Smart Aggregates Containing Piezoceramics: Fabrication. and Applications

Transcription

1 Smart Aggregates Containing Piezoceramics: Fabrication and Applications 1, 2, Chris Choi 1, 3 1. NSF- REU Researcher for the Department of Mechanical Engineering, University of Houston, USA 2. Undergraduate Student; Department of Civil Engineering, University of Maine 3. Undergraduate Student; Department of Mechanical Engineering, University of Houston Graduate Student Advisors: Hai Chang Gu PhD. Student; Department of Mechanical Engineering, University of Houston Claudio Olmi PhD. Student; Department of Mechanical Engineering, University of Houston Faculty Advisors: Dr. Gangbing Song; Faculty-in-charge, Smart Materials and Structures Laboratory, University of Houston Dr. Yi-Lung Mo; Professor of Civil Engineering REU Sponsor, University of Houston I. Abstract The piezoelectric effect is a useful way to monitor the health of cast concrete. PZT patches can be embedded in small concrete blocks called a smart aggregate then cast in larger structures for concrete strength monitoring. They are designed to monitor the early age concrete strength development, impact detection and structural health. Piezoceramic materials exhibit the phenomenon of piezoelectricity. Piezoelectricity describes the event of generating an electric charge in a material when subjecting it to mechanical stress, and conversely, generating a mechanical strain in response to an applied electric field. PZT is an acronym for Lead Zirconate Titanate, which is a commonly used piezoelectric ceramic material and is the piezoelectric ceramic material used in this project. Smart aggregates are designed for civil structures such as buildings and bridges. II. Introduction Concrete is such a widely used building material around the world, it is important to monitor its health to prevent disaster. The idea of imbedding PZT patches in concrete is still relatively new. One goal in this research is to continue the study of the effectiveness of smart aggregates. Page 1 of 11

2 PZT patches can be an effective way to measure concrete health. However, the lack of mass production techniques makes it hard to be installed in numerous structures. Another goal of this project is to construct a reusable form for casting smart aggregates for their use in testing. Also we plan to test the strength properties of the smart aggregates and their effect on overall structure strength. In addition, we will explore some new piezoelectric smart aggregate techniques, 3D sensors and protected sensors with RFI Shielding. III. Smart Aggregate Construction Our Piezoelectric material for this project was Lead Zirconate Titanate (PZT). For this research, we cut 10-by-10 square millimeter patches of PZT from a large wafer. These patches were then soldered to a 24/2 (24 American Wire Gauge (AWG), 2 Conductors) shielded communication cable that was connected on the other end to a BNC connecter for computer connection for instruments such as an oscilloscope. We decided to use common PVC pipe caps as our reusable molds to cast the smart aggregate. The caps are approximately 1-1/2 in diameter and 2 in depth. We were able to purchase these at a local hardware store. The standard mode size would be easy to duplicate for mass production. A 1/8 diameter hole was drilled in the bottom of the PVC pipe cap and a custom-made circular aluminum plate was fabricated and placed in the pipe cap. The idea was to push the concrete out after it had been without destroying the mold and allowing the mold to be used again. However the concrete had a tendency to stick to the walls of the pipe cap and removal became quite difficult. Later we used motor oil when casting to help loosing the concrete but removal was still difficult. Future research will need to be done to determine an easier mold removal strategy. To insert the wire with the PZT chip on the end, a small notch was cut in the side of the pipe cap. The wire was placed in the notch with the PZT chip inside the pipe and concrete was poured in, filling the inside of the pipe cap. The smart aggregates were left to harden for at least 24 hours. All of our smart aggregates were cast with the concrete mix specified in Table 1. A schematic of the smart aggregate mold is shown in Figure 1. See Figures 3-8 for a pictorial look of making smart aggregates. Subsequent to preparing our first smart aggregates we monitored their capacitance for the first 5 days after casting (see graph Figure 2). At first, the capacitance was erratic but it eventually leveled off to around 6 nano-farads (nf). The optimal capacitance of the PZT patches is six nano-farads. All of our piezoelectric patches were designed with a capacitance between nano-farads (nf). Page 2 of 11

