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Experimental Techniques

USING ULTRASOUND FOR IN-SERVICE ASSESSMENT OF RENDERED WALLS

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Flores-Colen, Inês; IST, DECivil de Brito, Jorge; IST, DECivil de Freitas, Vasco; Faculdade de Engenharia da Universidade do Porto, Civil Engineering;

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Keywords:

Technical Article

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Complete List of Authors:

EXT-T-1255.R2

Nondestructive Testing

Composites, Civil Structures Testing

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Re Society for Experimental Mechanics, Inc.

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USING ULTRASOUND FOR IN-SERVICE ASSESSMENT OF RENDERED WALLS Inês Flores-Colen1∗∗ Jorge de Brito2; Vasco Peixoto de Freitas3 1

Assistant Professor, Department of Civil Engineering, Architecture and Georresources, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal, [email protected]

2

Full Professor, Department of Civil Engineering, Architecture and Georresources, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal, [email protected]

3

Full Professor, Department of Civil Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal, [email protected]

ABSTRACT

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Ultrasound has been widely used for the diagnostic testing of structural elements, but little research has been done and no specific standard exist on its application to non-structural materials such as mortars. These coat-

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ings are usually applied on walls with more than one layer; therefore, the ultrasound technique needs more investigation to better understand the results for rendered walls and in-service conditions. This paper discusses

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the use of this technique, using the indirect method, when assessing the performance of cement-based mortars. Experimental work was carried out in the laboratory and in-situ for this purpose. It was possible

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to relate the pulse velocity to the mechanical properties of the mortars, proposing criteria to support the inservice performance.

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It was also possible to characterise the presence and severity of various anomalies, mainly discontinuities. Finally, when this technique was combined with others (on rendered walls or on extracted samples) the

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in-service diagnostic testing under real service conditions was less susceptible to subjectivity. The results showed that this technique has a great potential to characterize the in-service performance of renders and to help the degradation diagnosis. The specific influence factors of this technique for renders should be emphasized and separated from applications in concrete. KEYWORDS: in-situ; performance; ultrasound; façades; renders.

∗

Corresponding author: Inês Flores-Colen Telephone: +351 218418742 Fax: +351 219497650

Society for Experimental Mechanics, Inc.


1. INTRODUCTION The in-service assessment of building façades has essentially been based on the visual inspection of existing anomalies. This enables a qualitative characterisation of the in-service performance of coated surfaces, but it has several drawbacks: the assessment is subjective (it depends strongly on the experience of the inspector); access to examine the degraded areas of the façade is difficult; any diagnosis may be doubtful if the anomaly is at some depth [1; 2]. In-situ testing helps to improve the accuracy of the diagnosis of the pathology and degradation of the façade by supplying quantitative parameters that prevent over-conservative assessments of its true condi-

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tion, which consequently increase the cost of repairs. The technique or techniques used in the diagnosis depend significantly on the material, human and financial resources available, the purpose of the inservice assessment and the time needed to collect and interpret the data [2; 3]. Our work resulted in a pro-

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posal for a method to assess the in-service mechanical, physical and chemical performance of rendered façades that integrates in-service inspection methods: visual examination, diagnostic aids, in-situ and laboratory tests, with sample collection [1; 4]. The various findings of this research derive from a combina-

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tion of several laboratory testing techniques (experiments on mortar prisms and small-scale models of mortar applied to brick) and in-service tests (experiments on rendered façades with various types of mortar and levels of deterioration).

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This article sets out to discuss the potential of using the ultrasounds technique for in-service diagnostic testing

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of coated surfaces with cement-based mortar. It proposes criteria to support the interpretation of the results so

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as to improve the characterisation of in-service performance by means of quantitative measurement parameters. The article is therefore organised in three main sections, after this Introduction. The first section presents the ultrasound technique; it describes the test procedure, the measurement parameters, their suitability for in-service diagnosis and the variability in interpretation of the results, based on the literature review. Section two presents and discusses the results of the laboratory experimental programme, and section three does the same for the in-service experiments. The main conclusions are then presented. This article breaks new ground by proposing criteria for the in-service assessment of the performance of areas with and without deterioration (cracking and disaggregation), for cement-based mortar. It should help to encourage the wider use of this in-service technique as an indirect measure of the deformability and mechanical characteristics of mortar applied to the exterior walls of buildings.


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2. THE ULTRASOUND TECHNIQUE IN IN-SERVICE DIAGNOSIS Ultrasound equipment is often used in the non-destructive assessment of various materials, particularly concrete, to determine the propagation speed of waves. The electro-acoustical transducers produce longitudinal waves (compressive P-waves) with particle displacement in the path direction and faster, providing more useful information. The technique has been extensively deployed in the in-service diagnosis of structural elements [5; 6; 7], though there has been little research on its application to non-structural materials, such as cementitious mortars

[8; 9].

This technique has been used along with other in-situ techniques

such as pendulum hammer and thermography [10; 11] to study the performance of rendered walls in real in-

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service conditions. However, there are no agreed quantitative criteria to support the interpretation of the in-service performance results that supplement the testing procedures described in the technical literature. Our work seeks to contribute to the proposed quantitative criteria to assess the performance and character-

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ise the deterioration of rendering applied to brick walls, under real service conditions.

