2 hour transit time
2 hour transit time
(OP)
I am working on a project in a remote location. The project is a reinforced concrete box culvert storm drain. The only concrete plant in the area is a 90 minute drive away plus another 10 - 30 minutes waiting to unload so that readymix will be 2 hours total in the mixer. The producer says they have suitable mixes with retarder added that can go that long and meet state DOT specs.
1)should I believe them and
2)anything special I should do in plans / specs / QC to make sure the finished concrete comes out ok?
1)should I believe them and
2)anything special I should do in plans / specs / QC to make sure the finished concrete comes out ok?





RE: 2 hour transit time
that being said, i'm not looking at dot specs so you'd better dig deeper. i believe aci would back up the situation you describe as long as the workability is not dramatically effected (again, without adding lots of water at the site).
i'm sure there's one or two others on this board that will be chiming in with their expertise.
RE: 2 hour transit time
May be unorthodox, but as an alternative, what about adding the prescribed amount of water on-site with an inspector present, assuming that this is a mix design? Let the mixer truck mix it on site. When they batch it at the plant, they just put the sand, gravel, admixtures, and water in the truck from a chute anyway - what's the difference? I am sure that the batch plant could prescribe the proper mixing time.
Mike McCann
McCann Engineering
RE: 2 hour transit time
RE: 2 hour transit time
Retarding the set of the concrete is also possible and done regularly, although 2 hours is a stretch and I agree that it will be 3 before it gets poured. My take is that if the temperature is not high (normal DoT limits) and the concrete flows good, then it should be OK, obviously with increased testing. Certainly now in winter low temperatures will help.
"If the batch plant can't guarantee that the mix will be acceptable to DOT specs, don't do it.", "i'd include language to make them back up their claims", "the contractor should be on the hook for the final product and to reject anything that shows up and is not "suitable". if the final product is not satisfactory or if cylinder breaks (and/or field core compressive strength breaks) are too low, then the contractor should be aware on the front that they may be taking it back out"
I would be careful with all those comments (no offence intended guys). If there is only one batching plant in the area you probably need them more than they need you, and if you start asking for guarantees and the producer thinks that there is a riks of getting loads rejected or being on the hook for bad concrete placed, he might just refuse servicing you. I would suggest working with him. My own experience has always been positive with people wanting to help (if approached with the right attitude).
RE: 2 hour transit time
Your site is remote enough to treat it like an island which is not joined to land by road, where you transport all the materials to the island by barge and mix on site.
RE: 2 hour transit time
You may want to check some historic ACI publications or old DOT retirees.
I do not remember the exact details, but for "transit mix" I had to verify the gradations, batch weights, water added and the number of revolutions (most trucks had revolution counters) the drum turned before leaving the yard. For site mixing, it was only a matter of verifying the batch weights. - Obviously, all mixing revolutions after adding water and concrete property testing was done at the construction site. All admixtures must be added on site.
One other item - Because aggregates are rarely bone dry, the moisture content of the fine aggregate is important since it can trigger hydration of cement upon batching if it is beyond the typical 1% to 1 1/2% of absorbed moisture. If the temperatures are high and the aggregate is wet, the cement content may have to be adjusted. Do not expect the uniformity you get with a modern central mix plant, but it is certainly do-able and can be controlled and verified.
Dick
RE: 2 hour transit time
There are a couple of companies that specialize in batching/supplying concrete in remote areas. One that comes to mind is GL Throop.
http://www.throop.com/gallery.html#
I once worked on a project where they batched the concrete on site using a volumetric procedure which conformed to ASTM C 685. The batching equipment was mounted on a truck and had to be calibrated.
RE: 2 hour transit time
I would say use the batch plant mix, work with the plant to withhold a portion of the mix water, add the remaining mix water once the truck is on-site, and place the concrete quickly. Test the concrete every day or 50 yds with each truck "observed". Make sure that only the with-held water is added on site. The batch plant may suggest a water reducer/retardar, that should help keep the w/c ratio lower.
RE: 2 hour transit time
1) the project will be constructed next summer
2) I suggested a mobile plant, but the supplier says no he can do this and does it regularly. Total amount is about 3,500 cubic yards
3) The contractor will be allowed to produce concrete any way he wants, but since there is only 1 company in the area providing concrete, they will dictate the method of concrete production
4) QA/QC will be per DOT and FAA requirements so we will be doing a lot of testing and the contractor will absolutely be held to minimum requirements or remove and replace the concrete.
5) air entrainment is required
6) there is a chance that the contractor may want to pump the concrete into the culvert wall forms...
My biggest concern is how to determine acceptance / rejection criteria when the truck arrives. standard procedures are to test for slump, temperature and observe the total number of rev's of the mixer and the total time in the mixer which is typically 90 minutes. Will these procedures be enough?
