A New Penetrometric Test for in Situ Mechanical Characterization of Historic Mortar: Preliminary Results on Different Hydrated Lime Mortar Specimens

Some existing non-destructive tools are used to characterize on site the lime-based mortar of historical buildings from a mechanical point of view. Those devices are not only based on the surface hardness but are able to inspect the mortar joints up to a few millimeters depth. Historical mortar joints are normally too thin to extract representative samples for mechanical characterization and often too weak and without sufficient cohesion, due to a hard significant surface decay. Mortar “sanding” in historic building (lack of cohesion in bedding mortar, reducing it to grains) can penetrate from 20 to 40 mm, without compromising the overall masonry mechanical behavior. This makes it difficult to obtain reliable data on their real consistency. A new device has been realized to analyze in situ mortar joints, adapting the rebound hammer with a pointy probe (conical tip, 4mm diameter) and able to reach a depth of penetration of 60-70 mm inside the test specimen. The system is based on the automated measurement of the penetration rate at each hammer blow, allowing to discriminate the effect of the most degraded and powdered external mortar. Thanks to a lowcost electronic data acquisition system connected to the hammer, all measurements carried out on site are recorded and made available for processing. The paper presents a first series of preliminary laboratory tests on different samples of lime mortar made ad hoc for a Round Robin Test. Two different methods were compared on laboratory samples of known compressive strength: static and dynamic penetrometric tests, correlated with the compressive strength obtained in the laboratory and supplied by another university. The resulting correlation between the two methods was promising enough to start a first experience on a historic building where previous diagnostic investigations were carried out and a stratification of mortar joints was detected.