3 new ways to test concrete: (2/3) strength of interfaces.

An interesting problem of great importance in concrete construction and concrete research is the evaluation of the interface bond between concrete layers. In concrete rehabilitation and repair, for example, when a new layer of concrete is applied over an existing substrate, shear strength between the new and the old concrete is an important design parameter. Several factors can influence bond strength between concrete layers: surface treatment on substrate surface, use of bonding agents, strength of the old and the new concrete, etc. The Nogueira Torsion Test (NTT) can be used to apply antiplane shear stresses to evaluate interface shear bonding using cores. If the core has no notch and failure takes place in the interface, shear strength of the interface between the old and new concrete can be calculated using τ_interface = P/A, where P is the maximum applied load, A = π·R^2 is the cross sectional area of the core, and R is the radius of the core. If the core has a circular notch in the interface, the bonding strength is given by τ_interface = (P/A_N)·(R/r), where P is the maximum applied load, A_N = π·r^2 is the area of the notched cross section, R is the radius of the core, and r the notched section radius.  Fig. 3 depitcs interface shear strength tests on concrete cores.