In-situ Wireless Monitoring of Fiber Reinforced Cementitious Composite Bridge Piers

High-performance fiber reinforced cementitious composites (HPFRCC) are a cement-based material in which a relatively low volume fraction of short fibers (steel, polymeric or carbon) are included in a cement matrix to produce a material that strain hardens under tension. Exhibiting high damage tolerance and tensile strain capacity on the order of 1 to 3%, HPFRCC materials can be used to reduce transverse steel reinforcement and associated reinforcement bar congestion in shear-dominated structural components designed for seismic loadings. To monitor the long-term performance of critical structural components within civil structures, low-cost monitoring technologies are necessary. In this study, a wireless structural monitoring system is proposed for in-situ monitoring of HPFRCC structural components subjected to reversed cyclic loading. A low-cost wireless sensing unit designed to collect data with high precision is adopted to monitor the response of an HPFRCC bridge pier column instrumented with a dense array of linear voltage displacement transducers (LVDTs), strain gages and accelerometers. The wireless sensing unit platform is shown to be capable of recording the response of a structural system with accuracies comparable to those of a laboratory data acquisition system employing traditional coaxial cables. INTRODUCTION Over the past three decades, great strides have been made in advancing fiber reinforced cementitious composite (FRCC) materials for use as a primary building material in civil structures. Fiber reinforced cementitious composites employ small fibers (steel, polymeric, carbon, among others) in a cement matrix to produce a material that has superior tensile properties in comparison to traditional concretes. To address the post-cracking strain softening behavior of ordinary FRCC loaded in tension, a new class of the material that exhibits tensile strain-hardening behavior has emerged in recent years. Termed high-performance fiber reinforced cementitious composites (HPFRCC), these novel materials contain a relatively low volume fraction (Vf < 2%) as a result of fiber geometry and fiber-matrix interface optimization [1]. When loaded in tension, the strain-hardening behavior of HPFRCC leads to the formation of a high-density of micro-cracks, as opposed to a few wide cracks typical of reinforced concrete structural components. Some performance features of HPFRCC materials include strength in shear and self-confinement. As a result, HPFRCC have been proposed for use in the design of earthquake-resistant structures prone to large inelastic deformations and/or high shear stress demands. Since concrete-based building codes require substantial steel reinforcement to provide concrete components with shear-strength and confinement, HPFRCC materials can be used to substantially reduce, or even eliminate, the amount of transverse reinforcement required in earthquake-resistant members. Additional material properties that render HPFRCC well suited for seimic Source: Proceedings of International Modal Analysis Conference (IMAC XXIII), Orlando, FL, January 31 – February 3, 2005. regions includes extreme ductility under both tension and compression and natural dissipation of large-amounts of energy when cyclically loaded. To date, a number of HPFRCC structural specimens have been tested in the laboratory setting, including beam-column connections, structural shear walls, coupling beams, and flexural members under high-shear [2]; with outstanding performance attained in the lab, there is growing interest in the use of HPFRCC in real structural designs. As HPFRCC begin to be applied in the field, opportunities exist to monitor their long-term performance under realistic loading scenarios not producable in the laboratory. Empirical response data recorded from HPFRCC structural components will lead to better understanding of the material’s merits and limitations. Wireless structural monitoring systems are an emerging sensor technology that can potentially be used to monitor the behavior of HPFRCC structural components in the field. Traditionally, structural monitoring systems have been designed to use coaxial cables for reliable communication between sensors installed in the structure and a centralized data repository where all measurement data is aggregated and processed. Suffering from high installation and maintenance costs, cable-based structural monitoring systems are largely reserved for use in critical structures (long-span bridges, dams and large structures) in regions of common seismic activity. To substantially reduce the costs associated with structural monitoring systems, wireless communications has been proposed for communication of sensor data in the system [3]. In addition to integrating wireless transceivers with sensors, microcontrollers are often included in the design of wireless sensors to operate the sensor and to locally process measurement data. This embedded computational power is capable of executing sophisticated data interrogation tasks such as those associated with system identification and damage detection analyses [4]. Wireless sensing units have made their way outside of the laboratory where they have proven themselves effective in monitoring the ambient and forced response of various field structures. For example, the Alamosa Canyon Bridge in New Mexico has been instrumented with seven wireless sensing units to monitor bridge accelerations to modal hammer blows and traffic loads. During the instrumentation study of the bridge, the wireless monitoring system was shown to have comparable accuracy to that of a commercial cable-based system [5]. A new low-power wireless sensing unit prototype, designed for use in structural monitoring systems, is proposed for monitoring the performance of HPFRCC structural components under extreme lateral loading. The study is intended to illustrate the potential of wireless sensing units to monitor the long-term behavior of structural components constructed of novel civil engineering materials, particularly in earthquake-prone regions. An HPFRCC bridge pier specimen is densely instrumented with linear voltage displacement transducers (LVDTs), strain gages and accelerometers (acting as inclinometers) to monitor the specimen’s response to displacement-controlled reversal cycles. The response data collected by the wireless sensing unit is compared to that collected by a traditional cable-based laboratory data acquisition system to validate its accuracy. This paper represents the first phase in a two-phase study that is exploring the fusion of low-cost wireless sensing technology with new building materials to monitor in-situ performance and automate the detection of structural damage consistent with material limit states. HPFRCC CIRCULAR BRIDGE PIERS Highways and bridge systems