1. Microwave Nondestructive Evaluation of Mortars with and without Sodium Hydroxide Inclusion
Ashkan Hashemi, Kristen M. Donnell, and Reza Zoughi
Applied Microwave Nondestructive Testing Laboratory (amntl)
Electrical and Computer Engineering Department
Missouri University of Science and Technology (S&T)
Rolla, MO 65409, USA
Kimberly E. Kurtis
School of Civil and Environmental Engineering
Georgia Institute of Technology
Atlanta, GA , USA
Iman Mehdipour, Kamal H. Khayat
Center for Infrastructure Engineering Studies
Department of Civil, Architectural and Environmental Engineering

2. Outline Introduction Microwave Approach Sample Preparation
Measurement Results
Summary
Future Work

3. Introduction What is ASR?
The concrete prism test, ASTM C1293, is considered among the more reliable laboratory test methods for assessment of potential for ASR
But this test requires the addition of sodium hydroxide (NaOH) to the mix water in order to accelerate the ASR gel formation
In this investigation, the influence of the NaOH addition is examined through microwave nondestructive dielectric constant measurements
What is ASR?
The chemical reaction between alkalis present in portland cement and certain siliceous minerals/rocks present in some aggregates is known as ASR
Si-OH + OH- + Na+, K+  Si-O-Na, K + H2O
Si-O-Si + 2OH- + 2Na+, K+  2(Si-O-Na, K) + H2O

4. Sufficient Reactive Silica
Introduction
Sufficient Reactive Silica
Sources
Opal
Tridymite
Cristobalite
Chert
etc.
Portland cement
Other cementing materials
Chemical admixtures
Wash water (if used)
Aggregates
External sources (deicing chemicals)
Sufficient
Alkali Concentration
Sources
Sustain chemical
reaction
Provide water for gel expansion
Sufficient Moisture
Role

5. Introduction ASR formation: ASR Gel Sio2
Alkali elements in cement paste react with reactive aggregates
Na+ K+
OH-
alkali-silica gel forms around and within the aggregate
ASR Gel
ASR gel imbibes water from its surroundings which leads to expansion and cracking
H2O

6. Microwave Approach
Interactions of materials with microwave signals is macroscopically described by the complex dielectric constant:
It is an intrinsic property of material, independent of measurement technique
Reflection and transmission coefficient as well as scattered signal properties are directly influenced by this parameter
Temporal dielectric property characterization of such mixtures can provide invaluable information about the materials properties and any changes within them
Permittivity
Loss Factor

7. Microwave Approach Pure water Vs. ionic water
𝜀 𝑤0 =𝑠𝑡𝑎𝑡𝑖𝑐 𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝜀 𝑤∞ =ℎ𝑖𝑔ℎ 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓 𝜀 𝑤
𝜏 𝑤 =𝑟𝑒𝑙𝑎𝑥𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑜𝑓 𝑝𝑢𝑟𝑒 𝑤𝑎𝑡𝑒𝑟
𝑓=𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
NaOH is an ionic solution which has higher ionic conductivity compared to pure water
Since ionic conductivity affects loss factor of the material, microwave dielectric constant measurements should distinguish between samples with and without NaOH
Ionic conductivity [S/m]

8. Microwave Approach Previous Work:
For cement-based materials, microwave nondestructive characterization techniques have been utilized for evaluating:
Cure state,
w/c, s/c and ca/c,
coarse aggregate content distribution, aggregate segregation, etc.,
cyclical chloride permeation in mortar along with extensive EM modeling for the same,
Carbonation [1], and
formation of alkali-silica reaction (ASR) gel in mortar with reactive and non-reactive aggregates [2]-[4]

9. Sample Preparation
Two different sets of mortar samples, one with and the other without NaOH but both containing an alkali-reactive aggregate, were cast and cured at hot and humid conditions
Dielectric constants of the samples were measured temporally at S-band (i.e., 2.6 – 3.95 GHz)
Completely-filled waveguide techniques was used for dielectric constant measurements R S X
109.2×54.6 mm
72.1×34 mm
22.8×10.1 mm
Measurement Setup

10. water-to-cement ratio (w/c) aggregate-to-cement ratio (a/c)
Sample Preparation
Samples were put in hot and humid conditions for 28 days. Every 2-3 days, the dielectric constant of the samples were measured during curing period
Curing conditions: 38° C ± 2° 85%<RH<95%
The measurements were conducted for two different alkaline-reactive aggregates
NaOH was added to the mixing water of both mixtures (with NaOH) for a total equivalent alkali content of 0.9% by mass of cement
Mix Design
Mix Proportions
Sample Type
Reactive I
Reactive II
Cement
Portland Type I/II
Aggregate
Las Placitas
Grand Island
water-to-cement ratio (w/c)
0.47
aggregate-to-cement ratio (a/c)
2.25

