A thermal comparator sensor for measuring autogenous deformation in hardening Portland cement paste

Transcription

1 Materials and Structures / Matériaux et Constructions, Vol. 36, December 2003, pp A thermal comparator sensor for measuring autogenous deformation in hardening Portland cement paste T. Østergaard and O.M. Jensen Department of Building Technology and Structural Engineering, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark ABSTRACT This paper describes a simple and accurate experimental device specially developed to measure autogenous deformation in hardening cement-based materials. The measuring system consists of a so-called thermal comparator sensor and a modular thermostatically controlled system. The operating principle of the thermal comparator is based on thermal expansion of aluminium. A particular characteristic of the measuring system is the fixation of the thermal comparator sensor to the deforming specimen. The modular system ensures effective thermostatic control of the hydrating cement paste samples. The technique allows continuous measurement with high accuracy of the linear deformation as well as determination of the activation energy of autogenous deformation. RÉSUMÉ Cet article décrit un appareil expérimental simple et précis spécialement conçu pour mesurer le retrait endogène dans des matériaux durcissants à base de béton. Le système de mesure consiste en un capteur thermique de comparaison et un système modulaire contrôlé de façon thermostatique. Le principe opératoire du capteur thermique est basé sur l expansion thermique de l aluminium. L une des caractéristiques particulières du système de mesure est la fixation du capteur thermique à l échantillon déformant. Le système modulaire assure un contrôle thermostatique des échantillons de pâtes de ciment hydratantes. La technique permet une mesure continue, de haute précision, de la déformation linéaire ainsi que la détermination de l énergie d activation du retrait endogène. 1. INTRODUCTION As the use of high-performance concrete, HPC, has increased, understanding of early age deformations of concretes has become more important [1-3]. Of special interest for HPC is the autogenous shrinkage and measurements of this. Different methods of measuring autogenous shrinkage exist. These methods can be allocated in two types of measurements: Volumetric and linear. Jensen [4] discusses these measuring techniques and suggests the linear technique as the most suitable for measuring autogenous deformation in hardening cement paste. For this purpose a special mould system consisting of corrugated tubes is suggested. Before set the corrugated moulds transform volumetric deformation into a linear deformation, and after set the corrugated tube allows measurement of linear deformation with negligible restraint [3]. 2. LINEAR DISPLACEMENT SENSORS For a specific application the selection of a sensor is based on its characteristics such as price, robustness, etc. Examples of linear displacement sensors are presented below [5]. 2.1 Potentiometric sensors The operating principle of potentiometric sensors is based on wire resistance, which relates linearly to the wire length. Editorial Note Prof. Ole Meilhede Jensen is a RILEM Senior Member and the Secretary of RILEM TC 196-ICC Internal curing of concretes and participates in RILEM TC 195-DTD Recommendation for test methods for autogenous deformation and thermal dilation of early age concrete. He is also a member of RILEM TAC (Technical Activities Committee) /03 RILEM 661

2 Østergaard, Jensen The displacement is coupled to a potentiometer, to measure resistance change caused by the wire length change. A good coil potentiometer can provide an average resolution of about 0.1% of the input full scale, while high-quality resistive film potentiometers may yield an infinitesimal resolution. Potentiometric sensors are simple and useful in some applications, but are based on a mechanical system leading to drawbacks such as motion resistance and heating of the sensor caused by excitation and friction. 2.2 Capacitive sensors A capacitive displacement sensor operates on a ratiometric principle involving two capacitors. The displacement results in a geometry change of the capacitor system and the displacement is thereby transformed into an electrical signal. Capacitive sensors are simple to use with the displacement expressed in terms of a voltage. However, one drawback is that the output is slightly influenced by the relative humidity. 2.3 Inductive sensors Inductive sensors operate based on electromagnetic induction. A magnetic flux coupling between two coils is altered by the movement of an object and subsequently converted into voltage. One common example of an inductive sensor is the Linear Variable Differential Transformer (LVDT). The advantages include a low susceptibility to noise, solid and robust construction, and infinitesimal resolution is possible. However, one drawback of LVDT s is that they require a mechanical coupling to the deforming specimen. 2.4 Optical sensors An optical displacement sensor may function based on a triangulation principle. A focused light beam forms a spot on the object. The image of this spot is recorded and this position is an indication of the distance to the object. The main advantages of optical displacement sensors are simplicity, the absence of the loading effect, and relatively long operating distances, which make them suitable for many sensitive applications. However, their main drawbacks are temperature range limitations as well as high cost for high resolution. 3. THE THERMAL COMPARATOR The thermal comparator sensor described in this paper is a simple and low cost sensor, specially designed for the linear measurement of autogenous deformation and thermal deformation of cement paste encapsulated in corrugated moulds. The thermal comparator sensor was developed during a recent project [6]. 3.1 Construction and mode of operation The thermal comparator sensor is shown in Fig. 1. It consists of an aluminium (Al) tube with a contact pin at the end, and a quartz tube with an adjustable screw at the end. The contact pin and the screw form a heat switch for the active temperature control of the Al tube. The Al tube is placed inside the quartz tube separated by Teflon bearings in order to ensure low friction. The Al tube and the quartz tube are separately fixated in opposite ends with a brace to the corrugated tube. Typically, positions of the braces are as shown in Fig. 1, about 350 mm apart. The Al tube is bifilar wound with a Ø 0.2 mm, 48 constantan thread on the outside for controlled, active heating. The working temperature range for the Al tube is typically 25 to 70 C. On the inside of the Al tube two thermocouples are placed at each third point for determination of the average Al temperature. The heat switch is connected with the constantan thread through a solid-state relay. Heating is on when there is no contact in the switch. This results in a temperature rise of the Al tube and, consequently, a thermal expansion. When contact is reached in the switch, heating is turned off. Measurements on a shrinking sample result in a gradual temperature lowering of the Al tube. If the temperature of the Al tube approaches the ambient room temperature it may be necessary to adjust the position of the heat switch. This is done with the screw, which allows the Al tube to heat up. The aluminium used for the thermal comparator is a 5052 alloy with a thermal expansion of 23.8 m/m K in the temperature range 20 C C [7]. In Fig. 1 the fixation of the quartz tube is placed directly under the heat switch. This eliminates the influence from thermal expansion of quartz. However, it is possible to reduce the distance between the two fixations without a noticeable contribution from quartz, as the thermal expansion coefficient of aluminium is approx. 40 times larger than the one of quartz. Fig. 1 The thermal comparator mounted on a corrugated tube filled with cement paste. When there is contact in the heat switch a solidstate relay cuts off heating of the aluminium tube, until contact is lost. Based on simple temperature measurements in the Al tube the deformation of the specimen is registered. 662

