MONITORING OF CONCRETE CURING USING NANOTECHNOLOGY/MEMS DEVICES

Transcription

1 MONITORING OF CONCRETE CURING USING NANOTECHNOLOGY/MEMS DEVICES Mohamed Saafi and Peter Romine Alabama A&M University USA Abstract Early age concrete properties and strength gain are mainly related to the hydration of cement and moisture transfer. In this paper we will present a new technique that is currently under development for monitoring concrete setting and hardening. The new monitoring technique consists of embedding Nanotechnology/MicroElectroMechanical System (MEMS) devices into concrete during construction to wirelessly monitor certain key parameters affecting the early age behavior of concrete structures such as in-place strength and shrinkage stresses. To test the feasibility of the proposed devices in monitoring temperature, moisture and early age shrinkage as well as to evaluate their durability when embedded into concrete, experimental tests were performed and preliminary results are presented in this paper. 1. Introduction Accurate field measurements of early age concrete properties, such as setting time, in-place strength gain and shrinkage stresses are crucial to operation scheduling and in situ quality control of concrete. For example, reliable measurements of in-place concrete strength can be used to determine when adequate strength has been obtained to allow removal of forms, shoring, transfer of prestressing, application of post-tensioning, and opening highways to traffic. In-place strength information allows each of these operations to proceed safely and at the earliest possible time leading to more economical construction. The development of concrete strength at early age mainly depends on the moisture diffusion and hydration temperature. Rapid loss of moisture could lead to insufficient strength development which in return could lead to the development of damage in concrete structures prior to loading [1]. At early age, concrete structures can also be subjected to deformation during the hydration process mainly due to moisture loss and hydration temperature [2]. If in the cooling phase, contraction of the concrete is hindered, tensile stresses are generated in the structure. These tensile stresses cause cracking when exceeding the tensile strength of the hardening concrete. Consequently, in situ measuring of early age shrinkage stresses can be used to estimate when shrinkage stresses begin to be generated as well as to estimate self-stressing and risk of premature cracking. When structures are constructed in successive phases, these measurements can help to improve the composition of concrete when necessary. Consequently, it is very important SAAFI, Nanotechnology, 1/14

2 to monitor the temperature and moisture distribution in concrete structures and shrinkage stresses to evaluate the setting and hardening process of Portland cement concrete for early age concrete strength gain monitoring and to evaluate the severity of shrinkage induced cracks. 2. Background Currently there are a few techniques in use for estimating the in-place strength gain and shrinkage stresses in concrete structures. The Windsor and Pullout probe tests are simple tests that are currently popular in the field. The requirements of these tests are given in ASTMC 803 [3] and ASTM C 900 [4] respectively. The Pullout test measures the ultimate load required to pull an embedded metal insert in the concrete structure. The Windsor probe test estimates the strength of concrete from the depth of penetration of a metal rod driven by a standard charge of powder. Another method for determining the setting behavior of concrete is the use of ultrasonic waves where velocity, amplitudes and frequencies have been used as an indicator of the change in elastic properties of concrete and hence the state of concrete [5,6]. However, the application of ultrasonic pulse velocity to determine the state of hydration in the field has limited success because this technique requires access to opposite sides of the structure. Another method such as the impact-echo method was used [7] to compare the measured wave velocity with the measured penetration resistance. The setting of concrete was defined as when the velocity begins to increase or when a specified wave velocity is reached. This method, however, may not be practical for early age concrete because concrete is in the form of viscous suspension at its very early age and thus be hard to impact. Microwave methods have also been used [8] to predict the in-place strength gain of concrete structures. This method uses microwaves to monitor changes in the dielectric properties produced by cement hydration. The dielectric properties of concrete provide an indication of the amount of moisture present in the system. The dielectric properties are then correlated with strength gain for a given concrete mixture in the laboratory. Concrete strength gain was also evaluated using the maturity method [9]. The hydration process of concrete is an exothermic reaction and the maturity method takes advantage of this fact to evaluate the strength development at early ages based on the temperature profile of concrete. The maturity profile is developed for concrete with given mixture proportions, from the observed temperature changes and the measured compressive strength obtained in the laboratory. The maturity method requires wired thermocouples to measure temperature mainly used to determine when concrete achieves minimum threshold strength to open the bridge deck to traffic. An ultrasonic technique that continuously monitors the setting and hardening of concrete has recently been developed [10]. The experimental procedure is based on high frequency ultrasonic measurements and consists of monitoring the wave reflection factor (WRF) at the interface between a steel surface and concrete. The WRF technique is a one-sided technique and uses correlation between the measured trends of WRF, elastic modulus and the hydration process of cement. In situ measurement of shrinkage induced stresses of concrete material at early age i.e. between 0 and 12 hours after mixing is a challenging task, and therefore, most shrinkage deformation have been measured starting from the time of removable of formworks using strain gauges and other devices. For in situ shrinkage deformation measurements, fiber optic sensors were recently explored [11]. When embedded into fresh concrete, shrinkage stresses shift the wavelength of the Bragg grating and when the fiber optic sensor is calibrated the shift in wavelength can be used to measure concrete deformation at early age. However, these sensors detect only strain within their own area and, moreover, since most available fiber optic sensors are limited in sensing cage lengths they can only detect local strain. Consequently, it is necessary to employ numerous sensing gauges to detect strain widely in a large-size structure leading to a complicated and SAAFI, Nanotechnology, 2/14

