Concrete curing is critical in producing precast concrete members. Great strides have been made recently toward better curing and in understanding what happens within concrete at it early stages.
The industry evolved from trusting plastic tarps loosely draped over a slab, to the cure monitoring devices of the 21st century. Elizabethtown College has been working on an ambitious project to develop a system to allow us to better understand concrete curing.
The system being developed monitors the chemical state of water, combining with portland cement, to form the solid hydrate in hardened concrete. The system does this by monitoring high-frequency rotations of the unreacted water molecule, as it slows to lower-frequency rotations as the molecule combines with portland cement.
By monitoring the disappearance of the unreacted molecule, the user can determine the amount of water that has been incorporated into the hydrate, a critical factor in determining compressive strength. By monitoring the appearance of lower-frequency rotations, the user also can monitor the formation of the developing structure, which may provide additional information.
The Time-Domain Reflectometry (TDR) system involves embedding a small inexpensive electrical sensor in the concrete product and interrogating with a very rapid electrical pulse. The returning waveform contains information about the state of the water molecule in the concrete mix as the material cures.
By analyzing this waveform, we can separate the amount of unreacted water from water that has combined or reacted with the portland cement. There are several components in our reacting-water spectrum, but we are focusing on one which has an immediate commercial application and whose interpretation is straightforward.
The fastest component in the returning waveform represents the unreacted water molecule, independent of all other components which cannot respond at these speeds. This component thus provides a measure of the instantaneous water/cement ratio as a function of cure time, a property which correlates with compressive strength. If we simply monitor this component with an embedded sensor, using a field-portable pulsing unit and software control, we can monitor the unreacted water concentration during curing.
Our strategy uses high microwave frequencies to isolate the unreacted water molecule at speeds at which no other molecule can respond. An inexpensive embedded sensor is used to probe the interior of the material where beam methods of interrogation cannot penetrate.
There are at least three potential applications in the concrete industry:
- Predicting Compressive Strength: There is a firm scientific basis for expecting a correlation between our signal and compressive strength, since compressive strength is known to vary with percent hydration, while our system measures the inverse of percent hydration, percent un-reacted water. A means of predicting compressive strength would reduce core-crush testing considerably, shortening wait times for multilayer structures and removing forms in pre-stressed structures.
- Verifying in-place water content: A means of verifying in-place water content would in itself be valuable, since a mix design is often “re-engineered” at the site, producing a water/cement ratio which is not optimum.
- Detecting moisture before surface coating application: A means of determining residual water before applying commercial surface coatings would be useful in preventing damage to these coatings.
The program developed from a similar program for monitoring cure in aerospace composites, funded the U.S. Army and Boeing Rotorcraft in the late 1990s. A Phase (Small Business Innovation Research (SBIR) grant was obtained from the U.S. Department of Commerce in 2000 to do some exploratory work in cement, which was continued with a Phase I SBIR from the National Science Foundation in 2002 to map out the actual signal evolution during cure.