2.1 Description of the test technique and measurement parameters The test technique follows ASTM C 597

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and consists of coupling a transmitting and a receiving trans-

ducer on the surface of the coating being studied. The transducers are positioned on the same or opposite

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faces in indirect or direct transmission, respectively. The direct method (cross probing) is the most reliable from the point of view of transit time measurement as well as path length measurement. The indirect method

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(surface probing) is the least accurate because the signal received is subject to errors due to scattering, but it is more commonly used since is the only one possible in some in-service conditions (e.g. in the assessment

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of rendered walls).Therefore, in this study an indirect method was used. The transducers are placed at previously defined positions and the distance between them is measured. The values of the times and distances makes it possible to calculate the pulse velocity of the waves in the tested material (known as apparent velocity when the indirect rather than the direct method is used). The equipment PUNDIT used is shown in Figure 1; it generates low frequency ultrasonic pulses and measures the time taken for them to travel from one transducer to the other through the material tested. Before starting the test, the functioning conditions of the apparatus are verified by means of a calibration bar. This operation consists of matching the reading of the digital display with the reference value (reference time 26.2 µs) indicated on the calibration unit via the adjustment knob.



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Figure 1 - Source of low frequency (50 kHz) ultrasonic waves and calibration of the apparatus The test consists of measuring the time it takes an ultrasound wave to travel through a given material over a fixed distance, with a set of measurements. The location of the transmitting transducer is fixed, the receiver

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location is changed by fixed increments along a path, and a series of transit time readings are made (Figure 2). The ultrasound pulse velocity (in km/s) is calculated as the quotient of the distance between the transducers (in two positions) and the time taken (transit time) [13]. A Vaseline white jelly (petroleum hydrocarbons

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chemical group) was used as coupling material of the transducers to the wall surface.

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Figure 2 - Placement of transducers 100 mm apart, for the indirect reading of the transit time per diagram 2.2 Suitability of the technique for in-situ use The suitability of this technique for in-service use takes into account the work needed beforehand, during and after the test. Therefore, the technique can be characterised as follows [2]: •

Average acquisition cost of apparatus (over €2 500);


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•

It is quick and easy to use (the test takes less than 1 h);

• It provides useful information about existing degradation (in terms of internal heterogeneities and discontinuities); the measurements made need further in-situ testing to better interpret the results; • It is a non-destructive method (but it can affect the appearance of the tested surface, depending on the material used to couple the transducers with the surface); • The apparatus is portable but needs a power supply point (which can have some autonomy if it is charged the day prior to inspection); • No special training is needed to handle or use the apparatus;

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• It provides a quantitative analysis in terms of the results for the mechanical performance; •

No subsequent laboratory work is required;

•

Its use depends to a great extent on the means of access (this technique needs access to the wall;

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therefore, the means of access (e.g. scaffolding) are relevant when certain areas of the façade need to be tested).

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2.3 Variability of results

With the direct method, in the laboratory, this test shows a coefficient of variation of about 2% to 2.5% in con-

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crete, accurate to ± 1% for distances between 100 and 300 mm [14]. The indirect method leads to less satisfactory results [11], as does its in-situ use. Among the ways authors have suggested to reduce this variability in struc-

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tural concrete elements are to carry out sufficient tests [15], take measurements with a smaller distance between transducers [16], and take several readings of the transit time while keeping the transmitting transducer fixed and

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changing the position of the receiving one with constant increments of distance on a straight line. The average velocity can be given by the slope of the linear regression that best fits the readings obtained [17]. The variability of the ultrasound technique when used on rendering has been studied in several in-service buildings [11; 18].

2.4 Interpretation of results There are no standardised performance criteria for the technique when applied to rendered walls, since the results depend on the type of mortar, testing procedures and inspection conditions. Several studies have looked at the relationship of this parameter with other properties of mortar such as porosity and permeability

[19]

in degraded and non-degraded specimens. Velocity is slowed by the scattering of the elastic



waves by voids in the mortar

[20]

. According to the manual of the PUNDIT equipment and other studies

mainly on concrete materials [21 - 24], this technique is particularly useful to determine crack depth in surfaces when the indirect method is used. Also several studies mention that the size, shape, orientation and arrangement of discontinuities (inclusions, cracks, air bubbles or other internal defects) strongly influence the wave parameters, both the velocity of longitudinal waves (P-waves), and the attenuation and frequency content [21 - 23]. The ultrasonic pulse transmits a very small amount of energy through air. Therefore, if a pulse traveling through the material arrives at an air-filled crack whose projected area perpendicular to the path length is larger than the area of the transmitting transducer, the pulse will diffract around the defect. Thus, the pulse travel time will be greater than that through similar material without any defect [23]. The

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same authors pointed out difficulties in the interpretation of the results in the case of small cracks and flaws or if they are filled with water or the crack tip is not well defined

[23]

. So the decrease of the pulse

velocity sometimes enables the measurement of cracks depth.

