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 [1]:
R = 4.489exp–0.106RPMs [1]
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 [2]:
RPMs = L0–L10 [2]
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
