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Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone
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Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras 5 6
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Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone
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Emilia Vasanelli a, Donato Colangiuli a, Angela Calia a,⇑, Maria Sileo a, Antonietta Aiello b
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a b
Institute for Archaeological and Monumental Heritage (CNR-IBAM.), via per Monteroni, 73100 Lecce, Italy University of Salento, Dept. of Engineering for Innovation, via per Monteroni, 73100 Lecce, Italy
a r t i c l e
i n f o
Article history: Received 17 September 2014 Received in revised form 4 February 2015 Accepted 9 February 2015 Available online xxxx Keywords: Physical–mechanical characterization Non-destructive test Soft building limestone Ultrasonic wave propagation Uniaxial compressive strength Water influence
a b s t r a c t UPV as non-destructive technique can effectively contribute to the low invasive in situ analysis and diagnosis of masonry elements related to the conservation, rehabilitation and strengthening of the built heritage. The use of non-destructive and non-invasive techniques brings all the times many advantages in diagnostic activities on pre-existing buildings in terms of sustainability; moreover, it is a strong necessity with respect to the conservation constraints when dealing with the historical–architectural heritage. In this work laboratory experiments were carried out to investigate the effectiveness of ultrasonic pulse velocity (UPV) in evaluating physical and mechanical properties of Lecce stone, a soft and porous building limestone. UPV and selected physical–mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. Factors such as anisotropy and water presence that induce variations on the ultrasonic velocity were also assessed. Correlations between the analysed parameters are presented and discussed. The presence of water greatly affected the values of the analysed parameters, leading to a decrease of UPV and to a strong reduction of the compressive strength. A discussion of the role of the water on these results is provided. Regression analysis showed a reliable linear correlation between UPV and compressive strength, which allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as the ultrasonic wave velocity. Low correlation between UPV and density was found, suggesting that other factors than density, related to the fabric and composition, also influence the response of the selected stone to the UPV. They have no influence on the UCS, that instead showed to be highly correlated with the packing density. Ó 2015 Published by Elsevier B.V.
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1. Introduction
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The conservation, rehabilitation and strengthening of historical buildings are increasingly required by modern societies, due to the need of preserving the memory of the past and keeping a record of cultural changes. Based on the appropriate knowledge and diagnosis of materials and structures, proper interventions can be adopted. According to the international charters of Athens cited by Venice [1], the diagnostic analysis of historical buildings should be carried out with the lowest degree of intrusion and with the fullest respect for their physical integrity, as per the principles associated with preserving objects of architectural and cultural heritage. To abide by these restrictions, the scientific community
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⇑ Corresponding author at: CNR-IBAM, University Campus, Prov.le Lecce Monteroni, 73100 Lecce, Italy. Tel.: +39 0832 422208. E-mail address: [email protected] (A. Calia).
has moved to propose alternative non-destructive testing (NDT) that is to be applied to the investigation of masonry and construction materials. Sophisticated non-destructive techniques – such as ground penetrating radar, thermography, sonic and ultrasonic tomography, and laser scanner survey – have been developed and improved throughout the years. By using an integrated approach it is possible to reconstruct the morphology of the masonry structures and to detect the presence of structural failures, such as cracks and voids, in addition to the presence of water coming from rising damp or from seepage, thus achieving an accurate and reliable identification and diagnosis of the construction, which is the basis for the restoration design [2–4]. In the field of characterization and diagnosis of ancient buildings, the knowledge of the physical–mechanical properties of the constituent materials and their in-use conditions is of crucial importance; however, being able to obtain samples of these materials is still a major obstacle. To this
http://dx.doi.org/10.1016/j.ultras.2015.02.010 0041-624X/Ó 2015 Published by Elsevier B.V.
Please cite this article in press as: E. Vasanelli et al., Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.02.010
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regard, the ultrasonic pulse velocity (UPV) method can be conveniently used to obtain a variety of information on the material properties and state of conservation. Ultrasonic velocities are related to the properties of the stone and they can thus be applied to assess the quality of the stones [5,6], to detect any presence of microscopic fissuring and cracks [7,8], to evaluate the material decay [9–12] and to assess the effectiveness of consolidation products [13,14]. In material characterisation, UPV testing has been reported by several authors as a useful and reliable non-destructive tool for assessing the mechanical characteristics of concrete [15–20]; it has also been suggested in the field of rock analysis for the estimation of elastic and strength properties, as well as of physical parameters of the stones [21–27]. Intrinsic factors of the stone – including density, porosity, grain size, oriented structures, microcracks, etc. – affect the elastic wave propagation; external parameters associated with humidity, temperature, and mechanical stress are also involved and it is important to study in what manner and how much their variation may modify the characteristics of the waves being measured trough a specific material. This aspect is of crucial importance when in situ measurements are made so that the results of the measurements are interpreted properly. In this paper UPV technique for the physical and mechanical characterization of Lecce stone – a soft and porous limestone used as construction material in Southern Italy – has been investigated under laboratory conditions. UPV measurements were performed and selected physical–mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. The study was aimed to find correlations between them, as well as to evaluate factors that can affect correlations, such as anisotropy and water presence. The final aim was to verify the reliability of UPV as a tool for the estimation of the in situ compressive strength of Lecce stone, thus limiting destructive tests for the mechanical qualification and structural diagnostic analysis of ancient masonries in which this material is used.
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2. Material description
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Lecce stone is a soft, fine-grained biocalcarenite commonly used in the past as construction material in Southern Italy. In this area, buildings across multiple districts that are made with this stone can be found, such as those within the city of Lecce and surrounding historic towns, where it is the almost unique material used in Baroque architecture and minor buildings despite its low durability [28]. Similar soft limestones can be frequently found elsewhere within the historic built heritage [29,30] due to their high availability, easy extraction, and workability. Microscopically, Lecce stone consists of fine microfossil fragments – mainly planktonic Foraminifera and fossil debris within a micritic groundmass (Fig. 1) that also contains dispersed clay minerals. Grain size is mainly around a few tens of microns. Variable fabric relies on stone frameworks of wackestone and packstone types [31], densely packed, but poorly cemented by fine calcite with microsparitic texture; this is intimately mixed into the groundmass and irregularly distributed. The frequent presence of bioturbations contributes to alter the textural homogeneity of the stone. Porosity is mainly intergranular; intragranular holes are sometimes present in the form of microfossil cavities. As reported in [28], porosity measured by Mercury Intrusion Porosimetry ranges between 30% and 43%, where the bulk of the pores (50–70%) is between 0.5 lm and 4–6 lm in radius; the remaining pores are mostly of smaller radii, up to 0.02 lm. The mineralogical composition of Lecce stone primarily consists of calcite with low magnesium content; a not carbonatic insoluble residue that accounts for 3% up to 11% in weight is made of clay minerals and amorphous
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Fig. 1. Thin section image (crossed nicols) of Lecce stone showing the densely packed structure mainly consisting of fine fossil remains within a groundmass with a poor microcrystalline cement.
compounds, quartz, and phosphatic grains. Clay minerals include glauconite, illite, kaolinite, chlorite, but also smectite which has an expandable lattice; their presence has been demonstrated to have an influence on the durability of Lecce stone [32].
