The kinetics of masonry bonding: The first hour

Two interdependent rates govern the properties of mortar and bond during the first hour of contact between masonry mortar and brick. They are (1) the rate of water movement into brick and (2) the rate of water movement out of the mortar under suction. The individual rates and the composite rate are equal to a constant times time{dollar}sp{lcub}0.5{rcub}{dollar}. The rate constant for any combination of brick water absorption fed by mortar water loss is successfully predicted from the permeability and water phase volume constants for the brick and the mortar determined experimentally.; Brick permeability and pore volume are determined using an extended initial rate of absorption test modified from ASTM C67. Consolidated mortar permeability and water loss are determined using an extended water retention test modified from ASTM C91. The overall rate allows prediction of the working time available to masons for any brick-mortar combination.; The flow properties of mortars are determined both with a flow table and with direct rheological measurements. The mortars have a Bingham plastic rheological character with essentially zero plastic viscosity. ASTM C109 flow table values and the Bingham plastic yield value correlate well if the flow table values are corrected for the mortar density.; The water movement rates in the first hour are significant to mortar strength and mortar-brick bond. The flexural strength of mortar bars is used to detect the effects of contact with a porous material, water immersion or sealed curing, air drying, age, and mortar type. The result is that extraction of water during the first hour always leads to better mortar strength for any given cure.; The removal of water from the mortar causes it to shrink or consolidate. The consolidation strain of mortar was compared to its drying strain. The gross movement of mortar during the consolidation strain period appeared to significantly degrade bond strength.; This paper demonstrates that mortar movement and not brick absorption is the significant mortar bond reducer and that this movement is predictable in amount and rate during the first hour of contact of mortar with brick.