3 expensive monitoring system. The cost of fiber optic sensors ($300/sensor) and their interrogation systems (approximately $50,000) prevented the use of these sensors in Civil Engineering as a standard method in material testing. All the methods described above have received little interest from the construction community mainly due to their serious drawbacks. These methods use extensive wiring systems to operate and necessitate special equipment to gain access to the structure during construction. In addition, these techniques perform localized measurements and the monitoring of large bridge structures and long pavement roads requires an extensive amount of time and effort leading to costly usage. Moreover, the current monitoring systems can be used only at certain and limited accessible locations in bridge decks, thus the removable of formworks, self-weight support and traffic opening is decided based on limited and insufficient information, compromising the structure s safety. Substructures such as concrete foundations and piles are inaccessible and their early strength cannot be evaluated using these monitoring systems. Finally, the systems described herein are based on surface measurements and do not measure the true in situ properties of the structure at early age such as moisture transfer, water-to-cement ratio and shrinkage induced stresses. Therefore, the objective of this paper is to introduce a cost-effective and reliable nondestructive technique using micro devices for early age monitoring of concrete structures. The devices are extremely cheap and are interrogated wirelessly to measure certain key parameters affecting the behavior of concrete at early age such as temperature, moisture and shrinkage stresses. Preliminary data for proof of concept is presented here. 3. Proposed Nanotechnology/MEMS Devices for Concrete Material Monitoring The proposed monitoring technique uses nanotechnology/mems to develop low cost, wireless devices that will be embedded into concrete to monitor the curing process and detect damages. Nanotechnology/MEMS refer to a collection of nano-micro-sensors and actuators, which can both, sense its environment and have the ability to react to changes in that environment with the use of microcircuit control. They include, in addition to the conventional microelectronics packaging, integrating antenna structures for command signals into micro-electromechanical structures for desired sensing and actuating functions. MEMS combine the signal processing and computational capability of analog and digital integrated circuits with a wide variety of nonelectrical elements, including pressure, temperature, chemical, stress/strain and accelerometer sensors. Nanotechnology/MEMS sensing techniques brings three advantages to its applications to civil infrastructures: miniaturization, multiple components and microelectronics. This technique offers promise for advancing both Non Destructive Evaluation (NDE) and condition based maintenance of structures. Another advantage is that the small size of sensors (typically less than 1 mm to as small as a few nanometers), allows them to be embedded into concrete structures without compromising the geometry, weight or inherent properties of structures. Nanotechnology/MEMS sensors contain some mechanical elements of which certain properties depend on specific (physical) environmental conditions. These elements are made to translate, deflect or deform as a result of a stimulus from the environment (physical or chemical) or an electrical signal. The most common mechanical elements are beams and cantilever beams and their deflection can be detected by measuring the stresses at the anchor point using doped piezoresistive regions or by measuring the change in the capacitance between some point on the beam and point opposite. These sensors also contain other components in the form of integrated circuit (IC) microsensors for example temperature, biological, chemical sensors, etc. These devices can detect the presence and concentration of organisms and compounds by measuring their effect on detectable parameters, such as resistance or capacitance, on selectively reactive or absorptive materials deposited on the device. Nanotechnology/MEMS technology represents SAAFI, Nanotechnology, 3/14

