Editor’s Note: This is part one of a two-part article. Look for the second installment in the July issue.
Every load of ready-mixed concrete comes with a delivery ticket full of useful information, if you know how to crack the secret code. Critical information includes identifying the mix, batch, and cumulative volume; help on checking yield; admixtures; and the amount of water that can be added within specs. The ticket also tells the batch time, which is important for predicting slump loss and setting time, and there is a lot of important cost information that will help you figure out what the concrete is worth (if you are delivering it) or how much you will have to pay (if you are receiving it).
Besides of all of this, if and when something hits the fan on your concrete project, the tickets are likely to be introduced as evidence, marked “Exhibits A, B, and C.” But despite this practical and economic value, concrete delivery tickets “just don’t get no respect.” When I needed some fresh tickets for an upcoming presentation, I visited the nearest construction site and found three of them blowing away in the wind!
The right mix
If you have ever placed concrete only to find out that you put the wrong mix in the wrong place, you already know how important it is to check the mix ID on the ticket. Look for the customer and jobsite info, special delivery instructions (back gate, east entrance) and the mix ID. Most producers have an ID code such as “4012” for non-air-entrained 4000 psi (@ 28 days) with #1 and #2 stone, or perhaps “4012AE” for a similar air-entrained mix. Special features such as fibers will often be in the mix designation as well.
Of course the busiest time onsite is when you are trying to get a pour started, which is when the concrete is being delivered. But if you don’t have the time to read the ticket to make sure that you are about to place the right mix, you have to ask yourself, “Do you have time later to deal with having placed the wrong mix?” If you only have one mix on the project, and your producer is shipping to you only, this may not be a big deal. But if you have a footing mix, slab-on-ground mix, column mix, and elevated-slab mix, and if your job is big enough that any of these can be placed on a given day, it pays to check the ticket.
The ticket also shows the volume of concrete in this load, and the total volume of the same mix shipped for this order today. It is important to know that these are calculated volumes, not measured volumes (more on yield later). This total cumulative volume is essential for managing your concrete order. Given the unevenness of subgrade, variable slab thickness, deformation of forms, settlement of shoring, and variable air content and yield of the concrete, it is tough to predict the exact volume of concrete to order. One common routine is to give the concrete producer an estimated total volume (say 100 cubic yards) and a hold value (maybe 96 cubic yards). But the trick is to make your final order before that last truck arrives. At the end of any given load, say 80 or 88 cubic yards, you can check the ticket, discover total yardage placed, estimate the volume to complete (maybe add ½ yard), and call in your final order.
he sales receipt
On smaller jobs the ticket serves as the only purchase order or sales agreement between the contractor and the producer. For that reason, it is common for the ticket to show price per cubic yard and total price per load. Extra charges may appear for items such as hot water in cold weather, extra cost for a partial load, or for long unloading times (overtime). It is common for the ticket to include text that describes these charges, and reading these terms and noting extra charges can reduce “sticker-shock” when the invoice shows up in the mail, because unless you are familiar with the terms under which the concrete was purchased, that invoice with all its extras can remind you of a cellphone bill or a rental car receipt.
For example, in the “mock-up” delivery ticket shown on page 21, the fine print indicates a split-load charge for orders less than 4 CY. This recognizes that the price structure for ready-mixed concrete includes the variable cost of the materials and the prorated cost of transport, plant and equipment, and driver. For small orders, the split-load charge helps to fairly cover these fixed costs.
Similarly, the concrete producer assumes a typical time onsite to discharge the concrete; five minutes per yard in our example. This also relates to the per-cubic-yard pricing of ready-mixed concrete by accounting for driver time and overall per cubic yard productivity. The example ticket indicates that the total discharge time recorded by the driver (and approved by the customer who signed the ticket) exceeded the allocated five minutes per CY. This overtime was charged at the rate announced in the fine print of $75 per hour.
Along with the printed “terms and conditions,” you are likely to find material safety data information along with statements that limit the producer’s responsibility for the finished product, given that control of the product passes to the contractor at the end of the chute. While it is common for an employee of the contractor to hastily scribble initials on the ticket when done with the truck, if you look closely that representative may have signed that he or she “inspected, approved, and received” the concrete. Those words make the scribbled signature more important than usually realized.
Slump loss and setting time
The ticket itself is a serial-numbered document to distinguish it from all others ever printed by that concrete producer. It will show the truck number, perhaps the driver’s name, and the date and time of batching. This is essential because most specs require that concrete may be discharged from the truck only up to 90 minutes (or sometimes less) after the batch time. Tickets may also show the times at which the truck left the plant, arrived onsite, began to discharge, and completed discharge. This information can be used to check spec compliance, as mentioned, determine any overtime truck charges, or to help explain or predict rate of slump loss or setting times.
Complaints from contractors about apparent truck-to-truck variability in slump, rate of slump loss, or setting time are not uncommon. But these characteristics are tied to the rate of hydration of the cement, temperature of the concrete, and the time at which cement and water first came into contact. Slump-loss and setting begin at batch time (printed on the ticket), but the contractor (or finisher on the deck) is far more aware of the time at which the concrete was placed.
For example, let’s say the first truck beat morning traffic and got to the site 15 minutes after batching, but the second truck carrying identical concrete had a 30-minute haul time. The second truck is likely to arrive at a lower slump, and is likely to appear to have a setting time 15 minutes shorter than the first. In cool weather this might not matter, but under hot, dry, sunny, or windy conditions, those might be 15 very important minutes. Of course, any actual batch-to-batch variation in slump or setting will just add to variable time-since-batching, but important clues are right there on the truck ticket.
This is demonstrated with the setting time data shown on the graphs at left. Using a variation on the ASTM C403 test, the pressure required to embed a boot-sized footprint into fresh concrete to a depth of ¼ inch is measured. This pressure is shown on the vertical axis. For this field test of concrete paving at a mall, the finishers could float the concrete when the penetration resistance was about 5 psi, and could apply the appropriate broom finish when the penetration resistance was about 15 psi. In the top graph, the horizontal axis shows time after placing the concrete, with about an hour difference in float-time between truck 60 and truck 28. The bottom graph shows that there was only about a 15-minute variation in batch-to-batch setting when time is measured from batch time instead of placing time. Note that the first truck of the day, #167, was clearly a load of slower-setting concrete anyway you measure it!
The information discussed so far constitutes what is known as the Delivery Ticket, and is not significantly different from the receipt that accompanies delivery of many other construction materials. The second article in the series will explore the Batch Ticket with its details of concrete mixture composition, often printed on the same piece of paper. As we will see, ASTM C94, Standard Specification for Ready-Mixed Concrete, makes it clear that detailed batch information must be provided only when specified. Inclusion of such data is therefore not automatically mandatory.
