Experimental study on sound and damaged mortar: Variation of ultrasonic parameters with porosity

Construction and Building Materials 23 (2009) 953–958

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Experimental study on sound and damaged mortar: Variation of ultrasonic parameters with porosity Zoubeir Lafhaj a,*, Marc Goueygou b a b

LML URA CNRS 1441, Ecole Centrale de Lille, Villeneuve d’Ascq, France IEMN DOAE UMR CNRS 8520, Ecole Centrale de Lille, Villeneuve d’Ascq, France

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 26 January 2008 Received in revised form 20 March 2008 Accepted 6 May 2008 Available online 24 June 2008 Keywords: Porosity Leaching Ultrasonic Degradation Mortar

The relationship between porosity and ultrasonic parameters of mortar was studied. In order to enhance the sensitivity of the ultrasonic wave to microstructural changes, a high frequency range, 0.6–1.2 MHz, was chosen. The material investigated consists of four mortar mixtures with water/cement ratio varying from 0.3 to 0.6. Samples were chemically damaged for 4 periods of time (4, 9, 16 and 25 days). All the samples were degraded using a chemically accelerated process with an ammonium nitrate solution. This procedure yields a wide range of porosity values, from 10% to 35%. This growth of porosity was mainly attributed to the portlandite dissolution followed by C–S–H decalcification. The experimental results obtained showed that for sound samples, the correlation between physical and acoustic parameters yields the expected trend: velocity decreases with porosity. Secondly, the increase of porosity with the duration of degradation did not affect this relation between acoustic parameters and porosity where the latter remains linear. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The cover of a concrete structure is a protective barrier against aggressive agents that may penetrate into the structure and induce corrosion of steel bars. Therefore, on-site evaluation of the quality of this cover is essential to predict, at an early stage, the durability of the structure and define a strategy of maintenance. Porosity is of main interest, as moisture and chemicals can penetrate through connected pores. In order to predict the long-term behaviour of cement based materials, an ammonium nitrate test has been developed. This solution is considered as more aggressive than water and accelerates the degradation by 300 times [1]. Elastic waves are directly influenced by the elastic parameters of the material they propagate in. In homogeneous, linear and elastic media, the compression and shear wave velocities, cL and cT, are related to elastic moduli

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 1m ; cL ¼ q ð1 þ mÞð1  2mÞ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 1 cT ¼ q 2ð1 þ mÞ

ð1Þ

where E is the dynamic Young’s modulus, m the Poisson ratio and q the density. As elastic moduli depend on porosity, this induces a relationship between porosity and ultrasonic velocity. Velocity is

* Corresponding author. Tel.: +33 3 20 33 53 65; fax: +33 3 20 33 53 52. E-mail address: [email protected] (Z. Lafhaj). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.05.012

expected to decrease as porosity increases [2]. Lafhaj et al. [3] found that, for porosity values ranging from 8% to 14%, the velocity–porosity relationship was approximately linear. In their study, the variation of porosity was obtained by varying the water/cement ratio. Using a micro-mechanical model established by Jeong and Hsu [4], Hernandez et al. [5] derived extremely precise estimates of porosity from measured ultrasonic velocity. However, this model has many parameters as inputs, and especially the pore geometry, which may not be a priori known. Recently, Hernandez et al. [6] further investigated the validity of their model on chemically damaged mortar samples. The range of porosity values was then significantly enlarged (from 15% to 30%). The present study intends to relate porosity of mortar samples with their ultrasonic parameters for both sound and degraded mortar. Mortar is chosen instead of concrete in order to limit excessive attenuation of ultrasonic waves due to diffusion by coarse aggregates. Mortar samples were fabricated with four different water/cement (w/c) ratios. Porosity of those samples was further varied by chemical degradation in ammonium nitrate. The joint variation of w/c ratio and duration of immersion in ammonium nitrate allows for a wider range of porosity values (10–35%) than in previous studies. Ultrasonic parameters (pulse velocity and attenuation) of compression and shear waves are obtained by broadband ultrasonic spectroscopy [7]. Compared to Hernandez et al. [6] where samples are water saturated, our measurements are made in direct contact on dry samples. In addition, shear wave velocity was measured for all degradation levels, which enables to estimate elastic moduli.

