The Impact of Low Cement Mortar on Wall Cracking

Using less cement in mortar can lead to significant structural issues, including wall cracking. This article explores the reasons behind these problems and provides guidelines for ensuring durable and crack-resistant constructions.

Why Less Cement Can Cause Issues

Mortar is a crucial component in the construction of walls, providing not only the bond between bricks or blocks but also contributing to the structural integrity. However, reducing the amount of cement in mortar can have detrimental effects.

Weaker Bond and Structural Integrity

The primary role of cement in mortar is to create a strong bond between the bricks or blocks. Reducing the cement content can result in a weaker bond, leading to insufficient structural integrity. This can cause the walls to be more prone to cracking and other structural failures.

Increased Porosity and Moisture Issues

A lower cement content can increase the porosity of the mortar, making it more susceptible to moisture infiltration. This can lead to expansion and contraction, particularly with changes in temperature. Moisture can cause the mortar to absorb and release water repeatedly, leading to cracks over time.

Shrinkage Issues

Mortar with inadequate cement content can shrink more during the curing process, leading to cracks as it dries. This shrinkage can be exacerbated by environmental factors such as temperature fluctuations and moisture, which can further compromise the structural integrity of the wall.

Load-Bearing Capacity

Walls that rely on insufficiently strong mortar may not be able to support the intended loads. If the mortar is too weak, it can result in potential structural failures, leading to safety hazards and costly repairs.

Cracking Patterns and Causes

Cracking in walls can be caused by movement, either in the bricks or blocks themselves or in the mortar. While bricks or blocks can crack due to movement, it is usually the mortar that experiences the most noticeable and problematic cracks.

Brick Movement Cracking

Walls can crack when they experience movement due to unstable foundations. For example, if a part of the footing sinks more than another part, the wall becomes poorly supported and can crack. Alternatively, the movement of concrete bricks can be due to their natural shrinkage and expansion processes.

Mortar Cracking

Mortar can crack due to various factors, including:
- Too much cement or poor choice of sand
- Insufficient cement content, which can lead to compressive failure or allow the bricks or blocks to move relative to each other, potentially preventing widespread and very noticeable cracking
- Excessive cement or dusty sand leading to greater drying shrinkage than necessary

Case Study: Mortar Cracking and Legal Implications

A case in point was a legal dispute decades ago involving a home owner who had laid all the blockwork for his new home. Mortar joints began cracking all over the place. He sued the block manufacturer, claiming it was all the manufacturer's fault. Upon analysis, the mortar indicated a very cement-rich 1:2 cement to sand ratio.

The homeowner was also suing the block manufacturer for damage to roof trusses that he had not been able to use. It emerged during the trial that he had been laying the blocks whilst off work for a long time with a supposedly damaged spine. The judge was not impressed. Not only was he being fraudulent, but he also failed to mitigate his losses by covering the roof trusses to prevent rain damage.

The judge, exasperated by the legal arguments between two young barristers, interrupted them, pointing to the two of them and saying: 'Look at yourselves!'
The case involved several amusing matters, one of which directly involved the writer. After several hours of cross-examination, the court revealed that the owner had not taken steps to prevent damage to his roof trusses.

This case underscores the importance of proper cement mix ratios and adherence to construction guidelines to avoid costly and unnecessary structural issues.