Editor’s Note: In Part 1 (June 2014) we discussed what constitutes the delivery ticket. This is not significantly different from the receipt that accompanies delivery of many other construction materials. Now we will explore the batch ticket with its details of concrete mixture composition, often printed on the same piece of paper.
Let’s begin with ASTM C94, Standard Specification for Ready-Mixed Concrete, a governing document that is included “by reference” in most construction specifications. C94 makes it clear that detailed batch information must be provided only when specified; inclusion of such data is therefore not mandatory. In the advent of performance-oriented mixture specifications under which concrete producers have additional freedom and additional responsibility, modern concrete producers depend on their ability to uniquely formulate mixtures that competitively meet all project requirements. While a producer may have a competitive advantage due to its plant and equipment, much of that advantage is gone when the composition of every load is boldly printed on each ticket for all to see.
The challenge, of course, is to provide sufficient information to uniquely identify the mix and satisfy the needs of the owner/specifier/purchaser, but not give the game away. After all, we don’t normally require full disclosure of all ingredients and proportions in our admixtures, curing compounds, or form release agents, or a full chemical analysis of steel, so why do we need to know the concrete ingredients?
Nonetheless, many delivery tickets do include this information when ingredients and proportions are either required by the specification or voluntarily included by the concrete producer. So let’s look at what’s included on our sample ticket.
Under the column heading “Design Qty” the batch ticket above shows pounds (lb) of aggregates per cubic yard. In this case the assumption seems to be that the aggregates are in a Saturated, Surface-Dry (SSD) condition, where the aggregates neither contribute nor absorb batch water from the concrete (to be discussed further). This is followed with pounds of cementitious materials plus pounds or gallons of water per cubic yard, along with dosage of admixtures. Since the batch weights on the ticket should match the most recent and approved version of the concrete mixture submittal, early in the job it is a good idea to compare the submittal with the tickets.
Getting down to details, note that in the U.S., chemical admixtures are often dosed in units of fluid ounces per 100 pounds (cwt) of cementitious material. On the example ticket, a water-reducing admixture is specified at 3.00 fl oz/cwt (/C on the ticket) in a mix with 564 lb cement per cubic yard, resulting in the total dose per yard being 3 x 564/100 = 16.92 fluid ounces, or only slightly more than the equivalent of one 16 ounce water or juice bottle-full of WRA per yard. For the air-entraining admixture (AEA) dosage of 1.05 fl oz/cwt, only 5.92 fluid ounces are required per yard, or about 12 fluid ounces (one soft-drink can) for this 2-yard load. This is only 0.75 pounds of AEA for the almost 7,500 pounds of concrete in this batch, for a weight ratio of 0.01%. This miniscule weight percentage is why liquid admixtures cannot be batched on a weight basis; they are usually measured-out in vials or graduated cylinders visible to the batch plant operator.
The next column to the right is labeled “Required,” which shows the batch weights converted to amounts required for the ordered volume of the whole load (in this case, 2 cubic yards). While it would seem that all one has to do is multiply the “Design” column per cubic yard by the target load size (2 cubic yards), two complications arise to prevent those who don’t have the password from making sense of the numbers in the “Required” column.
The first complication is the use of cumulative batch weights, which combines the weights of each individual ingredient as it is physically added to the holding container at the ready-mix plant. This is seen clearly where 720 lb/CY of “Gravel 1” per yard is multiplied by 2 to obtain 1440 lb released into the weighing container. Then 1065 lb/CY of Gravel 2 is doubled to make 2130 lb for the 2 CY. This is added on top of the 1440 lb already there for a total of 1440 + 2130 = 3570 lb of combined Gravel 1 and Gravel 2. We then apply exactly the same arithmetic to the sand, but first we have to account for aggregate moisture conditions.