90 Minute Rule
90 Minute Rule
(OP)
Why does the 90 minute rule to discharge concrete from a truck mixer begin from time of loading the truck at the plant?
Why is time a concern before the concrete is mixed?
Why is time a concern before the concrete is mixed?





RE: 90 Minute Rule
Zambo
RE: 90 Minute Rule
The studies have shown that the concrete will begin to loose slump at approx. the 90 min range. This also depends on the ambient temp.
RE: 90 Minute Rule
RE: 90 Minute Rule
The question I was really asking was if concrete is first mixed at the site, why does the 90 minutes (US guideline) actually begin from the time of batching at the plant. This is a requirement in New York State DOT specifications.
Zambo is saying that the mixing begins at the plant (unless otherwise specified), but in my projects in New York, there is always mixing at the site, 100 revs for about 10 minutes.
So is this additional mixing, or is this first mixing. I always thought this was first mixing...hence why worry about time from the plant to the site? Of course if it additional mixing, then no issue.
RE: 90 Minute Rule
RE: 90 Minute Rule
Thread194-61290
I doubt that the first mixing is being carried out at site. Perhaps your contractor is adding an admixture at site and this is the reason for the additional mixing (self compacting concrete sometimes uses site added admixtures)
The specification you quote allowing 90 minutes does seem a short time, can this increase if a retarder is used?. If the concrete truck gets stuck in traffic then you must get big trouble. Still I guess there's no point in arguing with New Yory State DOT.
Zambo
RE: 90 Minute Rule
Retarders, admixtures, absolutely "verboten". Cold joints not withstanding, an engineer who allows a new load of concrete to be placed after 90 minutes or someother minor spec violation (and is observed ... sometimes at a distance by binoculars) is history, especially if he comes from a consulting engineer's office.
Anyway, back to my original question, there definitely is job site mixing at my projects, with agitation along the way to the site, but is there also plant mixing for the same batch, thus requiring the time limit?
Based upon the Ready-Mix web site, the answer seems to be the customer chooses what he wants, plant, transit, or job-site mixing, (one time mixing only)
Perhaps the answer is that the truck drums are not perfectly clean and dry. Therefore because of contamination in the drums, a time limit needs to be set.
RE: 90 Minute Rule
The mix time refered to in the NY highway specifications is from the moment that the mix water is added, which starts the chemical reaction. The intent of the 90 min cutoff point is to prevent cold joints, retempering and the like.
They (the State DOT) force the best case to prevent the worst case. Generally, I have found, if there is a valid reason to deviate from the specifications, the "project engineer", when all the facts (pros and cons) are presented will allow reasonable deviation from the specification, provided detailed and accurate documentation is kept in the Project Diary.
RE: 90 Minute Rule
RE: 90 Minute Rule