Driving by one of the ramps leading onto the East-bound Brooklyn Queens Expressway in New York I noticed a case of severe brick spalling on one of the masonry walls supporting the ramp, as shown in the picture below. What is happening is that the faces of the bricks on the wall are coming off – spalling – as a result of the compressive and shear forces acting on the bricks as a result of the load they are supporting. What is really interesting is that despite the relative weakness of the bricks, the mortar, the supposed weak link of the chain has remained quite intact. How could this be?
When discussing the best mortar:sand ratio in the field it is often heard that too much sand can make the mixture poor and weak or even whether Portland cement or mortar should be used. The role that lime plays in a mortar mixture is often little understood, and the strength requirements placed on mortar are exaggerated. As reported here the compressive strength of bricks ranges from 1k to 15k PSI, whereas here we see that Type I Portland cement can achieve a strength of 2.8k PSI at 7 days’ curing time.
This means that depending on the bricks we select for the job, we might be setting up the wall for failure by using a mixture that is too strong. If anything, then, we should be using mixtures that are weaker – or at least present higher flexibility – than brick to allow the mortar to absorb movements in the wall, whether caused by load or vibration. And this is where lime comes into play. Portland cement is manufactured through a complicated process that involves blending limestone, a rock composed primarily of calcium with other Silicon based rocks. Lime, however, is derived fundamentally from Limestone. Calcium being much softer than Silicon (for an example compare chalk versus sand), provides the right amount of plasticity to the mix.