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3. Experimental methods
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In this research, nineteen blocks (50 25 25 cm) coming from three different quarries of the Lecce district were tested. Cubes having 70 mm sides were cut from the blocks, and 151 specimens were obtained in total. Each cubic specimen (Table 1) is identified by a capital letter (A–N) followed by the number related to the block of provenance (1–19). The specimens are organized in four groups. Group1and Group 2 include specimens used to test density, UPV and UCS in dry and wet conditions, respectively. The specimens belonging to Group 3 were used for to assess the anisotropy by UPV measurements and its influence on the UCS. Group 4 was used to compare the density, UPV and UCS in dry and wet conditions of samples coming from the same block. All the specimens were oven dried at 70 °C until constant weight measurements were reached [33]. Apparent density in dry and wet conditions was determined by mass volume ratio. Saturation of the samples was obtained by immersion in deionized water at room temperature, according to UNI EN 13755 [34]. The weight was determined by means of a digital balance with a precision of 0.1 g. The dimensions of each specimen were exactly determined for the calculation of the apparent volume. All three dimensions of each specimen were measured by means of an analogic caliper with a precision of 0.01 mm. Four measurements were taken for each direction and then the mean value was calculated.
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3.1. UPV measurements
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The ultrasonic pulse velocity (UPV) test was performed according to ASTM D2845-05 [35] on dry and saturated samples. No coupling agents were used for the UPV measurements, in order to avoid the possible penetration into the pores before saturating the specimens. Velocities were measured by the direct transmission method using an Epoch 4plus (Olympus) instrument and probes with a frequency of 1 MHz. They were recorded in each direction (x, y, z) of the cubic specimens and expressed as the average of three readings.
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Table 1 Specimens for each block and grouping.
Group 1 (GP1):
184 185 186 187 188
Group 3 (GP3):
Group 4 (GP4):
In UPV measurements, the frequency of the transducers, the pulse velocity, the minimum lateral dimension of the specimen, and the grain size of the rock are interrelated parameters. In order to obtain accurate measurements the relation (1) should be satisfied [35]:
189 191
Group 2 (GP2):
DP5
v f
P 5d
ð1Þ
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where D is the minimum lateral dimension of the specimen, v is the pulse velocity, f is the frequency of the transducers, and d is the average grain size of the rock. Since v and d are intrinsic parameters of the stone, a transducer frequency of 1 MHz and a cubic specimen size of 70 mm satisfy the relation (1). An example of A-scan is reported in Fig. 2. The estimated relative error of the velocity measurements, obtained by combining the relative errors on time and thickness measurements, is about 0.3%. UPV measurements were performed on all the samples of the four groups in Table 1. Specimens belonging to GP2 group (blocks 13–19) were used to evaluate the UPV variation in the presence of water, performing the test on each of them in dry and saturated conditions.
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3.2. Uniaxial compressive test
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The compressive test was performed according to the UNI EN 772-1 standard [33] by means of a universal testing machine
Fig. 2. A-scan diagram obtained from the UPV measurement on a stone specimen.
.
(METROCOM Engineering spa) with a load capacity up to 200 kN and a load speed of 0.2 mm/min. The effect of the anisotropy was evaluated. The anisotropy values were calculated [36] for each sample as:
UPVmax UPVmin Anisotropy % ¼ UPVmax
ð2Þ
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212 214
where UPVmax and UPVmin are, respectively, the maximum and minimum values of the UPV measured along the three axes. The samples from 4 blocks (blocks 1–4, GP3group in Table 1) that showed the highest values of the anisotropy along z direction were used. Seven specimens from each block were tested with the application of the load both parallel and perpendicular to the z axis. In order to assess the influence of the water on the compressive strength, saturated and dry samples belonging to the same block were compared. The samples of the GP4 group (blocks 13–16) in Table 1 were used; for each block, five specimens were tested in each dry and saturated condition.
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3.3. Data analysis
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The values of the measured parameters were expressed for each block as the average of the measured or calculated values of the corresponding specimens; considering the number of the data, the dispersion was expressed in terms of the mean deviation. Comparisons between the mean values were based on a twotailed Student’s t-test, performed by Excel (MicrosoftÒ, Excel 2013). This test allows determining if two independent data sets, normally distributed, are significantly different from each other. A confidence interval of 99% was used to establish significance. Principal Component Analysis (PCA) [37] was performed by SIMCA (Umetrics, Sweden; Version 13.0). Before the multivariate statistical analysis, all the variables (UPVx, UPVy, UPVz and density) were scaled to unit variance (UV) in order to give equal importance to all of them.
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4. Results and discussion
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4.1. Principal Component Analysis (PCA)
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Principal Component Analysis (PCA) is a well-known statistical technique useful to identify patterns in data, by reducing the dimensionality of the data while retaining most of the variability in the dataset. It is based on the replacement of the original variables by a smaller number of their linear combinations (principal compo-
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nents). They identify directions, along which the variation of the data is maximal. Plotting of the resulting data allows a visual assessment of similarities and differences between samples, thus determining whether they can be grouped. A preliminary PCA of the density data and of the UPV values recorded along the x, y, and z directions for each sample of the GP1 group was performed; the first two Principal Components considered (t[1] and t[2]) accounted for 96.2% of the total variance. The score plot (Fig. 3) clearly shows a clustering of the specimens (total number 80) belonging to the same block. Since PCA is an unsupervised analysis, this result shows high homogeneity between the samples of the same block. On the basis of this outcome, the mean values of the analysed parameters were assumed to be representative of each whole block. The differences among the blocks are more significant compared to those among the samples of a single block. This supports the heterogeneity of the stone that was previously outlined.
4.2. Dry condition
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In Table 2, the mean values and the relative mean deviation for the apparent density, UPV, anisotropy, and compressive strength measured for each block in dry conditions are reported. Relative Average Deviation (RAD) is also reported for the apparent density (RADq) and compressive strength (RADUCS). As can be seen, the density of the blocks varies from 1.427 to 1.751 g/cm3 with low scatter levels for each of them; density values between 1.647 and 1.664 g/cm3 were measured for most of the blocks. Following the classification of the ultrasonic velocity (Table 3) [38], low ultrasonic velocities were recorded; the mean values of UPV vary considerably between the blocks from a minimum of 2496 m/s to a maximum of 3540 m/s and they are comparable to the velocity found in similar soft rocks [30,39,40]. Conversely, the dispersion of the data recorded for each block is low, supporting their homogeneity. The anisotropy values are mostly between 1.2% and 6.7%; higher degrees of anisotropy (13–18% approximately) were measured only for blocks 1, 2 and a variety of causes of anisotropy in rocks can be identified, namely orientation of crystals, grains, cracks and pores, stratification, and lamination structures [41]. In sedimentary rocks, such as the Lecce stone, the plane of anisotropy can be identified as the sedimentary plane. On the basis of the UPV data in Table 2, this corresponds to the xy plane, where the recorded values of UPV are higher than those measured along the perpendicular z direction. The compressive strength was generally found between 16.4 and 22.2 MPa (Table 2), with low to moderate levels of scatter (2.0–8.3%). Blocks 1, 2, and 4 show particularly higher values of the compressive strength (27.2–30.1 MPa), while lower values were recorded (11.3 and 8.6 MPa respectively) for blocks 10 and 12. The influence of the anisotropy on the compressive strength was investigated on the four blocks of the GP3 group (blocks 1–4 in Table 2), showing the highest degree of the anisotropy. A total
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Table 3 Classification of the ultrasonic velocities [38].
