4-axle front discharge ready-mix truck
4-axle front discharge ready-mix truck
(OP)
I am preparing a report that discusses the failure of a foundation wall due to the close proximity of the ready-mix truck when delivering the basement floor slab concrete. The truck took one heck of a ride into the basement.
This was a bare foundation, approximately 24' X 50' X 8' high, fully backfilled, with no floor framing in place at the top of the foundation wall. I will be interviewing the driver tomorrow morning to better understand the position of the truck relative to the foundation wall.
Does anyone on this forum have any insight regarding the load distribution to the front axle and how it changes as the discharge of concrete begins? Remember that a substantial portion of the 10 CY in the drum moves up the inner ribs and closer to the open end of the drum as discharge begins.
I'm well aware of the issues associated with back-filling a bare foundation wall, as well as the danger of high levels of ground surcharge immediately adjacent to the walls. At this point I'm really more interested in the load distribution to the truck's axles.
Ralph
Structures Consulting
Northeast USA





RE: 4-axle front discharge ready-mix truck
Of you know what plant delivered the mud, they should have the load specs on the truck axles.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: 4-axle front discharge ready-mix truck
Mike, I'll know more later this morning. No doubt the truck manufacturer has the axle loads for over-the-road, as they'd need that info for the various DOTs. I'm interested in how those loads redistribute as concrete is discharged (NOT something one would do as you're driving along the road).
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
First, the backfill should not be done without the wall being braced and/or the floor system installed first. Putting a heavy load (irregardless of the distribution) is also contratrary to logic and common sense.
That is a very and bad practice even for a 8' basement wall on residential construction. The basement walls are not usually designed or contemplated to be subjected to loads beyond normal service loads and the diaphragm action of a proper floor system is usually relied on.
Around here, the basement floor is one of the last things done since the walls are braced. The first floor system is installed and all mechinical and plumbing is in place before the basement slab is poured/placed. Some codes require 4" of concrete slab to be placed against the base of the walls above the finished floor to insure no movement and as a prevention of sliding due to the lateral loads.
The distibution of the loads will be highly variable and depend if any trailing axels for load distribution are activated. The truck distribution loads are never based on the wide range of the slope of the truck in relationship to the wall (sloping above or sloping down) - a horizontal zero slope is likely.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: 4-axle front discharge ready-mix truck
Dick Your comments are all valid. The drivers of these rigs know the rig and where it can and cannot go. But they know little about what should and should not be around a residential foundation.
Typically in this area, a residential footing is formed & poured on Monday. Wall forms are set on Tuesday - if it's a simple layout, they pour late Tuesday. Forms are stripped on Wednesday. Walls are damp-proofed on Thursday and, if the contractor really cares, foundation drains are installed also. By now, the dirt guy wants to move on, so he backfills on Friday. The framer won't work unless the walls are backfilled, 'cuz it's too difficult to frame out the floor with the foundation surrounded by an open excavation.
When the foundation walls crack, it's "bad concrete." None of this is right, but it is more common than should be permitted. In this case, they were placing the basement floor slab before the first floor was framed. More common is to place the floor after all of the framing is roughed-in. BTW, the basement floor slab thickness appeared to be 2-3" thick. No thickened slab for the eventual lally columns. Go figure.
I do realize that the axle loads will be based on the truck sitting level. I have to start somewhere. I hesitate to think of the calcs involved for the front wheel loads as the truck is repositioned for the next portion of floor and the driver hits the brakes. Hopefully I won't need to go there.
I'm in the Unfortunate position of having to document that the truck should not have been where it was. Intuitively we all know that it's stupid to backfill right away, never mind drive a ready-mix truck with 9 CY of concrete to within 2 feet of the foundation wall. It will be up to the attorneys and the guy in the black robe to determine who will pay for the damages.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: 4-axle front discharge ready-mix truck
Mike You are quite correct. But try to convince a pickup truck / wheel barrow foundation contractor to spend that money for a better finished product. Never mind trying to convince them that reinforcing steel is a good thing.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
Daniel,
I have a running email conversation going with Terex. It's an Advance FD4000. It may be a moot point, as circumstantial evidence points to the liklihood that the wall was cracked before the truck arrived on site.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
beton1 I completely agree. I suspect my client (the ready-mix producer) will not be completely happy with my conclusions and recommendations. An unreinforced wall (save for one #4 at the top) sitting on a simple strip footing is not a retaining wall, and should not be expected to behave as such. Especially with a surcharge load on the order of a loaded ready-mix truck (and with no floor framing in place at the top of the foundation wall). It is my opinion that the driver should have been instructed by his employer in how to avoid overloading the ground next to a new foundation. If he is directed to bring the truck closer, the contractor should be required to sign-off and accept the potential consequences. Just like adding water on-site to increase the slump.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
I have a problem with anyone backfilling an unbraced wall and then requesting the basement floor poured without a first floor system (acting as a diaphragm and construction platform in place) unless there were other reasons. Dmping the blame of a truck driver doing a routine delivery without any engineering experience does not seem appropriate, but the engineering may say otherwise.
