By Phil Cutler, P.E.
Excessive water can destroy a concrete mix design. Discovering the detrimental effects can be easily seen in a small-scale experiment using compressive-strength cylinders and a common mix design.
For one cylinder, increase the amount of mix water and keep all other parameters the same. Then, take the pair of cylinders and break them. With all other factors controlled, you’ll easily see your concrete strength reduces with increasing amounts of water. If you prefer to believe industry resources, simply view Figure 12-3 from the Portland Cement Association manual “Design and Control of Concrete Mixtures, 15th Edition.” The figure clearly illustrates that as the quantity of mix water increases (or water-cement ratio increases), concrete strength decreases. Figure 12-3: Approximate relationship between compressive strength and water-cementitious materials ratio for concrete.The general rule is “an extra gallon of water per yard reduces the compressive strength of your concrete mix by approximately 500 psi.” You wouldn’t expect anyone to dump a gallon into a mixer, but the tricky thing about water is that it can hide. Excess moisture in fine aggregates can easily account for a gallon of water.
So the real question is: Do you use moisture probes or nationally recognized ASTM manual test methods for determining moisture? We could make strong cases for both methods. However, with either choice, accurate accounting of moisture in aggregates is essential to keep your mix on target and within specified limits.
Aggregate moisture determination by testing
ASTM C70, “Standard Test Method for Surface Moisture in Fine Aggregate” and ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying” are the two manual methods used by plants to measure aggregate moisture content. These tests are typically performed daily by the quality control technician and may occur more than once per day depending on aggregate consumption, the prevailing weather conditions, whether or not bins are exposed to the elements and project-specific requirements.
The plant’s technician pulls the appropriate sample of each aggregate to be tested, usually prior to the first batch of concrete made each day, and performs the tests according to the detailed procedure. The ASTM C70 evaluations may be determined by mass or by volume of the sample. ASTM C566 uses the sampled aggregate mass against the same sample following drying.
Once the moisture of the aggregates has been determined, these values are typically given to the batcher for entry into the batching computer or to make the appropriate manual proportioning changes.
Aggregate moisture determination by probes
The use of moisture probes for determining aggregate moisture has been widely used as an accepted practice in precast plants across the country for more than 25 years. There are several types of probes on the market today. These technologies allow automatic moisture evaluations of materials.
According to Robin Shepherdson, president of Scale-Tron Inc., three types of moisture probe technologies are used by precasters. Resistance-type probes measure the resistance of an electric potential between two locations. However, this measurement method is not always accurate since it depends on the impurities, or minerals, present in the water.
“For example, whether the sand has been dried and then wetted again makes a difference between measuring the body of the pile and the surface,” Shepherdson said. “The measurement will show up at a higher value for that portion than the rest.”
Microwave-type probes do not rely on measuring impurities, making it a better option to use, he said. They use frequency to sense the amount of moisture.
Lastly, radar-type probes consist of guided radar waves to facilitate measurements. The probe sends radar waves into the sand and measures the reflection returned. By measuring the reflection points, a sine wave, or data pattern, is then created to guarantee an accurate moisture reading.
Probes can be placed in bins just under the discharge gates and above the weigh belt or directly in the path of the aggregates after they have dropped onto the belt. Probes can also be placed in the mixer.
“The important thing to remember using any of these technologies is to place the probe in the aggregate flow region, the moisture reading needs to be averaged for the entire batch and you want to keep the aggregate compacted to have an even density,” Shepherdson said. “Follow these three steps and you’ll get accurate data every time.”
Probes do not eliminate the need to perform manual tests for verification of aggregate moisture. However, they significantly reduce the frequency of manual tests. One of the biggest advantages over manual methods is they obtain values of aggregate moisture as they are batched in real time or with very minimal delay. This information is then fed directly into the automated mixing water batch controller of the plant. Automated input allows the plant to make fine adjustments to mix proportions and leads to a much narrower set of final batched concrete tolerances.
NPCA Plant Certification aggregate moisture requirements
Aggregate moisture requirements are addressed in Section 5.2.2 of the NPCA Quality Control Manual for Precast Concrete Plants. Subsection 22.214.171.124 covers conventional and dry-cast concrete and points out that aggregate surface moisture content shall be determined at least once per day in accordance with ASTM C70 or by alternate methods such as moisture meters, probes or ASTM C566. The subsection goes on to state that when aggregate bins are fitted with moisture probes or meters, aggregate surface moisture content shall be determined a minimum of once per week in order to validate probe calibration. Subsection 126.96.36.199 covers self-consolidating concrete and also requires validation of probe calibration once per week.
Moisture tests must be performed in accordance with ASTM C70 or ASTM C566.
For SCC processes made without moisture probes or meters and automatic mixing water adjustment systems, the aggregate surface moisture content must be determined at least once per day prior to the batch and once every four hours after the first batch. In addition, the language states that slump flow testing will take place for every three batches of SCC produced. This is to ensure that the moisture fluctuations of aggregates are accounted for correctly.
For additional information on moisture probes, please contact the NPCA technical services department at (800) 366-7731.
Phillip Cutler, P.E., is NPCA’s director of quality assurance programs.