Influence of substrate moisture state and roughness on interface microstructure and bond strength: Slant shear vs. pull-off testing

Abstract

There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.

Functional and adhesive properties of cement-based overlays modified with amorphous silica nanospheres

“…Until now, it has been shown in literature on the basis of research that the mechanical treatment of the surface of the concrete substrate by shot-blasting improves the level of bonding of the cement mortar overlay with this substrate. [5][6][7] In turn, Silfwerbrand and Beushausen [8] , Silfwerbrand et al [9] and Sadowski et al [10] showed, based on tests, that the removal of the cement laitance from the surface of concrete substrate improves the level of bonding between the cement mortar overlay and the substrate. It is also known from literature that the use of a bonding agent on the surface of the substrate before the application of the overlay improves the level of the bonding between the cement mortar overlay and the concrete substrate.…”

A cohesive model to predict the loading bond capacity of concrete structures repaired/reinforced with HPFRC/UHPFRC and stressed to mixed mode

“…Nevertheless, unrealistic loading conditions are applied to the interface. The failure of the interface depends on the angle of the plane with respect to the load 3 . In addition, the test is relatively insensitive to the surface preparation and roughness, since bond failures occurred only for smooth surfaces [2,17].…”

“…Dry-type surface was reached by curing the substrate surface for 14 days at laboratory conditions of 23 ± 2, 55% ± 5% RH, as also seen in [3]. During the curing, surface substrate was covered with a plastic sheet, in order to…”

A cohesive FE model for simulating the cracking/debonding pattern of composite NSC-HPFRC/UHPFRC members

“…Bond test configuration designed by authors: (1) substrate, (2) overlay,(3) interface, (4) slip point measured by LDTV, (5) bearing plates, (6) loading device,(7) force transducer, (8) rigid support…”

“…The whole range of possible moisture conditions expected in the field was taken into account in series investigated, in particular dry, 75% and SSD. Dry-type surface was reached by curing the substrate surface for 14 days at laboratory conditions of 23 ± 2, 55% ± 5% RH, as also seen in[3]. During the curing, surface substrate was covered with a plastic sheet, in order to slow down the carbonation process.…”