4 great potential application in concrete structures. For example, the use of MEMS sensors in concrete structures would increase the understanding of hydration and damage evolution. MEMS sensors could be used to detect crack initiation in specimens under loading or to monitor the setting of concrete by detecting the development of hydration products. The incorporation of these sensors in concrete structures would allow for real-time monitoring of the structure, providing information on durability and early damage detection. The proposed monitoring system consists of embedding MEMS devices into concrete materials during construction to continuously and wirelessly monitor the performance of concrete structures during curing (such as temperature, moisture and shrinkage stresses) as well as when these structures are put in service. The main advantage of the proposed monitoring technique is the low cost and the wireless sensing capability. Due to their batch fabrication, the current cost of each proposed sensor is $30 and the cost of the wireless interrogation system is approximately $800. However, it is expected that after full development of this monitoring technique, the cost of the devices will significantly be reduced to $5 per device and to $400 for the wireless interrogation system. Figure 1 illustrates the proposed wireless monitoring system. This system includes an interrogator probe sensor device using wireless communication and a circuitry capable for performing this function. The interrogator includes a computer, a transceiver and an interrogator antenna. The communication between the interrogator and the embedded system is carried out by a radio frequency (RF) radiation. The embedded system contains rectifier, modulator, memory with identification (ID), logic, MEMS sensors and antenna. Figure 1: Proposed Wireless System for Concrete Curing Monitoring The MEMS sensors are being designed to measure crucial parameters affecting the performance of concrete during curing such as moisture, temperature and shrinkage stresses. Each embedded device including electronic components and communication antenna will be packaged in the form SAAFI, Nanotechnology, 4/14

5 of an aggregate with a size of approximately 15 mm in diameter. These smart aggregates are passive and do not require power. During operation, the interrogator s transceiver emits high frequency electromagnetic wave signals to the embedded devices. Each embedded device antenna receives the RF signal and rectifies it to obtain a DC power using rectifier. The rectified power is used to power the MEMS sensor, which makes a measurement of the current temperature, moisture and stress. The logic also receives power for the rectifier, reads memory for the sensor device s ID, reads any MEMS data or reads data in a memory component used to store an event, and provides a digital signal to the modulator. The modulator modulates the antenna backscatter in response to the interrogation signal from interrogator. The antenna and the transceiver of the interrogator obtain the response signal from the embedded device. The response signal includes ID and sensor data from the embedded sensor device, and reports the ID and sensor data to the computer. The identification (ID) tag is used to identify a particular device providing the response. This allows an embedded device to be distinguished from a number or other devices as would be encountered in an array of devices on a system. The identifier tags are small devices that contain an identification code that can be read remotely using the interrogator. In the case of an array of sensors, the idea is to sense one or more parameters of interest at one or more locations, such as various levels of moisture at graduating depths in a concrete structure, and then read sensor data out along with the device's identification code. The ID code and digital response of the embedded device also provides a means of automatically logging data entry corresponding to the status of each sensor device. This may also include logging the corresponding location in the structure. In some cases, the position and depth of each embedded device in a structure may be deduced from the amplitude and phase variations of the communicated signal. These quantities vary with depth, material permittivity and permeability, and moisture content. The curing monitoring system may thus also include algorithms that determine the depth and position of each embedded device within a structure such as the depth and position of device in concrete. The remote monitoring can be carried out in different ways using the wireless monitoring instrument. For example, Figure 2 shows a typical monitoring technique for a pavement. An interrogator is being carried by a vehicle, such as a highway maintenance car, truck or any other suitable moving vehicle. The interrogator includes an antenna disposed below the midsection of the vehicle that allows wireless communication with the sensors embedded within the roadway. Radio frequency power, transmitted on an RF wave from the interrogator is used to power wireless transponders in each sensor device. The radio frequency power may also be used to power a sensor and sensor reading, as well as logic in the device included in a processor or microchip. Each device may then be individually powered and interrogated as the vehicle passes. Logic in each device may then tell the sensor device to return its identification code and a sensor reading to interrogator. The interrogation using interrogator and the vehicle comprises driving the truck over a roadway at a suitable speed. The upper speed of vehicle may depend on processing delays to send and receive a signal from a particular device. These delays may include a delay for a probing signal to reach a particular device, a delay for the device to generate a response, a delay for the response signal to reach interrogator, and any other processing or wireless transmission delays. Regardless of the speed of the vehicle, inspection of devices using the proposed system allows convenient (highway personnel sitting in the vehicle) and less-intrusive (to traffic) methods of road inspection. The vehicle also can include a twoantenna system containing a transmitting antenna at the front of the vehicle and receiving antenna disposed at the rear. The two antenna system allows a wireless device within the roadway to receive a probing signal from the front transmitting antenna, generate a response, and transmit a response signal back to the rear receiving antenna. This two-antenna system allows for increased speed of the vehicle since delays in communicating with a device are compensated by the distance between the front and rear antennas. The monitoring system shown in Figure 2 reduces the time and costs of roadway curing inspections. In some cases, the sensor devices respond to an SAAFI, Nanotechnology, 5/14

