By Claude Goguen, P.E., LEED AP
We throw the term “quality” around quite a bit when we describe our industry, our companies and our products, and yet quality means different things to different people. How would you respond to the statement that McDonalds makes a quality product? Yes, we’re talking about the Big Mac, Quarter Pounder and Filet-O-Fish.
Granted, when discussing quality, most people would not immediately conjure images of the Golden Arches, but consider this: You can go to a McDonald’s in Seattle or Miami, in Helsinki or Tokyo, order a Big Mac and it will taste the same. Moreover, you could go into a McDonalds in your home town today, next week, next month and next year, and that Big Mac will taste the very same every time. Whether or not you indulge in this famous sandwich is irrelevant, because the fact remains that McDonalds has an incredible system that ensures consistency among all its eateries around the world. That is a very important aspect of quality in mass production – consistency.
And now to bring this all back home, in our discussion about quality, there is little difference between the Big Mac and the Big Stack of manholes in your yard awaiting delivery. Do your customers feel confident that the products they receive from you will be the same every time, regardless of which plant, project or time of year? They should, despite the variables you may experience over the long haul with cement, aggregates and admixtures.
The thing with mass production
The advent of mass production has shifted the emphasis to the reproducibility of such things as the size, shape and strength of a product. Every project starts with a specification, either written or verbal. This is how the owner tells you what he wants. From that point until the product is delivered, there are many steps involving many people and many materials. Each of those steps involves some potential for variability, and provides for a challenging task to keep them all in check in order to deliver a consistent product.
We can’t eliminate variability, so we must ask ourselves how we can minimize it, and how much of it we can accept before specific actions are needed.
Many factors can influence variability (see Figure 1). It starts with your materials. In order to deliver a consistent product, you must demand and expect consistency from your vendors. You can be sure that McDonald’s product vendors have minimized their variability considerably.
You expect your purchased products to adhere to strict specifications such as ASTM C150 for cement, C33 for aggregates or C1017 for admixtures. You test your aggregates for moisture content. You test your well water. You store your materials in a consistent manner. You know that using aggregates from a bin under a large shady oak, compared with aggregates from a bin in the sun, may provide additional challenges to maintaining consistency due to moisture content and temperature.
In order to control variability in your raw materials, you must closely monitor the physical and chemical characteristics of each delivery. Look at each load of aggregates. Pick it up in your hand to see and feel how clean it is. This daily routine will make it easier to spot differences. Check your gradations, compute your fineness modulus and track both. Minor changes in fineness modulus from, let’s say, a 2.9 to a 2.7 could elicit some complaints from the production floor as the aggregates will be finer and, therefore, workability could be compromised.
You can also control variability in some raw materials by monitoring the mill certificates that accompany the deliveries. Your cement, for example, comes with a certificate containing a lot of data. It’s good to pay special attention to your C3S (tricalcium sulfate) and C2A (dicalcium aluminate) percentages from load to load, as these two constituents influence early strength and control your initial set.
Also worthy of tracking is your SO3 content. This is the sulfate that is added to the clinker in the manufacture of portland cement to retard the hydration of the aluminate phase. This is limited to 3% as per ASTM C150, but if this number varies from 2.3 to 2.7 between loads, it may have an effect on your initial set as well.
You can also track the Blaine Fineness. This number is a measurement of the surface area of your cement particles. The larger this number, the finer your cement. The finer your cement, the faster your cement will react. This is why high early cements will have Blaine Finenesses ranging near 2,440 ft2/lb or 500 m2/kg as opposed to 1,710 ft2/lb or 350 m2/kg for general purpose cement. By tracking these numbers from lot to lot, you will be able to see fluctuations and take action if necessary.
You should also look at your supplementary cementitious material certificates and track certain values. If you’re using fly ash, you should pay special attention to the LOI number, or the Loss on Ignition. LOI is calculated by heating up a cement sample to 900 to 1,000 C (1,650 to 1,830 F) until a constant weight is obtained. The weight loss of the sample due to heating is then determined. A high loss on ignition can indicate prehydration and carbonation, which may be caused by improper and prolonged storage or adulteration during transport or transfer. The large portion of unburned material left over will be carbon. We want to make sure we have minimal carbon residue in our fly ash. Carbon is very porous, and the more we have in our fly ash, the more risk we have of it soaking up our admixtures, especially air entrainments. Carbon can absorb air-entraining admixtures to different extents, making it more difficult to routinely impart the correct amount of entrained air into concrete.
For slag, keep an eye on the Blaine Fineness, and track your SAI (slag activity index) as well. This is how the slag is tested for compressive strength and compared to the reference for portland cement. If the SAI is 75%, that means the strength of the slag is 75% of the strength of the cement. If we’re opting for cement replacement, we want the SAI to be as close to 100% as possible.
Next we consider the batching or processing stage. Your materials could be very consistent, yet the manner in which you load your materials into the mixer could negate all of that. The loader should be very careful about how the aggregates get from the pile into the hopper. He should pick uniformly across the front of the pile so that the gradations and moisture content of the pile going into the hopper are as close as possible to those same values obtained in testing. Picking from the top and then bottom of the piles can seriously alter those values, increasing the variability of the batching process.
Mixing times need to be consistent as well as delivery and placement of the concrete. Calibration of batching equipment on a regular basis can ensure that weights or volumes of materials are closely controlled. Consolidation through internal and external vibration needs to be conducted the same way regardless of who is doing it that day. Here’s where continued training becomes very important. Relying on documented procedures used by one experienced worker to train a new person on using internal vibrators ensures that the new worker uses the same correct (or perhaps incorrect) methods. Plant-wide training for all workers minimizes that risk.
Curing products in the same environment can be challenging depending on the layout of your plant, but you must strive to provide consistent temperature, humidity and time regardless of climate.
Finally, we have the testing. It is critical that testing of concrete be performed consistently using the appropriate ASTM methods. Making cylinders with two layers one time and three layers the next can have an impact on the strength results. It is important when pulling a representative sample of aggregates from the stockpile that you pull it from different locations and not just from the surface. The same goes for sampling of fresh concrete.
Once again, consistent and periodic training for all QC personnel is a sure way to minimize variability in testing methods. Also, periodic calibration of testing equipment needs to be done to eliminate the risk factor.
Careful monitoring, tracking, training and calibration go a long way in minimizing variability and increasing quality. In the next issue, we will look at ways to measure and track variability.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services and Sustainability.