Progress in science and technology allows manufacturers to develop new construction materials, some of which are user and environmentally friendly, as well as less labor intensive than their predecessors. One recent innovation is the use of adhesives in lieu of conventional mortar as defined in the building codes for masonry construction. One of the main advantages of masonry construction with an adhesive in lieu of conventional mortar is expedited construction with reduced construction time. While the ultimate strength of mortar is typically assumed to be reached in 28 days, construction with masonry adhesive can be put in service within hours as recommended by the adhesive manufacturer.

Another advantage of the adhesive as compared with conventional mortars is product consistency. Conventional mortar can have a large variance in properties because of variations in the mixing procedure resulting from many parameters, including human error. Adhesives are typically premixed in a controlled manufacturing location. Other advantages are likely to be ease of application, reduced clean-up, no mixing requirement for one-part adhesives, and ease of transportation. Although the basic disadvantage of the masonry adhesive is its inability to serve as a leveling agent equivalent to mortar because of relatively thin adhesive bed joints, there is a significant reduction in labor costs which may result in increased productivity.

Masonry construction with conventional mortar

Mortar has been used in masonry construction since the beginning of civilization. The main advantages of conventional mortar are the ease of obtaining constituents and the ease of finding skilled personnel to mix and use it. Another advantage is that conventional mortar works as a leveling agent (ease of applying the mortar in bed joints to keep masonry walls leveled) for installations where there are irregularities in the masonry unit dimensions (ASTM C 90 allows masonry unit dimensions to have a tolerance of ±1/8 inch).

Kinds of mortar—ASTM C 270, referenced by Section 2103.8 of the International Building Code (IBC) and Section R607.1 of the International Residential Code (IRC), refers to three kinds of mortars. The traditional one is portland cement/lime mortar. Mortars containing only cement and sand have a high compressive strength and low water retentivity, but are also vulnerable to cracking and rain penetration. Mortars containing only lime and sand have just the opposite properties. A combination of cement and lime provides balanced properties (ASTM C 270). Hence, cement/lime mortars utilize cement and lime blends. The other two mortar kinds that are referenced by ASTM C 270 are mortar/cement and masonry/cement mortars, which are readily formulated to produce mortars conforming to specifications.

Types of mortar—Four major mortar types are defined by ASTM C 270: Types M, S, N, and O. In accordance with ASTM C 270, mortars can be specified in one of two ways, but not both: by proportion or by property. The proportion method allows the mortar producer to follow the mix-ratio guidelines set forth in Table 1 of ASTM C 270. If the material meets the proportioning specifications, mortar testing is not required. This is due to years of experience with mortar. The property specification option allows the producer to develop a specific mix design, which may differ from the proportioning specifications, by requiring the mortars to achieve the property requirements in Table 2 of ASTM C 270. This approach requires laboratory testing of mortar.

The strongest mortar among all four mortar types is Type M mortar because of its high cement-to-lime proportion. Type M is specifically suitable for structures that are below grade and in contact with the soil, such as foundations and retaining walls. Type S mortar is the second strongest mortar. It is mostly used in above-grade exterior walls that are exposed to severe environmental conditions. Type S can also be used in lieu of Type M in interiors and all load-bearing structures. Type N is a moderate-strength mortar. Because of its high lime content, Type N has a high workability property. Type N mortars are intended for use in bearing walls that are above grade, and for load conditions with low stresses, such as partitions and exterior walls where members are not subjected to harsh environmental conditions. Because this mortar is weaker in strength and contains more lime, it is more flexible and reduces cracking in applications such as veneer construction (Filloramo, 2006). Type O is the lowest-strength mortar. It is typically used in repair work so as not to create a mortar that is stronger than the walls or units where it is being placed. Because of its low strength, Type O is not used in walls or units that may be subjected to freezing and thawing cycles. Also, ACI 530-05/ASCE 5-05/TMS 402-05 limits Type O to applications within low seismic zones.

Masonry construction with masonry adhesive

Adhesives for masonry construction are evaluated by ICC Evaluation Service, Inc. (ICC-ES) using Acceptance Criteria AC362 (Acceptance Criteria for Adhesives for Masonry Construction). The object of AC362 is to demonstrate that masonry adhesive can be an alternative to Types M, S, N, and O portland cement/lime masonry mortar.

AC362 states that the adhesive is for use in the head and bed joints of the masonry; however, the first course of masonry must be set into a setting bed of code-complying mortar applied to the concrete foundation. AC362 was developed to be applicable to masonry construction designed in accordance with the requirements of IRC Section R606 or IBC Section 2107 (Allowable Stress Design), IBC Section 2108 (Strength Design of Masonry), or IBC Section 2109 (Empirical Design of Masonry), or the ACI 530-05/ASCE 5/TMS 402 Masonry Code for use under either the IBC or IRC, as permitted in IBC Section 2109.1 and IRC Sections R404.1 and R606.1.