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The propagation of ultrasonic or sound waves occurs through small localised deformations that are propagated. The propagation speed of the sound waves can be related to the dynamic modulus of elasticity [25], which is

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related to the ability of the render to absorb deformations (the smaller the first, the larger the second). In-service studies also show the multiplicity of factors that can affect the interpretation of ultrasound results

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when studying the mechanical performance of renders. Santos et al. [26] concluded that the results are affected by the presence of cracks and moisture on the wall. However, the combination of visual inspection, thermogra-

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phy, rebound hammer, moisture meter mapping and hammer tapping reduced the uncertainty of the in-service diagnosis. Additionally, Galvão et al. [27] studied several factors that can influence the in-service analysis, main-

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ly the type of render, presence of cracks, paint finishing, inspection conditions, distance between measurements and contact material used. In this study the cross-reference with the rebound hammer results was helpful for the interpretation of in-service assessment. According to the literature, the results of ultrasound measurements and calculated velocities depend on multiple factors: intrinsic characteristics of the mortar (porosity, apparent bulk density, modulus of elasticity...), presence of anomalies (cracks, loss of adherence, and loss of cohesion...) and also other aspects related with the technique and inspection procedures on-site. In this context, erroneous interpretations and bad decisions can be made due to oversimplification of the interpretation (for example disregarding some of the factors involved). To improve the results’ interpretation, the methodology should include the conjunction of several lab and field


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techniques. In this paper, two experimental programs were used. The first one, in the lab, was carried out to study the correlations between ultrasound measurements and other mechanical and physical characteristics of the render. The laboratory study does not include the analysis of degraded specimens. A second program included in-service testing of rendered walls in real conditions with some anomalies. In this last experimental program, the measurements occurred in degraded and non-degraded areas on the wall, and other in-situ measurements were also made using complementary techniques. This methodology is detailed in sections 3 and 4.

3. LABORATORY EXPERIMENTAL PROGRAMME

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3.1 General characterization

The laboratory experimental programme was designed to study the relationship between the in-service measurement parameters and the performance characteristics of the mortars and to establish reference parameters. The procedure to mix the samples followed EN 196-1[28]. Then, standard prims (three sam-

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ples per measured property, sized 4 x 4 x 16 cm3 in most tests, 2.5 x 2.5 x 28.5 cm3 in the dynamic elastic modulus test and 2.5 x 2.5 x 2.5 cm3 in the open porosity test) and nine small-scale models (1.5 cm layer

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of mortar applied on a ceramic brick over an area of 48 x 19 cm2) were produced (Figure 3).The produced mortars, applied in a single layer, are currently used in rendering of brick cavity-wall façades of buildings

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and are described next (Table 1), percentages in terms of ratio of water and mixing proportion in mass of binders and aggregates; •

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PM - cement-based coloured pre-dosed mortar - with white Portland cement (type I), air lime,

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sand, admixtures (0.12-0.50% of hydrophobic agent, 0.0025-0.0050% of air entrained agent, 0.05-0.15% of water retention agent - cellulose ether, 0.10-5% of pigments) and 1-2.5% of lightweight fillers; •

PL - cement-based pre-dosed mortar (to be used with final paint finish) - with white Portland cement (type I), air lime, sand, admixtures (0.10-0.50% of hydrophobic agent, 0.05-0.10% of water retention agent - cellulose ether) and addition (0-0.1% fibres);

•

PP - cement-based pre-dosed mortar (more compact, for when ‘heavier’ coatings are used, such as ceramic tiles - with white Portland cement (type I), sand, admixtures (0.10-0.5% of hydrophobic agent, 0.05-0.10% of water retention agent - cellulose ether);

•

PC - traditional or made-on-site mortars - with white Portland cement (type I) and sand, without



any admixtures or additions; •

PB - traditional or made-on-site mortars - with white Portland cement (type I), air lime and sand, without any admixtures or additions.

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Figure 3 - Standard prisms of rendering mortar (left), and small-scale models (centre and right)

The traditional PC and PB were prepared for comparison purposes, with similar characteristics to made

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on-site mortars that are normally applied as multi-coat systems. The laboratory tests were carried out only at 28 days, the age that is the common reference for laboratory

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studies. The purpose was to characterize the mechanical behaviour at that age. It was not the aim of the laboratory study to characterize the evolution of each mechanical property. 3.2 Apparent propagation velocity of ultrasonic waves

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The test procedure to determine the apparent propagations velocity of ultrasonic waves is set out in the American standard ASTM C597 [12], for concrete. The general procedure is described in section 2.1 (De-

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scription of the test technique and measurement parameters). In this experimental program, the transduc-

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ers were placed on five points on the surface of the render (indirect or surface transmission) with a distance between them of 100 mm (Figure 4), equal to the minimum recommended for concrete elements in the indirect method. The measurements occurred in one path in two directions (A to B and B to A). The apparent ultrasonic pulse velocity was determined between each two points (at distances 100 mm, 200 mm and 300 mm in both directions). Table 2 shows the individual results of the pulse velocity, the average of the results for each render and also the equations and definitions.