11. Measurements - I Reactive aggregate type I – Permittivity
Since same material was used in both set of samples, the only difference between them is due to NaOH
Samples with NaOH show higher permittivity compared to low alkali samples

12. Measurements - I Reactive aggregate type I – Loss Factor
As it was expected, for samples with NaOH higher values of loss factor was measured
This increase in loss factor in high alkali samples is attributed to higher ionic conductivity of the samples

13. Measurements - I Reactive aggregate type I – Mass Change
NaOH accelerates ASR formation, ASR creates microcracks and tends to imbibe water from it surrounding; hence, more water is absorbed from environment to high alkali samples compared to low alkali

14. Measurements - I
Also, more cracks were created in samples with NaOH  indication of ASR
Visible cracks on surface

15. Petrography Without NaOH With NaOH
I applied stain (uranyl acetate) to both samples. Both samples had a general fluorescence, similar to samples exposed to 1 molar solution of NaOH. Samples with NaOH had cracking apparent at its top finished surface (the face not touching the mold when cast) and did appear to have more aggregate with fluorescing cracks see image 3. (bottom half, center of image, two medium sized gray aggregates with green cracks) and image 4. (bottom half, center of image, one medium sized stained aggregate with bright green crack ).
 it is normal to see cracking in all the samples. It does look like there is perhaps more gel and cracking in the sample with NaOH, which agrees with the change in mass data

16. Measurements - II
The reason for repeating the measurements was to measure some mechanical properties of the two samples and see if they can be correlated to microwave measurements
It is reported that the higher alkali content in the cement, the lower the ultimate strength of the corresponding sample*
To evaluate this, the compressive strength of the new set was measured as well as dielectric constant measurements
*Reference: Smaoui, N., et al. "Effects of alkali addition on the mechanical properties and durability of concrete." Cement and concrete research 35.2 (2005):

17. Measurements - II Reactive aggregate type II - Permittivity
The same behavior was observed for the other type of reactive aggregate
Permittivity, loss factor, and mass change followed the same trend as the previous case

18. Measurements - II Reactive aggregate type II – Loss Factor
The same behavior was observed for the other type of reactive aggregate
Permittivity, loss factor, and mass change followed the same trend as the previous case

19. Measurements - II Reactive aggregate type II – Mass Change
The same behavior was observed for the other type of reactive aggregate
Permittivity, loss factor, and mass change followed the same trend as the previous case

20. Measurements - II Reactive aggregate type II – Compressive Strength
 the lower strength in the high alkali case can also be related to the more rapid reaction of the cement, which can give a coarser microstructure and a lower strength paste
As it was expected, high alkali samples showed less compressive strength compared to the low alkali samples
This can be attributed to microcracks and possible ASR gel production in samples with NaOH

21. Measurements - II Cumulative heat evolution Low Alkali High Alkali
Trend: The graph shows a faster reaction and a greater extent of reaction during the first 3 days of hydration for the high alkali samples
Reason: NaOH accelerates the reaction of the cement, which creates more bound water - in the hydrated cement paste - during these early periods

22. Summary and Future Work
For better correlation between lab test and field performance, it is critical to understand the effect of NaOH in concrete mix
Due to sensitivity of microwave signals to ionic conductivity of materials, microwave dielectric constant measurements manifested the difference between the two types of samples
Microwave dielectric measurements can also be correlated to mechanical properties of samples such as compressive strength
Future studying of other chemical properties such as rate of reaction and microstructure of the two different sets of samples, can further expand applicability of microwave nondestructive techniques in material characterizations

23. Acknowledgements
This work was supported by the National Science Foundation (NSF), as a Collaborative Grant between Missouri University of Science and Technology (S&T) and Georgia Institute of Technology, under Award No Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.

24. References
[1] Hashemi, Ashkan, et al. "Microwave detection of carbonation in mortar using dielectric property characterization." Instrumentation and Measurement Technology Conference (I2MTC) Proceedings, 2014 IEEE International. IEEE, 2014.
[2] Hashemi, A., et al. "Microwave NDE method for health-monitoring of concrete structures containing alkali-silica reaction (ASR) gel." 40TH ANNUAL REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Incorporating the 10th International Conference on Barkhausen Noise and Micromagnetic Testing. Vol No. 1. AIP Publishing, 2014.
[3] Hashemi, A., et al. "Effect of humidity on dielectric properties of mortars with alkali-silica reaction (ASR) gel." Instrumentation and Measurement Technology Conference (I2MTC), 2015 IEEE International. IEEE, 2015.
[4] Hashemi, A.; Horst, M.; Kurtis, K.E.; Donnell, K.M.; Zoughi, R., "Comparison of Alkali–Silica Reaction Gel Behavior in Mortar at Microwave Frequencies," Instrumentation and Measurement, IEEE Transactions on , vol.64, no.7, pp.1907,1915, July 2015.

25. Thank You!