Redi-mix Truck Batch & Placement Times 1

[DGOESSL]

Hello All:

I am researching specific code references (either ACI or ASTM) highlighting industry and public works standards for the placement of non-structural concrete, specifically for sidewalk, curb and apron construction.

We have a situation where sidewalks poured in NY state in July 2008 are showing severe signs of flaking and scaling. Core samples were taken showing strong breaks, however the surface is failing. The design mix is 4,000 psi,3" slump and 5.0 % air entrainment.

We are in the early stages of making legal claim against the contractor since he is not willing to replace the work.

Our claim is that the delivery and placement times of the material well exceeded 1.5 hours and we suspect that this may have lead to retempering the mix so it could be floated and tooled properly.

Can anyone out there give me some insight? Any input is greatly appreciated.

Dave G.

 

[Ron]

Get a copy of all the delivery tickets. The delivery was most likely required to conform to ASTM C94.

Flaking a scaling are generally finishing problems. Those can be brought about by excess placement times and retempering.

The delivery tickets should tell you if water was added at the site and how much. It will also tell you the batch time and the discharge time or time of arrival on site. Some suppliers also record the time they leave the site. All will help you in this evaluation.

The concrete sample data sheet should tell you the sampling times, temperature and slump of the concrete. The testing lab should also have recorded if water was added.

There are physical tests that can still be done to determine the probable cause of the scaling/flaking. A petrographic examination of a core cut from the concrete will tell you if the air content is right, if it was retempered, if it was "overworked" at the surface, if there was excessive bleed water, and a variety of other things.

This is a prime example of believing that concrete strength alone can be used to accept or reject concrete. Compressive strength is only an indicator of durability....there are many other factors.

Good luck.

 

[concretemasonry]

Ron hit most of the points very well.

Was there required to be an inspector on-site to verify properties and timing? Most sidewalk work I have seen does not justify the kind of expense the owner/municipality is willing to pay for.

Most decent ready mix/transit mix concrete suppliers use batching systems that give the actual batch weights and batching times. This may be on the delivery ticket, but should be available.

A good supplier uses a system that will provide a time line of the truck leaving the plant, job arrival, waiting time, discharge start, discharge end, clean-out and leaving site times for every load. This should be available from the supplier, if it exists.

If there is no on-site observations, the notes on arrival, waiting time and water adding may not have been made on the delivery ticket/receipt. Often, a site superintendent will not make any notes if he had problems causing a delay. Some contractors will also "tip" a driver if there are not radio/recording controls on the truck.

I worked several years as a concrete plant inspector and bridge inspector and saw a wide range of situations. A good supplier will not allow "tip" because that upsets the delivery to other projects.

Batching the concrete properly is usually not a problem, but delivery is one of the biggest costs (possibly 20%) and variables that can affect scheduling later in the day, so good producer will maintain good records. - A description of ready-mix concrete from averge large sophisticated supplier is that it is the "most expensive way to sell aggregate".

 

[conceng]

If this is on its way into legal territory, I'd say you will definitely need at least one full petrographic examination of a core. It may add to the strength of your claim and provide a more solid base. It may also indicate the problem has very little to do with batch time in the truck. It's hard to believe that they would have such a rampant over time problem. The supplier would certainly take exception to tying up trucks too often.

A full petro will give you the air void structure parameters to determine if how much air there is and if the air void structure is suitable to provide freeze-thaw durability. They should look at the surface paste to determine if there was loss of air due to overfinishing, or surface addition of water. They should also look at carbonation depth and microcracking to determine if the surface may be porous because of lack of curing.

My experience with low air contents is that the scaling is barely perceptible the first year if the concrete was otherwise properly proportioned, placed, finished and cured.

Oh yeah, ASTM C 94 provides 1 1/2 hours unless the owner allows an extension as long as the concrete can be properly placed, consolidated and finished without the addition of water.

Most agencies I have worked with use 1 1/2 hours for non-air entrained concrete and drop it down to 1 hour for air entrained concrete due to the risk of excessive loss of entrained air with time.

Greg

 

[dik]

Petrographic is OK... but, from the short period of time, sounds more like a finishing problem. Any aggregate issues in your locale?

Must have a lot of the sidewalk, else, litigation is an expensive remedy and not necessarily successful... You can replace a lot of sidewalk for a $1M...

Dik

 

[conceng]

A petro report would give you some basis for your claim and would likely speed up the negotiation process if youyou.

When they cut and tested the cores, did they run unit weights? We often try to estimate the air content by gravimetric method based on the unit weight. Not as good as a petro, but tells you if you're at least in the ball park.

Greg