RE: 2 hour transit time
RE: 2 hour transit time
RE: 2 hour transit time
as far as only having one plant to choose from, then you might as well ask them to write the specs if the answer is that you must accept whatever they send. that's rubbish in my opinion and you might as well say you're willing to accept unacceptable results. i would presume that picking a competant supplier is the contractor's problem...not yours. however, if the supplier is willing to go the extra step and put some guarantees on their products, then it's worth trying. no reasonable engineer would expect such a situation to be perfect, but there's got to be limitations and the contractor/supplier must be responsible for the final product. even though i haven't had a lot of exposure to mobile plants, my vote would be for that since it'll be summer time. and i'll bet that hot weather batching procedures will be the silver bullett if mobile batching is not an option (if assuming the 2 hour time).
one other point: many of you probably read or were involved in my discussion about low cylinder breaks on a job where the contractor first failed to provide the onsite storage box/initial storage conditions then provided an insulated box and blamed me (testing firm). to help keep consistency in the cylinder results (which is different than the field strength), point out section 1.6.? (i think) of aci 301 where the contractor supplies/maintains initial curing conditions. while i'll maintain that the higher initial temps likely doesn't make that big of a deal at 28 days, it's often used as a "bargaining tool" by the contractor to explain low break concrete and try to avoid having to tear it out. while the initial curing temps may play with the break strengths at certain days, i feel it's overblown when the outside temps are only 90 deg (maybe even 100). in other words, have a preconstruction meeting and lay it on the contractor to provide the initial curing storage so that it doesn't get dumped in your lap to deal with.
here's the link to my thread.
http://www
the results (in my opinion based on all the test results) were that the mix just didn't perform...it was the cheapest mix the contractor was willing to pay for and didn't perform well in the weather (whether hot or mild).
RE: 2 hour transit time
1. night pour for low temps, low traffic, and less distractions for the contractor from other trades. Night pours also usually better planned in my opinion.
2. they'll mix design with ice and retarder.
3. if you want more assurance, buy/request a trial batch that will mix in the hopper/truck for a couple of hours before they make the cylinders as part of the material submittal. have the owner's tester involved in this.
4. When it gets closer to the pour date.... Contingency plan with contractor.... truck rejection protocol, cold joint construction after what time and how, and go over there plan (what equipment and where, how many trucks working or truck turnover times, sampling location, waste discharge pump/truck location, any site conditions that will affect the pour)
RE: 2 hour transit time
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: 2 hour transit time
RE: 2 hour transit time
RE: 2 hour transit time
RE: 2 hour transit time
When in doubt, have the supplier perform a trial batch where he can mix up a load in the yard, monitor the slump and air loss for 90 to 120 minutes, then test and cast cylinders. If successful, he may not even have to waste it. He may be able to use it in a residential footing.
RE: 2 hour transit time
On several occasions on the Discovery Channel I've seen the use of Liquid Nitrogen to cool concrete for different reasons. One was the Oakland Project and the other was a large project similar to yours where high ambient and long waits were the problem.
There is a good general discussion of LN2 cooling of concrete with the Oakland Project being the center of the discussion. One of the shows it look like the LN2 Cooling unit was a permanent part of the mix plant and the other looked as if the LN2 unit was a temporary/portable fixture.
http:/
RE: 2 hour transit time
Also, as you are air-entraining this mixture, you'll want to monitor the air content, preferably in a hardened state not just with an air meter. Air-entraining agents stabilize the bubbles that develop when mixing. The air content will fall with time and the trend will be towards a coarser bubble structure. Redosing on-site is not always effective and an air meter may not capture this. This will make the concrete freeze-thaw susceptibile and reduce the lifespan of the structure being constructed. Perform a test batch at the ready-mix plant, cast a cylinder after initial mixing is done, set the ready-mix truck aside for 2 hours or so and cast another sample. Have the cylinders tested for hardened air void parameters per ASTM C 457. I would also throw in a couple of production checks as well through the project. The test is not easily done and can be expensive. Given that the nearest plant isn't close, I assume the nearest concrete petrographer is probably a lot further a way. God Bless FedEx though...
I would be deeply concerned about mixing the rock, sand, cement, and admixture and then adding the water on-site. Admixtures can be sucked into dry aggregates rendering them useless. The last thing you need is to transport 10 CY of concrete 2 hours and then find out you can't get the air content in spec. (I might add, at the time you find out it isn't working, you could already have 100 CY in transit).
RE: 2 hour transit time
RE: 2 hour transit time
Another thought -
We have very high day time temperatures, and quite often the concrete plant will use ice to reduce the mix temperature, as well as start pours at 0100.
uwrockguy is right - test batching is the way to go!