3 Figure 1 - Schematic of location and position of PZT patch inside pipe-cap for concrete casting C a p a c it a n c e v s. T im e Capacitance (nf) A 1 B 1 C E la p s e d T im e ( M in ) Figure 2 Capacitance of PZT patches after cast in smart aggregate Page 3 of 11

4 Figure 3 Cutting the PZT patch from a larger wafer Figure 4 Soldering the 24/2 shielded connection cable to the PZT patch Figure 5 Cable after the BNC connector has been soldered to other end of cable Figure 6 Waterproofing PZT patch Figure 7- Patch and mold before concrete poring Page 4 of 11

5 After we had made over twenty regular smart aggregates (one PZT chip, concrete only), we then constructed 3D smart aggregates and smart aggregates with RFI Shielding. We created the three dimensional (3D) smart aggregate by cutting three slots in the pipe cap each 90 degrees apart (see figure 9). The PZT patches were each put in three different ways to represent three different planes [x-y, y-z, and x- z]. These sensors can measure waves coming from all different angles. Conversely, the 3D smart aggregates are much harder to construct because of the amount of work keeping the Figure 8- Concrete poured in to the mold angles lined up correctly during casting. The other new mart aggregate we made was an RFI shielded aggregate. Using 100-mesh copper with a.0022 wire diameter the smart aggregate is designed to block radio frequency interference (RFI) that could impede the sensors ability to communicate with other sensors during health monitoring (see figure 10). Setup for creating the shielded aggregate is the same as with regular smart aggregate except before concrete pouring the mesh is lined against the pipe cap walls and on the bottom against the aluminum plate. After the concrete is poured and fills the pipe cap an additional piece of copper wire mesh is placed on the top of the concrete and the entire aggregate hardens into one structure. xz-plane yz-plane xy-plane Figure 9 Diagram of 3D smart aggregate y z x Figure 10 Cut RFI shielding copper mesh before inserted in to mold Page 5 of 11

6 Regular w/ UH logo Engraved 3-D RFI Shielded IV. Testing Procedure Figure 12 Location of smart aggregate in a 6 diameter 12 tall cylinder Figure 11 Types of smart aggregate made A. Experimental Setup With the first batch of smart aggregates we constructed two SA cylinders, one with normal concrete and one with self-compacting concrete, each with two smart aggregates. In addition, we made four cylinders without smart aggregates. Each cylinder mold is six inches in diameter and twelve inches in height. Two holes are drilled in the side of the mold at four inches and eight inches in height for the BNC connectors and wire to go through (see Figure 12). Table 1: Mix Proportions for Smart Aggregate Type III Cement lbs Fine Aggragte lbs Water lbs BASF Superplasicsizer 3 grams Total: 2.5 lbs Table 2: Mix Proportions for Concrete Cylinders Cement (Type 1/2) 28 lbs Sand (Unsifted) 74 lbs Coarse Aggregate 82 lbs Water 17 lbs Total (Approx.): 200 lbs To make a cylinder with smart aggregate the concrete must be poured in three stages. First, the concrete is poured to the four inch mark and one smart aggregate is placed horizontally in the concrete with the wire through the drilled hole. At this point if the concrete is not self compacting the mix is prodded 25 times with a rod before the next part is poured in. When prodding the concrete we try not to touch the smart aggregate to avoid tilting of the aggregate. Then, another four inches of concrete is poured on top of the first smart aggregate. The second smart aggregate is placed in the same manner as the first one with prodding if necessary. Finally, the rest of the concrete is poured to fill out the cylinder. Page 6 of 11