Fig. 3. Score plot of the PCA (2 components, R2Xcum(2PC) = 96.2%). The analysis was performed on the data (UPVx, UPVy, UPVz and density) of the overall 80 specimens of GP1 group. The shape of the symbols refers to the quarry provenance, while the color identifies each block. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Class
V (m/s)
Definition
1 2 3 4 5
<2500 2500–3500 3500–4000 4000–5000 >5000
Very low velocity Low velocity Middle velocity High velocity Very high velocity
Table 2 Values of the apparent density (q), UPV, anisotropy, UCS and Relative Average Deviation (RAD) for each block in dry conditions. Block
q (g/cm3)
RADq (%)
UPVx (m/s)
UPVy (m/s)
UPVz (m/s)
UPVmean (m/s)
Anis. (%)
UCS (MPa)
RADUCS (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1.751 ± 0.004 1.716 ± 0.010 1.632 ± 0.009 1.750 ± 0.002 1.660 ± 0.002 1.650 ± 0.004 1.647 ± 0.004 1.663 ± 0.003 1.719 ± 0.005 1.499 ± 0.006 1.641 ± 0.015 1.427 ± 0.006 1.664 ± 0.008 1.660 ± 0.007 1.657 ± 0.003 1.659 ± 0.005
0.21 0.57 0.55 0.13 0.10 0.23 0.23 0.18 0.28 0.43 0.89 0.42 0.46 0.39 0.16 0.32
3693 ± 25 3579 ± 51 3327 ± 36 3360 ± 12 3120 ± 12 3100 ± 15 3153 ± 21 3360 ± 33 2931 ± 22 2546 ± 26 2781 ± 29 2513 ± 31 2844 ± 28 2822 ± 28 2854 ± 28 2822 ± 23
3714 ± 28 3535 ± 64 3314 ± 40 3343 ± 13 3085 ± 14 3054 ± 16 3156 ± 24 3373 ± 13 2943 ± 23 2585 ± 72 2769 ± 30 2528 ± 13 2853 ± 32 2844 ± 21 2870 ± 16 2851 ± 47
3214 ± 13 2953 ± 30 3108 ± 23 2930 ± 5 3014 ± 15 2980 ± 16 3038 ± 16 3195 ± 11 2844 ± 11 2516 ± 29 2753 ± 27 2447 ± 35 2738 ± 33 2739 ± 25 2732 ± 22 2711 ± 33
3540 ± 9 3356 ± 29 3250 ± 30 3211 ± 7 3073 ± 4 3045 ± 3 3116 ± 6 3310 ± 12 2906 ± 13 2549 ± 38 2767 ± 28 2496 ± 24 2812 ± 25 2802 ± 21 2818 ± 21 2795 ± 27
13.8 ± 0.6 17.7 ± 1.9 6.7 ± 0.7 12.9 ± 0.2 3.4 ± 0.8 3.9 ± 1.0 4.2 ± 0.9 5.6 ± 0.7 3.5 ± 0.9 3.0 ± 1.7 1.2 ± 0.5 3.7 ± 1.1 4.3 ± 1.2 3.9 ± 0.7 4.8 ± 0.8 5.0 ± 1.8
30.1 ± 0.6 28.8 ± 0.9 22.2 ± 0.8 27.2 ± 0.7 20.5 ± 0.8 19.4 ± 1.2 20.0 ± 1.0 19.9 ± 0.6 21.0 ± 0.8 11.3 ± 0.4 16.4 ± 0.6 8.6 ± 0.7 20.7 ± 1.7 21.3 ± 1.4 19.7 ± 1.5 19.6 ± 0.9
2.0 3.2 3.5 2.5 3.8 6.2 5.2 2.9 4.0 3.3 3.7 7.9 8.3 6.7 7.4 4.7
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301 302 303 304 305 306 307 308 309 310 311 312 313
Block
Anisotropy (%)
Compressive strength (//)(MPa)
Compressive strength (\) (MPa)
1 2 3 4
13.8 ± 0.6 16.0 ± 1.5 6.5 ± 0.9 13.1 ± 0.4
29.6 ± 0.3 26.3 ± 0.7 22.2 ± 1.1 26.8 ± 0.6
30.9 ± 0.8 28.8 ± 0.9 23.4 ± 1.1 27.8 ± 1.4
number of 56 samples was used; seven specimens from each block were tested with the load applied along the z axis and the same number of samples were loaded along the perpendicular direction. Anisotropy and compressive strength values are reported in Table 4. A slight dependence of the compressive strength on the direction of the applied load was found. The Student’s t-test P-values (Fig. 4), which indicate the probability of obtaining statistically different mean values of the compared datasets, were calculated considering a significance level (a) of 0.01. The mean values of the compressive strength with the load applied along the parallel and the perpendicular directions with respect to the z axis are not statistically different for blocks 3 and 4 (P-values > 0.01); they
Fig. 4. Compressive strength with the load applied along the parallel and the perpendicular directions with respect to the z axis. Student’s t-test P-values are shown.
Fig. 5. Correlation between UPVmean and apparent density in dry condition.
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differ in the case of blocks 1 and 2 (P-value 6 0.01), though the difference is less than 2.5 MPa, namely <10%. Therefore, hereinafter the compressive strength values were reported without considering the direction of the anisotropy. Only a rough correlation (R2 = 0.53) between the measured values of UPV and apparent density was obtained (Fig. 5). Conversely, a good degree of correlation of the compressive strength with the dry density was found (R2 = 0.84, Fig. 6) that is in accordance with what was obtained by other authors for similar high porous limestones [30,42]. A moderate correlation was found between compressive strength and UPVmean in dry condition (R2 = 0.73, Fig. 7). The good correlation between UCS and density demonstrates a strict link between the mechanical properties of the stone and its bulk properties. Taking into account the petrographical characteristics, they mainly arise from packing rather than from cementation. On the contrary, ultrasonic velocities seem to be also influenced by factors other than density. UPV is very sensitive to the microstructure of the stones; as it was found for the anisotropy, more so than the mechanical resistance, it can be affected by the variations of the fabric features of Lecce stone. The composition of the stone related to the presence of clay minerals can also contribute to the different velocities of propagation, being that the clay content is an important parameter in reducing wave velocities. An
Fig. 6. Correlation between compressive strength and apparent density in dry and saturated conditions.
Fig. 7. Correlation between compressive strength and UPVmean in dry and saturated conditions.
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increase in clay content of a rock is usually associated with a decrease of velocity [43–46].
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4.3. Water influence
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Seven blocks (GP2 group of Table 1) were analyzed to evaluate the effect of the water on the density and on the UPV of the stone. Five samples were cut from each block and the above parameters were measured on the same samples in dry and saturated conditions. In Table 5 the values of the density of dry and saturated blocks and those of the water absorption coefficient (Ab) [34] are reported. They show noticeable increases of the density in saturated conditions, related to high water absorption coefficients (Ab), ranging from 11% to 18% in weight. A reliable correlation (R2 = 0.92) between dry and wet density was found (Fig. 8). In Table 6 UPV and anisotropy values in dry and saturated conditions are reported. In the presence of water a decrease of the UPV was recorded along the three directions, where the decrease is slightly higher along the z-axis. The decrease of the UPVmean was found between
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Table 5 Density of dry and saturated blocks and water absorption coefficients (Ab). 3
3
Block
Density-dry (g/cm )
Density-sat. (g/cm )
Ab (%)
13 14 15 16 17 18 19
1.668 ± 0.006 1.659 ± 0.006 1.669 ± 0.003 1.669 ± 0.003 1.758 ± 0.003 1.612 ± 0.004 1.611 ± 0.005
1.926 ± 0.006 1.915 ± 0.003 1.929 ± 0.004 1.928 ± 0.003 1.957 ± 0.003 1.892 ± 0.007 1.906 ± 0.004
15.50 ± 0.09 15.45 ± 0.19 15.59 ± 0.08 15.54 ± 0.14 11.29 ± 0.08 17.3 ± 0.4 18.3 ± 0.2
Fig. 8. Correlation between dry and saturated density of the blocks.