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A B

Figure 4 - Measurement of ultrasonic waves transit time by indirect method on rendered brick model, the path length has a total of 300 mm and two directions: A to B and B to A, with the transmitter on A or B

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position respectively

3.3 Discussion of results of the laboratory tests Table 1 shows the apparent propagation velocity values range from 2760 m/s to 3791 m/s for the mortars

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analysed, which is the same order of magnitude as reported by other authors [29]. The coefficients of variation of the results were no more than 13% (Table 2), which seems acceptable for this kind of test, which uses

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the indirect method to measure the propagation time. Based on the results from Table 1, the laboratory work established the relationship (R2 > 0.6, power trend) between apparent wave propagation velocity (measura-

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ble on the brick + render) and characteristics determined by standardised tests (normally used in the technical specifications [1, 4]). An increase of apparent pulse velocity corresponds to an increase of the compres-

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sive strength (R2 = 0.62, power trend) and of the apparent bulk density (R2 = 0.94, power trend) obtained from geometric measurement. On the other hand, an increase of apparent pulse velocity leads to a decrease

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of the open porosity (R2 = 0.68, power trend) determined by the Archimedes principle (samples taken from the models and immersed for a minimum of 7 days with a pressure of 88 cm using a pressure pump) and of the diameter of the notch in Martinet Baronnie impact test (R2 = 0.96, linear trend) that consists on impacting a steel sphere that is equivalent to an energy of 3 Joules[30]. The results have also shown a good correlation between apparent ultrasonic pulse velocity and dynamic elastic modulus (R2 = 0.88, exponential trend, with Vapparent < 3.5 km/s ⇒ Ed < 10 000 MPa for a first group of mortars, Figure 5), despite using indirect method (surface transmission). From the literature, the ultrasonic waves are directly influenced by its elastic parameters (relation between shear velocities and Young’s modulus for homogeneous material), and the elastic modulus depends on porosity, therefore the pulse velocity decreases when porosity increases [19].



The laboratory results show that mortars with good mechanical performance (more compact, with higher internal and superficial strengths, less deformability capacity and less internal voids) also have higher apparent pulse velocity values. Therefore, this technique has the potential to help the characterization of the mechanical performance of the applied mortars on a substrate, especially when the use of other standardized techniques is not possible. Therefore, this technique has the potential to be integrated in a field diagnosis when these limitations occur.

PP

16000

2

Rexp = 0.8804

14000

± 1000 MPa

12000 10000 8000 6000 4000 2000 0 0

0.5

PC PM

1

± 0.1km/s

PL

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Dynamic elastic modulus (MPa)

18000

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1.5

2

2.5

3

3.5

4

Apparent ultrasonic pulse velocity (km/s)

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Figure 5 - Relationship between apparent ultrasonic pulse velocity and dynamic elastic modulus from laboratorial test results 3.4 Proposed criteria

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The mechanical and physical performance of these five mortars was studied in the lab using 13 techniques and 23 parameters, as detailed in [1]. The limits proposed are based on these results. In this paper,

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the focus is only on one of those parameters (apparent ultrasonic pulse velocity). In this section, the proposed quantitative criterion intends to support the use of ultrasound technique in in-service performance assessment, revealing the suitability of this technique for field diagnosis. Thus results of the experimental tests enabled the mortars studied, mostly cement-based, to be split into two main groups (I and II) with quite distinct performance features. Group I contains all the pre-dosed cementitious mortars (one-coat, either pre-coloured or to be painted as a final finish), the majority of lower density pre-dosed mortars, with an apparent bulk density of less than 1550 ± 150 kg/m3. Group II contains cementitious traditional mortars (made on site), with apparent bulk density of more than 1550 ± 150 kg/m3. But some of the pre-dosed mortars, especially those intended to receive heavier finishes, could


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belong to either group, depending on the manufacturer’s formulation. The distinction between the two groups is important when, in an in-situ assessment, the type of mortar applied is not known and so comparison with the characteristics specified by the designer or declared by the manufacturer in the design and supply stages is not possible. A differentiating value for the apparent propagation velocity of the ultrasonic waves has therefore been proposed for the two groups (Vap = 3.3 ± 0.4 km/s), that is, based on this study it is held that the apparent propagation velocity of the ultrasonic waves of the group I mortars is lower than the reference value, and that of the group II mortars is higher. It was not possible, in the laboratory study, to assign minimum limit

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values to group I or maximum ones for group II because while the number of mortars represents the most common ones, it is limited (there may be others on the market that have better mechanical performance due to the incorporation of various additions and/or admixtures). In addition, the laboratory study does not include the analysis of degraded specimens, which could have led to much lower values.

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4. IN-SITU EXPERIMENTAL TESTS

4.1 General characterization

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After the laboratory work, a number of inspections of different rendered façades of different ages that

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were exposed to real in-service conditions were carried out. The in-service programme set out to assess the application of the test technique and determine the relevant in-service parameters and their potential for improving in-situ diagnosis.

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The case study comprised 42 buildings and around 100 exterior facings coated with pre-dosed or traditional mortar and with a variety of finishing coatings [1, 31]. The ultrasounds technique was used in 8 case studies (EC01, EC02, EC04, EC05, EC06, EC11, EC12, EC13), usually on more than one rendered wall surface and with different orientations (Table 3). In general, the readings were taken on paths located in a current area, with increments in a straight line of distances of 100 or 200 mm between the transmitting and receiving transducers. A total of 89 paths (see column “No. of paths” in Table 3) were analysed, varying in length from 300 mm to 1100 mm. On some surfaces it was possible to analyse different paths in non-degraded areas by visual assessment, or with several anomalies (dirt, cracking, disaggregation or pulverulence), characterised by additional diagnostic tools (e.g. magnifying glass, binoculars, optical microscope) (Figure 6, left). The total number of measurements on each path varied depending on the



case study (Figure 6, right), but at least two measurements were taken per path (forward and back).

