Editor's note: The following is an excerpt from the book, Self-Consolidating Concrete: Applying what we Know, by Joseph Daczko, published by CRC Press, 2012. This portion examines the importance of controlling moisture in the aggregate that will be used for producing self-consolidating concrete.
ONE OF THE most common concerns during field production of self-consolidating concrete (SCC) is the adequate control of aggregate free moisture. Uncontrolled, excess moisture will increase the water-to-powder ratio, decrease the mixture plastic viscosity and potentially lead to excess fluidity, instability, segregation and bleeding.
Fine aggregate moisture has been found to be stable at levels as high as 6 percent to 8 percent, which could amount to 50 to 70 kg/m3 (85â€“120 lb/yd3) of water delivered through fine aggregate for a mixture using 850 kg/ m3 (1450 lb/ yd3) of sand. It is therefore critical to have an accurate dayto- day and within day knowledge of moisture contents.
To help with this consistency, some producers store aggregates in covered locations so as to minimize the impact of weather changes. The photo on the right shows covered aggregate storage with a misting system used to maintain consistent moisture. The photo page 18 shows an unprotected aggregate stockpile and the application of water through a sprinkler system.
Both producers are attempting to maintain more consistent moisture, but the uncovered system will have a higher degree of variability. Whatever the system in use, one must be aware of the efficacy of that system and plan material monitoring and QC activities accordingly.
Moisture monitoring methods
Various methods and equipment are currently used for the control of moisture contents and can be categorized as either manual or automatic. Manual methods include standard test methods, such as ASTM C 566, "Test Method for Total Evaporable Moisture Content of Aggregate by Drying," and ASTM C 70, "Standard Test Method for Surface Moisture in Fine Aggregate."
These methods require a technician's effort to test and calculate the moisture content of the aggregates. They can be used when producing SCC as long as significant vigilance is used in monitoring concrete performance. The difficulty with these methods is that the moisture is measured at a single point in time and is assumed to be relatively constant between tests. If potential exists for the moisture content to vary, such as before, during and after a rain storm, a rigorous testing protocol, which would include more frequent moisture and concrete testing, should be in place to ensure consistent performance.
The quality control manual for the National Precast Concrete Association (NPCA) states that surface moisture content shall be physically tested once a day prior to the first SCC batch, even when moisture probes are in use. If inline moisture meters are not used, both the Precast/Prestressed Concrete Institute and NPCA require that the moisture be manually measured at the beginning of each batching operation and every four hours of continuous batching or at any time a change in moisture content becomes apparent. A very robust mixture should be developed for production facilities that do not use in line moisture meters.
Automatic moisture determination methods include the use of moisture probes in aggregate bins, as well as in the mixing vessel (usually only found in mixers in precast plants). Moisture probes in aggregate bins are generally configured to provide feedback to the batching system which automatically adjusts added batch water to compensate for the aggregate moisture.
According to one study, moisture probes have been shown to be reasonably accurate for fine aggregate but not as accurate for coarse aggregate. The interim guidelines from PCI however, require that moisture probes be capable of measuring changes of 0.5 percent in the moisture content of both coarse and fine aggregates.
ACI 304 states that moisture probes should be recalibrated to oven dried samples monthly or whenever the slump (workability) of the concrete produced is inconsistent. Studies have suggested that this calibration procedure should be done with great care, in particular when obtaining the sample for oven drying. The sample should be obtained from an area as close to the probe as possible. The use of accurate, well-calibrated aggregate moisture probes provides real-time adjustments during the batching and mixing process, leading to more uniform production.
In a recent quality benchmarking survey, the NRMCA asked producers how often they measure aggregate moisture and how often they calibrate their moisture probes. Eighty-nine percent of the respondents said that they either check the moisture at least daily or use moisture probes. When it came to calibrating the moisture probes, 29 percent said they did it weekly, 29 percent said they did it monthly, 21 percent said they did it quarterly, while 14 percent said they did it annually or greater.
Moisture probes can also be installed in the concrete mixing vessel. These probes determine the moisture content of the concrete mixture once all materials are batched, allowing for trim water addition at the end of the cycle. They can be placed in the mixer either in a fixed position or as a revolving probe attached to the mixing arms.
One published study on mixer moisture probes included an experiment incorporating two different water content measurements. The results showed that the probe location (fixed versus revolving) provided different levels of repeatability and total error in water content prediction with the revolving probe being more precise.
During concrete batching, the measured total water content will change as materials are added. It will also change as the batch is mixed and becomes more homogenous. Accurate moisture measurements with in-line mixer moisture meters can take up to 30-45 seconds after all material is batched for the reading to stabilize.