6 interrogator in the range of tens of milliseconds. Regardless of the response time of a sensor device, the ability to probe the devices with a truck or other moving vehicle significantly reduces time and costs of inspections. Figure 3 shows another possible monitoring technique for a bridge deck. The proposed system relies on a hand-held or portable interrogator carried by a person. Using wireless techniques, the interrogator communicates with sensing devices. The interrogator produces a probing signal that penetrates into concrete. In response to the probing signal from interrogator, each device makes a sensor reading. Circuitry within device may convert the sensor measurement into a signal output by a transponder in the device. The transponder then returns a response signal to interrogator that includes the device's ID and the sensor reading. The interrogator thus allows a person to poll the embedded devices and obtain moisture, temperature and shrinkage stresses in a convenient manner. Figure 4 shows a monitoring technique for a pile foundation, where a compact hand-held system can be used to interrogate the MEMS sensors embedded into a pile for curing and concrete setting monitoring. Figure 2: Curing Monitoring of a Pavement Road Figure 3: Curing Monitoring of Bridge Deck Figure 4: Concrete Pile Curing Monitoring 4. Preliminary Experiments Preliminary experiments were carried out to evaluate the feasibility of embedding MEMS devices into concrete for moisture, temperature and stress monitoring as well as to evaluate the durability and survivability of these devices in a concrete corrosive environment. The curing and damage process of concrete is significantly affected by the temperature and moisture level. Therefore, a sensor system for continuous monitoring of internal relative humidity and temperature is highly desirable for both during construction and afterwards. The proposed MEMS sensor for internal humidity and temperature consists of four bulk micromachined microcantilever beams (120 x 380 microns) capable of measuring simultaneously temperature and moisture in materials (Figure 5). The MEMS sensor is based on a shear stress principal for measuring water vapor, in which the microsensor chip combines a proprietary polymer sensing element and Weatstone Bridge piezoresistor circuit to deliver two DC output voltages that are linearly proportional to relative humidity from 0 to 100% RH full scale and to temperatures from 30 o C to +100 o C. A water vapor sensitive nanopolymer film is bonded to the top of each cantilever beam and designed to SAAFI, Nanotechnology, 6/14

7 expand and contract during exposure to water vapor. In addition, each microcantilever beam contains an embedded nanostrain gauge (resistor) to measure the beam stress by the piezoresistive effect. The four strain gauges are electrically connected together into a Wheatstone Bridge circuit configuration directly on the sensor chip. Bridge excitation is provided by a 1.2V DC voltage source that is external to the sensor chip. During sensor operation, ambient water vapor molecules in the material absorb into the sensing film surface producing shear stresses at the film/beam interface which cause the cantilever beam to deflect. This deflection is measured as a resistance change in the embedded strain gauges and is linearly proportional to the shear stress. Consequently, the water vapor concentration is transduced into a proportional differential R voltage change in the bridge circuit. The change in the resistance of the sensor due to R moisture is given by the following equation: R 1 3 k σ s k R Eh Eh Eh = + = 4 σ s (1) where k is sensor gage factor, E and h are the modulus of elasticity and the thickness of the cantilever beam respectively and σ s is the moisture induced shear stress at the film/beam interface. To measure temperature, an on-chip semiconductor (thermistor) temperature sensor is bonded to the top of the chip. The MEMS chip is packaged in the form of an aggregate to resist environmental attacks where the die is protected by a polymeric coating, and the chip is embedded in a protective jacket. The size of the packaged sensor is 5 mm in diameter and 4 mm in thickness (Figure 5). As shown in Figure 6, the MEMS sensor for concrete shrinkage stress monitoring is composed of eight nanosensor rosettes patterned on n-type (111) silicon membrane of 4 mm x 4 mm with a thickness of 8 µm and an on-chip temperature. The sensor is capable of measuring principal and shear stresses in concrete material. This eight-element dual-polarity rosette contains p-type and o n-type sensor sets, each with resistor elements making angles of φ = 0, ± 45,90 with respect to the horizontal axis. The eight sensors are electrically connected together into a Wheatstone Bridge circuit configuration directly on the sensor chip. Bridge excitation is provided by a 1 DC voltage source that is external to the sensor chip. Silicon was used to produce the MEMS sensor with a size of 1mm x 1mm as shown in Figure 6. The stress measuring is based on the piezoresistive theory. To measure temperature, an on-chip semiconductor (thermistor) temperature sensor is bonded to the top of the chip. The MEMS chip is packaged in the form of an aggregate to resist environmental attacks where the die is protected by a polymeric coating, and the chip is embedded in a protective jacket. The size of the packaged sensor is 5 mm by 5 mm and 2 mm in thickness (Figure 6). The manufacturing of the sensors was performed using a combination of standard and customized semiconductor processing steps. Manufacturing begins with standard Complimentary Metal Oxide Semiconductor (CMOS) processing steps. Each microsensor starts out as a blank silicon wafer. Typical CMOS steps performed on the wafer include chemical vapor deposition (CVD), oxidation, doping, diffusion and metallization. Photolithography and chemical wet etching are used to pattern and form the silicon platform and measurement structures of the sensor. A hybrid CMOS process is performed to deposit, pattern and activate the polymeric sensing element. The last hybrid process step is plasma etch to release the micro-cantilever beams from the surrounding silicon. Commercial microelectronics and related packaging technologies is used to provide SAAFI, Nanotechnology, 7/14