Figure 6 - Measuring micro-cracking with optical microscope (left), and measurements on surfaces at 13

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and 35 months (right)

The pulse velocity calculations in each path used the procedure described in section 3.2, but in this case the areas with and without anomalies were told apart. The in-situ results are summarised in Table 3. The

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laboratory study described in section 3 was carried out only to characterize the expected mechanical behaviour of mortars, without degradation. Thus the proposed criteria (section 3.4) intended to support in

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field diagnosis in non-degraded areas (with no visible anomalies column in Table 3). However, the in-situ assessments described in this section intend to show the influence of anomalies on the pulse velocity

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values. With this in-service experimental program, the usefulness, variability and potential of ultrasound technique to study the in-service mechanical performance of the render are therefore discussed.

4.2 Discussion of results of in-service tests

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The results summarised in Table 3 enable us to conclude that the variability of the technique should be analysed as follows: •

From what is associated with repeated readings on the same path - the maximum coefficient of variation obtained was 16% and corresponds to the variability of the technique. It is slightly higher than that obtained in the laboratory (with the same procedure), which was 13%, as mentioned. These minor differences could be linked to the differing in-service exposure conditions.

•

From what is linked to the results for different paths on the same surface - the maximum coefficient of variation reached 45%, which expresses the variability of the sampling, that is, the different in-service behaviour of the render on the same surface under the same in-service conditions. This variability characterizes the variation of the state of degradation, together with eventual differences in the appli-


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cation conditions of the render in the same wall. But this value is still below the 50% reported by Gron et al. for in-situ tests [32], and in fact the service factors contribute considerably to the variability of the in-service analyses, which does not occur in controlled laboratory conditions). It was possible to see sound areas (no visible anomalies) on most of the surfaces, except in two cases of generalised degradation. Examination of the values for these areas (Figure 7) showed that the technique confirmed the expected mechanical behaviour in terms of apparent propagation velocity for EC01 and EC02 (PM with Vap < 3.3 ± 0.4 km/s, group I; TC with Vap > 3.3 ± 0.4 km/s, group I). The levels for EC04, EC11, EC12 and EC13 had lower than expected values, bearing in mind the type of mortar and respective group (I or II). In the

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case of EC04, despite being from group I, the PM render showed lower values of pulse velocity by around 2.0 km/s. The EC11, EC12 and EC13, the cementitious traditional renders, with expectable values higher, showed pulse velocity below 3.3 ± 0.4 km/s. Thus the technique can indicate the presence of some internal deterioration that was not detected in the visual inspection. In these circumstances, the technique may be inconclusive if used

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on its own, but it can enhance the reliability of an analysis if used in conjunction with other in-situ or laboratory techniques, with the testing of extracted samples. Indeed, the use of other tests confirmed the flawed in-service

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behaviour of some of the rendered surfaces studied [1]. The pendulum hammer, impact Martine Baronnie test and pull-off tests were also carried out in the field diagnosis of the rendered walls. In some cases, the open

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porosity, the dry bulk density and the compressive strength were determined in the lab by testing extracted samples [1, 4]. In this context, the ultrasound technique in conjunction with other in-situ techniques can reduce

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the uncertainty of the in-service diagnosis in terms of the expected mechanical behaviour. One example is the coloured pre-dosed mortar applied to the EC04 walls (Figure 7), where it is concluded

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that there is defective adherence of the mortar to the base which has led not only to widespread cracking, visible on the surface, but to internal cracking too (detected by ultrasound).

The in-service programme also made it possible to associate percentages of loss of apparent propagation velocity of the waves, depending on the anomaly (divided in nine types A 1 to A9, Table 3 and Figure 8), taking the sound areas identified visually as reference (Table 3, column “initial good areas”). Figure 8 summarises the reductions in the apparent pulse velocity by anomaly type. For example, for anomaly A5 (cracking 0.1 mm to 0.2 mm), the reductions of pulse velocity when compared with the good areas results are: for EC11 - 31% for F/S wall; 18% for F/O wall; 44% for F/N wall; 19% for F/E wall; for EC12 - 43% for F/S wall; 32% for F/O wall; 30% for F/N wall. The coefficients of variation for all the cases studied vary from



4% (for A9 - transition zones between materials) to 62% (for A4 - micro-cracking from 0.05 mm to 0.1 mm or areas with blistering), which shows how hard it is to obtain a direct relationship between the anomalies that are visible and their depth in the coating. It is even harder for superficial anomalies such as microcracking up to 0.1 mm wide or in places with surface dirt, where there were also velocity values that did not lead to decreases relative to the sound areas (these values are highlighted in Table 3).

Vap (km/s)

Apparent pulse velocity in-service rendered walls 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

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10

15

20

25

Rendered walls (sound areas, without visible anomalies)

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Figure 7 - Summary of average apparent propagation velocity of ultrasonic waves in 23 facings of 6 case studies (EC01, EC02, EC04, EC11, EC12 and EC13), in areas deemed sound on visual assessment, for

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different mortar coatings (pre-dosed coloured, PM, for painting, PL, and traditional cementitious, TC, or with cement and hydraulic lime, TB)

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Decrease relative to visually sound areas (%)

Reduction of apparent pulse velocity of ultrasonic waves by anomaly 0 -10 -20 -30 -40 -50 -60