7 With our first batch we were not able to get any structural data for the cylinders containing the smart aggregates, due to a testing failure. We then cast six more cylinders; three with smart aggregates (two smart aggregates in each cylinder, one as a sensor and one as an actuator) and three without smart aggregates using the 200 lb. mix specifications outlined in Table 2, again with each cylinder weighing 30 lbs. B. Results We allowed the cylinders to harden until Day 10 when we broke them out of the molds and capped them with pipe capper. The cylinders were labeled cylinders 1-3 without smart aggregate and cylinders 1-3 with smart aggregate. We tested Cylinder 1 without smart aggregate on Day 13 and planned to test the rest on that day too, however the strain gauge broke and we had to wait for a replacement. We were able to test the remainder of the cylinders on Day 18. We performed a stress-strain test on all the cylinders except Cylinder 3 with smart aggregate which was tested in compression only. The results of the test are shown in Table 3. The cylinders were tested on the Universal Compression Testing Machine with a steady load rate of 500 lbs/sec. We computed the stress and the strain by using the formulas in equations 1 and 2 respectively. The stress-strain curves for the four cylinders tested using the stress-strain control mode on Day 18 can be found in Figures Load Load Load Stress = = = 2 2 (1) Surface Area d 6 π π 4 4 Deflection Deflection Strain = = (2) Original Length 16 The strength of the Concrete can be scaled to the 28 day standard using the time-strength equation for concrete proposed by committee 209 of the American Concrete Institute: ' where c ( t) ' ' t f c ( t) = fc(28) (3) t f is the compressive strength at time t, t is the age of concrete in days and ' f c(28) is the concrete compressive strength at 28 days. The scaled results using equation 3 are shown in Table 4. Page 7 of 11

8 Table 3: Summary of tested cylinders (Strength at time of testing) Cylinders Without Smart Aggregate Cylinders With Smart Aggregate Cylinder # Days Ultimate Strength (KSI) Cylinder # Days Ultimate Strength (KSI) Average 3.66 Average 3.80 Table 4: Summary of tested cylinders (Scaled to 28 day standard) Cylinders Without Smart Aggregate Cylinders With Smart Aggregate Cylinder # Days Ultimate Strength (KSI) Cylinder # Days Ultimate Strength (KSI) Average 4.03 Average 4.07 Figure 13 Stress Strain curve of cylinder 2 without smart aggregate Stress Strain Curve Stress (KSI) Strain Page 8 of 11

9 Figure 14 Stress Strain curve of cylinder 3 without smart aggregate Stress Strain Curve Stress (KSI) Strain Figure 15 Stress Strain curve of cylinder 1 with smart aggregate Stress Strain Curve Stress (KSI) Strain Page 9 of 11

10 Figure 16 Stress Strain curve of cylinder 2 with smart aggregate Stress - Strain Curve Stress (KSI) Strain Figure 17 Stress Strain Curves for all cylinders Tested on day 18 in stress-strain mode Stress Strain Curves for both Specimens NC 2 NC 3 SA 1 SA Strain Page 10 of 11

11 VI. Conclusions and Future Work Our research concluded that it is possible to build smart aggregates cheaply and efficiently. We also showed it is possible to make smart aggregates with 3D sensors and smart aggregates with RFI shielding. Our testing showed that the smart aggregates imbedded in the concrete cylinders had no negative effect on the ultimate strength of those cylinders compared to the cylinders without smart aggregate. Future research efforts will include the improvement of the smart aggregate molds, particularly the smart aggregate removal from the molds. In addition, more research should be done to determine the sensor s accuracy in the concrete and their use in larger more realistic structures. Also, work will need to done to determine the feasibility of installing 3-D smart aggregate in cylinders and structures along with the effectiveness of the RFI shielding. VII. Acknowledgments The research study described herein was sponsored by the National Science Foundation under the Award No. EEC The opinions expressed in this study are those of the authors and do not necessarily reflect the views of the sponsor. VIII. References 1. Gu, H, G Song, H Dhonde, Y L. Mo, and S Yan. "Concrete Early-Age Strength Monitoring." Smart Materials and Structures 15 (2006): Song, Gangbing. "Basics About Piezoceramic Materials." Intelligent Structural System. University of Houston: Department of Mechanical Engineering. 3. Song, Gangbing, Haichang Gu, and Yi-Lung Mo. Smart Aggregate. 4th China-Japan-US Symposium on Structural Control and Monitoring, 16 Oct. 2006, University of Houston. 4. Song, G., Gu, H. and Mo, Y.L., Smart Aggregates: a Distributed Intelligent Multi-purpose Sensor Network (DIMSN) for Civil Structures, Proceedings of the 2007 IEEE International Conference on Networking, Sensing and Control, London, UK, April, 2007: Page 11 of 11