8 sensor die in a package that is easy to handle with existing printed circuit board assembly equipment as well as easy to package with other micro-devices. Figure 6: MEMS Sensor for Moisture and Temperature Monitoring Figure 7: MEMS Sensor for Shrinkage Stress Measuring Concrete cylinders of 152 mm by 305 mm with water to cement ratio of 0.45 and 0.53 were used to evaluate the performance of the MEMS moisture/temperature sensor. Each MEMS was SAAFI, Nanotechnology, 8/14

9 embedded in the middle of the specimen and placed in a curing room at room temperature of approximately 21 o C at RH = 50. In addition, concrete beams of 100 mm by 100 mm by 450 mm with a water to cement ratio of 0.5 were used to evaluate the capability of the MEMS stress sensor in measuring shrinkage stresses. Each stress sensor was embedded in the middle of each beam to measure longitudinal and transversal shrinkage stresses. The concrete beams were removed from molds after 5 hours of curing to accelerate the shrinkage. The moisture, temperature and shrinkage stresses were monitored continuously using a data acquisition system with wire connections for proof of concept. 5. Preliminary Results Figure 8 and 9 shows typical voltage output of the sensor for the first 20 days of curing. Concrete temperature and room temperature recorded by thermocouples are also shown in the figures. As can be seen in Figure 8, the temperature output voltage increased to its maximum of 0.15V after 10 hours of curing. This maximum voltage corresponds to the peak of the maximum temperature generated by the hydrated cement paste. The output voltage then decreased and remained constant with voltage peaks due to the change in the room temperature as evidenced by the thermocouple outputs. Figure 8 revealed that the sensor output and concrete temperature recorded by the thermocouple exhibited a similar trend, indicating that a relationship between the temperature and the sensor voltage output exist, and this unique property will be used to calibrate the MEMS temperature sensor. Figure 9 shows the response of the MEMS humidity sensor where the demolding initiated a brief drop in the sensor output at a time of 1 day. After the temperature and vapor pressure reached equilibrium, the output voltage increased reaching its maximum after 2 days before gradually decreasing as drying occurred. Figure 9 also shows that the internal concrete temperature affected the sensor output where the sudden drop in the internal concrete temperature generated sensor output voltage peaks. Experimental tests are currently underway to evaluate the sensor s sensitivity and drift as well as to develop calibration equations where the effect of the temperature will be incorporated. The MEMS temperature and humidity responses are shown in Figures 10 and 11 for a water to cement ratio of 0.45 and As expected, specimens with water to cement ratio of 0.53 exhibited a higher voltage response during the hydration process than those with a water to cement ratio of However, water to cement ratio appeared to have no significant influence on the temperature sensor response after the main hydration process is completed. Water to cement ratio has no effect on the MEMS humidity response during the first 48 hours, however sensor outputs decreased faster in specimens with water to cement ratio of This response is attributed to the rapid loss of water from specimens with relatively low water to cement ratios. SAAFI, Nanotechnology, 9/14

10 Figure 8: Typical MEMS Temperature Sensor Response Figure 9: Typical MEMS Humidity Sensor Response SAAFI, Nanotechnology, 10/14