954

Z. Lafhaj, M. Goueygou / Construction and Building Materials 23 (2009) 953–958

2. Methods 2.1. Sample preparation In order to obtain various porosity values, mortar samples were fabricated with different water/cement (w/c) ratios ranging from 0.3 to 0.6. The samples are cylinders of 37 mm diameter and 70 mm height. They are cored from larger cylinders preserved over 28 days in water saturated with lime at a constant temperature of 20 °C and then rectified to obtain parallel end faces. The mortar used is of a fine grain type, made up of Portland cement CPA CEM I 52.5 from Origny and Hostun sand. Table 1 below gives the proportions of the various constituents for a given w/c ratio. 2.2. Chemical degradation The degradation of the cement based materials by ammonium salts was studied by several authors [1,8,9] and recognized as potentially aggressive. All agree on the mechanisms of degradation: ammonium in a basic medium such as cement based materiTable 1 Proportion of the mortar constituents (w/c = 0.4) Constituent

Proportion (kg/m3)

Water Cement Fine sand (0.16–2 mm) Coarse sand (0.8–3.15 mm)

256 639 966 414

als dissociates out of gas ammonia and releases protons, thus involving the dissolution of the portlandite. Based on the characteristic of the high solubility of ammonium nitrate, its dissolution in great quantity increases the reaction [10,11]. In order to accelerate the leaching of cement based materials by water, a system of chemical degradation was designed (Fig. 1a), based on the immersion of samples in an ammonium nitrate solution (6 Mol/l). The use of nitrate is justified by the fact that this last degrades the cementing matrix in a way similar to water but with an accelerating factor of 300 [12,13]. Each sample is placed in the solution with an agitator for periods of 4, 9, 16 and 25 days. Agitators were used to homogenize the solution during all the duration of degradation. The measure of pH is monitored and the solution in which the samples were immersed is renewed regularly (on average every 2 days) to reach a stable pH near 8.0. These precautions are necessary to be able to compare the degradation between all the mortars tested, as leaching kinetic is strongly affected by experimental conditions. After each degradation period, the samples were washed with demineralised water to extract remaining nitrate salts, thus avoiding the formation of expansive gel. Finally, both end faces, which are more degraded, were removed to obtain specimens with circumferential degradation and each cylinder was cut into five 10mm thick disks. Both the thickness of the degraded layer and the total diameter of each disk were measured (Fig. 1c). 2.3. Porosity measurement Porosity of both sound and degraded samples was measured by the gravity method, using vacuum saturation. This is a well

Fig. 1. Experimental device of chemical degradation: (a) schematic procedure; (b) photo of the set up and (c) picture of the degraded sample.

955

Z. Lafhaj, M. Goueygou / Construction and Building Materials 23 (2009) 953–958

established technique, as it has the advantage to be fast and easy to apply. Moreover, it is considered to provide a good estimate of porosity of materials such as mortar or concrete. It consists of submitting the sample to a moderate oven-drying at a temperature of 60 °C [12]. The drying is stopped when the weight of the sample remains constant, which is achieved when its variation does not exceed 0.01%. The weight of the dried sample, denoted as Mdry, is then measured with an accurate balance. Afterwards, the specimen is immersed under vacuum in a container filled with distilled water. The weight of the sample is measured at different times until it becomes stable; the sample is then considered as fully saturated with water and its weight denoted as Msat. In addition, the volume of the sample (Vvol) was precisely measured with a pycnometer. Finally, Porosity (p) was determined using the following formula:

p¼

M sat  Mdry qw V vol

ð2Þ

where qw is the volume density of water. Then, the porosity of the degraded zone is easily deduced from the total porosity of the sound and degraded sample and the thickness of the degraded zone. 2.4. Ultrasonic measurements Transducer

Broadband ultrasound spectroscopy was used to obtain ultrasonic parameters of the samples. This technique can be applied either in transmission mode, with a pair of transducers, one transmitter and one receiver, or in pulse-echo mode with a single transducer working as both transmitter and receiver. The transmitted ultrasonic pulse is wideband, with a 1 MHz central frequency and a 60% bandwidth. As the analysis is performed in the frequency domain, ultrasonic parameters are estimated from 0.6 to 1.2 MHz. A Panametrics 5055 pulser–receiver is used to send a high voltage pulse to the transmitter and to amplify the received signal. The amplified signals are then acquired by a digital oscilloscope and transferred to a PC for further analysis. A thin layer of coupling agent is applied between the transducer face and the sample. It is either a silicone gel (Sofranel D coupling agent) for longitudinal waves or a highly viscous liquid (Sofranel SWC coupling agent) for shear waves. Pulse velocity is measured in pulse-echo mode by estimating the time-of-flight between the first backwall echo s1(t) and a reference signal s0(t) synchronous with the excitation pulse (electrical crosstalk signal)

