Measuring a slab's internal humidity: Researchers develop a new tool to concrete mixes

Concrete Producer, The, Jan, 2005 by Rick Yelton

Some say the only fun visitors have in many rural towns is watching flesh paint dry. Even though the University of Illinois at Urbana-Champaign may be in a small town, researchers there may have the last laugh on how interesting watching stuff dry can be.

At ACI's meeting in November, Zachary Grasley, a graduate student in the school's Department of Civil and Environmental Engineering, presented a new probe and software system that measures a hardened concrete's internal humidity decreases with time.

It makes watching concrete dry the in-thing. In recent years, ready mixed concrete producers selling to flatwork contractors have been at the point of attack to develop mix designs that cure quickly with curling. This is especially true for contractors pouring gym floors or other special surfaces.

But providing a fast-drying floor has its problems. ACI's "Control of Cracking in Concrete Structures" identifies relative humidity as a factor affecting drying shrinkage. The lower the relative humidity, the greater the ultimate shrinkage and rate of shrinkage. This work may answer how contractors can provide enough humidity during the initial phase of concrete set to reduce the likelihood of plastic shrinkage, and not increase the drying time to delay floor covering placement.

Grasley's team is conducting studies to eventually create modeling of drying stresses to improve the ability to predict and prevent shrinkage cracking. They plan to develop test methods with their system to monitor curing effectiveness and eventually to qualitatively rank curing methods.

Yet there's also a field side to the research. The system allows contractors to monitor recently placed concrete floors for internal relative humidity, so the owner knows when floor coverings can be applied. "Now there's a definitive method to prove that the concrete has dried, despite ambient conditions," says Grasley.

He also believes the instrumentation system will become a useful tool in predicting curling of concrete pavements/slabs and structural monitoring for bridge decks, dams, etc.

A system for lab and field

Initially, Grasley and students Matthew D'Ambrosia and Robert Rodden wanted to design a system that was lab-quality and field user-friendly, focusing first on real-world problems.

Measuring a concrete slab's internal relative humidity isn't exactly well-known because there wasn't an economical method that allowed engineers to compare lab results with field experiences. This is changing because Grasley and his team found an inexpensive sensor that was both rugged enough for field duty, and small enough not to be obtrusive.

Unlike bulky and expensive varieties, the probe transmits readings digitally. "This was a significant advantage, as we lengthen instrument cables without sacrificing accuracy," Grasley says. Researchers can place the probe in fresh concrete with a high pH without damaging it. It also measures temperature.

There are no plans to commercialize the system yet. But Grasley's team hopes universities and testing labs build their own internal humidity testing devices using the manual developed in the course of the research.

-- RICK YELTON

Contact Zachary Grasley at the Concrete Materials Research Group at 217333-2465, or e-mail grasley@uiuc.edu.

Download the instruction manual for constructing a pc-dependent version of datalogger and free software at https://netfiles.uiuc.edu/dlange/www/RHSystem.html.