3 Materials and Structures / Matériaux et Constructions, Vol. 36, December 2003 Deformation of the specimen, s (m/m), is as a very good approximation assumed to be proportional to the temperature change of the Al tube: l e T (1) Al s Al Al ls where, l Al [m] is the distance between the fixation of the aluminium tube and the heat switch, l s [m] is the distance between the two fixation points, Al [m/m K] is the thermal expansion coefficient for the aluminium and T Al [K] is the temperature change of the Al tube. For the set-up shown in Fig. 1 Equation (1) transforms into: e 23.8 T µm/m (2) s Al The working temperature range is approximately 45 C, which equals a linear full scale of about 1100 µm/m for the set-up shown in Fig. 1. However, by moving the position of the quartz tube fixation the linear full scale of the thermal comparator sensor can be increased. If the quartz tube fixation is positioned at the middle of the corrugated tube the linear full scale is doubled to more than 2000 µm/m, cf. Equation (1). Especially before setting the deformation of cement paste will exceed the linear full scale [4]. To measure these deformations adjustment of the heat switch is thus necessary. However, this may not be required for the measurement of autogenous deformation of the set cement paste, or for the measurement of activation energy of autogenous deformation, since these deformations are much smaller. In any case a redesign of the heat switch may enable an automatic reset facility. The apparatus is set up in a module system, where a row of measuring units can run simultaneously. One or more units can be removed or reinserted during measurement without interfering with the remaining units. In each unit thermostated water flows along the specimen for effective thermostatic control. The temperature of the water is controlled by an adjustable heat source and by cold-water flow. This allows a very fast and controlled change in temperature of the samples, as is needed e.g. for measuring the activation energy of autogenous shrinkage. The thermostatic control is shown in Fig. 2. A special thermoformed plastic lid is positioned between the thermal comparator and the water. This minimizes Fig. 3 Three measuring units with thermal comparators. Each unit can be operated individually. A Pt-100 reference temperature is also measured for increased accuracy. evaporation of the water and thermal interaction between the thermal comparator and the sample. Three units with thermal comparators mounted on top of hardening cement paste samples are shown in Fig Heat switch A well-defined geometry and stable, inert surface of the heat switch are important to ensure noise-free measurements. An example of a measurement with a rough surface of the heat switch is presented in Fig. 4. The Fig. 2 Sketch of thermostated water flow. The temperature of the thermostated water is controlled by cold-water flow and an adjustable heat source. Thermostated water flows along the specimen. Fig. 4 Measured unrestrained deformation during sealed hardening of cement paste with w/c = 0.3 and 20% silica fume addition (top curve). During the 11 days of measurement the temperature was varied between 18 C to 22 C as shown by the bottom curve. The measurement was started after 24 hours of sealed curing at 20 C. The severe noise on the deformation measurements was caused by a rough surface of the heat switch. 663

