How to Specify Clay Structural Masonry f ’m = 4000 psi in the Midwest
Did you know that simply stating f’m for your masonry walls is not enough? What is the best way to specify f’m for your clay masonry wall?
This article describes the 3 key items you need to include in your General Notes and Specifications to correctly specify clay masonry strength.
When specifying the compressive strength for clay or concrete masonry on your project, it is not sufficient to indicate f’m only. The clay or concrete unit strength, mortar type, and grout strength are key items to include in your General Notes and/or Specifications. This article will focus primarily on specification for clay masonry, and we have provided similar guidance for concrete masonry strength in the SE Insight article titled ‘How to Specify Concrete Masonry’.
According to the masonry code, TMS 602 Specification for Masonry Structures, designers can use the Unit Strength Method for determining the masonry design strength, f’m. The two components needed to use the Unit Strength Method are block strength and mortar type. A particular f’m can be achieved by either: 1.) using stronger block and weaker mortar, or 2.) using weaker block and stronger mortar. Therefore simply stating a minimum f’m on construction documents does not indicate to contractors what block or mortar should be used. Also, by only specifying f’m, the required strength of grout is left unknown.
The key components for the strength of clay masonry walls are the hollow structural clay units. Clay masonry units are available as either common/face brick or hollow brick. Hollow bricks allow for reinforcing to be added within its cells similar to concrete masonry block. More information can be found at Brick Industry Association (BIA) gobrick.com under technical Note 41.
The common compressive strength in Midwest for hollow brick, f ’brick as determined by ASTM C652 tests, is 8,250 psi or higher. Higher strengths above 11,500 psi are also available from some manufacturers.
The next component that needs to be specified is the mortar. There is a lot of confusion over mortar strength and its effects on f’m. The common mistake is to believe that masonry is only as good as its weakest element – the mortar. It’s important to remember that mortar only makes up a small percentage of the overall wall as most of the material in a wall is higher strength clay masonry. There is also confusion over testing; mortar tests are done in non-absorptive molds that result in a higher moisture content and less strength than mortar placed in a masonry wall between clay units. Properties of mortar such as bond strength and workability are more important in many cases than compressive strength. TMS 602 clearly defines the strength of the wall to be more than the strength of the mortar. Mortar strength has been shown to be a relatively unimportant factor in determining f’m. In TMS 602, Table 1 (section 1.4B.2.a) shows the type of mortar and unit strength can be used to find the assembly compressive strength, f’m. TMS 602 also refers to the prism testing that was done (Figures SC-1 and SC-2) which supports the f’m values shown in the table. Therefore it is also important to recognize that f’m is dependent on the type of mortar, not the mortar strength – the requirements for the mortar strength are set once one selects the
mortar type per ASTM C270 specification. The two most common mortars to use in structures are Type S and Type N. Type S has benefits to strength and durability that make it ideal for walls that have structural load demands, such as bearing walls, exterior walls, shear walls, fire walls, stair shaft walls, elevator shaft walls, etc. Type S is the common mortar type for structural reinforced masonry walls. Therefore, if your clay masonry wall is designed as a structural reinforced wall to resist gravity and/ or lateral loads, Type S is the recommended mortar type to specify. If your clay wall is a non-structural unreinforced veneer or partition wall, Type N is the mortar type preferred by contractors for its high workability.
Following the recommendations above, designers should research available brick strengths and specify accordingly. For example, f’brick=11500psi and Type S mortar for structural clay masonry. Using the Unit Strength Method, the resulting f ’m will be 4,000 psi or higher; see Table 1.
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