C L;T ¼

e dt

Buffer

s1(t)

s2(t)

Sample

s1(t) s2(t)

ð3Þ

where e is the sample thickness, dt the time delay between signals s0(t) and s1(t) (Fig. 2a). A better precision could have been achieved by considering time delay between multiple backwall echoes, but a single echo was generally observed in degraded samples, due to excessive attenuation. Attenuation is measured using the buffer rod method [14]. First, the backwall echo from an aluminium buffer is recorded without sample, yielding a reference signal with amplitude spectrum denoted as S0(f). Second, the sample was coupled to the buffer (Fig. 2b) and signals s1(t) and s2(t) were recorded (Fig. 2c). After time windowing, their amplitude spectra S1(f) and S2(f) are obtained. Finally, attenuation was derived from the following equation:

(

" #) 1 S0 ðf Þð1  Rðf Þ2 Þ aðf Þ ¼ ln 2e S2 ðf Þ with Rðf Þ ¼ SS10 ðfðf ÞÞ is the buffer/sample reflection coefficient.

ð4Þ

Fig. 2. Measurement of pulse velocity and attenuation: (a) estimation of time delay Dt, (b) measurement setup, (c) reflected pulses.

Since the frequency content of the received signals is varying slightly from one sample to another, attenuation is presented as the slope in dB/cm/MHz of the a(f) curve within the 10 dB bandwidth of signal s2(t). 3. Experimental results In order to investigate the effect of porosity on ultrasonic parameters, each sample of mortar was subjected to the three kinds of tests previously described: porosity, pulse velocity and attenuation measurement. For each w/c ratio and duration of

956

Z. Lafhaj, M. Goueygou / Construction and Building Materials 23 (2009) 953–958

degradation, an average of measurements performed on five samples taken from the same core is presented. 3.1. Porosity vs. w/c ratio and time of degradation Fig. 3 displays the variation of porosity with w/c ratio for sound and degraded samples. As expected, porosity increases linearly with w/c ratio. As an example, in sound samples, porosity increases from 12% to 18% when the w/c ratio goes from 0.3 to 0.6. On the same figure, the variation with w/c ratio of the total porosity measured for different duration of degradation (4, 9, 16 and 25 days) was reported. It can be noted that for all the durations of degradation, total porosity increases with w/c ratio, which demonstrates that leaching had as consequence of an increase of the porosity. This increase of porosity is more significant for samples degraded

40

35

Sound 4days 9 days 16 days 25 days

Porosity (%)

30

25

20

15

10 0.2

0.3

0.4

0.5

0.6

0.7

W/C

10

0.65

9

0.6

8

0.55

7

0.5

Degraded 4 days

Degraded 9 days

W/C

Degraded thikness (mm)

Fig. 3. Variation of porosity with water/cement ratio for sound and degraded mortar.

for 25 days. As an example, for samples degraded, respectively, 4 days and 25 days, the relative increase of porosity measured when the w/c ratio increases from 0.3 to 0.6 goes from 6% to 12%. Finally, it can be noted that the linear variation of porosity with w/c ratio remain valid for both sound and degraded samples. Fig. 4a illustrates the evolution of the degraded thickness with the square root of time for different w/c ratios. It appears clearly that degraded thickness increases with the square root of time and this is true for all w/c ratio values. This growth, which may be attributed to the high diffusion coefficient of these samples, can be characterized by two phases: a very sharp variation in the first step followed by a stabilization. The latter is observed when the duration of degradation reaches 25 days. The evolution of the degraded zone can be explained by the decalcification of portlandite, which leads to micro porosity and by the decalcification of the c–s–h hydrates which leads to macro porosity. These decalcifications affect the hydraulic properties of mortar, mainly porosity that increases with the degradation time. In order to improve the interpretation of these results, fig. 4b presents the variation of the degraded thickness with w/c ratio at different durations of degradation (4, 9, 16 and 25 days). First, it can be observed that for given time of degradation, the degraded thickness increases linearly with water/cement ratio and for all the duration of degradation investigated. As an example, for samples with water/cement equal to 0.3, the degraded thickness goes from 2.5 to 8.16 mm when the duration of degradation increases from 4 days to 25 days. This tendency remains the same for other w/c ratios. On the other hand, it can be observed that the linear variation of degraded thickness with w/c ratio remains valid for sound and degraded samples. Generally, in these experimental studies, water porosity was measured for sound and damaged samples. As expected, total porosity increases with w/c ratio and time of degradation (Fig. 3). The maximum porosity increase induced by 25 days degradation is about 35%. The question is whether this variation is caused merely by a volume increase of the degraded material or additionally by a porosity increase within the degraded material. From the measured thickness of the degraded layer, an estimate of porosity of this layer was made. It shows that porosity in the degraded layer varies only slightly, between 30% and 35%. Therefore, the observed global porosity increase is mainly due to an increase of the degraded volume.