5% and 10%. On the contrary, a slight increase of the anisotropy was found for the saturated samples, with a difference between dry and wet conditions ranging from 1% to 3%. A good correlation (R2 = 0.86) was found by the regression analysis between the mean values of UPV measured on the blocks in dry and saturated conditions (Fig. 9). The decrease of UPV recorded in the presence of water does not have an immediate explanation. An increase of UPV in the presence of water has been shown for various types of rocks [6,23,47] and it is consistent with the fact that the ultrasonic wave velocity is higher in water than in air, and thus in water filled pores. Indeed, the phenomenon is rather complex. The effect of the water saturation on the increase of UPV has been found to be higher for low porosity rocks than for high porosity rocks [48,49]. The latter can show a great ranges of velocities when the rock is saturated, some being lower than those measured in dry conditions [5,39,46,50,51]. The propagation of the elastic waves in water-saturated porous media follows complex mechanisms that depend on many physical and chemical factors [52]. It is influenced by the porosity of the rock and by the ratio between the wave velocity within the mineral skeleton and the wave velocity within the fluid in the pores [53]. Water can influence the velocity within the rock skeleton. It can weak the calcium carbonate ionic bonds that hold the stone grains together leading to the weakening of the solid/solid contacts between the grains with the consequent decrease of the ultrasonic wave velocity in the stone skeleton. This effect, that has been reported by Atzeni et al. [39] for a similar highly porous limestone, could have great incidence on the microstructure of the Lecce stone, consisting of a particle framework densely packed but weakly cemented by poor and fine microsparitic calcite mixed to the groundmass; it could contrast the increase of the P-wave velocity in the pores filled by water, leading to the overall decrease of the velocity. A contribution to this result of the clay minerals present
Fig. 9. Linear correlation between UPV in dry and saturated conditions.
Table 6 UPV and anisotropy values measured for each block in dry and saturated conditions. Block
UPVx (m/s)
UPVy (m/s)
UPVz (m/s) Dry
UPVmean (m/s)
Anis. (%)
UPVx (m/s)
UPVy (m/s)
UPVz (m/s) saturated
UPVmean (m/s)
Anis. (%)
13 14 15 16 17 18 19
2863 ± 14 2854 ± 27 2823 ± 27 2829 ± 21 3402 ± 23 2687 ± 31 2677 ± 48
2908 ± 64 2861 ± 26 2869 ± 14 2894 ± 13 3439 ± 30 2688 ± 19 2657 ± 24
2764 ± 19 2738 ± 13 2760 ± 16 2756 ± 15 2954 ± 9 2659 ± 26 2630 ± 28
2845 ± 17 2818 ± 18 2817 ± 5 2826 ± 11 3265 ± 10 2678 ± 26 2654 ± 33
5.4 ± 2.2 4.7 ± 0.7 4.0 ± 0.5 4.8 ± 0.8 14.3 ± 0.8 1.3 ± 1.1 1.9 ± 0.8
2721 ± 20 2727 ± 30 2700 ± 21 2719 ± 16 3090 ± 48 2466 ± 66 2446 ± 37
2772 ± 47 2703 ± 36 2730 ± 27 2758 ± 39 3111 ± 33 2421 ± 39 2440 ± 33
2579 ± 31 2531 ± 17 2589 ± 25 2584 ± 25 2574 ± 11 2375 ± 39 2389 ± 18
2691 ± 8 2654 ± 24 2673 ± 15 2687 ± 18 2925 ± 27 2421 ± 47 2425 ± 22
7.1 ± 2.5 7.4 ± 0.9 5.4 ± 0.8 6.3 ± 1.7 17.4 ± 0.9 3.8 ± 1.0 3.6 ± 0.6
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Fig. 10. Compressive strength measured in dry and saturated specimens.
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within the Lecce stone is not to be excluded. Water can change the structure of clay minerals with an expandable lattice, by promoting swelling phenomena; ultrasonic velocity in expandable clay minerals is lower in wet conditions than in dry ones [54,55] and their presence in rocks contribute to the UPV decrease in saturated conditions [45]. The effect of water on the compressive strength was evaluated on four blocks (GP4 group of Table 1). The presence of water lead to a strong reduction of the compressive strength (Fig. 10), ranging from 44% to 50%. Similar decreases have been found by several authors for highly porous limestones [39,56,57]. The weakening of the strength of the interactions between the stone particles related to the effect of the water on the rock framework – already discussed in relation to the UPV results – could account for the reduction of the elastic properties and of the compressive strength of the stone. Good correlation between the compressive strength and the saturated density was found, with R2 = 0.82, close to the value measured in dry conditions (Fig. 6). Finally, regression analysis between the UCS and UPV mean values in saturated conditions showed a high value of R2, equal to 0.82. The slopes of the two regression lines, obtained in dry and saturated conditions (Fig. 7), are not equal, showing that the presence of water influences UPV and compressive strength in a different way. In fact, both UPV and compressive strength are lower in the presence of water but a stronger reduction can be appreciated on the compressive strength.
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5. Conclusions
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This study investigated the potential of using UPV as a simple and economical non-destructive technique for the physical and mechanical characterization of a highly porous and soft building limestone (Lecce stone), in order to contribute to sustainable low-impact investigation procedures for the analysis of historic masonries. Investigations were carried out in laboratory conditions; depending on the sample size, the measurements were performed using the appropriate frequency of 1 MHz. They were aimed to acquire necessary knowledge on the stone response to the test with reference to some basic parameters, prior to the in situ application for the mechanical and physical characterization of the masonry ashlars. Anisotropy and presence of water as factors influencing the ultrasonic wave propagation were investigated. Correlations between UPV, density and uniaxial compressive strength (UCS) were assessed in dry and saturated conditions by the regression analysis.
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Low UPV and compressive strength values that were obtained are consistent with the high porosity and weak cementation of Lecce stone. A slight anisotropy was detected by UPV and it was proved to have no significant effects on the compressive strength. A quite different response to the ultrasonic wave propagation compared to low porosity stones was found in the presence of water. Saturated stone exhibited lower UPV values than dry stone and a remarkable reduction of UCS, as well. The water’s influence on the chemical bonding forces between the highly packed but poorly cemented particles of the stone skeleton was suggested as leading to the weakening of the solid/solid interfaces between the grains and the consequent reduction of both UPV and mechanical resistance. A contribution of the clay minerals to the decrease of UPV in saturated conditions was also suggested. A reliable statistical correlation between UPV and UCS was found in both dry and saturated conditions; it allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as ultrasonic wave velocity, thus limiting destructive sampling of materials within buildings. A low statistical correlation was found between UPV and density, showing that also other factors affect UPV within the Lecce stone. Fabric variations and variable presence of clay minerals were suggested to account for these results, according to the high sensitivity of ultrasonic waves to detect intrinsic stone features related to textural and mineralogical variations; these do not reflect on the mechanical resistance, which results mainly from the packing density. A strong linear correlation with the density was found for UCS in dry and wet samples. Finally, the study contributes to better insight into the response of high porous and soft limestones to ultrasonic wave propagation and the reliability of the UPV technique in evaluating their compressive strength.
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Acknowledgement
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This research was supported by Puglia P.O. 2007-2013 FESR funds.
