Quality control (QC) measures apply to virtually any manufactured product including the mixing of ingredients use in the production of concrete. Among these is precise control of moisture in the ingredients. Concrete moisture content is specified in standards issued by the ASTM and CSA*. In this post we describe how Barkman Concrete, headquartered in Steinbach, Manitoba, uses a high-resolution benchtop moisture analyzer to assure quality control while reducing the time to conduct critical moisture analysis.
Here you will learn about:
- Concrete ingredients and the water-to-cement ratio
- Sources of moisture in concrete ingredients
- Concrete moisture analysis at Barkman
- Barkman experience with a benchtop moisture analyzer
- How a benchtop moisture analyzer works
Garry Funk is Research and Development Manager at Barkman Concrete (www.barkmanconcrete.com), voted in 2018 as one of Canada’s best-managed companies. Garry explains “Concrete at its most basic is a simple mix of small aggregates (sand), larger aggregates (gravel), Portland Cement and water.
“Sand and gravel,” he says, “are the structural filler while Portland Cement and water provide the ‘glue’ to hold the mixture in its hardened state.”
As an indication of quality, a property called compressive strength is often used. As Garry points out, “Most people would see compressive strength as a quality qualifier and there is a strong correlation between compressive strength and how concrete is used.
“But,” he says, “more and more in the industry are also using the moisture content of the concrete, or more specifically the weight of water and the weight of cement powder. This is called the water-to-cement ratio in the mix. The ratio is viewed as a reasonable measure in addition to compressive strength for other properties such as durability.”
Folks outside the industry may think of cement and water as separate materials and that analyzing moisture in the cement would be easy. But there is more involved.
Sources of Moisture in Concrete Ingredients
“The aggregates used almost always hold some water and this moisture that they bring to the mix needs to be measured and accounted for in the water-to-cement powder calculation as well,” he says.
“Another consideration for the aggregate moisture content is that the material proportions are most often measured by weight. To ensure that we are mixing up the same volume of concrete every time, we need to ensure that the same volume of aggregates are added and the weight of the water is compensated for.
“For example, if the mix recipe calls for 1000 kg of sand and the moisture content of the sand is 5%, 1050 kg of sand will be target weight for dosing to add 1000kg of sand.”
Concrete Moisture Analysis at Barkman
At Barkman, moisture content of small and large aggregates is most often measured by devices placed in the aggregate stream as they are being dosed.
This equipment needs to be constantly checked to ensure it is properly calibrated. The process is often done in a number of ways all of which take anywhere from 30 minutes to an hour and longer.
“To meet both ASTM and CSA test procedures for determining the moisture content of aggregates, these time consuming methods must be used,” Garry says. “For those of us involved in mixing concrete, conducting day-to-day and sometimes hour-by-hour testing is very time consuming.
“That’s what led us to investigate a benchtop moisture analyzer to help speed the process while meeting ASTM and CSA requirements.
“It is the tool we use to provide quick and accurate feedback that allows us to keep our moisture measuring equipment running at its best without shutting down the batching and mixing process.”
Barkman Experience with a Benchtop Moisture Analyzer
“We have used benchtop analyzers in our lab for a number of years during which we worked to assure ourselves that the results using small samples would reconcile with traditional test methods,” Garry points out. “This was essential in order for us to trust its accuracy.
“The concern we had was that in the standard test methods for moisture content significantly larger sample amounts are dried out than what can be dried out by the benchtop moisture analyzer.
“To illustrate, our standard lab sample was 1000g, but with a moisture analyzer we use 50g. As you can appreciate this is a substantial difference.
“Another point we addressed is the importance of taking representative samples. This applies to any sample used in moisture analysis but with the smaller sample size this consideration is even more important.
“Over a period of parallel testing we developed enough confidence to move exclusively to a moisture analyzer. We use a Kern MLS 65-3A moisture balance we purchased from Tovatech. With it we can usually get a reading in about 6 -7 minutes vs. 30 minutes to an hour.
“We are also able respond to concerns much more quickly and easily run more tests in a tighter time frame.
“While we continue to run tests using the standard protocols the Kern moisture analyzer lets us run tests more quickly, allowing us to maintain a higher level of precision in this measurement.”
How a Benchtop Moisture Analyzer Works
In general they calculate moisture content by the thermogravimetric method, also termed loss of weight on drying (LOD). The sample is weighed before and after the test, the difference in weight being the amount of moisture removed during the test.
We describe here the use of the 65 gram capacity Kern MLS 65-3A halogen moisture analyzer used at Barkman. It has a readout (the weight/percentage shown on the control panel) of 0.001g (0.01% moisture), offers a drying temperature of 50°-160°C (1°C increments) and can store 49 complete drying programs for easy recall.
Please note that this is a general description of conducting moisture analysis and does not reflect how Barkman Concrete uses their unit.
Before testing, set up the Kern moisture balance in a level and protected area according to the operating instructions. If it has been moved or environmental conditions change significantly it will have to be recalibrated. This is accomplished by following simple instructions in the user manual.
- Be certain to take representative samples to be tested. If time elapses before the test
- Keep samples in an airtight container to prevent them from absorbing ambient moisture.
Moisture Content Testing:
As noted above testing procedures can be programmed into the MLS 65-3A moisture balance so that they do not have to be reset each time a routine moisture check is conducted.
- Place a sample plate on the sample plate holder and tare the moisture balance.
- Select the correct drying profile – standard, gentle, step or rapid.
- Set the desired drying temperature and shut-off criteria (when a specified time has elapsed, when the sample reaches a constant weight per user-defined time period, or manually).
- Carefully and evenly spread the recommended sample weight on the sample pan.
- Closing the moisture analyzer lid will start the analysis.
The halogen heater quickly drives off moisture while the balance displays the status of each stage of the process.
Moisture analysis automatically terminates when the set shut-off criterion is reached. The final moisture content is posted on the unit’s display panel along with the heating mode, temperature and time. Data can be transferred via an RS232 interface for record keeping.
For more information on moisture analysis see our page on how a moisture analyzer works.
Please contact the scientists at Tovatech for unbiased advice on selecting a moisture analyzer to meet your specific requirements, and for information on its operation and maintenance.
*American Society for Testing and Materials; Canadian Standards Association