it is not readily available product in your area, you could end up paying a premium for it, whether you actually needed it or not. maybe normal weight is ok or 2,000 psi lightweight is ok for this job. don't over specify material properties, it costs your project money that could have been used eleswhere.

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