My wife and I both love historic architecture so Italy naturally seemed like the perfect destination for our honeymoon. After spending two weeks in Cinque Terre, a string of historic seaside villages along the Italian Riviera coastline, we ended our stay in Milan. The city of Milan has always been considered an industrial powerhouse and, as a result, was heavily bombed during WWII. Of the few remaining historic structures left standing, the most impressive is undoubtedly the Duomo di Milano.
Construction of Milan’s Duomo officially began in 1386 and it remains the fifth largest Christian church in the world. Visiting Milan’s Duomo was a truly jaw-dropping experience: it boasts 135 spires, 96 gargoyles and it is said to contain more integral statues than any other building in the world with an amazing 3,159 in total. As one considers the magnificence of this structure it is also worth considering a far less obvious but equally important feature, its mortar.
It is easy to overlook mortar when appreciating a building such as Milan’s Duomo. Mortar represents only a small and singular component of a structure and our attention is naturally drawn to a building’s broader architectural features. However, it is remarkable to consider that for over 600 years mortar has not only acted as the glue, which holds Milan’s Duomo together but it has been responsible for protecting its intricately carved masonry units from the effects of settling and thermal expansion. What I find even more remarkable is that historic lime mortars such as those employed in Milan’s Duomo are relatively weak and rarely have compressive strengths in excess of 900 pounds per square inch.
It has been my experience that contractors, building owners, and specifiers place too much emphasis on a mortar’s compressive strength when selecting bedding or pointing mortar. While it is true that mortars must be strong enough to carry a wall’s compressive load, a mortar’s most critical mechanical role does not reside in its compressive strength but rather its ability to accommodate small structural movements by means of deformation under stress.
Imagine you have a marshmallow on the table in front of you and you are pressing firmly on it with your palm. Are you visualizing the sides of the marshmallow bulging out as it squishes into a pancake? The uni-axial (single direction) compressive stress exerted onto the marshmallow has created tensile strain within its mass as it stretches outward and squishes flat. In this example, the marshmallow exhibits a high degree of elasticity and is able to deform significantly without breaking apart.
Although mortar has significantly greater rigidity than a marshmallow, it behaves similarly when under compression. ASTM C109, The Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, determines the compressive strength of mortars in the same way as our marshmallow example. In ASTM C109 a compressive force, measured in pounds per square inch (PSI), is applied to a cured two-inch cube of mortar until the mortar distends and fails under tensile strain. The compressive strength of the mortar is determined by the PSI that was applied to the cube of mortar at the moment it failed.
The tensile strength of a mortar is proportionate to its compressive strength; therefore the stronger a mortar is under compression the more resistant it is to tensile strain. Lime binders, on the other hand, generally have substantially lower compressive, and therefore, tensile strength and require comparatively low compressive stress to generate deformation. Rather than a sudden rupture characteristic of Portland cement failures, softer limes compact and eventually crumble at their point of failure. This very gradual and controlled failure is one example of lime mortar performing its job as a sacrificial element in a building.
Let’s revisit our marshmallow experiment and envision how it would behave if the marshmallow was contained in a cylinder. As you compress the marshmallow from above you would feel resistance. The compressive pressure of you pushing downward on the marshmallow forces the mass to press against the walls of the cylinder. The tensile strain formed within the marshmallow induced by its desire to stretch outward is now counteracted by its confinement in the cylinder. Under confinement, tensile stress is converted into horizontal compressive forces redirected towards the center of the mass in a mechanical function known as tri-axial stress.
Like the marshmallow confined in a cylinder, mortar is confined by and bonded to surrounding masonry units. As mortar distends from the joint under compression it is restrained by the friction caused at the bond interface between the mortar and adjacent masonry units resulting in tri-axial stress and shear strain at the bond. The point of failure under shear strain in well-bonded masonry will occur within the material that has the least tensile strength. When high strength hydraulic lime or Portland cement rich mortars are used, the point of shear failure under tri-axial stress may occur in the weaker masonry unit and not the more easily replaceable mortar joint. (Hansen, Navarro & Van Balen 2008)
Tri-axial stress in mortar is not only a consequence of compressive loads being applied to masonry but also from thermal dilation of individual masonry units. All masonry units (such as brick, stone or terra cotta) possess varying degrees of thermal dilation by which they increase in volume during exposure to rising temperatures. Masonry units, which have a greater rate of thermal dilation than the mortar will expand and compress mortar joints.
