Monument Future

Conclusions

Portable XRF provides a nondestructive method of acquiring data on its geographical distribution and rate of growth. The Mn/Fe counts ratio can be used to distinguish the Mn-rich varnish from other types of surface deposits. The direct Mn and indirect Fe/Fe ratio methods can be used to estimate the layer thickness and hence the growth rate. Patches of urban rock varnish have been identified by pXRF on buildings across the northern United States from Washington to New York City to Minneapolis. These patches have typically been observed on red Triassic sandstone. However, they have also been found growing on older Carboniferous sandstone in New York City’s Central Park. Growth rates estimated from datable patches on the Smithsonian Castle and nearby gate posts are in the range of 83 ± 2.0 to 95 ± 2.4 nm/yr. This is significantly higher than the maximum rate of 40 nm/yr observed for desert varnish.

166

Acknowledgements

The authors would like to thank Bill Rebel of American Engineering Testing, Inc., for providing the XRF analysis of the James Hill House varnish sample.

References

Dorn R. I. 2007. Rock varnish. In: Geochemical Sediments and Landscapes. Nash D. J., McLaren S. J. (eds). London, Blackwell, pp. 246–297.

Grissom C., Aloiz E., Vicenzi E., Livingston R. A. 2018. Seneca sandstone: A heritage stone from the United States. In: Global Heritage Stone: Worldwide Examples of Heritage Stones, GSL Special Publication 486. London, Geological Society of London.

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Macholdt D. S. et al. 2017a. Black manganese-rich crusts on a Gothic cathedral, Atmospheric Environment 2017:205–220.

Macholdt D. S. et al. 2017b. Characterization and differentiation of rock varnish types from different environments by microanalytical techniques, Chemical Geology 459:91–118.

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McNeil J. A., Cecil F. E. 2014. X-ray fluorescence measurements of manganese in petroglyphs and graffiti in the Bluff, Utah Area, Colorado School of Mines, unpublished manuscript. http://inside.mines.edu/~jamcneil/XRF_Report_Bluff_Ut.pdf

Merrill G. P., Matthews E. B. 1898. The Building and Decorative Stones of Maryland. Baltimore, MD, Maryland Geological Survey.

Ochsner J. K. 1982. H. H. Richardson: Complete Architectural Works. Cambridge, MA, MIT Press.

Peck G. 2013. The Smithsonian Castle and the Seneca Quarry. Charleston, SC, The History Press.

Sharps M. C., Grissom C. A., Vicenzi E. P. 2020. Nano-scale structure and compositional analysis of manganese oxide coatings on the Smithsonian Castle, Washington, DC, Chemical Geology 537: 119486.

Vicenzi E. P., Grissom C. A., Livingston R. A., Weldon-Yochim Z. 2016. Rock varnish on architectural stone: Microscopy and analysis of nanoscale manganese oxide deposits on the Smithsonian Castle, Washington, DC. Heritage Science 4:26.

167

MICRODRILLING RESISTANCE MEASUREMENTS SYSTEM AND MORTAR PENETROMETER: TWO METHODS FOR EVALUATING IN SITU MORTAR RESISTANCE

Barbara Sacchi

1

, Emma Cantisani

1

, Teresa Salvatici

2

, Carlo Alberto Garzonio

2

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

 – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

 VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1

 Institute for Heritage Science – National Research Council of Italy (CNR-ISPC), Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy

2

 Department of Earth Sciences, University of Florence, 50121 Florence, Italy

Abstract

In situ evaluation of mortars’ resistance and consolidant treatments’ performance in ancient buildings is an essential argument in their conservation. Moreover, the mechanical properties of an ancient mortar are directly connected with the state of preservation of the building or the structure in which it is included, and it is not a parameter easy to be determined, especially for the masonry joints. Mechanical methods often request sampling and laboratory tests, while only few micro destructive techniques are known as penetrometric tests. They can be either static (with action of a pressure drills) or dynamic (with action of a percussion drills) and commonly usable in situ. Among them, Microdrilling Resistance Measurement System and Mortar Penetrometer are two important methods for determining the resistance of a mortar.

