EXPERIMENTAL DETERMINATION OF THE RELATIONSHIP BETWEEN POROSITY AND SURFACE WAVE PARAMETERS OF FULLY AND PARTIALLY SATURATED CEMENT PASTE

Transcription

1 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, 29 EXPERIMENTAL DETERMINATION OF THE RELATIONSHIP BETWEEN POROSITY AND SURFACE WAVE PARAMETERS OF FULLY AND PARTIALLY SATURATED CEMENT PASTE Fethi SOLTANI 1, Zoubeir LAFHAJ 1, Marc GOUEYGOU 2 1 LML UMR CNRS 817, Ecole Centrale de Lille, Villeneuve d Ascq, France, zoubeir.lafhaj@ec-lille.fr 2 IEMN DOAE UMR CNRS 852, Ecole Centrale de Lille, Villeneuve d Ascq, France Abstract The aim of this paper is to investigate the effect of porosity on surface wave velocity in cementitious materials. The considered material is cement paste. Fully and partially saturated states are studied. The surface wave velocity is measured using a specific automatic setup developed for this experimental study. The obtained results show a sensitivity of surface wave to porosity in cement past. The effect of water content on surface velocity is analysed. A decrease of wave velocity is observed between the fully and partially saturated states. Résumé L objectif de l article est d étudier l effet de la variation de la porosité sur la vitesse de propagation de l onde de surface dans un matériau cimentaire. L étude consiste à mesurer la vitesse de l onde de surface à l aide d un dispositif automatisé dédié spécifiquement à ce travail expérimental. L étude est réalisée sur des dalles de pâte de ciment entièrement et partiellement saturées. Les résultats obtenus montrent la sensibilité de l onde de surface à la porosité dans la pâte de ciment. L effet de la teneur en eau à été constaté par une diminution de la vitesse quand l état de saturation passe de 1% à 7%. Keywords Surface wave velocity, Porosity, Saturation, Cement paste, Experimental. 1. Introduction The lifetime of concrete structures is often related to the capacity of concrete to prevent the penetration of aggressive agents (sulphates, chlorides, CO2 ) in its porous structure. The aptitude of concrete cover to resist to these external chemical attacks is conditioned by its properties of transfer (porosity, permeability and diffusivity). These physical properties are related to the characteristics of the porous environment (the shape of the pores, distribution of the sizes of the pores and tortuosity) [1]. These parameters can be qualified as indicators of durability, provided however that one has the adequate methods to determine them [2]. Several methods of in situ measurement of these physical properties were developed. As an example, the method of Figg consists in measuring the permeability of the concrete cover [3]. Btcris is used to determine the amplitude of the hydrous and gas exchanges between concrete and the ambience in order to estimate the velocity of deterioration of the properties of concrete skin. These methods have many advantages mainly their reproducibility and their easy to use and their low cost. However, they are limited in terms of precision and they are semidestructive.

2 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, 29 It is thus necessary to develop new methods of investigation that are able to quantify the properties of the concrete, in particular the transfer properties. On the other hand, ultrasonic methods have been used since mid 194 s for concrete testing. The principle of these methods is to estimate quantitatively the properties of materials by measuring the velocity and the attenuation of the ultrasonic waves in a given frequency band. These techniques are largely used to quantify elastic properties of the materials [4], to detect cavities and cracks [5]. Several research works were undertaken to study the relationship between concrete properties and parameters of ultrasonic waves (P and S) [6], [7]. These studies highlighted the sensitivity of the parameters of propagation to the porous structure of the concrete. The present work deals with the investigation of cement paste using surface wave (Rayleigh wave) velocity. Variation of experimental surface wave velocity with porosity for fully and partially saturated cement paste was studied. Principal results are presented and analyzed. 2. Materials Cement paste was chosen instead of mortar and concrete as it allows better resolution (higher frequencies) and lower dispersion of ultrasonic measurements. In order to obtain various porosity values, cement paste samples were fabricated with different water/cement (W/C) ratios. In addition, the samples were either partially or fully saturated with water. Fully saturated (1%) and partially saturated samples (7%) were studied. The study of other partial saturation states, including the dry state of cement pastes is in progress. The samples were cast into rectangular slabs of dimensions (5x25x1) cm 3. They were cored from bigger rectangular slabs (1x5x1) cm 3 preserved over 28 days in a water solution of lime at a constant temperature of 2 C. The cement paste used is made of a fine grain type, made up of cement CPA CEM I The mixture is prepared with five different w/c ratios, ranging from.3 to.7 with a.1 increment. Table 1 gives the compositions of those samples. Super-plastifier was used in a proportion of 1% of the cement weight. Table 1. Composition and properties of cement paste specimens Samples W/C Cement (kg/m 3 ) Water (kg/m 3 ) Adjuvant (kg/m 3 ) CP CP CP CP CP Experimental setup 3.1. Porosity Porosity was measured by the gravity method [8], using vacuum saturation. This method consists of saturating the cement paste sample under vacuum. Once it is fully saturated, it is weighted with centigram precision and its volume V is determined by hydrostatic weighing. Then, the sample is submitted to moderate oven drying at a temperature of 6 ±2 C. The drying is stopped when the weight of the sample remains constant. Fig. 1 shows the decrease

3 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, 29 of normalized mass loss. The weight of the dried sample is obtained after 21 days of drying. Porosity, p, is then determined using the following formula: M sat M dry p = (1) ρ V Where ρ w the unit mass of water, V is is the volume of sample, M dry and M sat denote the weight of the dried and fully saturated samples, respectively. w Normalized mass loss CP3 CP4 CP5 CP6 CP Time(Days) Figure 1 Mass loss of cement paste 3.2. Ultrasonic measurements In order to ensure a reproducible coupling, surface wave velocity was measured using an ultrasonic air-coupled device. High frequencies were used to increase the sensitivity to material heterogeneity, such as porosity and microcracks. The transmitter is fixed; the receiver is placed on an automated rail and moved with constant step (.5 cm) along the axis of the transmitter [9]. The emitted signal is a frequency modulated sine (chirp) with a spectrum between 15 and 6 khz; the received signal presents a dominant frequency around 35 khz. In this study, surface wave velocity is determined from 1 successive signals (Fig. 2.a) as: 1 V S = (2) D Where D is the slope of the regression line between time of arrival of the received wave and transmitter-to-receiver distance (Fig. 2.b).

