Читать книгу Monument Future - Siegfried Siegesmund - Страница 251

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 (R² 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).