MSI Time-Domain-Reflectometry (TDR) Concrete Cure Monitoring provides a new approach to cure monitoring in concrete and cementitious materials [1-7].  Using an ultrafast pulse and inexpensive capacitance sensor, our TDR system follows rotation of individual chemical states of water participating in the hydration reaction. The system monitors formation of bound-water hydration products, as well as unreacted free-water to form the hydrated state.

The system uses a pulsed microwave dielectric measurement called Time- Domain- Reflectometry (TDR) to follow molecular dynamics over an extremely wide frequency range. Chemical states which are separated include:   A free-water component near 10 GHz representing chemically unreacted water which decreases continuously during cure. A medium-relaxation component around 100 MHz representing water attaching to developing microstructure which increases during early cure.  A low-relaxation component around 1 MHz representing local ion migration in developing microstructure which increases during early cure.  An ion-conductivity component below 1 MHz representing long-range ion drift through developing structure which decreases continuously with cure.

Monitoring of Cement Hydration by Broadband TDR Dielectric Spectroscopy (2004).* Monitoring of Hydration Processes in Cement Materials by Broadband Time-Domain-Reflectometry Dielectric Spectroscopy (2007)

A unique aspect of the system is that the measurement is performed the time domain, where sensor response can be separated from other effects by propagation delay.  Data can be either transformed to a microwave frequency spectrum for scientific-quality analysis, or interpreted directly in the time domain for robust field-grade control.  The same system is used in both cases and scaled-up or scaled down as appropriate.

Frequency-Domain Cure Monitoring

A typical TDR frequency spectrum for Portland Cement is shown below.  Data is presented as frequency-dependent permittivity, starting at 10 kHz on the left and ending at 10 GHz on the right for both real and imaginary components. The signal is repeated several times during the cure process, starting at 8 minutes with the red trace and ending at 20 hours with the brown trace.  The material is a standard portland cement obtained from the NIST Cement and Concrete Reference Laboratory (CCRL) and mixed in a water/cement ratio of 0.4.

The TDR spectrum separates into 3 easily-identifiable features for both real and imaginary permittivity. On the right is the free-water response, representing water in an unattached state which falls during the period.  In the middle is an intermediate-frequency response representing local ion migration and water attaching to developing microstructure, which grows during the period.  On the left is the ion conductivity, representing ions moving through the microstructure which falls during the period.

Dielectric modeling reveals additional details [3]. Standard dielectric models including Debye and Cole-Davidson are simultaneously fit to both real and imaginary permittivity using shared model parameters. The intermediate- frequency feature separates into 2 components, with a 1 MHz component representing ion migration in developing microstructure and 100 MHz component representing water attaching to developing microstructure. The low, medium, and free-relaxation components can now be tracked continuously during the cure process.

The free- and bound-water response are only obtainable by high-frequency methods such as TDR.  The free-water falls with cure and represents water loss to hydration. The bound water rises with cure and represents formation of early microstructure. We thus monitor loss of reacting and formation of developing microstructure simultaneously.  

Time-Domain Cure Monitoring

A very simple application is to monitor the direct TDR transient from which the frequency spectra are derived.  Early portions of the pulse around 100 picoseconds represent 10 GHz behavior containing the free-water response, while latter portions of the pulse around 50 nanoseconds represent 20 MHz behavior containing the bound-water response.  The two signals can thus be monitored as a function of cure time, producing simple stripchart-grade indicators of free- and bound-water response.  Typical results for CCRL Portland cement are shown below.

The figure shows the transient stable during the initial dormant period, and then showing a loss of free-water and the growth of bound-water at around 10 hours. Then bound water reaches a peak around 20 hours, and then begins to fall as water becomes more tightly-bound and the TDR frequency spectrum broadens.  The free- and bound-water signals continue to show a slow decrease to 500 hours.

Comparison with Hydration Measurement

The free-water transient signal can be directly compared with the water loss of hydration. The  amplitude represents the free-water permittivity where only unreacted water is active, so the measured signal represents the volume fraction of unreacted water in the cement paste, which is the inverse of the degree of hydration.  For CCRL cement the degree of hydration is known from a measurement of non-evaporable water content, and is compared with the direct-transient amplitude below. 

The above describes some specific examples of TDR Cure Monitoring System and its application. Some general features, advantages, and benefits of the system are listed below

Microwave frequency sensing.	Monitors free- and bound-water molecular dynamics.	Provides more information on water loss to hydration and formation of hydration products than lower cost solutions.
Pulsed sensor interrogation.	Isolates sensor response by propagation delay.	Simplifies microwave calibration procedure. Easy to install and use.
Coaxial pin sensor	Matches impedance over wide range of frequency and cure state	Rugged and low-cost. Provides ability to analyze molecular dynamics over a wide range of frequency and cure state.
Fourier transform analysis.	Provides free- and bound-water molecular rotation spectrum.	Provides analysis of free- and bound-water molecular dynamics at a level research laboratories expect.
Direct transient analysis.	Provides stripchart-grade indicators of free- and bound-water spectrum.	Provides information on water loss and formation of hydration products quickly and in a simplified manner.
Portable electronics package.	Provides small portable unit with LCD or PC display.	Portable and robust electronics suitable for field use. Monitors multiple sensors simultaneously.

National Science Foundation 

Our work is being carried out jointly with the Microwave Materials Lab at Elizabethtown College, Elizabethtown PA. A recent NSF award to the microwave lab is as follows:

Award #0700699 - 1 May 2007

CMMI Division of Civil, Mechanical, and Manufacturing Innovation
ENG Directorate for Engineering

Monitoring of Hydration in Cement Systems by
Broadband Time-Domain-Reflectometry Dielectric Spectroscopy

Abstract

Prior work in our laboratory demonstrated a continuous monitoring of the dielectric relaxation spectrum in hydrating portland cement over the frequency range 10 kHz to 8 GHz from initial mixing to several weeks cure. Measurements were made using broadband Time-Domain-Reflectometry Dielectric Spectroscopy with a 35 ps stimulus and an embedded capacitance sensor. Three fundamental signals were identified, corresponding to unreacted free water, bound-water attaching to developing microstructure, and grain polarization. The three signal components were fit to appropriate molecular models as a function of cure time and monitored throughout the process. The result is 1) a free-water relaxation which monitors the disappearance of water into hydration and thus follows percent hydration, and 2) a bound-water relaxation which monitors water attaching to developing microstructure and thus monitors formation of this microstructure, and 3) a grain-polarization relaxation which monitors development of this microstructure.

Copyright © 2007 Material Sensing & Instrumentation, Inc