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References
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Contents lists available at ScienceDirect
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Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone
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Emilia Vasanelli a, Donato Colangiuli a, Angela Calia a,⇑, Maria Sileo a, Antonietta Aiello b
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a b
Institute for Archaeological and Monumental Heritage (CNR-IBAM.), via per Monteroni, 73100 Lecce, Italy University of Salento, Dept. of Engineering for Innovation, via per Monteroni, 73100 Lecce, Italy
a r t i c l e
i n f o
Article history: Received 17 September 2014 Received in revised form 4 February 2015 Accepted 9 February 2015 Available online xxxx Keywords: Physical–mechanical characterization Non-destructive test Soft building limestone Ultrasonic wave propagation Uniaxial compressive strength Water influence
a b s t r a c t UPV as non-destructive technique can effectively contribute to the low invasive in situ analysis and diagnosis of masonry elements related to the conservation, rehabilitation and strengthening of the built heritage. The use of non-destructive and non-invasive techniques brings all the times many advantages in diagnostic activities on pre-existing buildings in terms of sustainability; moreover, it is a strong necessity with respect to the conservation constraints when dealing with the historical–architectural heritage. In this work laboratory experiments were carried out to investigate the effectiveness of ultrasonic pulse velocity (UPV) in evaluating physical and mechanical properties of Lecce stone, a soft and porous building limestone. UPV and selected physical–mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. Factors such as anisotropy and water presence that induce variations on the ultrasonic velocity were also assessed. Correlations between the analysed parameters are presented and discussed. The presence of water greatly affected the values of the analysed parameters, leading to a decrease of UPV and to a strong reduction of the compressive strength. A discussion of the role of the water on these results is provided. Regression analysis showed a reliable linear correlation between UPV and compressive strength, which allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as the ultrasonic wave velocity. Low correlation between UPV and density was found, suggesting that other factors than density, related to the fabric and composition, also influence the response of the selected stone to the UPV. They have no influence on the UCS, that instead showed to be highly correlated with the packing density. Ó 2015 Published by Elsevier B.V.
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1. Introduction
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The conservation, rehabilitation and strengthening of historical buildings are increasingly required by modern societies, due to the need of preserving the memory of the past and keeping a record of cultural changes. Based on the appropriate knowledge and diagnosis of materials and structures, proper interventions can be adopted. According to the international charters of Athens cited by Venice [1], the diagnostic analysis of historical buildings should be carried out with the lowest degree of intrusion and with the fullest respect for their physical integrity, as per the principles associated with preserving objects of architectural and cultural heritage. To abide by these restrictions, the scientific community
54 55 56 57 58 59 60 61 62 63
⇑ Corresponding author at: CNR-IBAM, University Campus, Prov.le Lecce Monteroni, 73100 Lecce, Italy. Tel.: +39 0832 422208. E-mail address: [email protected] (A. Calia).
has moved to propose alternative non-destructive testing (NDT) that is to be applied to the investigation of masonry and construction materials. Sophisticated non-destructive techniques – such as ground penetrating radar, thermography, sonic and ultrasonic tomography, and laser scanner survey – have been developed and improved throughout the years. By using an integrated approach it is possible to reconstruct the morphology of the masonry structures and to detect the presence of structural failures, such as cracks and voids, in addition to the presence of water coming from rising damp or from seepage, thus achieving an accurate and reliable identification and diagnosis of the construction, which is the basis for the restoration design [2–4]. In the field of characterization and diagnosis of ancient buildings, the knowledge of the physical–mechanical properties of the constituent materials and their in-use conditions is of crucial importance; however, being able to obtain samples of these materials is still a major obstacle. To this
http://dx.doi.org/10.1016/j.ultras.2015.02.010 0041-624X/Ó 2015 Published by Elsevier B.V.
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regard, the ultrasonic pulse velocity (UPV) method can be conveniently used to obtain a variety of information on the material properties and state of conservation. Ultrasonic velocities are related to the properties of the stone and they can thus be applied to assess the quality of the stones [5,6], to detect any presence of microscopic fissuring and cracks [7,8], to evaluate the material decay [9–12] and to assess the effectiveness of consolidation products [13,14]. In material characterisation, UPV testing has been reported by several authors as a useful and reliable non-destructive tool for assessing the mechanical characteristics of concrete [15–20]; it has also been suggested in the field of rock analysis for the estimation of elastic and strength properties, as well as of physical parameters of the stones [21–27]. Intrinsic factors of the stone – including density, porosity, grain size, oriented structures, microcracks, etc. – affect the elastic wave propagation; external parameters associated with humidity, temperature, and mechanical stress are also involved and it is important to study in what manner and how much their variation may modify the characteristics of the waves being measured trough a specific material. This aspect is of crucial importance when in situ measurements are made so that the results of the measurements are interpreted properly. In this paper UPV technique for the physical and mechanical characterization of Lecce stone – a soft and porous limestone used as construction material in Southern Italy – has been investigated under laboratory conditions. UPV measurements were performed and selected physical–mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. The study was aimed to find correlations between them, as well as to evaluate factors that can affect correlations, such as anisotropy and water presence. The final aim was to verify the reliability of UPV as a tool for the estimation of the in situ compressive strength of Lecce stone, thus limiting destructive tests for the mechanical qualification and structural diagnostic analysis of ancient masonries in which this material is used.
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2. Material description
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Lecce stone is a soft, fine-grained biocalcarenite commonly used in the past as construction material in Southern Italy. In this area, buildings across multiple districts that are made with this stone can be found, such as those within the city of Lecce and surrounding historic towns, where it is the almost unique material used in Baroque architecture and minor buildings despite its low durability [28]. Similar soft limestones can be frequently found elsewhere within the historic built heritage [29,30] due to their high availability, easy extraction, and workability. Microscopically, Lecce stone consists of fine microfossil fragments – mainly planktonic Foraminifera and fossil debris within a micritic groundmass (Fig. 1) that also contains dispersed clay minerals. Grain size is mainly around a few tens of microns. Variable fabric relies on stone frameworks of wackestone and packstone types [31], densely packed, but poorly cemented by fine calcite with microsparitic texture; this is intimately mixed into the groundmass and irregularly distributed. The frequent presence of bioturbations contributes to alter the textural homogeneity of the stone. Porosity is mainly intergranular; intragranular holes are sometimes present in the form of microfossil cavities. As reported in [28], porosity measured by Mercury Intrusion Porosimetry ranges between 30% and 43%, where the bulk of the pores (50–70%) is between 0.5 lm and 4–6 lm in radius; the remaining pores are mostly of smaller radii, up to 0.02 lm. The mineralogical composition of Lecce stone primarily consists of calcite with low magnesium content; a not carbonatic insoluble residue that accounts for 3% up to 11% in weight is made of clay minerals and amorphous
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Fig. 1. Thin section image (crossed nicols) of Lecce stone showing the densely packed structure mainly consisting of fine fossil remains within a groundmass with a poor microcrystalline cement.
compounds, quartz, and phosphatic grains. Clay minerals include glauconite, illite, kaolinite, chlorite, but also smectite which has an expandable lattice; their presence has been demonstrated to have an influence on the durability of Lecce stone [32].