11 Figure 10: MEMS Temperature Sensor Response for Different Water/Cement Ratios Figure 11: MEMS Humidity Sensor Response for Different Water/Cement Ratios SAAFI, Nanotechnology, 11/14

12 The response of the embedded MEMS stress sensor due to free shrinkage is given by Figure 12. In this Figure, the change in the resistance of the resistors and concrete temperature are presented. As can be seen, there was a positive increase in the resistance of each resistor up to a maximum value, then remained constant for a period of 10 hours before gradually decreasing (negative increase) as drying occurred. The increase in the resistances is attributed to the expansion of the cement paste due to the hydration temperature (ettringite formation), whereas the decrease in the resistances is attributed to the shrinkage of the concrete due to loss of moisture. Figure 12 shows that the maximum positive changes in the resistance occurred when the hydration temperature reached its maximum and the rate of change in these resistances depends mainly on the orientation of the resistors within the concrete. Resistors parallel to the longitudinal axes of the beam (angle = 0 o ) exhibited the highest decrease in their resistance due to shrinkage and resistors parallel to the transversal axis of the beams exhibited the lowest decrease. Resistor orientation had a slight effect on the positive change during expansion. Figure 12: MEMS Stress Sensor Response The change in each resistor was used to calculate the principal shrinkage stresses using the piezoresistive theory developed for the sensor as shown in Figure 13. As shown, a brief drop in the tensile stresses σ 1 and σ 2 (stresses due to cement expansion) occurred when the specimens were removed from the molds. After the temperature and vapor pressure reached equilibrium, the measured tensile stresses increased slightly before gradually decreasing indicating that the shrinkage began thereby generating internal compressive stresses and these compressive stresses increased as the concrete dried. The compressive stress in the longitudinal axis increased more rapidly than that in the transversal axis. SAAFI, Nanotechnology, 12/14

13 Figure 13: Measured Shrinkage Stresses 6. Conclusions In this paper, a cost effective wireless monitoring technique to monitor the behavior of concrete at early age is presented. Preliminary results indicated that the proposed nanotechnology/mems sensors have a great potential application in concrete structure to continuously monitor their behavior with a minimum of cost and labor. Based on the obtained experimental results it was found that the proposed MEMS survived the concrete corrosive environment and internal and external stresses. Also it was found that the MEMS outputs reflect the change in the concrete properties and can be used to measure moisture content, temperature and shrinkage-induced stresses with a high sensitivity. Research is currently in progress to further evaluate the durability of the sensors and determine their sensitivity and calibration equations. Analytical models to predict the strength gain using the sensors outputs as well as the wireless monitoring technique are also currently being developed for field application. 7. Acknowledgement The authors would like to acknowledge the financial support of the National Science Foundation Grant CMS References 1. Kim, J. K. and Lee, C.S., Moisture diffusion of concrete considering self-dessication at early ages, Cement and Concrete Research, 29 (2000) Persson, B., Self-desiccation and its importance in concrete technology, Materials and Structures 30 (199) (1997) ASTM C803, Standard test for penetration resistance of hardened concrete, Annual Book of ASTM Standards (1994) SAAFI, Nanotechnology, 13/14

14 4. ASTM C900, Standard test method for pull out strength of hardened concrete, Annual Book of ASTM Standards (1994) Boutin, C. and Arnaud, L., Mechanical characterization of heterogeneous materials during setting, European Journal of Mechanics 14 (4) (1995) Keating, J., Hannant, D.J. and Hipper, A.P., Correlation between cube strength, Ultrasonic pulse-velocity and volume change for oil-well cement slurries, Cement and Concrete Research, 19 (1989) Pessiki, S.P. and Carino, N.J., Setting time and strength of concrete using the impact-echo method, ACI Material Journal 85 (1988) Moukwa, M., Brodwin, M., Christo, S., Chang, J. and Shah, S.P., The influence of hydration process upon microwave properties of cements, Cement and Concrete Research 21 (1991) Carino, N.J., The maturity functions for concrete made with various cements and admixtures, ACI Material Journal 89 2 (1992) Kolluru, V.S., Mohsen, J.P., Shah, S.P., Ultrasonic technique for monitoring concrete gain at early age, ACI Material Journal 99 5 (2000) Slowik, V., Schlattner, E. and Klink, T., Experimental investigation into early age shrinkage of cement paste by using fibre Bragg gratings Cement and Concrete Composite (in press). SAAFI, Nanotechnology, 14/14