In the present work these two useful techniques are compared, in order to find a correlation between them which help to control and determine the resistance of a mortar, in addition to define the state of preservation or the effectiveness of the application of a consolidant product on a mortar surface.

Keywords: Mortar’s resistance, mechanical tests, consolidation, microdestructive techniques

Introduction

Mechanical characterization of stone materials (i.e natural stones, mortars, bricks) belonging to historic building is a very important issue for structural evaluation, prevention of risk and definition of best restoration practices. In practice it is very difficult to perform destructive test on single elements of masonry of historic building due to high cost and impact on constructions (Boschi et al. 2016; Del Monte et al. 2020).

The effort to have a sufficient amount of samples for the application of mechanical tests in laboratory (i. e. UNI EN 1015-11 2019; UNI EN 12504-1 2019; UNI EN 12390-13 2013; UNI EN 12390-6 2010) or the difficulty to reproduce exactly an ancient mortar with similar composition, microstructure and state of conservation, forces the use of in situ tests. Most of used methods (penetrometric, percussion or rotational techniques) provide data that are indirectly correlated to the strength through measurements like resistance to abrasion, to compression and to penetration, micro sandblasting, etc (Fratini et al. 2006; Yang et al. 2016; Pelà et al. 2018).

Microdrilling Resistance Measurement System is a micro-destructive static technique used in built heritage to assess the superficial cohesion of natural and artificial stone materials, both in laboratory 168and in situ. The instrument is constituted by a modified drill in which a load cell allows to measure drilling resistance of the material under examination, maintaining constant rotational speed and penetration rate (Delgado et al. 2002; Pamplona et al. 2007; Nogueira et al. 2014; Nogueira et al. 2018).

Mortar penetrometer is a dynamic penetrometer and is comparable to an adapted Schmidt rebound hammer, in which the impact plunger is replaced by a metal pin. In this type of test, calibrated impact mass is used to insert a metal pin into the mortar, measuring the penetration depth. This penetration depth corresponds to compressive strength of mortar according to a penetrometer correlation curve.

In this paper we performed both in situ and laboratory tests on ancient mortars in order to identify a correlation among data obtained with drilling and penetrometric techniques.

Similar attempt to correlate drilling measurements with non destructive techniques, was performed by Costa et al. 2010 and Noguiera et al. 2014, comparing DRMS data and ultrasonic tests, concluding that ultrasonic velocity, measured in direct mode, is in good agreement with the drilling results, although expressed by the average values of the distributions with great variations.

Microdrilling measurement system (DRMS)

Drilling Resistance Measurement System (DRMS) measures the force that the tested material opposes to perforation, keeping constant penetration rate (ν) and rotational speed (ω). The first applications of this technique date back to the beginning of 20th century to study the weak points of stone surfaces (Pamplona et al. 2007). Nowadays drilling resistance instrument is used both for natural and artificial stones, particularly for mortars, even if the application on heterogeneous materials is still a discussed and studied argument from many researchers (Costa et al. 2010, Del Monte et al. 2008, Delgado Rodrigues et al. 2016, Dumitrescu et al. 2017). Recent devices were developed by SINT Technology, and DRMS Cordless became quite widespread. In such instruments, drill bit diameter can vary between 3 and 10 mm, the penetration rate (ν) between 1 and 80 mm/min, the rotational speed (ω) between 20 and 1000 rpm, and the maximum achievable load is 100 N.