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3. Experimental methods
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In this research, nineteen blocks (50 25 25 cm) coming from three different quarries of the Lecce district were tested. Cubes having 70 mm sides were cut from the blocks, and 151 specimens were obtained in total. Each cubic specimen (Table 1) is identified by a capital letter (A–N) followed by the number related to the block of provenance (1–19). The specimens are organized in four groups. Group1and Group 2 include specimens used to test density, UPV and UCS in dry and wet conditions, respectively. The specimens belonging to Group 3 were used for to assess the anisotropy by UPV measurements and its influence on the UCS. Group 4 was used to compare the density, UPV and UCS in dry and wet conditions of samples coming from the same block. All the specimens were oven dried at 70 °C until constant weight measurements were reached [33]. Apparent density in dry and wet conditions was determined by mass volume ratio. Saturation of the samples was obtained by immersion in deionized water at room temperature, according to UNI EN 13755 [34]. The weight was determined by means of a digital balance with a precision of 0.1 g. The dimensions of each specimen were exactly determined for the calculation of the apparent volume. All three dimensions of each specimen were measured by means of an analogic caliper with a precision of 0.01 mm. Four measurements were taken for each direction and then the mean value was calculated.
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3.1. UPV measurements
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The ultrasonic pulse velocity (UPV) test was performed according to ASTM D2845-05 [35] on dry and saturated samples. No coupling agents were used for the UPV measurements, in order to avoid the possible penetration into the pores before saturating the specimens. Velocities were measured by the direct transmission method using an Epoch 4plus (Olympus) instrument and probes with a frequency of 1 MHz. They were recorded in each direction (x, y, z) of the cubic specimens and expressed as the average of three readings.
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Table 1 Specimens for each block and grouping.
Group 1 (GP1):
184 185 186 187 188
Group 3 (GP3):
Group 4 (GP4):
In UPV measurements, the frequency of the transducers, the pulse velocity, the minimum lateral dimension of the specimen, and the grain size of the rock are interrelated parameters. In order to obtain accurate measurements the relation (1) should be satisfied [35]:
189 191
Group 2 (GP2):
DP5
v f
P 5d
ð1Þ
204
where D is the minimum lateral dimension of the specimen, v is the pulse velocity, f is the frequency of the transducers, and d is the average grain size of the rock. Since v and d are intrinsic parameters of the stone, a transducer frequency of 1 MHz and a cubic specimen size of 70 mm satisfy the relation (1). An example of A-scan is reported in Fig. 2. The estimated relative error of the velocity measurements, obtained by combining the relative errors on time and thickness measurements, is about 0.3%. UPV measurements were performed on all the samples of the four groups in Table 1. Specimens belonging to GP2 group (blocks 13–19) were used to evaluate the UPV variation in the presence of water, performing the test on each of them in dry and saturated conditions.
205
3.2. Uniaxial compressive test
192 193 194 195 196 197 198 199 200 201 202 203
206 207
The compressive test was performed according to the UNI EN 772-1 standard [33] by means of a universal testing machine
Fig. 2. A-scan diagram obtained from the UPV measurement on a stone specimen.
.
(METROCOM Engineering spa) with a load capacity up to 200 kN and a load speed of 0.2 mm/min. The effect of the anisotropy was evaluated. The anisotropy values were calculated [36] for each sample as:
UPVmax UPVmin Anisotropy % ¼ UPVmax
ð2Þ
208 209 210 211
212 214
where UPVmax and UPVmin are, respectively, the maximum and minimum values of the UPV measured along the three axes. The samples from 4 blocks (blocks 1–4, GP3group in Table 1) that showed the highest values of the anisotropy along z direction were used. Seven specimens from each block were tested with the application of the load both parallel and perpendicular to the z axis. In order to assess the influence of the water on the compressive strength, saturated and dry samples belonging to the same block were compared. The samples of the GP4 group (blocks 13–16) in Table 1 were used; for each block, five specimens were tested in each dry and saturated condition.
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3.3. Data analysis
226
The values of the measured parameters were expressed for each block as the average of the measured or calculated values of the corresponding specimens; considering the number of the data, the dispersion was expressed in terms of the mean deviation. Comparisons between the mean values were based on a twotailed Student’s t-test, performed by Excel (MicrosoftÒ, Excel 2013). This test allows determining if two independent data sets, normally distributed, are significantly different from each other. A confidence interval of 99% was used to establish significance. Principal Component Analysis (PCA) [37] was performed by SIMCA (Umetrics, Sweden; Version 13.0). Before the multivariate statistical analysis, all the variables (UPVx, UPVy, UPVz and density) were scaled to unit variance (UV) in order to give equal importance to all of them.
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4. Results and discussion
241
4.1. Principal Component Analysis (PCA)
242
Principal Component Analysis (PCA) is a well-known statistical technique useful to identify patterns in data, by reducing the dimensionality of the data while retaining most of the variability in the dataset. It is based on the replacement of the original variables by a smaller number of their linear combinations (principal compo-
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nents). They identify directions, along which the variation of the data is maximal. Plotting of the resulting data allows a visual assessment of similarities and differences between samples, thus determining whether they can be grouped. A preliminary PCA of the density data and of the UPV values recorded along the x, y, and z directions for each sample of the GP1 group was performed; the first two Principal Components considered (t[1] and t[2]) accounted for 96.2% of the total variance. The score plot (Fig. 3) clearly shows a clustering of the specimens (total number 80) belonging to the same block. Since PCA is an unsupervised analysis, this result shows high homogeneity between the samples of the same block. On the basis of this outcome, the mean values of the analysed parameters were assumed to be representative of each whole block. The differences among the blocks are more significant compared to those among the samples of a single block. This supports the heterogeneity of the stone that was previously outlined.
4.2. Dry condition
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In Table 2, the mean values and the relative mean deviation for the apparent density, UPV, anisotropy, and compressive strength measured for each block in dry conditions are reported. Relative Average Deviation (RAD) is also reported for the apparent density (RADq) and compressive strength (RADUCS). As can be seen, the density of the blocks varies from 1.427 to 1.751 g/cm3 with low scatter levels for each of them; density values between 1.647 and 1.664 g/cm3 were measured for most of the blocks. Following the classification of the ultrasonic velocity (Table 3) [38], low ultrasonic velocities were recorded; the mean values of UPV vary considerably between the blocks from a minimum of 2496 m/s to a maximum of 3540 m/s and they are comparable to the velocity found in similar soft rocks [30,39,40]. Conversely, the dispersion of the data recorded for each block is low, supporting their homogeneity. The anisotropy values are mostly between 1.2% and 6.7%; higher degrees of anisotropy (13–18% approximately) were measured only for blocks 1, 2 and a variety of causes of anisotropy in rocks can be identified, namely orientation of crystals, grains, cracks and pores, stratification, and lamination structures [41]. In sedimentary rocks, such as the Lecce stone, the plane of anisotropy can be identified as the sedimentary plane. On the basis of the UPV data in Table 2, this corresponds to the xy plane, where the recorded values of UPV are higher than those measured along the perpendicular z direction. The compressive strength was generally found between 16.4 and 22.2 MPa (Table 2), with low to moderate levels of scatter (2.0–8.3%). Blocks 1, 2, and 4 show particularly higher values of the compressive strength (27.2–30.1 MPa), while lower values were recorded (11.3 and 8.6 MPa respectively) for blocks 10 and 12. The influence of the anisotropy on the compressive strength was investigated on the four blocks of the GP3 group (blocks 1–4 in Table 2), showing the highest degree of the anisotropy. A total
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Table 3 Classification of the ultrasonic velocities [38].