6

0.45 0.4

5

Degraded 16 days

W/C = 0.3 W/C = 0.4 W/C = 0.5 W/C = 0.6

4

0.35 0.3

3

Degraded 25 days

2 1.5

0.25 2

2.5

3

3.5 1/2

Time

4

(days)

4.5

5

5.5

2

3

4

5

6

7

8

9

10

Degraded thickness (mm)

Fig. 4. Evolution of the degraded zone in the mortar: (a) degraded thickness variations of water/cement ratio versus duration of degradation and (b) evolution of the degraded thickness with water/cement ratio.

957

Z. Lafhaj, M. Goueygou / Construction and Building Materials 23 (2009) 953–958

3400

2200

Sound samples

3200

Transversal wave pulse velocity (m/s)

Longitudinal wave pulse velocity (m/s)

Sound samples

3000

2800

2600

2400

Degraded samples (25 days)

2200 10

15

20

25

30

35

2000

1800

1600

Degraded samples (25days)

1400

1200 10

40

15

Porosity (%)

20

25

30

35

40

Porosity (%)

Fig. 5. Variation of ultrasonic pulse velocity versus porosity. (a) Longitudinal wave and (b) shear wave.

3.2. Pulse velocity and attenuation vs. porosity Fig. 5 shows the ultrasonic pulse velocity versus porosity for longitudinal and shear wave, respectively. Again, the expected trend is observed: velocity of both waves decrease as porosity increase. Only the results obtained on sound and total degraded mortar are analysed. The results concerned with intermediate state of degradation present a high dispersion and difficulty for interpretation. This may be due to the fact that, although the samples were washed after chemical degradation, some of the ammonium nitrate solution remained inside the pores. Previous studies [3] have shown that the presence of water inside pores tends to increase velocity in mortar. In our case, the presence of ammonium nitrate solution may interfere with the decrease of velocity related to increasing porosity. It can observed that for both longitudinal and shear waves, the experimental values obtained for samples degraded 25 days are one and a half time lower than those obtained for sound samples. This result is interesting and shows that a simple method based on the measurement of UPV at successive duration may be used to monitor the degradation of concrete in structures. Table 2 presents the results of linear regression performed on UPV for both longitudinal and shear waves. To evaluate these results a linear regression (CL,T = ap + b) is performed on the curves presented on Fig. 5. Results are presented in Table 2. All values of the regression coefficient R are found to be higher than 0.96, which means that the measured variation of UPV versus porosity is correctly described by a linear relationship in the range of porosity considered. It can be noted here that the porosity considered was varying in time. Fig. 6 presents the evolution of shear wave attenuation versus porosity for sound samples. Due to excessive attenuation, no backwall echo (signal s2(t) in Fig. 2b and c) could be recorded in degraded samples, so attenuation was estimated in sound samples Table 2 Results of linear regression of data from Fig. 5a and b

Longitudinal wave Shear wave

a

b

R

3859 3012

3821 2552

0.96 0.98

Attenuation slope (dB/Cm/MHz)

12

10

8

6

4

2

12

13

14

15

16

17

18

19

Porosity (%) Fig. 6. Shear wave attenuation vs. porosity in sound samples.