In my 40 years experience I have seen situations the are created by accelerating work to get draws from a financial institution and then use the cash for payroll and operating expenses and not pay for some sub-contractors and materials for 90 days. Of course that is business and not engineering.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: 4-axle front discharge ready-mix truck
Yes, the surcharge was probably way too high. Is a ready-mix driver trained to know that? Was there an alternative location from which to discharge without a pump? The decision to place by direct discharge was not made by the driver, but by someone else. What responsibility does that entity bear? Was this action anticipated in design (obviously not), or should it have been considered?
Don't ya just love litigation? Just when you think you have an answer......
RE: 4-axle front discharge ready-mix truck
Ron... No litigation - yet. A 50' straight run of 10" thick X 8' high unreinforced wall (save for one #4 at the top) will shrink...and it will crack. Backfilling with some rather large cobbles doesn't help. Then positioning the truck right next to the wall?
Who directed the truck driver seems kinda vague right now. I suspect he did it on his own to help the guys placing the slab. Probably a poor choice. From what I understand, the discharge rotation of the drum would have shifted the load's center of mass towards the right side - the same side that was closest to the wall.
One would think that the ready-mix producer would have some guidelines for their drivers.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
The cg of the mass will move quite a bit with the drum rotation, depending on whether the issue occurred at mixing speed or discharge speed.
Further, after the mixing speed run-up at the site, the drum rotation reverses for discharge. Watch a drum mixer sometime...you'll see how much the rock on their chassis/suspension.
RE: 4-axle front discharge ready-mix truck
Have you ever encountered any specs on maximum mixing rotational speed? I have found that once the super-p starts to have an efect, the concrete paste tends to foam excessively. Unfortunately, the foam created doesn't really dissipate.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
Foaming could be caused by mixing blade issues in the transit mixer. Check National Ready Mix Concrete Association (NRMCA) criteria.
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
RE: 4-axle front discharge ready-mix truck
Point taken of course,but over here we have a strict system regarding building sites involving safety and risk assessment training. To get on to a site you must have the appropriate licence or documentation, ie worker, machine operator, delivery, inspector or visitor. You then have to go through an induction, which can take 1 hr, pointing out the specific risks, vehicle movements and general dos and don'ts of the site. You then sign it saying you understand and accept it. The responsibility then reverts to the signee,but, as you rightly pointed out Ron, you can't "disclaim negligence". For instance, if the safety officer identifies a danger, then you can't just warn people about it, you have to make it safe.
There is a "Chain of Responsibility" which creates a very visible paper trail. To find the broken link, just follow the trail!
I think there was a lot of things wrong here before the concrete truck even got to the site. 50'long wall,8'high,backfilled and unreinforced? Over here that may have been classed as an excavation deeper than 5' with a high risk of sudden failure, therefore no workers are allowed in it without the appropriate shoring!
There is a system of risk assessment which works from 1 to 5 ie. highly likely to unlikely and 1 to 5 ie.a risk of death to a papercut. That situation,if there were workers down the hole, should have been rated a 1.
Can't get that unreinforced wall out of my mind,is that normal practice? Even if it is eventually fixed top and bottom[?], what about the middle?
RE: 4-axle front discharge ready-mix truck
Tryan, Here in the Northeastern US this practice is far too common. There doesn't seem to be anything explicit in the relevant codes to prevent it from occurring.
It is not unusual to see footings formed and placed on Monday, footing form stripped and wall forms installed on Tuesday (or even late Monday), walls placed late Tuesday or Wednesday morning, wall forms stripped late Wednesday or Thursday morning, damp-proofing applied on Friday, and the walls back-filled on Saturday if the excavator needs to move to the next one. Perhaps an extreme example, but not too far from reality.
Keep in mind that this is in the category of residential construction (single family home), so there's little oversight by any officials or design professional. The risks taken by the builder in the process are amazing, with little thought given towards the long-term quality of the work. Those that do the wood framing that should be in place before the walls are back-filled would scream very loud at having to deal with a foundation that has not been back-filled, as it makes their work more difficult.
Having close ties to a ready-mix producer, I hear the horror stories about how they get blamed for bad concrete because the foundation wall cracked. Never mind the fact that the foundation guy added lots of water to increase the slump so the concrete flows farther and the site guy backfilled with ordinary fill as soon as the damp-proofing is dry.
Not right, but far too common.
Ralph
Structures Consulting
Northeast USA
RE: 4-axle front discharge ready-mix truck
I would recommend that any heavy loads like this stay outside the 45 degree zone of influence (i.e. at least 8 foot in your case).
RE: 4-axle front discharge ready-mix truck
Trucks are typically stocked with 4 or 5 chutes. If I were in your shoes, i would also investigate whether access was provided to place concrete for the slab.
Getting complex on the analysis of the truck surcharge doesn't seem appropriate since there are also complexities that get simplified out in the geotechnical retaining wall calcs too. Why not just run it as a flat simple truck? It's the simplest and most favorable to your client, unless you can show a slope away from the wall. I assume you are doing all of this to establish a load on the wall to compare pound-for-pound against the loads the contractor put on the wall by backfilling early for some sort of percent damages at fault scenario.