Mortars also exhibit their own rate of thermal dilation independent of the masonry units. Lower strength lime mortars typically have less thermal dilation and their thermal expansion is limited by the masonry units when confined in a mortar joint. In this scenario lime mortars can function as a cushion, absorbing thermal expansion of the units as the mortar itself deforms under stress. In contrast, stronger Portland cement mortars are less able to absorb the thermal dilation of the surrounding masonry units. Moreover, Portland cement mortars exert their own thermal dilation, which can be greater than that of traditionally softer masonry units. In both cases, excessively strong Portland cement mortars can induce shear strain on weaker masonry units leading to potential failure of the units themselves. (Hansen, Navarro & Van Balen 2008)
Once again referring to our marshmallow example, this time you have assembled a plunger fashioned from a wood disk cut to fit precisely into the cylinder that contains the marshmallow. The disk is attached to a long rod and you are inserting the plunger into the cylinder and pressing down firmly on the marshmallow. Are you able to envision the marshmallow compressing slightly? Although the cylinder restrains the marshmallow from bulging outward, its mass still experiences a slight reduction in volume due to a phenomenon known as pore collapse mechanism.
Hardened mortars are a culmination of intertwined microscopic crystalline structures interlaced with a matrix of pores, air bubbles, micro cracks and other anomalies. When a mortar becomes loaded under tri-axial compression its pores, voids and cracks become weak points, which allows the binder to give way through pore collapse. Deformation in mortar resulting in pore collapse is typically a consequence of either distortion of the mortar’s crystalline structure or by the formation of micro cracking around voids. (Tjioe & Borja 2014)
Crystalline deformation occurs when shear stress causes a slippage of some of the atomic and molecular bonds that form the mortar’s crystal structures. Stronger highly hydraulic mortars (with hydraulically formed crystals) have a higher propensity for crystalline deformation, which typically results in only partial pore collapse under normal loads and the overall deformability of the mortar is reduced. Softer less hydraulic mortars have weaker molecular bonds and are more prone to microfracturing, which results in an increase in pore collapse and overall greater mortar deformability as the mass compacts. A Stanford University study by Martin Tjioe and Ronaldo Borja concluded that micro fracturing is a “softer form of deformation” than crystal deformation. For this reason, high strength hydraulic Portland cement mortars with crystal deformation characteristics exhibit a rapid and brittle failure under significant tensile strain. (Tjioe & Borja 2014)
Highly hydraulic mortars with greater tensile strength (ie. Portland cements) require proportionately higher tri-axial stress to induce crystal deformation and pore collapse. In some cases the excessive tri-axial stress required to achieve pore collapse in Portland cement mortars produces shear strain at the masonry’s bond that can exceed the tensile tolerance of historic masonry units and the units themselves may fail. By contrast, softer lime mortars require much less stress to induce deformation and very little strain is imposed on surrounding masonry units. (Wiggins 2018) (Insert picture of sheer at bond)
Elasticity vs. Plastic Deformation
Under stress a marshmallow compresses and, when relieved of that pressure, it returns to its original volume and shape. A mortar’s capacity to deform and rebound without being damaged is characterized by its modulus of elasticity. Highly hydraulic mortars that experience only partial pore collapse as a result of crystal deformation have a comparatively high modulus of elasticity. Mortars with higher elasticity exert additional sheer stress on surrounding masonry units under deformation. Although highly elastic mortar has some benefits in new construction, its unyielding characteristics make it less desirable for restoration work where soft masonry units may be susceptible to damage caused by shear strain.
Less hydraulic lime mortars are more prone to permanent deformation (plastic deformation) as a result of micro cracking and greater pore collapse. Under plastic deformation a mortar experiences greater overall volume reduction as it compresses under stress. Ultimately, masonry structures constructed with low strength lime mortars that exhibit plastic deformation are better able to adapt to differential rates of settlement and thermal movement than stronger hydraulic mortars. This ability is best exemplified by nearly all-historic masonry structures built prior to 1900, which were constructed using lime mortars without employing movement joints. (Costigan & Pavia 2013)
Mortar selection determined principally by high compressive strength ultimately disregards other more significant performance characteristics that are critical to the preservation of historic masonry structures. In restoration, a mortar’s mechanical role does not lie in its strength but rather its ability to deform under stress and accommodate movements within the structure.
Masonry construction is responsible for the world’s most impressive historic structures and Milan’s Duomo is just one of many examples. The scale of the Duomo and its breathtaking artistry inspires awe among people from all cultures and generations. Yet, what is just as impressive as the Duomo’s intricately carved facade is how effective its mortar has served to preserve the structure for over 600 years. That fact that Milan’s Duomo has withstood centuries of stress from settling and thermal expansion is a real testament to the mechanical elements of the building’s mortar. So the next time you find yourself gazing at beautiful historic masonry remember to also admire its mortar.