AC362 outlines the qualification testing and analysis requirements for masonry construction with adhesives, such as fixed-value tests (masonry compression strength, flexural bond strength, out-of-plane load and creep tests) and comparative tests (freeze-thaw resistance, high-temperature resistance, wet-dry cycling resistance, ultraviolet light resistance, and water penetration and leakage). AC362 also provides manufacturing specifications and quality control during manufacturing.

An evaluation in accordance with AC362 is used to develop an evaluation report (ESR) that offers code officials an independent resource for demonstrating code compliance, and structural design engineers a resource for preparing a design. ICC-ES has issued an evaluation report (ESR-1968) on a masonry adhesive evaluated in accordance with AC362. The evaluation report provides a description of the adhesive, information regarding its uses, design guidelines for the designer (including seismic limitations), installation requirements, assembly conditions and curing guidelines for the contractor, and special inspection requirements for the code officials.

Masonry adhesive—The masonry adhesive considered in ESR-1968 is formulated to bond concrete masonry units. It has been evaluated for use as an alternative to Type M, S, N, and O portland cement/lime masonry mortar. The masonry adhesive is a solvent-free, single-component polyurethane, moisture-cured structural adhesive. It has an open time life (time to allow adjustments of the unit after it has been placed) of 15 minutes at 75° F and 50 percent relative humidity (RH). Hardening time is 72 hours. It has an 18-month storage life when stored in unopened containers at temperatures between 65° F and 95° F. It can be applied with a standard caulking gun.

Conclusion
Findings associated with ICC-ES evaluation report ESR-1968 may be summarized as follows:

• Design of masonry construction with the adhesive is to be the same as masonry design with a specific conventional mortar used in comparison tests.
• Until further research is available, use of adhesives is limited to unreinforced concrete masonry, non-prestressed masonry construction, and non-fire-resistance-rated construction.
• The stronger the bond of the mortar, the less able mortar is to accommodate building movement (Hendry, 2001). The bond strength of the masonry unit/masonry adhesive joint is much stronger than that of the masonry unit/conventional mortar joint. The IBC does not provide guidance regarding the use of masonry with adhesives in high seismic categories. Consequently, use of masonry adhesive in plain (unreinforced) masonry wall construction has only been evaluated for use in Seismic Design Categories A and B until a seismic testing procedure is developed and adopted into AC362 through ICC-ES Evaluation Committee public hearings.
• Because the adhesive bed-joint of masonry units is very thin, the bed-joints cannot accommodate joint reinforcement or anchors for anchored veneer installed in the bed-joint.
• To prevent detrimental effects of high/low temperature on the performance of masonry construction utilizing an adhesive, such as effect on creep performance, masonry construction with adhesives is currently limited to temperatures that are within the temperatures limits addressed in the qualification tests. Also, cold-weather and hot-weather construction of masonry walls is required to follow the general guidelines provided by the applicable codes, such as the 2006 IBC and ACI 530-05/ASCE 5-05/TMS 402-05.
• Concrete masonry walls crack as a result of drying shrinkage, temperature fluctuations, and carbonation. These shrinkage cracks occur when masonry is restrained from movement. Consequently, masonry walls constructed with mortar or adhesive require vertical separators called control joints. NCMA document TEK-10-1A considers shrinkage cracks in concrete masonry as a non-structural issue and requires control joints only in walls where aesthetics or water penetration is of concern. AC362 recommends using control joints in accordance with the requirements of NCMA TEK 10-2B and TEK 10-3.

References

International Building Code (IBC), International Code Council, 2006.

International Residential Code (IRC), International Code Council, 2006.

ICC-ES Acceptance Criteria AC362. Adhesives for Masonry Construction. ICC Evaluation Service, Inc., February 2007.

ICC-ES Evaluation Report ESR-1968. Mason Bond F6200F. ICC Evaluation Service, Inc., Issued Nov. 1, 2007.

ACI 530-05/ASCE 5-05/TMS 402-05. Building Code Requirements for Masonry Structures. 2005.

ASTM C 270. Specification for Mortar for Unit Masonry. 2004; Vol. 04-02, Philadelphia, PA.

ASTM C 90. Specification for Loadbearing Concrete Masonry Units. 2003; Vol. 04-02, Philadelphia, PA.

Filloramo R. Mortar Testing. Structure; May 2006.

Hendry AW. Masonry Walls: Materials and Construction. Construction and Building Materials, 2001, Vol. 15, pp. 323-330.

NCMA TEK 10-1A. Crack Control in Concrete Masonry Walls. National Concrete Masonry Association. 2005.

NCMA TEK 10-2B. Control Joints for Concrete Masonry Walls-Empirical Method. National Concrete Masonry Association. 2005.

NCMA TEK 10-3. Control Joints for Concrete Masonry Walls-Alternative Engineer Method. National Concrete Masonry Association. 2005.

Mahmut Ekenel, Ph.D., is a senior evaluation specialist for the ICC Evaluation Service in Whittier, Calif. He can be reached at mekenel@icc-es.org.



Masonry wall construction with masonry adhesive.