Remote Jobsites

[JedClampett]

We occasionally have projects that are far from batch plants, exceeding a 90 minute travel time. It seems like we always have the idea of adding the water at the site, but it's extremely difficult to implement. Why can't the mixer trucks have the exact amount of water for the mix in a tank or something, put it in with the dry ingredients at the site, mix it up and place it? It's still a batch plant employee, so the "chain of custody" is intact.
There must be a good reason, but I can't figure it out.

 

[MikeHalloran]

I saw one on the road recently. It looked a little like an agricultural seed or fertilizer dispenser, comprising several hoppers mounted over a cylindrical chamber that must contain some sort of auger/mixer, with an Archimedes pump at the very rear.

My sister works for one of the few concrete companies who will sell them component materials; the trucks can only mix a yard or so at a time, so they don't compete directly with the big trucks.

I'd bet somebody is working on a larger version as we speak...



Mike Halloran
Pembroke Pines, FL, USA

 

[concretemasonry]

Much of it depends on the estimate of the amount of water loaded into tank, a guess of the amount of water in the fine aggregate(sand). The coarser aggregates usually have the water that in absorbed and is relatively consistent on a daily basis. If the load is agitated during transit, the aggregate AND cement will be warmer and drier that it is after a 15 minute drive. - That makes controls difficult to enforce since there a certain amount of guessing. Proper handling and storage of fine aggregates can go a long way toward uniformity.

When I has in college, I spent 4 summers (5 year curriculum) as a concrete inspector and often had to be at the plant at 3:00 AM to grab samples, especially if there was rain the night before because the storage was on the ground in very large piles and lower parts were totally saturated around the underground conveyors to the elevated plant that only had 100 tons or so that was drained. There are unique situations where you cannot accurately predict the amount of water needed unless you leave the choice up to the driver of the truck.

I inspected concrete in plants and on site for 4 summers while in college and I realize there are no absolutes just because there is a specification. - It was a great experience. I got of hours and overtime since I worked the big jobs over well over 500 yards with an early start (3:00AM for me and 6:00 0r 7:00 AM for the pour) and then the smaller jobs later to give me plenty of 12-16 hour days. There was no way, I could have made a test procedure for a job that gave nothing better than a shot in dark for the water to be carried, but the modern trucks with metered water is an improvement for records that must be recorded.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[TXStructural]

Short answer is because the rocks aren't dry and you will end up with a mess. Even relatively dry rock (the phrase is SSD - saturated, surface dry) will have enough water to cause cement to stick to the surface and start the hydration process. The ASTM rules for transit time are being changed as I type, but I don't know exactly what the changes will be.

 

[msquared48]

At first, I thought this idea had merit, but it'll sounds like a lot of changes/improvements need to be implemented in the batching process before that can happen. The new trip times will be interesting.

I also wonder if further latitude will be given to the drivers. Interesting to note that none are engineers, but they have the last say on the delivered quality of the product. Too many are cowboys in my opinion.


Mike McCann, PE, SE (WA)

 

[Ron]

TXstructural is correct. In addition, there is usually some small amount of wash water remaining in the drum from the last batch...not enough to change the W/C ratio, but enough to cause clumping and balling of cement when attempting to batch dry.

Further ASTM C94 time starts upon first introduction of water to cement....for SSD aggregate conditions, that's when the cement hits the aggregate.

 

[BigH]

Does your mixes use retarders?

 

[msquared48]

"Does your mixes..." Really BigH!!!

Mike McCann, PE, SE (WA)