A1

A2

A3

A4

A5

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A6

A7

A8

A9

Anomalies

Figure 8 - Reductions in the apparent propagation velocity of the ultrasonic waves by anomaly type (T1 = dirt; T2 = micro-cracking ≤ 0.05 mm; T3 = pulverulence or disaggregation at surface or peeling paint; T4 = microcracking, 0.05 mm to 0.1 mm or areas with blistering; T5 = cracking 0.1 mm to 0.2 mm; T6 = pulverulence or medium disaggregation; T7 = cracking 0.2 mm to 0.5 mm; T8 = pulverulence or major disaggregation, at depth; T9 = discontinuities up to the base (transition zones between materials) or detachment


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It was concluded from the analysis that ultrasound is very sensitive to existing cracking and allow an indepth assessment. Regarding disaggregation, the velocity decreased by between 25% and 50%, which seems to indicate different degrees of pulverulence or disaggregation in the render. In the cases examined, some areas had blistering and poor adherence of the render to the base. Despite the decrease (of 25% in the part with blistering), the number of cases showing this anomaly was too small to let it be concluded that this technique could be used to assess the adherence of the mortar to its base (mostly ceramic brick, in the façades studied). Nor were other aspects in the results of the in-service programme conclusive, in particular the influence of several layers of coating (as in the traditional mortars) or that of the type of

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surface finish (in this case, a coat of water-based paint on the final finishing of some surfaces). In addition, analysis of the case studies showed the great sensitivity of this technique relative to the moisture content at the surface of the surfaces, even in inspections in dry weather [1], as reported by other researchers [33] when applying it to concrete elements. The surface moisture meter was measured using a moisture meter device (Figure 9).

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For example, for EC06 the surface moisture content on a wall was mapped with an area of 1.2 m x 1.4 m.

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54 measurements of surface moisture were carried out. Figure 10 indicates the average value of surface moisture at the top (MC = 22%) and the bottom (MC = 35%) of the target area. In this figure the pulse

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velocity is also indicated. These results showed differences of 13% in surface moisture, and 2.3 km/s in the apparent propagation velocity of the ultrasonic waves.

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Finally, it was still possible to analyse some parameters in EC01 over time, and measure the apparent propagation velocity of the ultrasonic waves at 13 and 35 months (Figure 5, right). It was found that the

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velocity fell 13%, including sound (no visible anomalies) and cracked areas, which could indicate some increase in the depth of cracking in the almost two-year period. It should be noted that in this period the average opening of cracks increased slightly, as ascertained by a crack comparing device. However, only continued monitoring can determine the actual increase in crack depth, in real in-service conditions, since the change found is of the same order of magnitude as the variability of this test technique, as mentioned earlier (maximum coefficient of variation from 13% to 16% for the results of applying the technique in the laboratory and in-situ, respectively, excluding the scatter associated with sampling).



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Figure 9 - Moisture meter device to measure the surface moisture content on the east façade of case study EC06, during specific inspection conditions (air temperature of 26.3 ºC and relative humidity 42.8%)

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Figure 10 - Map with ultrasound (pulse velocity Vap) and moisture meter (surface moisture content Hsup) results for a degraded area on the east façade of case study EC06 5. CONCLUSIONS Standardised procedures have been established for using ultrasound to test concrete elements and these may be extrapolated to the assessment of in-service rendered façades (though the technique is not yet widely used for this). Our study assessed the potential of this technique to find information useful to inservice diagnosis. In this paper, two experimental programs are presented. The first one, in the lab, was carried out to study the correlations between ultrasound measurements and other mechanical and physical characteristics of the renders. The laboratory study does not include the analysis of degraded specimens. A second pro-


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gram included in-service testing of rendered walls in real conditions with some anomalies. In this last experimental program, the measurements occurred in degraded and non-degraded areas on the wall, and other in-situ measurements were also made using complementary techniques. Although the apparatus is easy to use and only moderately expensive, there are some limitations in terms of performance analysis as it is significantly influenced by the inspection conditions, such as means of access (e.g. scaffolding) and existing degradation (internal cracking that cannot be seen on the surface and high moisture content at the surface). The in situ results show that there is a relationship between the decrease of the apparent velocity com-

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pared with the velocity measured in areas without visible problems and the type of anomaly: microcracking of 0.05 mm and surface loss of cohesion led to a 10% decrease; cracking between 0.05 mm and 0.2 mm led to a 20 - 30% decrease; cracking of 0.2 mm to 0.5 mm and significant loss of cohesion led to a 4-50% decrease, and values above 50% were found for discontinuities that reached the substrate.

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Furthermore, in the laboratory, the propagation velocity of the ultrasonic waves was found to have good dependency relationship with other mechanical characteristics, in particular the dynamic modulus of

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elasticity (R2 = 0.88, exponential trend), and compressive strength (R2 = 0.62, power trend). It can thus be regarded as an indirect method to extrapolate the mechanical performance of coatings in-situ, even though

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the in-service correlations were considerably lower for the case studies analysed. Our study also made it possible to propose and discuss, in the laboratory, expected limits for two groups

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of cementitious mortars, and after that assessing through real case studies. The groups were: I - less compact renders with apparent bulk density of less than 1550 ± 150 kg/m3 and apparent pulse velocity below

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3.3 ± 0.4 km/s; and II - more compact renders, with apparent bulk density above 1550 ± 150 kg/m3 and pulse velocity above 3.3 ± 0.4 km/s. The groups and limits proposed are also supported by the conjunction of other techniques that are detailed in reference [1]. Also, these criteria have been used in other laboratory and in-service performance studies of renders, helping the diagnosis, especially in conjunction with other in-situ techniques, such as the pendulum hammer, and laboratory analysis of samples to determine compressive strength, porosity and apparent bulk density. It is expected that this approach will help to encourage research that is more performance based, with a view to the assessment and long-term monitoring of performance, which is only possible if there are parameters that can be measured in-situ. With this approach the reliability of the in-service anal-



ysis is greatly enhanced, at relatively little extra cost.