In this study ancient mortars were examined, and drilling tests were performed both in situ on bedding mortars and in laboratory on drilling core samples extracted from some Tuscan monuments (see Case Studies paragraph). The used instrument is a Cordless Drilling Resistance Measurement System by SINT Technologies S. r. l. (Italy). On the base of preliminary tests, the chosen parameters were 300 rpm/20 mm/min for in situ measures and 150 rpm/20 mm/min for tests on core samples. Three holes were performed on each area in situ, while two holes were made on each core sample. A 5 mm diameter Fischer SDD widia drill bit was used.

Penetrometer

Penetrometer gives information about the conditions of preservation and homogeneity of a mortar. The penetrometer consists of a striking mass connected through a spring to a striker, in which a pin is inserted. The pin, subjected to constant dynamic shocks advances inside the mortar joint which pushes and compresses the mortar along its path.

The resistance that the mortar offers to the pin advancement is proportional to the mechanical strength of the material (Łątka and Matysek, 2018). The pin is made of steel, has a diameter of 3 mm and a conic tip at an angle of 25°.

The instrument used is a Penetrometer RSM-15 Version 0.1 of Diagnostic Research Company (DRC Srl) and it is supplied with a KIT including a striker, an automatic measuring body, a manual comparator, a series of tips and measuring tips and support accessories for measuring driving depth. The impact energy is 4.55 Nm, the impact mass is 835 g and stroke of 82 mm.

The result is the needle penetration depth (RPMs, mm), after a number of impacts defined according to the type of procedure used (in the study case 10 impacts). It’s possible to calculate the compressive 169strength (R, in MPa) with a correlation curve and equation of the instrument :

R = 4.489exp

–0.106RPMs

In the case studies two penetrometer tests on each core sample were carried out. The test was executed verifying previously the length of the initial steel needle L0 (80 mm), then applying n. 10 impacts, removing the penetrometer, leaving the needle in the mortar and calculating L10, the length of the needle outside the mortar and finally calculating the absolute penetration depth :

RPMs = L0–L10

Case studies

DRMS and penetrometric tests were applied directly on site on bedding mortars of bricks in San Francesco Church, in Pisa (Tuscany, Italy ) and on drilling core samples of nucleus of walls (Fig. 1) coming from San Francesco Church (Pisa), Giotto’s Bell Tower and Palazzo Vecchio (Florence, Italy).

San Francesco Church (Pisa, Italy), built between XIII and XV century, is a very important catholic church in Italy, now undergoing to an extensive restoration plan.

Figure 1

: Examples of drilling core samples subjected to drilling (left side) and penetrometer (right side) tests.

During the most recent diagnostic campaign promoted by Opera del Duomo of Florence and conducted on the Giotto’s Bell Tower to investigate the history, the structures and the materials of external façades and masonry, some samples of mortars were obtained. The mortar samples were taken from the foundations and from the masonry of the II and V levels of the bell tower, using a core drill.

Core samples of foundation structures from Palazzo Vecchio (headquarters of municipality of Florence town) were also tested.

Tested mortars were prepared before the measurements: for San Francesco Church on site case study, the plaster of the wall was removed, in order to have bedding mortars of bricks exposed; the core samples were cleaned from coring residues.

The chemical, mineralogical and petrographic studies carried out on these mortar samples allowed us to classify these mortars as aerial lime mortars or weakly hydraulic mortars. They have a fine grained aggregate (mean grain size ranges from 500 to 200 mm, maximum grain size from 1 mm to 1.2 mm) with a prevalently silicatic composition and a binder/aggregate ratio from 1/1 to 1/3.

Results and Discussion

Microdrilling tests should be performed on quite homogeneous materials, while bedding mortars are heterogeneous mixtures of materials, expecially when cut from nucleous of historical walls built in different centuries. Nevertheless, having the aim to compare the mechanical resistance that a mortar opposes to the penetration of a rotating drill and an advancing pin, it was decided to express drilling results by average values of resistance, without any kind of correction. Sometimes indeed the distribution presented great variations, but it was considered that the resistence of a mortar to penetration is connected both to binder and to aggregate, and to the different composition, grain size and grain size distribution of aggregate, too. The average value of drilling resistance was calculated on the first 10 mm from the surface for two main reasones. First of all, the same depth was always reached from penetrometer measurements, too. Moreover, a greater depth could have been difficult to be reached without bumping into brick or stone fragments, since both in situ and from core drilling, the mortars are bedding mortars of old and irregular walls.

