The time it takes a concrete slab to dry enough to be able to install a floor covering is a major factor in the schedule of most construction projects. Accurately predicting drying time can potentially save a lot of time and money. Unfortunately, there are so many variables that a simple formula like the “rule of thumb” (it takes one month for each inch of thickness to dry) just doesn’t work very well.
VARIABLES THAT AFFECT DRYING
One of the variables that affect drying time is the type of concrete. There are many types of concrete with very different rates of drying. Here are just a few:
- Standard
- Accelerated Set
- Flowable
- Self-Consolidated
- High Strength
Standard, or normal concrete, has been around for centuries. It’s the standard mix of Portland cement, aggregate, and water.
Accelerated set concrete is made with rapid set cement, which has a higher limestone content than Portland cement. It cures in 24 to 72 hours.
Self-consolidated concrete (also called flowable concrete) is highly fluid and requires no vibration for compaction. It has a high water-to-cement ratio.
High strength concrete has a higher compressive strength than standard concrete. It’s made by using a lower water-to-cement ratio and adding silica fume to make a stronger bond to the aggregate.
These concrete types have different rates of drying primarily because of their different water-to-cement ratios.
In addition to the concrete type and water-to-cement ratio, more variables come into play, like aggregate type, exposure to sunlight, and proximity to walls and structural beams.
Conditions in the ambient air also play a big part in determining how long concrete takes to dry. Ambient conditions are kept relatively constant by HVAC during the drying process, but they still need to be closely monitored in case something goes wrong. Any changes could significantly delay drying.
All of this means you can’t depend on the rule of thumb to be an accurate estimate of concrete drying time.
HOW DO YOU KNOW WHEN THE CONCRETE IS DRY?
No matter how you estimate the drying time, your estimate doesn’t tell you if the concrete is dry. All it really tells you is that it’s time to test the concrete. And if your initial test shows that it isn’t dry, you need to give it some more time and test it again. How much more time is pretty much your best guess based on past experience. You continue this process until the test results show that the slab is dry enough to receive the floor.
The unpredictable nature of this process can cause schedules to slip and costs to mount up. Until recently, this was the best process for drying a concrete slab.
TREND ANALYSIS
A better way to conduct the drying process is to start moisture testing as soon as the slab is considered cured. (Usually about 30 days.) Then on a regular schedule, you test the slab, take readings of the ambient conditions, and log the data. A detailed log of the moisture and ambient data helps you to spot problems immediately and correct them. So, for instance, you might notice that drying in the slab has slowed down. Or maybe there’s a change in the ambient conditions. If there’s any problem, you can correct it sooner and more easily identify what’s responsible.
The data log also allows you to perform trend analysis. Trend analysis is simply looking at the data on a chart or in a table, seeing a trend, and projecting where you think the concrete is dry enough to install the floor covering. If you see the slab is going to be dry sooner than you expected, you might be able to reschedule dependent tasks earlier.
As you gather data from more projects, you’ll be able to make more accurate projections, schedule tighter, and save time and money.
THE TOOLS FOR TREND ANALYSIS
The tools for this process must be fast, accurate, and easy to use for repeated testing. For the most accurate data logging, they must have integrated digital data collection capability.
The Wagner Meters Rapid RH® L6 system provides ASTM F2170 compliant moisture testing, integrated with advanced data collection technology.
The Rapid RH L6 Smart Sensors come with a NIST-traceable certificate of calibration and install directly in the concrete.
The Rapid RH Total Reader® is used to transmit the data via Bluetooth® to the DataMaster™ L6 app, which runs on any iOS or Android device. The DataMaster L6 app stores, displays, reports, and emails the data in PDF format. Backup copies of the data are stored in the cloud and in the sensors that are permanently installed in the slab. This all-digital path from the sensor to the final report, plus automatic backup ensures the highest data integrity and safety.
To monitor ambient conditions, the Wagner Meters Smart Logger™ can record ambient temperature and relative humidity for 300 days of replaceable battery life or up to 12,000 readings. The Smart Logger communicates via Bluetooth® to the Smart Logger app to download, store, display, report, and email data in PDF format.
CLOSING THOUGHT
The powerful new technology of data logging and trend analysis can give you a real professional edge when your job depends on concrete drying time.
- Data logging means you can detect problems earlier, make corrections, and determine responsibility, protecting you from liability.
- Trend analysis helps you to better predict drying times and schedule tighter.
- These tools can help you become a sought-after professional who can save time and money on the concrete drying process.
about the author:
Jason Spangler, Wagner Meters’ flooring division manager, has more than 25 years’ experience in sales and sales management across a broad spectrum of industries. He has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture test. Jason has extensive industry involvement, including the National Wood Flooring Association (NWFA), the International Certified Flooring Installers Association (CFI), and is vice chairman of associations for The Flooring Contractors Association (FCICA). Call Wagner Meters at 855.350.7715 and ask for Jason or visit www.wagnermeters.com.
Modern Contractor Solutions, December 2019
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