Fig. 3. Score plot of the PCA (2 components, R2Xcum(2PC) = 96.2%). The analysis was performed on the data (UPVx, UPVy, UPVz and density) of the overall 80 specimens of GP1 group. The shape of the symbols refers to the quarry provenance, while the color identifies each block. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Class
V (m/s)
Definition
1 2 3 4 5
<2500 2500–3500 3500–4000 4000–5000 >5000
Very low velocity Low velocity Middle velocity High velocity Very high velocity
Table 2 Values of the apparent density (q), UPV, anisotropy, UCS and Relative Average Deviation (RAD) for each block in dry conditions. Block
q (g/cm3)
RADq (%)
UPVx (m/s)
UPVy (m/s)
UPVz (m/s)
UPVmean (m/s)
Anis. (%)
UCS (MPa)
RADUCS (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1.751 ± 0.004 1.716 ± 0.010 1.632 ± 0.009 1.750 ± 0.002 1.660 ± 0.002 1.650 ± 0.004 1.647 ± 0.004 1.663 ± 0.003 1.719 ± 0.005 1.499 ± 0.006 1.641 ± 0.015 1.427 ± 0.006 1.664 ± 0.008 1.660 ± 0.007 1.657 ± 0.003 1.659 ± 0.005
0.21 0.57 0.55 0.13 0.10 0.23 0.23 0.18 0.28 0.43 0.89 0.42 0.46 0.39 0.16 0.32
3693 ± 25 3579 ± 51 3327 ± 36 3360 ± 12 3120 ± 12 3100 ± 15 3153 ± 21 3360 ± 33 2931 ± 22 2546 ± 26 2781 ± 29 2513 ± 31 2844 ± 28 2822 ± 28 2854 ± 28 2822 ± 23
3714 ± 28 3535 ± 64 3314 ± 40 3343 ± 13 3085 ± 14 3054 ± 16 3156 ± 24 3373 ± 13 2943 ± 23 2585 ± 72 2769 ± 30 2528 ± 13 2853 ± 32 2844 ± 21 2870 ± 16 2851 ± 47
3214 ± 13 2953 ± 30 3108 ± 23 2930 ± 5 3014 ± 15 2980 ± 16 3038 ± 16 3195 ± 11 2844 ± 11 2516 ± 29 2753 ± 27 2447 ± 35 2738 ± 33 2739 ± 25 2732 ± 22 2711 ± 33
3540 ± 9 3356 ± 29 3250 ± 30 3211 ± 7 3073 ± 4 3045 ± 3 3116 ± 6 3310 ± 12 2906 ± 13 2549 ± 38 2767 ± 28 2496 ± 24 2812 ± 25 2802 ± 21 2818 ± 21 2795 ± 27
13.8 ± 0.6 17.7 ± 1.9 6.7 ± 0.7 12.9 ± 0.2 3.4 ± 0.8 3.9 ± 1.0 4.2 ± 0.9 5.6 ± 0.7 3.5 ± 0.9 3.0 ± 1.7 1.2 ± 0.5 3.7 ± 1.1 4.3 ± 1.2 3.9 ± 0.7 4.8 ± 0.8 5.0 ± 1.8
30.1 ± 0.6 28.8 ± 0.9 22.2 ± 0.8 27.2 ± 0.7 20.5 ± 0.8 19.4 ± 1.2 20.0 ± 1.0 19.9 ± 0.6 21.0 ± 0.8 11.3 ± 0.4 16.4 ± 0.6 8.6 ± 0.7 20.7 ± 1.7 21.3 ± 1.4 19.7 ± 1.5 19.6 ± 0.9
2.0 3.2 3.5 2.5 3.8 6.2 5.2 2.9 4.0 3.3 3.7 7.9 8.3 6.7 7.4 4.7
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301 302 303 304 305 306 307 308 309 310 311 312 313
Block
Anisotropy (%)
Compressive strength (//)(MPa)
Compressive strength (\) (MPa)
1 2 3 4
13.8 ± 0.6 16.0 ± 1.5 6.5 ± 0.9 13.1 ± 0.4
29.6 ± 0.3 26.3 ± 0.7 22.2 ± 1.1 26.8 ± 0.6
30.9 ± 0.8 28.8 ± 0.9 23.4 ± 1.1 27.8 ± 1.4
number of 56 samples was used; seven specimens from each block were tested with the load applied along the z axis and the same number of samples were loaded along the perpendicular direction. Anisotropy and compressive strength values are reported in Table 4. A slight dependence of the compressive strength on the direction of the applied load was found. The Student’s t-test P-values (Fig. 4), which indicate the probability of obtaining statistically different mean values of the compared datasets, were calculated considering a significance level (a) of 0.01. The mean values of the compressive strength with the load applied along the parallel and the perpendicular directions with respect to the z axis are not statistically different for blocks 3 and 4 (P-values > 0.01); they
Fig. 4. Compressive strength with the load applied along the parallel and the perpendicular directions with respect to the z axis. Student’s t-test P-values are shown.
Fig. 5. Correlation between UPVmean and apparent density in dry condition.
5
differ in the case of blocks 1 and 2 (P-value 6 0.01), though the difference is less than 2.5 MPa, namely <10%. Therefore, hereinafter the compressive strength values were reported without considering the direction of the anisotropy. Only a rough correlation (R2 = 0.53) between the measured values of UPV and apparent density was obtained (Fig. 5). Conversely, a good degree of correlation of the compressive strength with the dry density was found (R2 = 0.84, Fig. 6) that is in accordance with what was obtained by other authors for similar high porous limestones [30,42]. A moderate correlation was found between compressive strength and UPVmean in dry condition (R2 = 0.73, Fig. 7). The good correlation between UCS and density demonstrates a strict link between the mechanical properties of the stone and its bulk properties. Taking into account the petrographical characteristics, they mainly arise from packing rather than from cementation. On the contrary, ultrasonic velocities seem to be also influenced by factors other than density. UPV is very sensitive to the microstructure of the stones; as it was found for the anisotropy, more so than the mechanical resistance, it can be affected by the variations of the fabric features of Lecce stone. The composition of the stone related to the presence of clay minerals can also contribute to the different velocities of propagation, being that the clay content is an important parameter in reducing wave velocities. An
Fig. 6. Correlation between compressive strength and apparent density in dry and saturated conditions.
Fig. 7. Correlation between compressive strength and UPVmean in dry and saturated conditions.
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increase in clay content of a rock is usually associated with a decrease of velocity [43–46].
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4.3. Water influence
341
Seven blocks (GP2 group of Table 1) were analyzed to evaluate the effect of the water on the density and on the UPV of the stone. Five samples were cut from each block and the above parameters were measured on the same samples in dry and saturated conditions. In Table 5 the values of the density of dry and saturated blocks and those of the water absorption coefficient (Ab) [34] are reported. They show noticeable increases of the density in saturated conditions, related to high water absorption coefficients (Ab), ranging from 11% to 18% in weight. A reliable correlation (R2 = 0.92) between dry and wet density was found (Fig. 8). In Table 6 UPV and anisotropy values in dry and saturated conditions are reported. In the presence of water a decrease of the UPV was recorded along the three directions, where the decrease is slightly higher along the z-axis. The decrease of the UPVmean was found between
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Table 5 Density of dry and saturated blocks and water absorption coefficients (Ab). 3
3
Block
Density-dry (g/cm )
Density-sat. (g/cm )
Ab (%)
13 14 15 16 17 18 19
1.668 ± 0.006 1.659 ± 0.006 1.669 ± 0.003 1.669 ± 0.003 1.758 ± 0.003 1.612 ± 0.004 1.611 ± 0.005
1.926 ± 0.006 1.915 ± 0.003 1.929 ± 0.004 1.928 ± 0.003 1.957 ± 0.003 1.892 ± 0.007 1.906 ± 0.004
15.50 ± 0.09 15.45 ± 0.19 15.59 ± 0.08 15.54 ± 0.14 11.29 ± 0.08 17.3 ± 0.4 18.3 ± 0.2
Fig. 8. Correlation between dry and saturated density of the blocks.
