PSI - Issue 10

I. Karatasios et al. / Procedia Structural Integrity 10 (2018) 211–218 I. Karatasios e tal. / Structural Integrity Procedia 00 (2018) 000 – 000

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Table 3. Mechanical and physical properties (n=3) of mortar mixtures after curing for 28 days, as well as adhesion strength in joined stone specimens determined by four-point flexural bending strength (4PB). Mixture Stone Strength Category 3-point flexural bending strength Compressive strength Open porosity

Adhesion strength 4-point flexural bending

Water Absorption Coefficient

2 sec 0.5 )

RM σ f (MPa)

RM σ c (MPa)

STRM σ f (MPa)

%

A (g/ m

RM1 RM2 RM3 RM4 RM5 RM6

High

6.10 1.78 2.74 1.48 1.58 3.04

55.85

2.5 0.8 1.0 0.3 0.5 0.8

10.30 33.05 24.52 39.93 40.32 27.93

57.5

Very Low Moderate Very Low Very Low Moderate

5.65

124.6 160.0 196.0 601.3 297.5

11.40

7.37 5.01

13.30

3.4. Failure mode and microstructure compatibility of mortars mixtures

All specimens from the three groups of stones failed at the stone-mortar interface, detaching small pieces from the stone surface (Fig.1). This type of failure indicates optimum adhesion and minimum potential for stone alteration during damage. The non-homogeneous matrix of the particular stone lithotypes enhances the possibilities for stone damage in case of stronger adhesion medium, especially in the areas close to discontinuities, clay veins, voids, etc. The interpretation of mechanical properties of adhesion mortars shows no obvious correlation between the compressive strength and the adhesion strength determined by 4-point flexural bending. It is easily observed in Table 3 that mixtures with different compressive strength values present very similar adhesion strength values. In contrast, 3-point flexural bending strength provides an index of the internal cohesion and an indirect estimation of tension strength of conservation mortars (RILEM (1980)), considering also parameters of microstructure (i.e. porosity) and chemistry (e.g. percentage of hydration completion). Similarly, 4-point flexural bending introduces in addition the parameter of adhesion strength to the stone substrate. Thus, the relationship between internal cohesion of repair mortars and bond strength between mortars and stone substrate can be investigated by corelating 3-point and 4-point flexural bending strength. Therefore, by selecting from each strength category the stones with the higher compressive strength values and then plotting together the 3-point and 4-point flexural bending strength values of mortars and stone-mortar specimens, a linear correlation is achieved (Fig.2). For the same samples (higher values of each strength category), the adhesion strength is around the one third of the bending strength of the same mixture (Fig.3), independently of the cementing material type. The linear relationship provided in Fig.2 can be used in similar projects in the future, providing a valuable tool for the design of mortar mixtures for joining archaeological stone fragments without reinforcement. To this direction, it is worth noting that the 4-point flexural strength depends primary to the 3-point flexural strength rather than the compressive strength of the mortars. Thus, high compressive strength values shouldn‟t be the mai n require ment of this type of conservation mortars.

Fig. 1. Representative pictures of the joined stone fragments (left: ST01/RM01, right: ST07/RM06) after four point bending test, indicating the failure at the stone-mortar interface, detaching small pieces from the stone surface.