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Fire Resistance Ratings of Concrete Masonry Assemblies

  • September 2020

INTRODUCTION

Concrete masonry is widely specified for fire walls and fire barriers because concrete masonry is noncombustible, provides durable fire resistance, and is economical to construct. Chapter 7 of the International Building Code (IBC) (ref. 2) governs materials and assemblies used for structural fire resistance and fire-rated separation of adjacent spaces. This TEK is based on the provisions of Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1/TMS 216 (ref. 1) , which outlines a procedure to calculate the fire resistance ratings of concrete masonry assemblies. The 2014 edition of the ACI 216.1/TMS 216 is referenced in the 2015 IBC for concrete and masonry materials. This TEK is based on both prescriptive details and tables as well as the calculated fire resistance procedure, which is practical, versatile and economical. The calculation procedure allows the designer virtually unlimited flexibility to incorporate the excellent fire-resistive properties of concrete masonry into a design. Included are methods for determining the fire resistance rating of concrete masonry walls, columns, lintels, beams, and concrete masonry fire protection for steel columns. Also included are assemblies composed of concrete masonry and other components, including plaster and gypsum wallboard finishes, and multi-wythe masonry components including clay or shale masonry units.

METHODS OF DETERMINING FIRE RESISTANCE RATINGS

Because full-scale fire testing of representative test specimens is not practical in daily practice due to time and financial constraints, the IBC outlines multiple options for fire rating determination:

  • standardized calculation procedures, such as those in the ACI 216.1/TMS 216 and in Section 722 of the IBC;
  • prescriptive designs such as those in Section 721 of the IBC;
  • engineering analysis based on a comparison with tested assemblies;
  • third party listing services, such as Underwriters Laboratory; and
  • alternative means approved by the building official per Section 104.11 of the IBC.

Of these, the calculation method is an economical and commonly used method of determining concrete masonry fire resistance ratings. The calculations are based on extensive research, which established relationships between the physical properties of materials and the fire resistance rating. The calculation method is fully described in ACI 216.1/TMS 216 and IBC Section 722, and determines fire resistance ratings based on the equivalent thickness of concrete masonry units and the aggregate types used to manufacture the units. Private commercial listing services allow the designer to select a fire rated assembly that has been previously tested, classified and listed in a published directory of fire rated assemblies. The listing service also monitors materials and production to verify that the concrete masonry units are and remain in compliance with appropriate standards, which usually necessitates a premium for units of this type. The system also is somewhat inflexible in that little variation from the original tested wall assembly is allowed, including unit size, shape, mix design, constituent materials, and even the plant of manufacture. More information on listing services for fire ratings is provided in CMU-FAQ 015-23 (ref. 16).

For prescriptive designs, the IBC provides a series of tables that describes requirements of various assemblies to meet the fire resistance ratings specified. The last two options listed above require justification to the building official that the proposed design is at least the equivalent of what is prescribed in the code.

CALCULATED FIRE RESISTANCE RATINGS

Background

The calculated fire resistance method is based on extensive research and testing of concrete masonry walls. Fire testing of wall assemblies is conducted in accordance with Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119 (ref. 3), which measures four performance criteria, as follows:

  • resistance to the transmission of heat through the wall assembly;
  • resistance to the passage of hot gases through the wall, sufficient to ignite cotton waste;
  • load-carrying capacity of loadbearing walls; and
  • resistance to the impact, erosion and cooling effects of a hose stream on the assembly after exposure to the standard fire.

The fire resistance rating of concrete masonry is typically governed by the heat transmission criteria. From the standpoint of life safety (particularly for fire fighters) and reuse, this failure mode is certainly preferable to a structural collapse endpoint, characteristic of many other building materials.

The calculated fire resistance rating information presented here is based on the IBC and ACI 216.1/TMS 216 (refs. 1, 2).