only. On average, attenuation slope is observed to increase with porosity, which is attributed to an increase of material heterogeneity. However, this increase is not linearly related to porosity and tends to stabilize from the second w/c ratio (0.4). The sharp increase between the first and second w/c ratio was already observed in [3]. Material heterogeneity, due to the presence of sand grains not much smaller than the wavelength, induces large error bars, thus preventing the three largest attenuation values to be distinguished. 4. Conclusions This study intended to relate physical parameters – mainly porosity – and ultrasonic parameters (pulse velocity and attenuation) of sound and damaged mortar samples. In sound samples, the expected trends were observed. Porosity of sound mortar increases linearly with the water cement ratio. Pulse velocity decreases linearly with porosity. Porosity of samples was further increased by chemical damage. The duration of degradation

958

Z. Lafhaj, M. Goueygou / Construction and Building Materials 23 (2009) 953–958

influences this growth of porosity which was higher for mortar degraded 25 days than those degraded for 2 days. The thickness of degradation increases with the square root time of degradation. No clear correlation could be obtained between porosity and acoustic velocity in all damaged samples. Despite a significant dispersion of experimental results, a linear relation between UPV and porosity obtained for sound samples and for the most degraded samples (25 days) was established. This study also highlights the difficulty of obtaining accurate estimates of ultrasonic parameters, because of the high variability of the material under study. The present study shows how ultrasonic parameters are expected to evolve as physical parameters related to damage vary. Our long-term goal is to allow quantitative interpretation of data acquired on-site using ultrasonic NDT techniques. To achieve this goal, the first step is to study the influence of parameters other than porosity on ultrasonic measurements. Those parameters include water content and aggregate properties (size distribution, volume concentration and elastic moduli). The second step is to estimate one of the material parameters, e.g. porosity, from ultrasonic measurements. Obviously, a single measurement is not sufficient to identify more than one unknown parameter. Therefore, ultrasonic measurements should be coupled with data obtained from other non-destructive techniques that are sensitive to the same material properties. Another solution is to take advantage of the whole bandwidth of the measured ultrasonic signals. This would require including frequency as an additional parameter in the relationships between ultrasonic parameters and material properties.

References [1] Carde C, Francois R. Effect of ITZ leaching on durability of cement-based materials. Cement Concr Res 1997;27:971–8. [2] Ould Naffa S, Goueygou M, Piwakowski B, Buyle-Bodin F. Detection of chemical damage in concrete using ultrasound. Ultrasonics 2002;40:247–51. [3] Lafhaj Z, Goueygou M, Djerbi A, Kaczmarek M. Correlation between porosity, permeability and ultrasonic parameters of mortar with variable water/cement ratio and water content. Cement Concr Res 2006;36:625–33. [4] Jeong H, Hsu DK. Quantitative estimation of material properties of porous ceramics by means of composite micro-mechanics and ultrasonic velocity. NDT&E Int 1996;29:95–101. [5] Hernandez MG, Izquierdo MA, Ibanez A, Anaya JJ, Gomez-Ullate L. Porosity estimation of concrete by ultrasonic NDT. Ultrasonics 2000;38:531–6. [6] Hernández MG, Anaya JJ, Ullate LG, Cegarra M, Sanchez T. Application of a micromechanical model of three phases to estimating the porosity of mortar by ultrasound. Cement Concr Res 2006;36:617–24. [7] Eggers F, Kaatze U. Broad-band ultrasonic measurement techniques for liquids. Measur Sci Technol 1996;7:1–19. [8] Lea FM. The action of ammonium salts on concrete. Mag Concr Res 1965;17: 115–6. [9] Lea FM. The chemistry of cement and concrete. 3rd ed. London: Arnold; 1970. [10] Carde C, Francois R. Modelling the loss of strength and porosity increase due to the leaching of cement pastes. Cement Concr Compos 1999;21:181–8. [11] Carde C, Francois R. Effect of the leaching of calcium hydroxide from cement paste on mechanical and physical properties. Cement Concr Res 1997;27: 539–50. [12] Loosveldt H. Etude expérimentale des comportements hydrauliques et poromécaniques d’un mortier sain ou dégradé chimiquement. PhD thesis. Ecole Centrale de Lille; 2002. [13] Agostini F, Lafhaj Z, Skoczylas F, Loosveldt H. Experimental study of accelerated leaching on hollow cylinders of mortar. Cement Concr Res 2007;37:71–8. [14] Papadakis EP, Fowler KA, Lynworth LC. Ultrasonic attenuation by spectrum analysis of pulses in buffer rods: Method and diffraction correction. J Acoust Soc Am 1973;53:1336–43.