2 hour transit time 5

[cvg]

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?

 

[msucog]

middle of winter might be ok...middle of summer, i'd be doubtful since the 2 hours would likely be closer to 3 under certain circumstances. if the mix doesn't lose workability in that time without having to add additional water, then it'll probably be ok. i'd include language to make them back up their claims though. and probably bump up the testing frequency (multiple slumps, additional cylinders, visual inspection, etc) to try and catch "bad" batches. 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. again, for 2 hours possibly in winter time, i wouldn't think you'd see horrible results as long as the contractor is coordinated and experienced enough to prevent delays.

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.

 

[msquared48]

If the batch plant can't guarantee that the mix will be acceptable to DOT specs, don't do it.

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

 

[msucog]

never personally dealt with onsite mixing but seems like aci or astm has documentation for that if my failing memory serves me correctly.

 

[kelowna]

Certainly mixing on site is a possibility. I have no experience with it. Although by putting the water all at the same time I can think that there is increased risk of lumps and improper mixing. At present our supplier batches in 3 parts each truck to avoid dry lumps.

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).

 

[hokie66]

Why don't you use a mobile batching plant? You will get better quality concrete by batching it there than driving it around for two hours before placing.

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.

 

[concretemasonry]

Site mixing is a definite possiblity. Back in the dark ages, when I was a state DOT concrete inspector some small plants were exclusive "transit mix", while others offered mixing on site. - Often, this is the only solution unless someone can move the construction site. The concrete is dry batched into the truck and water added in the batching yard or on site.

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

 

[henri2]

For complaince issues check ASTM C 94 Section 11.7. Amongst other requirements, it deals with situations where concrete exceeds the 1-1/2 hour time limit.

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.

 

[GeoPaveTraffic]

Given that you are placeing concrete in the middle of winter for a box culvert...

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.

 

[cvg]

I should clarify a few things
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?

 

[henri2]

During summer, the concrete supplied may not conform to ASTM C 94 Sec 11.7. In order to CYA, go with a supplier who has a mobile plant. As I pointed out, there are several suppliers who handle these types of projects..and they are prepared to work all over the country.

 

[DRC1]

If you can keep the temperature below 90 degrees, the mix should be fine. The 90 min travel time is a conservative estimate of how long it will take the mix to reach that temperature. In the summer, that can be exceeded in under 90 min. necesitating proceedures such as using ice. Assuming the plant is a reputable plant, you will get your best quality concrete by batching the concrete at the plant and placinging it before it gets too hot. The truck should be able to dump in 10 min. if it is being pumped. If we pour in the summer we will start early (5-6:00) so that the concrete is placed by noon. We cover with poly and if it is hot out, use sprikler hoses to cool with water once the mix has a little set. The problem is that the DOT probably has 90 min as a travel time requirement and the question is will they accept a longer travel time on this project if the temperatue is in spec? That will need to be dicussed with them, but again the redi-mix, esp. with all the chemistry in the mix these days, will give you the best quality.

 

[msucog]

if i recall correctly, the 90 min is a guideline...the consistency without additional water is the key. in the summer, the 90min may be 45-60min. in the winter time, it might be 120-150min...all depending on the mix. if temps get too high, the stuff will hydrate like crazy so hot weather batching by the plant will be a critical step. you should review aci 301 and 305 for additional help on setting the guidelines.

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.eng-tips.com/viewthread.cfm?qid=195292&page=1

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).

 

[DarthSoilsGuy]

you'll be fine.

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)

 

[dicksewerrat]

Make up a test concrete placement to look at the cylinder breaks, time the cold joint set, etc. Or tell the contractor to get a portable plant. what size are these boxes? can they be prefabbed in a factory?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[turnitup]

I would get it all in writing. I was a P/M on a job site some time back, and was amazed at how fast the sales people of some suppliers would try to make a sale at any cost. If they will stand behind it at all cost's, then let them do the job. If not, have you considerd precast structures? Might be an alt.

 

[msucog]

prefab...my vote is to get the thing precast and delivered. i guess if the contractor can't meet the requirements without the designer having to prostitute themselves, then spend the money for precast. that slipped my mind. if the contractor can meet the specs (with some slack given primarily to the placement time window as long as consistency is not effected) then they can save a few bucks. i'm with dicksewerrat.

 

[BigH]

Interesting discussion. Of course, many of my recent jobs were such that there was no "batch plant" anywhere to be had - even within hundreds of miles. We did everything in the 1/4 yd mixers. At times we had two of them. Depends on how many metres you are pouring in a day. If you are doing 25 m or so at a time - no more, they work great. you can batch and pour within 10 minutes or less. With proper set-up, you can really push the work, have good control on weights of materials, etc. Our bridges won't be falling down due to the concrete. If you must do large pours, mobile plants would be the ticket.

 

[conceng]

Sorry to jump in so late, but I nobody has mentioned using a hydration stabilizer such as BASF/Master Builders Delvo Hydration Stabilizer. I know it's been successfully used consistently in applications that you are describing. You may need to contact a local admixture representative to help determine the proper dosage for the particular cement source you may be dealing with.

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.

 

[unclesyd]

Way out of my league.

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://www.allbusiness.com/manufacturing/nonmetallic-mineral-product-manufacturing/521313-1.html