Picking the Proper Mortar for Adhered Veneers

Different formulations are best suited for different situations

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TESTING FOR QUALITY CONTROL

Anyone who has written specifications knows that the key to quality control is to cite the appropriate industry standards and require that the specified materials provide evidence of meeting the appropriate level of performance through testing. In the case of mortars, there are several relevant testing standards listed as follows:

ASTM C270 “Standard Specification for Mortar for Unit Masonry” This standard covers all mortar for unit masonry, not just adhered veneer. Specifically, “this specification covers mortars for use in the construction of non-reinforced and reinforced unit masonry structures. Four types of mortar are covered in each of two alternative specifications: (1) proportion specifications and (2) property specifications.”
ASTM C1714 “Standard Specification for Preblended Dry Mortar Mix for Unit Masonry” This focused specification “covers masonry mortars whose materials and design requirements are governed by ASTM C270 (listed above) but are preblended dry in a factory instead of produced from individual raw materials delivered to the jobsite. The tight control of ingredient ratios possible with preblended dry mortar is also covered.”
ASTM C482 “Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste” The intent is that “This test method covers the determination of the ability of ceramic tile, ceramic mosaic tile, quarry tile, porcelain tile, and paver tile to be bonded to Portland cement paste.” As such, it is focused on tile being used as the veneer.
ASTM C1780 “Standard Practice for Installation Methods for Cement-based Adhered Masonry Veneer” The stated scope is “This practice is intended to provide accepted procedures to designers and installers of cement-based adhered masonry veneer in residential and commercial construction. This information is meant to complement the specific installation instructions provided by manufacturers of cement-based adhered masonry veneers and recognized building codes but is not meant to replace them. This practice covers the installation of cement-based adhered masonry veneer units for application as adhered veneer to exterior and interior walls, columns, landscape structures, and other structures suitable to receive adhered veneer.”
CSA A179 “Mortar and grout for unit masonry” This Canadian Standard specifies requirements for “a) mortar for use in bedding, jointing, and bonding of masonry units; and b) grout for use in filling core, cell, or cavity space in unit masonry construction.”
ANSI A118.4 “A118.4 American National Standard Specifications for Modified Dry-Set Cement Mortar” This standard covers the physical properties of thin-set mortar but not its composition. Some states are requiring compliance of ANSI A118.4 for improved performance over ASTM C482.
ANSI A118.15 “American National Standard Specifications for Improved Modified Dry-Set Cement Mortar” In recent years, the number of available thin set mortars which comply with the requirements of ANSI A118.4 has increased. Since there is a wide range of applications and features of ANSI A118.4 mortars, the need to distinguish standard-performing mortars from high-performing mortars became necessary. Hence, this standard provides another level of classification for thin-set mortars. The main distinguishing feature between the two categories of mortar is the 28-day shear bond strength requirements and a focus on the use of silica sand.

It is important to recognize that the ASTM and CSA standards may be able to be met using site-mixed products, but the ANSI standards referenced here cannot be met on-site. Rather, they can only be met by using factory-preblended products that can be tested in controlled laboratory conditions.

Based on all of the above, we focus next on the three primary preblended products that can be used in a variety of settings for adhered veneers.

TYPES OF PRE-BLENDED MORTARS

As noted, mortars can be pre-blended in any number of mixes to suit different conditions and project requirements. Within that variety, there are three primary types of mixes, each with its own general makeup and intended use. These are summarized in the following sections.

Standard Adhered Veneer Mortar (AVM)

The standard AVM is a versatile and durable mortar intended for thin veneers. It is formulated for lightweight manufactured thin veneer masonry units (e.g., manufactured stone, thin brick, etc.) over wood frame and steel stud construction. It typically contains an optimized blend of dry cementitious materials, masonry sand, and performance-enhancing admixtures specifically designed for use as a scratch coat, bond coat, and joint grout. When formulated and mixed cohesively, it can provide consistent workability, anti-sag capabilities, high water retention, and excellent bond strength. The final wall system produced is generally a non-load bearing, aesthetically pleasing exterior veneer or interior finish ideal for residential and light commercial construction.

AVM is generally designed to be installed with about a ½-inch layer for good bonding and is compatible with the characteristics of most masonry thin veneer units. It is also generally available in standard and custom colors for joint grouting of the masonry veneer. AVM products are available that meet ASTM C270, CSA A179, and ASTM C1714 applicable requirements.

Photo courtesy of SPEC MIX

Following the correct industry standards and specifications for mortar helps assure quality control and predictable performance in exterior and interior conditions.

Polymer Modified Adhered Veneer Mortar (PMAVM)

Mortar that is modified by adding in polymers (PMAVM) is an advanced adhered veneer mortar. It is intended for use in bonding manufactured stone veneer, natural thin-cut stone, and thin brick to cementitious substrates including scratch coats, CMU, and properly prepared concrete. It is formulated to achieve low sag, high productivity, and a strong bond between the veneer products and the cementitious substrate. It also provides excellent workability, water resistance, efflorescence mitigation, and durability. The anti-slip properties of PMAVM make the installation of large natural thin-cut stone easy while holding units in place for an extended time.

PMAVM is also a thick bedding mortar that can be used for scratch coat, bond coat, and jointing between adhered stones/ bricks. It has higher flexural bond strength and better adhesion than standard AVM due to the polymer admixtures in the mix. It meets ASTM C270, CSA A179, and ASTM C1714 applicable requirements plus the ANSI 118.4 standards when tested in a pre-blended condition.