Equivalent Thickness

Extensive testing has established a relationship between fire resistance and the equivalent solid thickness of concrete masonry walls, as shown in Table 1. Equivalent thickness is essentially the solid thickness that would be obtained if the volume of concrete contained in a hollow unit were recast without core holes (see Figure 1). The equivalent thickness is determined in accordance with Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140 (ref. 4), and is reported on the C140 test report. If the equivalent thickness is unknown, but the percent solid of the unit is, the equivalent thickness of a hollow unit can be determined by multiplying the percent solid by the unit’s actual thickness.

The equivalent thickness of a 100% solid unit or a solid grouted unit is equal to the actual thickness. For partially grouted walls where the unfilled cells are left empty, the equivalent thickness for fire resistance rating purposes is equal to that of an ungrouted unit. For partially grouted walls with filled cells, see the following section. Loadbearing units conforming to ASTM C90 (ref. 5) that are commonly available include 100% solid units, 75% solid units, and hollow units meeting minimum required face shell and web dimensions. Typical equivalent thickness values for these units are listed in Table 2.

Filling Cells with Loose Fill Material

If all cells of hollow unit masonry are filled with an approved material, the equivalent thickness of the assembly is the actual thickness. This also applies to partially grouted concrete masonry walls where all ungrouted cells are filled with an approved material.

Applicable fill materials are: grout, sand, pea gravel, crushed stone, or slag that comply with ASTM C33 (ref. 6); pumice, scoria, expanded shale, expanded clay, expanded slate, expanded slag, expanded fly ash, or cinders that comply with ASTM C331 (ref. 7); perlite meeting the requirements of ASTM C549 (ref. 8); or vermiculite complying with C516 (ref. 9).

Wall Assembly Fire Ratings

The fire resistance rating is determined in accordance with Table 1 utilizing the appropriate aggregate type used in the masonry unit and the equivalent thickness.

Units manufactured with a combination of aggregate types are addressed by footnote C, which may be expressed by the following equation (see also the blended aggregate example, below):

Blended aggregate example:

The required equivalent thickness of an assembly constructed of units made with expanded shale (80% by volume), and calcareous sand (20% by volume), to meet a 3-hour fire resistance rating is determined as follows. From Table 1:

Multi-Wythe Wall Assemblies

The fire resistance rating of multi-wythe walls (Figure 2) is based on the fire resistance of each wythe and the air space between each wythe using the following equation:

For multi-wythe walls of clay and concrete masonry, use the values in Table 3 for the brick wythe in the above equation.

Reinforced Concrete Masonry Columns

Concrete masonry column fire testing evaluates the ability of the column to carry design loads under standard fire test conditions. Based on a compendium of fire tests, the fire resistance rating of reinforced concrete masonry columns is based on the least plan dimension of the column as indicated in Table 4. The minimum required cover over the vertical reinforcement is 2 in. (51 mm).

Concrete Masonry Lintels

Fire testing of concrete masonry beams and lintels evaluates the ability of the member to sustain design loads under standard fire test conditions. This is accomplished by ensuring that the temperature of the tensile reinforcement does not exceed 1,100°F (593°C) during the rating period. The calculated fire resistance rating of concrete masonry lintels is based on the nominal thickness of the lintel and the minimum cover of longitudinal reinforcement (see Table 5). The cover requirements protect the reinforcement from strength degradation due to excessive temperature during the fire exposure period. Cover requirements may be provided by masonry units, grout, or mortar. Note that for 3 and 4 hour requirements, not enough cover is available for 6-in. (152 mm) masonry; however, if a special analysis indicates that the reinforcement is not necessary or not needed, such as when conditions for arching action are present, the cover requirements may be waived. See TEK 17-01D (ref. 11) for lintel design and conditions for arching action.

Control Joints

Figure 3 shows control joint details in fire-rated wall assemblies in which openings are not permitted or where openings are required to be protected. Maximum joint width is 1/2 in. (13 mm). Although these details are not directly in the IBC, they are included by reference in ACI 216.1/TMS 216.

