Understanding the Compressive Strength of Mortar

The compressive strength of mortar is a critical factor in determining the overall durability and performance of masonry structures. It can vary based on several factors, including the mix design, curing conditions, and the application of the mortar. This article delves into the different types of mortar and their compressive strengths, along with insights from relevant masonry codes.

Types of Mortar and Their Compressive Strengths

Mortar is a mixture of cement, lime, water, and sand, used in construction to bind masonry units together. The compressive strength of mortar can range from 2.4 to 3000 psi (1.6 to 21 MPa), depending on the type and application. Below are the typical compressive strength ranges for different types of mortar:

Type M Mortar

Compressive Strength: 2500 to 3000 psi (17 to 21 MPa) Typically used for more demanding applications such as foundations and load-bearing walls

Type S Mortar

Compressive Strength: 1800 to 2400 psi (12 to 17 MPa) Used for general masonry applications

Type N Mortar

Compressive Strength: 750 to 1500 psi (5 to 10 MPa) Primarily used for non-load-bearing walls and non-critical applications

Type O Mortar

Compressive Strength: 350 to 600 psi (2.4 to 4.1 MPa) Used for tuck-pointing and pointing of existing masonry

These values are typically measured after 28 days of curing, which is the standard curing period in construction. However, the specific strength of the mortar can also be influenced by factors such as the water-to-cement ratio, the type of sand used, and whether any additives are included in the mix.

Impact of Mortar Strength on Masonry Structures

The compressive strength of mortar is crucial for ensuring the structural integrity of masonry buildings. However, the effect of mortar strength on the overall strength of masonry structures is relatively minor. In many cases, mortar strength is not even assessed; instead, the focus is on using 'deemed to satisfy' mortar ratios. For instance, in the case of stack bonded piers made of bricks and mortar, the entire structure is often tested to ensure compliance.

Specific Applications of Mortar

Brick Laying: In brick masonry, the mortar used for laying bricks should provide at least 70% of the compressive strength of the bricks. This typically translates to a minimum compressive strength of 1.5 MPa for the mortar. Filling Material: When mortar is used merely as a filling material or for surface regularizing, the compressive strength is less critical. The workability and consistency of the mortar are more important. Structural Elements: If mortar is used as a component of a load-bearing structure, a professional civil engineer should be consulted to analyze the project and ensure structural integrity.

Relevant Excerpts from AS 3700 2011 Masonry Code

For detailed information, it is advisable to refer to the Australian Standard AS 3700 2011, which provides guidelines for the design, construction, and testing of masonry structures. Some relevant excerpts include:

AS 3700 2011: P9.2, Clause 9.2 Mortar mix design: The use of appropriate mortar mix designs is essential to achieve the desired compressive strength and workability.

AS 3700 2011: Section 5.3.2.4: The selection of mortar strength should be based on the requirements of the masonry units and the intended application.

Conclusion

Understanding the compressive strength of mortar is crucial for designing and building resilient masonry structures. The type and strength of mortar greatly influence the durability and performance of masonry buildings. By considering factors such as water-to-cement ratio, sand type, and additives, builders can ensure that the mortar they use provides the necessary support and longevity to the structure.