Acknowledgements Thanks are due to the FCT and to the ICIST - IST research centre for their support.

References [1] Flores-Colen I, de Brito J., Freitas VP. On-site performance assessment of rendering façades for predictive maintenance, Structural Survey 2012, 29 (2): 133-146. [2] Flores-Colen I, de Brito J, Freitas VP. Expedient in situ test techniques for predictive maintenance of

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rendered façades. Journal of Building Appraisal 2006, 2 (2): 142-156. [3] Flores-Colen I, de Brito J, Branco FA. In situ adherence evaluation of coating materials. Experimental Techniques 2009; 33 (3): 51-60.

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[4] Flores-Colen I, de Brito J, Freitas VP. In-service parameters from façade rendering mortars: Bulk density and open porosity determined from samples collected in situ. Structural Survey 2012; 28 (1): 17-27.

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[5] McCann DM, Forde MC. Review of NDT methods in the assessment of concrete and masonry structures. NDT & International 2001, 34(2): 71-84.

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[6] Achenbach JD. Quantitative nondestructive evaluation. International Journal of Solids and Structures 2000, 37(1-2): 13-27.

[7] Mahmood TM. Use of combined ultrasonic and rebound hammer method for determining strength of

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concrete structural members. Concrete International 1981, 3(3): 25-29.

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[8] Subramaniam KV, Lee J. Ultrasonic assessment of early-age changes in the material properties of cementitious materials. Materials and Structures 2007, 40(3): 301-309.

[9] Reinhardt HW, Grobe CU, Herb AT. Ultrasonic monitoring of setting and hardening of cement mortar - A new device. Materials and Structures 2000, 33(0): 580-583. [10] Santos C; Matias L; Magalhães AC; Veiga MR. Application of thermography and ultra-sounds for wall anomalies diagnosis. A laboratory research study. International Symposium Non-Destructive Testing in Civil Engineering 2003, Berlin, DGZIP, CD. [11] Santos LA, Flores-Colen I, Gomes MG. In-situ techniques for mechanical performance and degradation analysis of rendering walls. Restoration of Buildings and Monuments. 2013, 19(4): 255-266. [12] ASTM. Standard test method for pulse velocity through concrete. ASTM C597: 2009. Philadelphia,


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American Society for Testing Materials. [13] CEN. Testing concrete - Part 4: Determination of ultrasonic pulse velocity. EN 12504-4: 2004. Brussels, Comité Européen de Normalisation. [14] Nepomuceno MC. Non-destructive concrete tests (in Portuguese). Synthesis work. 1999. Covilhã, UBI, 428 p. [15] Malhotra VM. Testing early-age strength of concrete in-place. Concrete International 1985, 7(4): 39-41. [16] Stergiopoulou C, McCuen RH, Aggour MS. Ultrasonic testing of concrete structures using indirect transmission 2008, 36(2): 8 p.

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[17] Laranja R.; de Brito J. Structural assessment of existing concrete buildings, Progress in Structural Engineering and Materials 2003, 5(2), pp. 90-98. [18] Galvão J, Flores-Colen I, de Brito, J. In situ testing to evaluate the mechanical performance of rendered facades - rebound hammer and ultrasound techniques. XII DBMC - International Conference on

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Durability of Buildings Materials and Components 2012, FEUP, Porto, CD. [19] Goueygou M, Lafhaj Z, Kaczmarek M. Relationship between porosity, permeability and ultrasonic

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parameters in sound and damaged mortar. International Symposium Non-Destructive Testing in Civil Engineering 2003, Berlin, DGZIP, CD.

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[20] Tan K, Chan B, Guan L. Ultrasonic evaluation of cement adhesion in wall tiles. Cement & Concrete Composites 1996, 18, pp. 119-124

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[21] Shiotani T, Aggelis DG. Wave propagation in cementitious material containing artificial distributed damage. Materials and Structures 2008, RILEM, DOI 10.1617/s11527-008-9388-4.

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[22] Aggelis DG, Momoki S. Numerical simulation of wave propagation in mortar with inhomogeneities. Technical Paper. ACI Materials 2009, 5 p.

[23] Naik TR, Malhotra VM, Popovics JS. Chapter 8 - The ultrasonic pulse velocity method. Handbook of NDT of Concrete. 2nd edition. Ed. V. M. Malhotra and N. J. Carino, CRC Press LLC, USA; 2004. [24] Sutan NM, Meganathan M. A comparison between direct and indirect method of ultrasonic pulse velocity in detecting concrete defects, Russian Journal of Nondestructive Testing 2003, 8(5), pp. 1-9. [25] BSI. Testing concrete. Determination of ultrasonic pulse velocity BS EN 12504-4:2004. London, British Standards Institution. [26] Santos LA, Flores-Colen I, Gomes MG. In-situ techniques of mechanical performance and degrada-



tion analysis of rendering walls. Restoration of Buildings and Monuments 2013, 19, 4, pp. 255-266. [27] Galvão J, Flores-Colen I, de Brito J. In-situ testing to evaluate the mechanical performance of rendered façades - Rebound hammer and ultrasound techniques. 12th DBMC International Conference on Durability of Building Materials and Components 2011, pp. 991-999. [28] CEN. Methods of testing cement - Part 1: Determination of strength. EN 196-1: 2005. Brussels, Comité Européen de Normalisation. [29] Veiga MR., Velosa A, Magalhães A. Experimental applications of mortars with pozzolanic additions: characterization and performance evaluation. Construction and Building Materials 2009, 23(1), pp. 318-327.

