Technical Tips: Use Workable Mortar

Mortar Characteristics and Performance
Every mason wants a workable mortar on his trowel every day, regardless of the type of unit he is working with or weather conditions. He's looking for a highly plastic mortar that's easy to spread, adheres to head joint surfaces, accommodates unit placement, has good board life, and facilitates consistent tooling. He wants mortar that is the same from hour-to-hour and day-to-day.

The project architect probably has a very different set of concerns about mortar performance. He wants a mortar that will establish a good bond, provide uniform color, and resist moisture penetration and freeze/thaw deterioration.

The mason is concerned with the plastic properties of the mortar while the designer and owner are concerned with the hardened properties. It may be surprising to both parties that the expectations of each can be achieved by following a few simple, common-sense principles. A workable, plastic mortar is key to providing completed masonry that has the properties the designer and owner require.

The architect should always select the weakest mortar that will fulfill the structural requirements for the project. In most cases a Type N mortar is sufficient. Where higher flexural values are required, a Type S mortar can be used. Focus quality-assurance procedures on verifying that the proper mortar materials and proportions are used. Permit the mason to control water content to optimize the workability of the mortar for the units and weather conditions encountered. The mason contractor should select a well-graded sand that does not contain excessive clay fines and carefully control mortar production by:

• Keeping the sand pile covered to prevent wetting or drying out.
• Establishing mortar proportions at the beginning of the work and posting the mix design on or near the mixer for everyone to follow.
• Using consistent mixing procedures.
• Always pre-wetting mortar containers before filling with fresh mortar.
• Controlling mortar temperatures (that is, cool materials if needed in hot weather, and heat mortar materials as required in cold weather), and
• Mixing mortar to keep pace with construction (don't let mortar set in mixer or mortar box for excessive periods of time).

Recognize that unit characteristics affect mortar performance. It important that the mason contractor know what kind of units he is working with and what their initial rate of absorption characteristics are. He should keep concrete masonry units dry and clay masonry units at uniform moisture levels. High absorption brick may require wetting before use.

Following these guidelines will furnish the mason with a workable mortar and the owner with quality construction.

The Basics On Mixing A Consistent Mortar
Mix mortar materials in a mechanical mixer. With the mixer running, add materials in this sequence:

• 2/3-3/4 of the required water, plus
• 1/2 of the required sand, plus
• Masonry cement or mortar cement, or
• Hydrated lime followed by portland cement, plus
• Remainder of sand, and
• Water required to reach workable consistency.

Mix mortar for not less than three minutes and not more than five minutes after the last materials have been introduced into the mixer.

Acknowledgment
We gratefully acknowledge the assistance of the Portland Cement Association and Jake Ribar, Principal Evaluation Engineer, Construction Technologies Laboratories, Skokie, Illinois in allowing us to reuse this information from Masonry Today, Volume 6, Number 2, Winter 96/97.