4-axle front discharge ready-mix truck

[RHTPE]

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

 

[msquared48]

What you are describing is probably a 10 yard Osh-Kosh. Had one deliver mud to my place a few years ago.

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

 

[RHTPE]

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

 

[concretemasonry]

Splitting hairs about the dynamic load distribution of of a semi-fluid load is really not that important.

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.

 

[RHTPE]

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

 

[msquared48]

That's what pumper trucks, and an additional $500.00 are for.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RHTPE]

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

 

[DTGT2002]

Ralph, I've had good luck interacting with Pump Companies and Crane Companies to get wheel loads and outrigger loads in similar situations - do you know the manufacturer of the truck?

 

[RHTPE]

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

 

[beton1]

Cracked wall or not, the truck should not have been that close. And the wall certainly is not capable, under the circumstances, to hold the truck or the lateral load imposed. The truck ending up in the basement puts responsibility, some or all, at the slab contractor or whoever directed the truck. Sounds like fun.

 

[RHTPE]

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

 

[concretemasonry]

Whoever wanted to have a basement floor poured at that time should also bear some of the responsibility (it could be the builder or the concrete contractor, possibly acting on orders given to him). The concrete supplier has no way of knowing the strength of the wall and may have assumed it was or was not designed as a cantilever retaining wall. Usually residential walls are really not designed, but are built by prescriptive codes/standards.

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.

 

[Ron]

While the loading fundamentals might be clear, the responsibility for the damage will be less so.

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......

 

[RHTPE]

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

 

[Zambo]

Truck drivers are not engineers, where they can and cannot drive should be made clear to them. Who should make it clear to them could be a grey area. The insurer will decide.

 

[cvg]

OSHA regulations apply here and the regulations specify that the contractor is responsible for construction site safety. It is clear that temporary shoring was not provided and should have been and lucky nobody was killed. The general contractor or possibly the excavation subcontractor has primary responsibility for the excavation safety. In addition, the readymix company is also required to follow OSHA safety regulations and apparently did not. They should also share the responsibility. Loading of the truck has nothing to do with any of this.

 

[Ron]

Ralph,
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.

 

[RHTPE]

Ron - I have, first hand for my own concrete work. I spec'd a low w/c mix with site-dosed super-p. In mix mode they rock & roll quite a bit.

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

 

[Ron]

Ralph. The mixing speed doesn't usually change, but the number of revolutions at mixing speed can and should be controlled. No more than 40 to 50 revs at jobsite and no more than 300 total revs from batching to discharge.

Foaming could be caused by mixing blade issues in the transit mixer. Check National Ready Mix Concrete Association (NRMCA) criteria.

 

[Tryan]

Where I come from [oz] the delivery docket quite clearly states "responsibility ends at the kerb",and the docket is a legal contract!