In addition to these prescriptive fire resistance rated control joints, other control joints may be permitted in fire rated masonry walls. For example, the IBC and ACI 216.1/1/TMS 216 include provisions for ceramic fiber joint protection for precast panels, which are similar to concrete masonry walls in that both rely on concrete for fire protection, and both are governed by the ASTM E119 heat transmission criteria (see Figure 4). The first two categories of aggregate types in Table 1 would correspond to the carbonate or siliceous aggregate concrete curve and the last two aggregate categories of Table 1 would correspond to the semi-lightweight or lightweight concrete curve. For example, for an 8-in. (203-mm) limestone aggregate concrete masonry wall with a maximum control joint width of 1/2 in. (13 mm), a 1 in. (25 mm) thickness (measured perpendicular to the face of the wall) of ceramic fiber in the joint can be used in walls with fire resistance ratings up to 3 hours, while a 2 in. (51 mm) thickness can be used in the joints of a 4-hour wall.

Steel Columns Protected by Concrete Masonry

Fire testing of a steel column protected by concrete masonry evaluates the structural integrity of the steel column under fire test conditions, by measuring the temperature rise of the steel. The calculated fire resistance rating of steel columns protected by concrete masonry, as illustrated in Figure 5, is determined by:

Effects of Finish Materials on Fire Resistance Ratings

In many cases, drywall, plaster or stucco finishes are used on concrete masonry walls. While finishes are normally applied for architectural reasons, they can also provide additional fire resistance. The IBC and ACI 216.1/TMS 216 include provisions for calculating the additional fire resistance provided by these finishes.

Note that when finishes are used to achieve the required fire rating, the masonry alone must provide at least one- half of the total required rating and the contribution of the finish on the non-fire-exposed side cannot be more than one-half of the contribution of the masonry alone. This is to assure structural integrity during a fire. The finish material must also be continuous over the entire wall.

Certain finishes deteriorate more rapidly when exposed to fire than when they are on the non-fire side of the wall. Therefore, two separate tables are required. Table 7 applies to finishes on the non fire-exposed side of the wall, and Table 8 applies to finishes on the fire-exposed side. For finishes on the non-fire exposed side of the wall, the finish is converted to an equivalent thickness of concrete masonry by multiplying the finish thickness by the factor given in Table 7. The result, Tef, is then added to the concrete masonry wall equivalent thickness, Te, and used in Table 1 to determine the wall’s fire resistance rating (i.e., the equivalent thickness of concrete masonry assemblies, Tea = Te Tef).

For finishes on the fire-exposed side of the wall, a time (from Table 8) is assigned to the finish. This time is added to the fire resistance rating determined for the base wall and nonfire-exposed side finish, if any. The times listed in Table 8 are essentially the length of time the various finishes will remain intact when exposed to fire (i.e., on the fire-exposed side of the wall).

When calculating the fire resistance rating of a wall with finishes, two calculations are performed, assuming each side of the wall is the fire exposed side. The fire rating of the wall assembly is the lower of the two. Typically, for an exterior wall with a fire separation distance greater than 5 ft (1,524 mm), fire needs be considered on the interior side only

Installation of Finishes

Finishes that contribute to the total fire resistance rating of a wall must meet certain minimum installation requirements. Plaster and stucco are applied in accordance with the provisions of the building code without further modification. Gypsum wallboard and gypsum lath are to be attached to wood or metal furring strips spaced a maximum of 16 in. (406 mm) o.c., and must be installed with the long dimension parallel to the furring members. All horizontal and vertical joints must be supported and finished.

UNCONVENTIONAL AGGREGATES

In recent years, manufacturers of concrete masonry products have been exploring the use of alternative materials in the production of concrete masonry units. Some of these materials have not been evaluated using standardized fire resistance test methods or have been evaluated only to a limited degree. Such unconventional materials, which are typically used as a replacement for conventional aggregates, may not be covered within existing codes and standards due to their novelty or proprietary nature.

