There's actually a meter for the concrete barrel on the truck, according to American Concrete Institute (ACI) not supposed to rotate more than 300 revolutions.
Slump is important for many reasons, one thing is for sure, the design mix's water/cementitious ratio if changed ( adding more water to the design mix on site than called for in the actual mix design ) can substantially reduce the compressive strength of the concrete.
For residential contractors, say using a 3000-4000 psi mix design, a seasoned contractor would know already what mix design to use with a slump that works best without adding more water, enough to weaken the mix. That experienced contractor will know the suppliers and what to order. If the ready mix truck does show up and it's too stiff, yet the batch plant having been calibrated correctly to provide the correct lbs of materials including correct amount of water, it is not advisable to allow the contractor to arbitrarily add water as they see fit to loosen the mix and increase the slump, increasing the water to cementitious material ratio will definitely weaken the material. It happens and materials are subject to rejection, it needs to be workable and allow time to place, without altering the mix design
Residential applications are not monitored like large commercial jobs, it is imperative to have an experienced concrete contractor in the residential sector, one that can be trusted and knows this material and how to deal with reputable suppliers.
Your uncle was definitely onto something there, concrete creates heat, usually noticed after it's placed when you touch the form, like on a wall, but in the barrel, never heard of that, but can assume he knew that if the barrel was warm, the concrete has started the hydration process, been in the truck too long and won't be able to be placed, finished in time, as well as it failing prematurely.
Concrete suppliers are well aware of punctuality for the most part, if you are doing any kind of pour in NYC, you don't want those trucks caught in traffic, even for some small sidewalk jobs, controlled inspections are required, engineer is sent to the plant to inspect/monitor the batching plant, and an inspector is on site taking test cylinders usually 4 per so many cubic yards placed, kind of forget how many per cu yd now, as well as performing inspections of reinforcing and placing the concrete, each pour requirtes a licensed P.E. to sign and stamp, on large jobs at the end, these reports are all reviewed by a city plan examiner prior to approval for certificate of occupancy on building jobs. Much of what is done on these large jobs can be scaled down and applied to the smallest of jobs like what is common on residential work. The right contractor is key to this on residential work as it's only them and a homeowner, neither may know much about the material from what I have seen and things can go wrong quickly, absolutely correct, get another contractor at the first sign of incompetence or lack of knowledge, can be very costly.
I investigated a claim recently, they just ommitted the reinforcing in a slab, and it was required on approved drawings, now it cannot be approved by the municipality for C of O, they now have to hammer it all out and do it over, had a superintendent type or inspector been on the job, it could have easily been avoided, even if you ate the cost of one or 2 trucks, was as strip mall, quite a large slab, stop work order would have prevented a large claim here.
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