Q: I'm a quality control manager for a mid-size ready-mix producer and am dealing with several quality issues which need resolution. I am having difficulties convincing the owner that he needs to increase funding for the quality system in order to produce more consistent concrete. How can I convince him?
A: Deciding how to fund a quality system depends on economics. The owner is responsible for maintaining profitability, and this includes how to fund a quality control system.
If you want to convince the owner to provide more funding to the quality system, you must produce a business plan and present the cost savings. If you can show that increasing control of the aggregate gradation allows you to save 30 to 50 pounds of cement per yard, this can be equated into dollars. If you can show you will reduce rejected loads or claims, those also result in savings.
Typically, there are several different ways to make concrete which will meet specifications for construction projects, all of which are based on how to manage the variations in production. Figure A illustrates how variability exists in concrete manufacturing.
The dot in the center represents the design strength of the concrete, in this case 4000 psi. The inner circle represents variability in the testing. The next circle represents the variability between batches of concrete. The outer circle represents variability in the raw materials used in each batch.
The sizes of the circles are not meant to represent the magnitude of the variation, as each may be different from producer to producer, or project to project. The circles are accumulative in each case. Each one of these variations is present in all concrete, and the measured compressive strength for a mixture will fall anywhere within these circles.
If we design concrete to the required specified strength, failures will occur due to the variabilities. In this illustration, it is highly likely that 50 percent of the measured strengths will fail to meet the specified strength.
This leads to Figure B, which more accurately represents how concrete is designed to deal with these variations.
The three main variations are also present in this figure, although they are skewed. This figure represents the over-design of a concrete mixture. ACI 301, Specifications for Structural Concrete, provides methods of developing the required target strength for over-design. Note that in this illustration, there is still a risk of measuring a strength which does not meet the specified strength.
Current industry standards allow for one low-strength result per 100 tests. By observing the arrows within the diagram, we can see that as we over-design concrete (moving our target strength to the right), we reduce our risk of failures but we increase costs.
This increase represents the cost of the concrete mixture. To achieve the target strength (over-design strength, not specified strength) the producer typically requires more cement or admixture. This higher target strength is required to limit risk of low compressive strength due to the variations. It is easy to observe that by making these variations smaller, we can save money in concrete mixes.
The money saved in this example is rather simplistic. Not all of the savings goes directly to the bottom line because of money needed to create, run, and manage a quality system required to reduce the variations. The ready-mix producer must examine the cost versus return on investment for a certain level of quality system.
Self-consolidating concrete is an exception to his rule. These mixes are highly sensitive to variations in aggregate gradations and water content (aggregate moisture). High-strength concrete (over 10,000 psi) is another example. Without a good quality control system, it would not be practical to produce such mixes.
The savings in the concrete mixture is just one reason to increase the producer's level of quality control. However, limiting the producer's risk of rejected concrete and claims is another issue worth addressing.
Concrete is perishable, and if profit is being lost because the material fails to meet specifications, either in the plastic or its hardened state, then the quality system needs to be adjusted to lower these claims.
One example of this question was posed to me years ago by a readymix producer. They wanted to know if they should put low-range water reducer in their residential driveway mix. Here are the questions I asked and their answers:
1. Do you have issues with failures and claims (scaled driveways)?
2. Do you want to lower the risk of said failures and claims?
3. Are you going to change the mix, take cement out, or replace it with fly ash or slag?
Answer: No. I concluded that by adding water reducer to the mix, they would only increas the cost of the mixture without additional benefit to the company.
A ready-mix producer's level of quality control is an economic decision unless they are making high-performance concrete. A producer must consider the savings that results from a quality control system.
Contributed by Braun Intertec. Visit www.braunintertec.com.