5% and 10%. On the contrary, a slight increase of the anisotropy was found for the saturated samples, with a difference between dry and wet conditions ranging from 1% to 3%. A good correlation (R2 = 0.86) was found by the regression analysis between the mean values of UPV measured on the blocks in dry and saturated conditions (Fig. 9). The decrease of UPV recorded in the presence of water does not have an immediate explanation. An increase of UPV in the presence of water has been shown for various types of rocks [6,23,47] and it is consistent with the fact that the ultrasonic wave velocity is higher in water than in air, and thus in water filled pores. Indeed, the phenomenon is rather complex. The effect of the water saturation on the increase of UPV has been found to be higher for low porosity rocks than for high porosity rocks [48,49]. The latter can show a great ranges of velocities when the rock is saturated, some being lower than those measured in dry conditions [5,39,46,50,51]. The propagation of the elastic waves in water-saturated porous media follows complex mechanisms that depend on many physical and chemical factors [52]. It is influenced by the porosity of the rock and by the ratio between the wave velocity within the mineral skeleton and the wave velocity within the fluid in the pores [53]. Water can influence the velocity within the rock skeleton. It can weak the calcium carbonate ionic bonds that hold the stone grains together leading to the weakening of the solid/solid contacts between the grains with the consequent decrease of the ultrasonic wave velocity in the stone skeleton. This effect, that has been reported by Atzeni et al. [39] for a similar highly porous limestone, could have great incidence on the microstructure of the Lecce stone, consisting of a particle framework densely packed but weakly cemented by poor and fine microsparitic calcite mixed to the groundmass; it could contrast the increase of the P-wave velocity in the pores filled by water, leading to the overall decrease of the velocity. A contribution to this result of the clay minerals present
Fig. 9. Linear correlation between UPV in dry and saturated conditions.
Table 6 UPV and anisotropy values measured for each block in dry and saturated conditions. Block
UPVx (m/s)
UPVy (m/s)
UPVz (m/s) Dry
UPVmean (m/s)
Anis. (%)
UPVx (m/s)
UPVy (m/s)
UPVz (m/s) saturated
UPVmean (m/s)
Anis. (%)
13 14 15 16 17 18 19
2863 ± 14 2854 ± 27 2823 ± 27 2829 ± 21 3402 ± 23 2687 ± 31 2677 ± 48
2908 ± 64 2861 ± 26 2869 ± 14 2894 ± 13 3439 ± 30 2688 ± 19 2657 ± 24
2764 ± 19 2738 ± 13 2760 ± 16 2756 ± 15 2954 ± 9 2659 ± 26 2630 ± 28
2845 ± 17 2818 ± 18 2817 ± 5 2826 ± 11 3265 ± 10 2678 ± 26 2654 ± 33
5.4 ± 2.2 4.7 ± 0.7 4.0 ± 0.5 4.8 ± 0.8 14.3 ± 0.8 1.3 ± 1.1 1.9 ± 0.8
2721 ± 20 2727 ± 30 2700 ± 21 2719 ± 16 3090 ± 48 2466 ± 66 2446 ± 37
2772 ± 47 2703 ± 36 2730 ± 27 2758 ± 39 3111 ± 33 2421 ± 39 2440 ± 33
2579 ± 31 2531 ± 17 2589 ± 25 2584 ± 25 2574 ± 11 2375 ± 39 2389 ± 18
2691 ± 8 2654 ± 24 2673 ± 15 2687 ± 18 2925 ± 27 2421 ± 47 2425 ± 22
7.1 ± 2.5 7.4 ± 0.9 5.4 ± 0.8 6.3 ± 1.7 17.4 ± 0.9 3.8 ± 1.0 3.6 ± 0.6
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Fig. 10. Compressive strength measured in dry and saturated specimens.
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within the Lecce stone is not to be excluded. Water can change the structure of clay minerals with an expandable lattice, by promoting swelling phenomena; ultrasonic velocity in expandable clay minerals is lower in wet conditions than in dry ones [54,55] and their presence in rocks contribute to the UPV decrease in saturated conditions [45]. The effect of water on the compressive strength was evaluated on four blocks (GP4 group of Table 1). The presence of water lead to a strong reduction of the compressive strength (Fig. 10), ranging from 44% to 50%. Similar decreases have been found by several authors for highly porous limestones [39,56,57]. The weakening of the strength of the interactions between the stone particles related to the effect of the water on the rock framework – already discussed in relation to the UPV results – could account for the reduction of the elastic properties and of the compressive strength of the stone. Good correlation between the compressive strength and the saturated density was found, with R2 = 0.82, close to the value measured in dry conditions (Fig. 6). Finally, regression analysis between the UCS and UPV mean values in saturated conditions showed a high value of R2, equal to 0.82. The slopes of the two regression lines, obtained in dry and saturated conditions (Fig. 7), are not equal, showing that the presence of water influences UPV and compressive strength in a different way. In fact, both UPV and compressive strength are lower in the presence of water but a stronger reduction can be appreciated on the compressive strength.
417
5. Conclusions
418
This study investigated the potential of using UPV as a simple and economical non-destructive technique for the physical and mechanical characterization of a highly porous and soft building limestone (Lecce stone), in order to contribute to sustainable low-impact investigation procedures for the analysis of historic masonries. Investigations were carried out in laboratory conditions; depending on the sample size, the measurements were performed using the appropriate frequency of 1 MHz. They were aimed to acquire necessary knowledge on the stone response to the test with reference to some basic parameters, prior to the in situ application for the mechanical and physical characterization of the masonry ashlars. Anisotropy and presence of water as factors influencing the ultrasonic wave propagation were investigated. Correlations between UPV, density and uniaxial compressive strength (UCS) were assessed in dry and saturated conditions by the regression analysis.
390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415
419 420 421 422 423 424 425 426 427 428 429 430 431 432 433
7
Low UPV and compressive strength values that were obtained are consistent with the high porosity and weak cementation of Lecce stone. A slight anisotropy was detected by UPV and it was proved to have no significant effects on the compressive strength. A quite different response to the ultrasonic wave propagation compared to low porosity stones was found in the presence of water. Saturated stone exhibited lower UPV values than dry stone and a remarkable reduction of UCS, as well. The water’s influence on the chemical bonding forces between the highly packed but poorly cemented particles of the stone skeleton was suggested as leading to the weakening of the solid/solid interfaces between the grains and the consequent reduction of both UPV and mechanical resistance. A contribution of the clay minerals to the decrease of UPV in saturated conditions was also suggested. A reliable statistical correlation between UPV and UCS was found in both dry and saturated conditions; it allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as ultrasonic wave velocity, thus limiting destructive sampling of materials within buildings. A low statistical correlation was found between UPV and density, showing that also other factors affect UPV within the Lecce stone. Fabric variations and variable presence of clay minerals were suggested to account for these results, according to the high sensitivity of ultrasonic waves to detect intrinsic stone features related to textural and mineralogical variations; these do not reflect on the mechanical resistance, which results mainly from the packing density. A strong linear correlation with the density was found for UCS in dry and wet samples. Finally, the study contributes to better insight into the response of high porous and soft limestones to ultrasonic wave propagation and the reliability of the UPV technique in evaluating their compressive strength.
434
Acknowledgement
466
This research was supported by Puglia P.O. 2007-2013 FESR funds.
467
References
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Please cite this article in press as: E. Vasanelli et al., Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.02.010
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