From the time that the water hits the cement to the time of discharge is 90 minutes. This is by law. ASTM C94, “Standard Specifications for Ready-Mix Concrete.” The State Department of Transportation determines the allowable times between batching (adding water to the cement, rock, sand mix) and discharge (when the mix is flowing out of the truck into your forms).
So let’s look at a typical pour. First you determine how much you need. This is a scientific guess. You always add about a 1/2 yard to your order. It is better to have too much than too little. The typical truck around here carries 14 yards. That is a full load.
For those of you who have the desire to cut off a concrete truck driving at 65 mph on the freeway, well the truck weighs 30,000 pounds empty. With a full load it can tip the scales at 60,000-70,000 pounds. Stay away from these trucks on the road. The stopping distance of a truck, particularly heavy ones like a concrete truck, can be estimated using various factors, including speed, weight, and the type of brakes used. While specific stopping distances can vary, a general formula for stopping distance is:
Stopping Distance = (Speed in mph / 10) x (Speed in mph / 10)
Using this for a concrete truck weighing up to 70,000 pounds traveling at 65 mph:
- Calculate the speed factor: (65/10) x (65/10) = 6.5 x 6.5 = 42.25 feet.
However, this is a rough estimate. Considering the weight and braking efficiency, heavier trucks typically require more distance to stop. Therefore, in practical scenarios, stopping distances can be significantly longer, often in the range of 100 to 150 feet or more, depending on conditions like road surface, load, and brake condition.
Back to our scenario. If you only need 7 yards, you pay extra for a short order to compensate the concrete company for truck time and batching costs. If you need 21 yards, you order 211/2 yards. They will send a full truck and what they call a floater truck. A floater is a truck with a full batch (14 yards) that supplies the additional mix to whoever needs it. That truck might supply 3 or 4 different sites that might have mis-ordered or needed additional mix to fulfill the load of 211/2 yards.
With me so far? You order and they tell you that you have a 10:00 pour time. That means that the truck MIGHT be there at 10. Let’s say that truck had an early pour of 7:00 and that he has another pour at 8:15. That means the truck gets to the 7:00 pour and hope that crew knows what they are doing, the forms are done, and that they do not have any stand time. Stand time is when the truck is not discharging mix. It is sitting there waiting to discharge a perishable product (remember the 90-minute rule, by LAW?), and the forms are not done, or the crew got stuck in traffic, or any number of reasons. Guess who pays for stand time? Hint, it is not the concrete company. So, the 7:00 pour is really a 7:20 pour and the truck gets back to the batching site, loads up for the 8:15 pour that is now an 8:50 pour. So, your 10 o’clock pour is going to run late. The opposite scenario happens also. Let’s say the crew at the 7:00 pour are mighty rock stars, and they finish early. No stand time, forms are done, and this crew is humpin’ and bumpin’. The crew gets the truck out of there by 7:20, the truck get back to the batching site, loads up, and the 8:15 pour is now a 7:40 pour. That means, that if this crew discharges all their product, your pour might move to 9:00 or even 8:30.
So you are at the job site, and it is 8:40, and you hear the gentle rumble of a fully loaded concrete truck turning the corner with your batch. Whether you are ready or not, you are pouring. The concrete is the king here. Yes, you can tell the driver you are not ready, and he is being the kindest of gentleman that he is, will completely understand as he starts the stand time clock on you. He will also tell you that he cannot guarantee the product (remember the perishable thing) in the most gentlest and soothing tones. After that shouting match with the driver, you begin to scream at your crew to move their butts, and all the while you are looking at your profit margin declining at about the same rate as the deterioration rate of the concrete mix sitting in the truck. Yes, you can add stuff to the concrete to slow down or speed up the dry time. That has consequences to the finished look of the concrete, especially if you are steel finishing. You want the best product, so you can do the best finish, and not get a huge amount of pitting, dropout, dry spots, cracking and all the other wonderful adjectives that can be applied to concrete.
Designers, you should be at the pour. You are part of quality control. You need to know concrete. If the job goes bad, you are a part of it. The client is pissed at everyone, including you. As a designer, you will live to love that call from the client telling how crappy the concrete looks and you should have been there. And you are wondering why you are getting the blame.
Concrete is a stressor. I did a pour of colored concrete at SRI, International one year. The designer specified an adobe-colored concrete and we were pouring 14 yards. We were ready, the forms were set, the crew was there, everything felt right. Colored concrete does not have the same dry time as regular. It dries way quicker. The mix was wrong. I knew it the minute it came out of the truck. But we poured anyway, and it flashed on us. That means it was drying before we could finish it. The next day we jack hammered 14 yards of concrete out and hauled it to the dump. The day after we were pouring again. I pitched a mighty wail to the concrete supplier, and he gave us the concrete to us free. I ate the labor to bust out the flashed concrete and the dump fee.
Concrete is ready when concrete is ready. It does not care about whether you are ready or not.
The Fork & Trowel 