4 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, Time (µs) a) Set of 1 received signals with a constant 5mm increase of the transmitter-receiver distance. µ s) ( gh o ffli m e T i Distance (cm) b) RW velocity estimation using regression of time-of-flight vs. transmitter-receiver distance Figure 2 Rayleigh wave pulse velocity measurement (W/C =.3) 4. Results and discussion 4.1. Porosity In order to improve the correlation between porosity and surface wave velocity, measurements of surface porosity are measured on cylindrical samples of 37mm of diameter and 12 mm of height. These samples were cored from the surface of the rectangular slab. The value of porosity selected is the average of three measurements. Fig. 3 illustrates the variation of surface porosity with respect to W/C ratio. As expected, porosity increases with W/C. Porosity values range from 22.81% for the lowest W/C ratio to 44.6% for the highest. 45 Surface Porosity(%) W/C Figure 3 Variation of surface porosity with water/cement ratio

5 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, Ultrasonic parameters Fig. 4 presents the variation of surface wave velocity with porosity of cement paste. The obtained results show a decreasing velocity when porosity increases. This is observed for both fully and partially saturation states. 22 Surface Velocity(m/s) Saturated Partially Saturated Surface Porosity (%) Figure 4 Variation of surface wave velocity with surface porosity It is observed that the decrease of the surface wave velocity is a boit 24% and 35% for the fully and partially saturated states, respectively, when porosity increases by 1%. Such decrease is due to the degradation of mechanical properties of cement paste. The propagation velocity of Rayleigh wave increases with water content. It can be noted that the deviation between the fully and partially saturation states is greater for higher porosities. For example, when W/C ratio increases from.3 to.7, the difference between the fully and partially saturated surface velocities grow from 1.1% to 16%. The saturation state of pore volume has a significant effect on surface wave velocity. This increase of the ultrasonic wave velocity with saturation degree was observed in previous studies. An experimental study carried out by Yaman et al. [7] showed that an increase of saturation degree in concrete imply an increase of static and dynamic moduli, and thus a greater rigidity of the material. The obtained correlation coefficients (R² (Saturated) =.89; R² (Partially saturated) =.98) are close to 1 and thus highlight a linear behavior between surface wave velocity and porosity in the cement paste. 5. Conclusions The presented work was carried out in order to study the effect of porosity variation on the surface wave velocity in the cement paste. The Rayleigh wave velocity was measured using a transducer with large band width (15 khz-6 khz) and a central frequency of 35 khz. The surface porosity was measured on 12cm depth samples. The water content variation in porous volume was considered. Two degree of saturation were studied: 7% and 1%. Studies on other saturation sates are in progress.

6 NDTCE 9, Non-Destructive Testing in Civil Engineering Nantes, France, June 3th July 3rd, 29 The experimental results obtained show a decrease of velocity when the porosity increases and an increase of velocity when saturation degree increases. A linear behavior between the surface wave velocity and porosity was observed. This study proved a significant variation of surface wave velocity with the cement paste porous structure. Finally, this work has shown the possibility to predict the mechanical and physical properties of cement paste skin using the ultrasonic non-destructive measurement method based on surface wave propagation. References 1. Buil M., Ollivier J.P., "Conception des bétons: la structure poreuse. La durabilité des bétons, collection de l association technique de l industrie des liants hydrauliques". Sous la direction de J. Baron et J-P. Ollivier, Baroghel-Bouny V., Ammouche A., Hornain H., "Analyse de la microstructure et propriétés de transfert", Revue Française de Génie Civil, n 2-3, 21, pp Figg J.W., "Methods for measuring the air and water permeability of concrete", Magazine of Concrete Research, Vol 25(85), 1973, pp Martin L.P, Dadon D. and Rosen M., "Evaluation of Ultrasonically Determined Elasticity- Porosity Relations in Oxide Zinc", The American Ceramic Society, Vol 79, 1996, pp Aggelis D.G., Shiotani T., "Repair evaluation of concrete cracks using surface and through transmission wave measurements", Cement and Concrete Composites, Vol 29, 27, pp Lafhaj Z., Goueygou M., Djerbi A. and Kaczmarek M., "Correlation between porosity, permeability and ultrasonic parameters of mortar with variable water/cement ratio and water content", Cement and Concrete Research, Vol 36, 26, pp Yaman I.O., Aktan H.M., Hearn N., "Active and non-active porosity in concrete: Part II. Evaluation of existing models", RILEM Newspaper of Materials and Structures. Vol 35 (246), 22, pp AFPC-AFREM, "Concrete durability: determination of apparent density and water accessible porosity (in French) ", 1997, Safinowski P., Piwakowski B., Balayssac J.P., Goueygou M. and Kosecki A., "Non destructive testing of the concrete by ultrasounds using an automated device", Diagnobéton, 27, May 1-11, 27, Aix-en-Provence.