170Microdrilling tests performed in situ on different areas of masonries of San Francesco Church in Pisa showed values of drilling resistance from 0.7 N as minimum to 6.0 N as maximum. Penetrometer measurements on the same areas showed a similar variability (resistance on 10 impacts from 0.7 MPa to 2.2 MPa), but the values did not show a linear correlation (Fig. 2), as in other studies was found for Drilling vs. Ultrasonic and/or Compressive Strenght (Costa et al. 2010).

Figure 2

: “Drilling resistance” vs. “penetrometer measurements” results.

A better, even if not fully satisfactory, corrispondence was shown by measures performed on drilling core samples extracted from the different historical sites previously described, in particular from Giotto’s Bell Tower, where values of drilling resistance start from 10.6 N as minimum value up to 69.4 N as maximum, and resistance on 10 impacts varies from 1.3 MPa to 4.1 MPa.

This variability is certainly due to the high number of variables that impact on this kind of measures on mortars (different binder, different aggregate and dimensions of aggregate, different direction of penetration of drill bit or pin, and so on).

Moreover, mortars coming from Giotto’s Bell Tower and from Palazzo Vecchio show higher values in respect to San Francesco’s Church mortars both from penetrometer measures and drilling tests. This is in agreement with the different composition of binder used to prepare mortars: a natural hydraulic lime was used for mortars of Giotto’s bell Tower and Palazzo Vecchio, an aerial lime for mortars of San Francesco’s Church. Moreover, the mortars of San Francesco’ Church are in situ and structural features and environmental parameters, as the moisture, may affect their mechanical behaviour. While the superficial moisture of the drill core mortars is around 1.4(±0.1)% for all samples, in situ mesaures of investigated surfaces revealed higher values: on the left wall of the nave the moisture values range from 0.2 to 2.4 %, on the right wall (having in fact a different conservation state) they reach values of 5.4 %.

This further underlines the difficulty in finding standard methods for quantify the mechanical resistance of such heterogeneous materials.

Therefore, having so different samples of mortars and consequently so variable data, other data processing methods were verified, and polynomial trendline was find as the best one. Even in this case, the correlation gets worse for high values of drilling resistance.

171In Fig. 3, a polynomial trendline correlation between “Drilling resistance” and “penetrometer measurements” results is showed: on the left side of the picture, all the data are included, while on the right side, the samples having a ratio value (“resistance to drilling” / “penetrometer resistance”) > 10 were excluded, leading to a considerable improvement of correlation (R

2

 from 0.79 to 0.91).

Figure 3

: Polynomial trendline correlation between “Drilling resistance” and “penetrometer measurements” results, including all samples (left) and excluding samples having ratio value “mean resistance to drilling”/“penetrometer resistance” > 10 (right).

Conclusions

In this work, drilling resistance measurement was compared for the first time to mortar penetrometer method, to find a correlation between two micro destructive techniques usable in situ for the evaluation of mechanical resistance of mortars.

Results of tests carried out on ancient mortars belonging to different monuments of different periods and locations proved to be promising.

A good polynomial correlation was showed by drilling and penetrometer data, expecially for low-strenght mortars.

Nevertheless, the study should take considerable advantages from a systematic investigation on a high number of samples having known characteristics. To this aim further work is required to enhance the quality of the collected and compared data, designing specific artificial mortars ad hoc and testing their mechanical data. This kind of data collection could be usefull for the application of the tests in situ in order to have a simple classification of type of mortars.

Another purpose for the future work is to enrich the collected data with compressive strenght and Ultrasonic pulse velocity tests.

References

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