THERE IS A WIDELY HELD and incorrect belief in the concrete industry that a producer's quality control (QC) department takes away from and does not add to the bottom line.
In reality, QC is not just an expense to the producer. This department's activities can directly increase the producer's profits. In addition, the final product batched at the plant will meet specifications, the concrete contractor will be satisfied, and the project owner will be pleased.
In his book, Quality is Free, noted QC guru Phillip Crosby defines the “cost of quality” not just as the cost of QC efforts, but also the cost of poor quality. This includes chargebacks, cost to replace defective product, failures, and lawsuits. He further asserts that every dollar spent on quality efforts will reduce the cost of poor quality by $3.
One way in which the QC department can increase profits is by correcting concrete mixtures for yield. Concrete is batched by weight, but sold by volume. Therefore, a means to convert a mixture from weight to volume must be used. In imperial units, to convert from weight to volume, we divide the weight by the specific gravity (SG) and by 62.4 pounds/ cubic foot. The proper equation is:
For metric users the equation is simpler:
An example of the equation for cement for imperial users is:
In case you are wondering, the 62.4 is the weight of one cubic foot of water.
To calculate the yield of a concrete mix, determine the volume of each material, add it together, and compare it to 27 cubic feet or 1 cubic meter. Typically, concrete producers will design mixes to yield 27 to 27.2 cubic feet, depending on corporate preferences. The producer is only required to produce 27 cubic feet, but producing 27.2 cubic feet of material will reduce customer complaints about under-yielding.Why don't all mixes yield properly?
Most concrete mixes are inherited from previous operations and then modified. There is no guarantee that the original mixes were designed properly, but if materials or their specific gravities have changed, the yield will change.
Many producers are already aware of the significance of proper yield. But even just 10 years ago, it would not have been uncommon to analyze a typical production concrete mix that yielded as much as 28 cubic feet. Usually if the mix under-yields, the customer complains to the concrete producer. If the mix over-yields, the customer won't complain because he is filling his forms faster at lower costs.Financial impacts
First, with an overyielding mix, the producer is sending out extra material. This is usually sand, but it could include other material as well. For example, if a mix is yielding 28 cubic feet and the sand costs $10 per ton, the producer is sending out an extra 163 pounds of sand at a cost of $1.21 per cubic yard. Cut the yield to 27 cubic feet, and that $1.21 goes straight to the bottom line.
In another example, if a producer used an incorrect mixing water content of 210 pounds/cubic yard instead of 270 pounds/cubic yard, the result would be a concrete mix with a very low slump. The person placing the concrete will typically add water to achieve the desired slump, increasing the water-cement ratio. This can lead to an over-yield of 1 cubic foot.
In this case, the over-yield is uniformly distributed over cementitious materials and aggregates by 4 percent. Just eliminating cement over-yield will result in cost savings of $1.20/cubic yard.
The second financial impact is on lost sales. If a smart customer realizes that a mix is over-yielding, he can reduce his order quantity. For a 1000-cubic-yard slab placement, if the customer realizes the mix is yielding 28 cubic feet, he can reduce his order to 964 cubic yards. At $100 per yard, the concrete producer has just lost $3600 in sales.
Shortly after my father and I started selling our quality control software, one of our early customers informed us that just by adjusting the yield on his mixes, he could save his company more than $200,000 per year. As long as the materials don't change, these savings occur every year without additional effort.Why don't producers correct their yields?
While this seems simple, imagine doing the calculation for 1000 mixes in one plant and then updating each mix manually in the batching software. Now expand that to 15 or 50 plants.
As you can imagine, updating mixes for yield used to take days or even weeks. Even when computers became widely used and spreadsheets and other software made performing the calculations trivial, updating thousands of mixes still took a tremendous amount of time.
Fortunately, most of today's modern comprehensive concrete quality control software can perform mass updates of concrete mixtures to adjust for yield in just minutes. For those programs that can integrate with dispatch or batch plant software, the whole process can be done in as little as 15 minutes. Today, there is no excuse for a concrete mix not to not yield properly.
The tools and training are available, the financial incentive is clear, and the need is dire. It makes no sense for a concrete producer to operate without a functional quality control organization.
James M. “Jay” Shilstone Jr. is a concrete technologist for Command Alkon Inc. He is a Fellow of ACI and an active member of ASTM and NRMCA committees. E-mail jshilstone@command alkon.com. This story is based on a presentation at the 2011 NRMCA ConcreteWorks conference by Shilstone and Karthik Obla, vice president, technical services, at NRMCA.
TIPs for QC
Other methods can have an even greater impact on a producer's bottom line. Spurred on by its P2P (prescriptive to performance) efforts, the NRMCA and its Research, Engineering & Standards committee have begun to address this and other issues related to the quality control operation. The TIP (Technology in Practice) series helps educate QC personnel on the proper way of performing various functions.
For those just starting out, the NRMCA's Technical Short Course provides training and allows attendees to take the exam for Concrete Technologist Level II and III. ACI Field Technician Level I certification is a must for all concrete technical personnel.
NRMCA has also developed Quality Management System for Ready Mixed Concrete Companies. This free 260-page book on creating a quality management system is available at the NRMCA website. Also, the Guideline Manual for Quality Assurance Quality Control is a guide to creating a quality plan. This is available at the NRMCA bookstore.
Once you have set up a proper quality control operation, it would be nice to get credit for your efforts. The NRMCA hosts the annual Concrete Quality Award, recognizing producers that have implemented quality plans.
Completing the Quality Award application requires assembling a significant amount of information. After completing the application, hold on to that data so you can respond to the NRMCA Quality Benchmark, a bi-annual survey that shows the range of quality efforts by varying sizes of concrete producers. The results for 2012 are being tabulated now, but the results for 2010 were reported in the January 2011 issue of the association's inFocus magazine.
NRMCA is also finalizing details of a Producer Quality Certification which will be a step above the existing NRMCA Plant Certification. It includes requirements for personnel, quality practices, management commitment, and customer focus. For more, visit www.nrmca.org.