It is recommended that the producer discuss this stabilization time with the moisture meter company's technical representative. Not allowing time for this stabilization to occur may lead to inconsistency in the fresh SCC properties. In one published study, data was presented on both high-performance conventional slump and SCC, showing the stabilization time (mixing time required to reach a stable mixer amp reading) as a function of water to cement ratio for mixtures with two different cement contents and two high-range water reducer dosages.
The study showed that mixes with a higher w/c ratio required shorter mixing (stabilization) times than lower w/c mixtures. The graph on page 20 estimates the total water content per cubic meter for the various mixture types reported and plots it against the stabilization time. Regardless of mixture type (SCC or conventional slump) the total water content of a mixture can be used as a relative indication of mixing time required for a given mixer.
A concrete producer developing or producing various SCC mixtures would now realize that an SCC mixture with lower water content will require longer mixing to produce consistent properties. All other things being equal, the more water per unit volume of concrete, the more easily mixed it will be.
The stabilization time can also offer an indication of the overall water content in the mixture and has the potential to be used as a second level determination of the accuracy of moisture measurements prior to the batch being discharged.
If one knows the stabilization time for the mixture being produced, a relatively longer or shorter stabilization time could then be related to lower or higher water content respectively. Although to use this measurement as such, a calibration procedure would have to be conducted to relate stabilization time to various water levels for a given SCC mixture.
Accurate control of aggregate-free moisture is an important part of any plan for producing consistent SCC. When possible, automatic monitoring of moisture contents is recommended when producing SCC mixtures. Also, some producers use both aggregate and in-mixer moisture probes for better control.
Other steps for ensuring robustness of the mixture in terms of minimizing the impact of unaccounted for moisture on fresh properties can also be taken. The use of viscosity modifying admixtures has been shown by numerous researchers to provide robustness to SCC mixtures, as have other mixture proportioning techniques. All of these should be considered as one develops a plant or project specific SCC program.
Joseph Daczko is product manager at BASF Construction Chemicals. For more information and to buy Self-Consolidating Concrete: Applying What we Know, visit http://go.hw.net/SCC-book.
Correcting for Water in Aggregate
Concrete is made of three main materials: aggregate, cement, and water. In automated production environments, these are normally measured by weight.
As the aggregates are stored outside and subject to environmental conditions, their moisture can vary, for example, with sand, between 1 percent and 13 percent. This adds weight to the material being weighed.
This has two effects. First, when you weigh a dose of aggregate, the actual weight of the dry aggregate in the dose is less than required. Second, there is water present in the dose.
The first effect, the low dry weight, can lead to the final yield of the batch being low. Also, the final mix design could be incorrect which, for self-consolidating concrete (SCC) particularly, makes it very difficult to get repeatability in the batches.
The presence of water in the dose means that when you add a fixed quantity of water into the mixer, you actually have too much water in the mix. This means you must increase the cement and SCC admixtures to compensate for the extra water.
By using moisture measurement in the sand and aggregate bins, you can virtually eliminate the effects of this moisture variation. When you weigh the aggregates, you can compensate for the weight of the water in the dose and add the correct quantities of raw materials.
By using a water control solution in the mixer, you can correct for any other variations in the moisture of the mix that can be introduced, for example, when transporting the material from the weighing system to the mixer, and get a very repeatable final product.
Using microwave moisture measurement is the most cost-effective solution for measuring moisture in concrete production environments.
Contributed by Neal Cass, technical sales manager for Hydronix, a manufacturer of moisture measurement products. To see Cass' Spring 2012 ACI Convention presentation, The Effective Use of Moisture Equipment to Control the Water Content During SCC Production, visit www.theconcreteproducer.com.
Abstract submissions for the Fifth North American Conference on the Design and Use of Self-Consolidating Concrete are due June 30. The conference takes place May 12-15, 2013, in Chicago.
The event will demonstrate significant results and outcomes of research and practice of SCC, and their benefits for the concrete industry, society, and the environment. It also will encourage a platform to further address challenges and strategies for advancing the use of SCC to promote broader dialogue and greater interaction between SCC researchers and users.
Topics will include chemical admixtures; mix design methodologies and procedures; rheology and workability; formwork pressure, casting, and construction issues; production, quality control, and specifications; and architectural and precast concrete.
The conference is hosted by the Iowa State University Institute for Transportation and Northwestern University's Center for Advanced Cement-based Materials. Visit www.intrans.iastate.edu/events/scc2013.
Web Extra:Spring ACI Convention Presentation: "The Effective Use of Moisture Equipment to Control the Water Content During SCC Production" by Neal Cass, Hydronix technical sales manager, March 2012 (PPT)