On the necessity of new hydrophobic treatment after repointing of water repellent masonry

The impregnation of the exterior surface of a masonry wall with a water repellent is a common intervention in
(historic) building renovation and maintenance. Such treatments, whilst degrading at the surface with time
under influence of ultra violet light, remain effective below the surface several decades after their application.
During renovation works of masonry previously treated with a water repellent, the question arises whether it is
necessary to repeat the hydrophobic treatment of the entire masonry after repointing. Opposing opinions exist
with this regard, but no research clearly supporting one or the other. This research investigates for the first time
the effect of hydrophobic treatment when applied on previously treated and repointed masonry walls. Small
masonry walls were subjected to rain periods in the laboratory and their water uptake and drying behaviour were
studied. Moreover, this laboratory research was followed by 30 months of outdoor exposition of the masonry
specimens. The following cases were considered: (1) wall treated with water repellent, (2) wall treated with
water repellent, followed by repointing but without new water repellent treatment, (3) wall treated with water
repellent, followed by repointing and retreatment. This was done for three different types of pointing mortar:
ordinary Portland cement and natural hydraulic lime with standard sand, and natural hydraulic lime with sand
with one grain size. The results show that, after prolonged rain periods, the water uptake by repointed but not
retreated masonry is comparable to that of untreated, non-hydrophobic masonry, whereas drying is considerably
slower. This leads to a high saturation degree in repointed but not retreated masonry, which, in turn, increases
the risk of damage to the masonry by e.g. frost. Therefore, retreating repointed hydrophobic masonry should
definitively be considered.