often require the use of tall piers and columns to support elevated roadways. In particular, circular pier sections are popular because they render the pier easy to construct and provide lateral strength independent of the loading direction [6]. In addition, spiral reinforcement is very effective in providing confinement to the concrete, enhancing member shear strength and displacement capacity. In the design of concrete bridge piers, longitudinal steel reinforcement bars are placed around the perimeter of the pier to provide flexural strength. Transverse reinforcement is then added to provide shear strength, confinement for the concrete, and to prevent or delay buckling of longitudinal reinforcement bars. For circular piers sections, the transverse steel reinforcement consists of a steel spiral that wraps around the longitudinal steel bars. When bridge piers are used in seismic regions, they must be designed to experience large inelastic rotations and, depending on their shear span-to-depth ratio, to also withstand large shear stress demands. For example, the California Department of Transportation (Caltrans) provides stringent seismic design criteria to ensure concrete bridge piers concentrate inelastic plastic deformation to well detailed plastic hinge regions [7]. The plastic hinge region is defined as the length of the column over which inelastic deformations occur, and is typically intended to form at the base of cantilevered bridge piers. Thus, at the base of the pier, tightly detailed transverse reinforcement is used since bridge piers containing widely spaced transverse reinforcement have been observed to fail in shear [8]. However, as greater amounts of steel reinforcement are required to provide sufficient shear strength and confinement, reinforcement congestion and concrete placing problems may arise that could impact the desired quality and performance of the pier in the plastic hinge region. While no official design guidelines have been adopted for HPFRCC materials, HPFRCC structural elements have been thoroughly studied in the laboratory. For example, HPFRCC have been used in the plastic hinge regions of flexural members unreinforced in shear. In laboratory tests, these HPFRCC flexural members sustained shear stresses as high as 2.8 MPa and plastic hinge rotations of up to 0.06 rad [9]. Test results like these motivate our interest in the use of HPFRCC for the design of cantilever bridge piers in earthquake-prone regions. The superior shear capacity and self-confining properties of HPFRCC materials render them ideal candidates for bridge piers designed to withstand the large displacement demands. LOW-COST WIRELESS STRUCTURAL SENSORS In recent years, wireless structural monitoring systems have been growing in popularity with current systems offering data acquisition quality comparable to that of traditional cable-based monitoring systems. The fundamental building block of a wireless structural monitoring system is the wireless sensing unit. Designed to acquire structural response data and to wirelessly transmit it to other wireless sensing units, the wireless sensing unit is not a sensor per se, but rather a node of a wireless data acquisition system to which analog sensors can be attached. This flexibility permits the wireless sensing unit to be used to record the response of structural systems using a plethora of sensing transducers (e.g. accelerometers, strain gages, among others). A large number of different wireless sensing units have been proposed for structural monitoring applications, but a new wireless sensing unit prototype proposed by Wang et al. [10] is selected for use in this study. This specific prototype has been optimized to attain the data acquisition performance demanded by structural monitoring applications and attains communication ranges consistent with the dimension of civil structures. With batteries the most probable power source in the field, the unit has been designed to minimize the overall power consumption of the unit so that battery life-expectancies are maximized. Designed from commercial off-the-shelf embedded system components, the wireless sensing unit’s architectural design is partitioned into three sub-systems. The first subsystem is the sensing interface which is responsible for the collection of response data from structural sensors attached to the wireless sensing unit. The sensing interface consists of the Texas Instruments ADS8341 analog-to-digital (A/D) converter. As a four channel A/D converter, four sensing transducers can be attached to the wireless sensing unit and read simultaneously. The conversion resolution of the ADS8341 is 16-bits; the A/D converter is also capable of collecting data at a maximum sample rate of 100 kHz. After data is collected and converted to a digital format by the sensing interface, measurement data is then managed by the unit’s computational core. As the second subsystem, the computational core is responsible for the aggregation of data and for local interrogation of measurement data using embedded numerical algorithms. The computational core is designed using the Atmel AVR ATmega 128 microcontroller. The ATmega 128 has been selected for this prototype because previous wireless sensing units proposed by Lynch [11] had been based on a different microcontroller within the Atmel AVR microcontroller family. The ATmega 128 operates at 8-MHz and is designed with an 8-bit architectural bus. With only 128 kB of flash memory (used for permanent storage of embedded software) and 4 kB random access memory (RAM) (used for temporary storage of measurement data), additional memory is needed. For extensive storage of time-history data an additional 128 kB of external RAM is included in the computational core design. The third subsystem is the wireless communication interface where data can be transmitted to and received from other wireless sensing units within the monitoring system. The Maxstream 9XCite wireless modem is selected for integration with the wireless sensing unit design. Operating on the 900 MHz radio frequency band, the modem is capable of data rates as high as 38400 bits per second and communication ranges of 300 m (line-of-sight). Unlike previous wireless sensing units proposed for structural monitoring, the Maxstream modem only consumes a moderate amount of battery energy during its operation. The complete architectural design of the wireless sensing unit prototype is presented in Figure 1. Maxstream 9XCite Modem 900 MHz Spread Spectrum Wireless Transciever Sensing Interface Computational Core Wireless Communication S tru ct ur al S en so rs (A na lo g O ut pu ts 0 -5 V ) SPI Port UART Port Texas Instruments ADS8341 4-Channel 16-bit Analog-to-Digital Converter 8-bit Atmel AVR Microcontroller Cypress CY62128 128 kB SRAM Parallel Port Figure 1 – Architectural design of the wireless sensing unit proposed by Wang et al. [10]