4 Østergaard, Jensen Fig. 5 The surface texture and material of the heat switch is of great importance to the quality of the measurements. The sketch shows a heat switch with a rough surface of the screw. A vertical movement of the pin during measurement will lead to noise on the readings, since electrical contact will be reached at many different positions. measured deformation curve has a large amount of noise, and even the temperature change after 9 days is not reflected in a measured deformation. A close-up of the malfunctioning heat switch is shown in Fig. 5. In the final design of the heat switch the screw has a plane end surface and the contact pin is spherically formed at the end. In addition, the screw and the contact pin are gold plated. The measurements shown in the following were carried out with this heat switch, improving the quality of the measurements significantly. 4. MEASUREMENTS Fig. 6 shows simultaneous measurements on two identical specimens, illustrating the repeatability of the measuring technique. In the experiment the temperature was abruptly changed at several points during the hydration course for calculation of activation energies. The deformation was defined as 0 at 24 hours. Note that a large part of the autogenous shrinkage already occurred before this time [3, 4]. Time of set was 8.5 hours after mixing. The activation energy of autogenous shrinkage was calculated based on Arrhenius equation: E a t i t t ti R ln ti 1 1 T2 T1 where R [8.314 J/mol K] is the gas constant, t i [h] is the time of temperature change, T 1 and T 2 [K] are the temperatures immediately before and after t i, and t [m/m h] is the deformation rate. The calculated activation energies are presented in Fig. 6 at each temperature change. The temperatures of the thermostated water and in the centre of the cement paste specimen are shown in Fig. 6. The temperature of the cement paste is higher due to heat of hydration. A close-up of the temperature curves after 7 days is shown in Fig. 7. A 4 C temperature change of the thermostated water takes approx. 20 min., whereas the (3) Fig. 6 Autogenous deformation measured simultaneously on two identical cement pastes with w/c = 0.3 and 20% silica fume addition (top curves). During the measurement the temperature was varied between 18 C to 22 C to enable determination of activation energy. Two almost identical simultaneously performed measurements are shown. Measurements were started after 24 hours of sealed curing at 20 C. The deformation was defined as 0 at 24 h. The bottom curves show temperature measurements. temperature change in the centre of the cement paste is further delayed by approx. 5 min. Accuracy of measurements Fig. 8 shows autogenous deformation after setting on two identical cement pastes. The registered maximum deviation between the two specimens amounts to 25 µm/m. This deviation, however, is related to differences between the samples, and not measuring accuracy of the sensor [4]. Before setting the deviation is even larger, e.g. 200 µm/m [4]. The fluctuations observed on the curves are related to the thermal comparator sensor. The standard deviation of these variations is 3.5 µm/m or about 1 µm. Fig. 7 Close-up of the temperature change after 7 days. Measurements were datalogged every 10 min. 664

5 Materials and Structures / Matériaux et Constructions, Vol. 36, December 2003 fully sufficient for measuring autogenous deformation and its activation energy in hardening cement based materials. With the thermal comparator sensor a 1 µm resolution of the measurements has been achieved. This is fully sufficient for the present application, particularly since the resulting measuring accuracy is dominated by sample variability. In the present set-up the thermal comparator has been used for determining the autogenous deformation and its activation energy as well as the coefficient of thermal expansion of hardened cement paste. However, the thermal comparator is not limited to the measurements of cement paste deformation. It can be used for measuring other types of deformation of solids and liquids. Fig. 8 Autogenous deformation of two cement paste specimens with w/c = 0.3 and 20% silica fume addition measured simultaneously at constant 20 C. 5. CONCLUDING REMARKS The thermal comparator sensor is developed specially for measuring autogenous deformation of cement paste in a corrugated mould system. An alternative measuring system based on LVDTs is described in [4]. This set-up requires a complete, thermally stable dilatometer frame of invar for supporting the samples and mounting the LVDTs. Compared with this system the thermal comparator sensor is inexpensive and a regular dilatometer is not needed since the sensors are mounted directly on the corrugated moulds. The thermal comparator sensor is mechanically delicate. However, this has not been a problem for the present application. On the other hand, it is an electrically very simple and robust construction. Compared with other sensor types the thermal comparator sensor has a very slow response time, since it is based on heat flow. For other sensor types this may entirely relate to the electrical system. The response time of the thermal comparator, however, is REFERENCES [1] Bentz, D.P. and Jensen, O.M., Mitigation strategies for autogenous shrinkage cracking, Cement and Concrete Composites (2003) (In press). [2] Aïctin, P.-C., Autogenous shrinkage measurement, Proceedings of the Int. Workshop of Autogenous Shrinkage of Concrete, Hiroshima, June 13-14, 1998, E.-I. Tazawa (Ed.), (E&FN Spon, London, 1999) [3] Jensen, O.M. and Hansen, P.F., Autogenous deformation and RH-change in perspective, Cement and Concrete Research 31 (12) (2001) [4] Jensen, O.M., A dilatometer for measuring autogenous deformation in hardening Portland cement paste, Mater. Struct. 28 (181) (1995) [5] Fraden, J., Handbook of Modern Sensors: Physics, Designs, and Applications, 2 nd Edn. (Springer, San Diego, 1996). [6] Østergaard, T. and Falkenberg, S., New measuring technique for determination of activation energy of autogenous shrinkage in hardening cement paste (only available in Danish), M.Sc. Thesis (Department of Building Technology and Structural Engineering, Aalborg University, Aalborg, 2000). [7] Bardes, P.B. (Ed.), Metals Handbook, Vol. 2, Properties and Selection: Nonferrous Alloys and Pure Metals, 9 th Edn. (American Society for Metals, Metals Park, Ohio, 1979). Paper received: May 16, 2003; Paper accepted: October 13,