Measuring the Impact Behavior of Fresh Mortars by Pressure Mapping

Abstract

Mortars and concretes are subjected to diverse rheological situations during their processing and application. Their behavior can be predicted for most situations by rotational rheometry and/or squeeze flow. However, very high strain rates that occur during impact are very difficult to be properly simulated using these methods. Especially for rendering mortars, impact behavior (during manual or spray application) is important not only regarding productivity and rebound issues but also because the initial mortar-substrate contact extension strongly affects the interfacial bond strength, which is one of the renders most critical hardened properties. The present work introduces initial developments towards an impact rheometry technique intended primarily for cementitious and other suspensions used in construction. The technique is based on a pressure mapping system that measures area and force with a thin and flexible sensor at 100 Hz. Mortars were applied manually, by continuous spraying, and by free fall. Results showed that in manual application, pressures up to 50 kPa took place, whereas in the mechanical procedure the pressure values were considerably lower (>5 kPa) since the force is distributed in small portions of material by the spray. Some of these results can be useful to compare projection techniques and equipment, for the development of nozzles, and to set free fall drop heights to better simulate manual applications and even allow more realistic molding procedures for mortar-substrate tensile bond tests.