90 Minute Rule

[PT999]

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?

 

[Zambo]

The concrete is mixed at the batching plant normally using a pan mixer with rotating blades which is much quicker than a drum mixer(this usually includes the addition of water) The truck mixer would normally only keep the drum rotating to stop the concrete from segregating not for mixing. I say normally because sometimes the mixing water is added later and the truck mixer must then rotate at a faster speed for mixing. If you search the threads you will find various posts about the speed of rotation of the drum for both transit and mixing purposes.

Zambo

 

[gatorn10ac]

Good point Zambo. But if you also read ACI and ASTM it says the time should/does start at the time the mix water is added. The 90 min rule is a guide line. If the temp. does not jump above specified limits and the contractor can use the concrete without adding water to re-temper the load he can continue to use it.

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.

 

[oldhand]

In the UK the time limit is 120 minutes from the addition of the mixing water. This rule is based on the asssumption that the concrete starts to set after 120 miniutes and takes no account of cement blends or additives. The difference in time lmits between the two countries may be due to the lower ambient temperatures in the UK. It is an arbitrary rule as tests have shown that concrete can be kept mobile by agitation for longer periods but I suspect that the mixing technique or speed may influence this. The basic rule should be if it is still mobile and can be compacted into the forms then the comcrete can be used. It is a brave man however who will defy the rule ast it can be expensive to rectify and if you have not followed the guide lines.....!

 

[PT999]

Thanks, Zambo, gatorn10ac, and oldhand

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.

 

[BigH]

Check out Neville's book Properties of Concrete. He has a good discussion. There are two types of mixing - central plant mix and truck mix. In the later, there is a 90minute rule about the time the moist aggregate can be in contact with the cement (in UK it is 2 hours). He talks about agitation of the mix and that 3 to 4 hours may be permitted without damage. Some specs also allow retempering but may have to add a bit of cement too besides just the water. So, check out Neville's book.

 

[Zambo]

take a look at the following thread on concrete delivery.

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

 

[PT999]

Thanks Zambo
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.

 

[Rjeffery]

PT999,

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.

 

[JCESSENEL]

I believe ASTM actually starts tha clock when the hydration process begins, either when the water is added to the cement or the cement is added to the aggregate which aggregate generally has some amount of water to react with the cement. I'm generally opposed to allowing water to be added at the site for retempering but if the slump and temperture are still OK I would accept the load up to 2 hours after initial batching of the cement.

 

[ravreyes1]

I am concreting a 500mm x 500mm section reinforced concrete beam with an span clear span of 3m between RC columns. How many days it would take before stripping the side forms and the soffit forms?