On-Site Magazine

Measuring moisture to deliver top-quality surfaces


Asphalt Construction Roads

Automated, instant moisture measurement helps achieve quality, durability and performance.


In the asphalt pavement industry, accurate moisture measurement is vital to producing and laying down a durable, quality surface. Most asphalt mixes contain petroleum additives and water, and the ratio of moisture to solids is critical to the curing process.

To pave roads and parking lots, hot mix asphalt (HMA) is widely used because it hardens quickly and provides good traction. This material is typically mixed at an asphalt plant before transportation to the site by truck or rail car, at which point operators dump the mix into hoppers and then dispense it before heavy rollers compact the surface.

During its journey, the moisture content in the HMA can vary due to environmental conditions or water vapour loss as the HMA mix is heated, stored and dispensed. This variability can be disastrous to the quality, finish, longevity, and even safety of the finished asphalt.

To ensure the moisture content remains consistent, operators would ideally test the moisture content at every step of the way. Unfortunately, moisture measurement tests are rarely or inconsistently conducted. In addition, many moisture meters require calibration, sampling, and time. Many of the tools are not portable or durable enough to be used on heavy construction job sites.


The availability of cost-effective, automated and instant moisture measurement devices is providing the industry with an option, however, and it an option that some contractors are using to ensure optimal moisture levels from the plant to the paving site.



One traditional approach to test for moisture content involves Loss on Drying testing. This uses a moisture balance to measure the total change in the material weight change of a mixture after drying. However, such tests typically require technicians to prepare a sample to be brought to a lab, and each test takes upward of 15 minutes to several hours to perform. And asphalt samples are not small. Larger samples require a big drying oven, a big pan, and often overnight drying.

To ensure consistent moisture content at HMA plants, the industry now uses Near-Infrared (NIR) light equipment for a highly accurate, non-contact, secondary-measurement method that delivers immediate, laboratory-quality moisture readings.

NIR moisture meters generate accurate instant measurement of solids, slurries and liquids without contact or sample preparation. Available in desktop, handheld portable and inline models, when used with analysis software, inconsistencies can be quickly detected and corrected.

For real-time testing of HMA, either during plant processing or dispensed to and from trucks or rail cars, inline tools can be used for 100 per cent inspection of all mixed materials without having to take samples offline for testing.

Moisture analyzers provide an instant, non-contact, non-destructive measurement of liquids and solids while the product is moving. For frequent spot checks of materials anywhere in the process, operators can use portable, handheld and instant moisture meters. The user simply points the instrument and the moisture content is instantly shown on a digital display.

Once technicians lay the asphalt, handheld units can also instantly measure not only the road surface but also sublayer moisture to ensure proper curing.

NIR composition analyzers work well for asphalt producers or contractors who want to analyze the composition of the mix as well and, as recycled materials become a larger percentage of the inputs for asphalt production, these analyses will also play a role in documenting the components of the mix.

Some can instantly verify the correct quantity of additives and binders, as well as recycled materials in the mix such as tire rubber, Reclaimed Asphalt Pavement (RAP), Reclaimed Asphalt Shingles (RAS), slags, glass, and even pig manure.

As the asphalt industry strives to improve the quality of its end product, automated instant moisture testing and composition analysis tools are available to play a role in achieving superior quality and performance, which will ultimately translate into greater profitability.


John Bogart is managing director of Kett US, a manufacturer of a full range of moisture meters and other analyzers.



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