Project coordinator: Associate Prof. Sara Pavia
Funded by: Office of Public Works (OPW) & Ecole Nationale des Travaux Publics de l’Etat Vaulx en Velin, France

Impact of lime mortar properties on masonry strength and behaviour

Lime mortars are increasingly used for building repair and new construction. Mortars influence masonry to such a great extent, that they can either enhance or adversely affect the quality and durability of masonry. This project studies the properties of mortar and masonry and their interactions in order to specify mortars that improve the quality of masonry. In addition, despite the importance of water content, it is often subjectively determined by the mason by assessing the mortar’s workability. This lack of explicit methodology results in a lack of consistency of mortar properties and field performance which adversely affect the large scale uptake of lime mortars into new building and mainstream technology (Figure - Variation of flexural bond strength with respect to water content (165, 185, 195 mm initial flow), hydraulic strength and water retention).

Graph chart of mortar type

The project studies how mortar water content, curing conditions, compressive/ flexural strength, elastic modulus, air content and water retention impact the flexural, compressive and bond strength of clay brick, sandstone and limestone masonries bound with lime and PC/lime mortars of diverse hydraulic strength and thus different stiffness and deformability.

Flexural strength masonry equipment

Flexural strength masonry tests parallel and perpendicular to the joints.

The relationships between bond strength and mortar hydraulicity, water content, workability and water retention can assist the production of mortars designed to reach high bond strengths, and this would improve the quality and performance of lime mortars in construction. Some research results (such as the control of mixing water using initial flow and the impact of varying water content) are being incorporated into masonry practice by OPW. The papers published have resulted in abundant citations and some of the results considered by CEN/TC51/W11/TG1 in the revision of Standard EN 459-2:2010 Building lime.

In order to optimise bond strength, mortars should be mixed to the maximum initial flow that still yields an adequate workability. For NHL2, this optimal flow is close to 165mm; for NHL3.5 slightly below 185mm, and for NHL5 it equals 185mm. These flows are coupled to high water retention and result in strong flexural bond. Regression analysis was applied to the experimental mortar and masonry characteristic compressive strength results, and equations for the estimation of the 28-day characteristic masonry compressive strength proposed. These experimental equations were compared with predictions by previous authors, and Eurocode 6 provided the best predictions not only for cement-lime but also for NHL brick masonry. 

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