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[30] Flores-Colen I, de Brito J, Freitas VP. Expedient render performance assessment based on impact resistance in situ determination. Construction and Building Materials 2009, 23 (9), pp. 2997-3004 [31] Flores-Colen I, de Brito J, Freitas V. Assessment of in-use performance parameters of rendering façades. in A State-of-the-art report on building pathology. CIB W86 Report. Publication 393, Ed. V. P. Freitas, Rotterdam; 2013.

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[32] Gron C, Falkenberg JA, Andersen JS, Borresen M, Pettersen A. Quality control manual for field

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measurements. NT Technical Report TR581: 2005. Helsinki, Nordic Innovation Centre, 86 p. [33] Ohdaira E, Masuzawa N. Water content and its effect on ultrasound propagation in concrete - the

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possibility of NDE. Ultrasonics 2000, 38(0): 546-552.

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TABLE CAPTIONS Table 1 - Results of testing programme for hardened rendering properties in renders, at 28 days [30] Table 2 - Individual results of ultrasound pulse velocity measured through the indirect method on smallscale models Table 3 - Visual appraisal and apparent pulse velocity measurements of in-service assessments

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Table 1 - Results of testing programme for hardened rendering properties in renders, at 28 days [30]

Render type

w/c

Mixing proportions in mass*

Fresh bulk density 3 (kg/m ) EN 1015-6

Tests on standard prisms

Tests on small-scale models

ρA 3 (kg/m )

RC 2 (N/mm )

Ed 2 (N/mm )

Popen

Ønotch

Vapparent

(%)

(mm) )

(km/s)

EN 1015-10

EN 1015-11

NF B 10-631

ASTM C20

Fe Pa 25

ASTM C597

PM 1.40 1: 0.2: 6 1440 1320 2.83 5250 23.38 0.31 2.76 PP 1.07 1: 0.2: 5 1840 1660 12.56 16150 9.86 0.54 3.79 PL 1.5 1: 0.25: 7 1580 1340 1.32 4825 33.64 0.38 3.06 PC 1.0 1:__: 5.5 1890 1660 5.92 11220 15.53 0.44 3.69 PB 1.0 1: 0.3: 4 1830 1720 5.96 13275 15.18 0.40 ** Legend: PM = coloured pre-dosed mortar; PP = high density pre-dosed mortar; PL = less compact pre-dosed mortar; PC = traditional mortar with cement as the only binder; PB = traditional mortar of cement and lime binders; ρ A = apparent bulk density; RC = compressive strength; E d = dynamic elastic modulus; P open = open porosity; Ønotch = diameter of the notch in the impact test; Vapparen t = apparent pulse velocity;*mixing proportions in mass = cement: lime: sand; **the measurement could not be performed because the render cracked and detached from the base during the impact tests (tests not included in this paper but which preceded the ultrasound tests).

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Table 2 - Individual results of ultrasound pulse velocity measured through the indirect method on smallscale models

Render type

Vapparent (km/s) _

Stdev (km/s)/ CV(%)

x PM PP PL PC

2.76 3.79 3.06 3.69

0.35/12.51 0.36/9.49 0.20/6.53 0.29/7.81

Vpath ABapparent (km/s) Path direction = A to B 100 mm

200 mm

300 mm

2.25 4.00 2.82 4.20

2.89 3.60 3.16 3.85

3.04 3.50 3.24 3.61

Vpath BAapparent (km/s) Path direction = B to A 100 mm 200 mm 300 mm 2.43 4.42 2.79 3.52

2.84 3.70 3.13 3.43

3.12 3.53 3.22 3.53

_

Legend: average ( x ) = arithmetic mean of the Vpath values for both directions AB and BA; Stdev = standard deviation that measures how widely values vary from the average value; CV=variation coefficient that represents the ratio between the standard deviation and the mean, and it is a useful statistic to compare the degree of variation from one data series to another; Vpath AB or BAapparent = pulse velocity between two points, direction AB or BA equal to the ratio of path length between transducers (d, in mm) and the transit time (T, in µs) given by the equipment; Equations (n= sample size):

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_

x = ∑ x n ; Stdev=

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−

∑ ( x − x) (n − 1) ; CV = St dev x ; V path apparent (km / s ) =

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d T


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Table 3 - Visual appraisal and apparent pulse velocity measurements of in-service assessments Case study

EC01

Render Type

EC02 EC04 EC05 EC06

PRC (PM) TC TC TC TC TB (C+CH) PRC (PL) PRC (PL) PRC (PM) PRC (PM) TC

EC10

TC

EC11 EC12

TC TC

Expected Age at Location group inspection Orientation

I

3

II

I

10