While test methods such as ASTM E119 define procedures for evaluating the fire resistance properties of concrete masonry assemblies, including those constructed using unconventional constituent materials, there has historically been no defined procedure for applying the results of ASTM E119 testing to standardized calculation procedures available through ACI 216.1/TMS 216. To provide consistency in applying the results of full scale ASTM E119 testing to established calculation procedures, CMHA has developed CMU-FAQ-013-23 (Ref. 15).

This guideline stipulates that when applying the fire resistance calculation procedure of ACI 216.1/TMS 216 to products manufactured using aggregate types that are not listed in ACI 216.1/TMS 216, at least two full scale ASTM E119 tests must be conducted on assemblies containing the unconventional material. Based on the results of this testing, an expression can be developed in accordance with this industry practice that permits the fire resistance of units produced with such aggregates to be calculated for interpolated values of equivalent thickness and proportion of non listed aggregate.

REFERENCES

  1. Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1- 14/TMS216-14. American Concrete Institute and The Masonry Society, 2014.
  2. International Building Code 2015. International Code Council, 2015.
  3. Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119-16a. ASTM International, Inc., 2016.
  4. Standard Methods for Sampling and Testing Concrete Masonry Units, ASTM C140-16. ASTM International, Inc., 2016.
  5. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-16. ASTM International, Inc., 2016.
  6. Standard Specification for Concrete Aggregates, ASTM C33-16e1. ASTM International, Inc., 2016.
  7. Standard Specification for Lightweight Aggregates for Concrete Masonry Units, ASTM C331-14. ASTM International, Inc., 2014.
  8. Standard Specification for Perlite Loose Fill Insulation, ASTM C549-06(2012). ASTM International, Inc., 2012.
  9. Standard Specification for Vermiculite Loose Fill Thermal Insulation, ASTM C516-08(2013)e1. ASTM International, Inc., 2013.
  10. Steel Column Fire Protection, TEK 07-06A. Concrete Masonry & Hardscapes Association, 2009.
  11. ASD of Concrete Masonry Lintels Based on the 2012 IBC/2011 MSJC, TEK 17-01D. Concrete Masonry & Hardscapes Association, 2011.
  12. Standard Specification for Concrete Building Brick, ASTM C55 14a. ASTM International, Inc., 2014.
  13. Standard Specification for Calcium Silicate Brick (SandLime Brick), ASTM C73-14. ASTM International, Inc., 2014.
  14. Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744-16. ASTM International, Inc., 2016.
  15. How is the fire resistance of a concrete masonry assembly calculated when using unconventional aggregates?, CMU-FAQ-013-23. Concrete Masonry & Hardscapes Association, 2023.
  16. What is the difference between fire resistance ratings for masonry assemblies obtained through IBC versus a listing service such as UL or FM?, CMU-FAQ-015-23. Concrete Masonry & Hardscapes Association, 2023.

Structural Design of Unreinforced Composite Masonry

  • November 2018

INTRODUCTION

Concrete masonry offers many textures, colors and sizes, along with choices in bond patterns and joint treatment making it an excellent choice for exterior and interior walls in residential, commercial and public buildings. Concrete brick can be used in both structural and veneer applications and is economical, durable, easy to maintain, fire resistant, and reduces sound transmission.

Multi-wythe masonry walls are classified as either composite or noncomposite depending on how the wythes interact. Connections between wythes of composite walls are designed to transfer stresses between the wythes, allowing the wythes to act as a single member in resisting loads. In contrast, for noncomposite or cavity walls each wythe individually resists the loads imposed on it. Concrete brick are used both in composite walls and as nonloadbearing veneer in cavity wall construction. Requirements for concrete brick veneers are summarized in Concrete Masonry Veneers, TEK 03-06C (ref. 1).

Standard Specification for Concrete Building Brick, ASTM C 55 (ref. 2), governs concrete brick and similar solid units. C 55 requirements are summarized in Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMUTEC-001-23 (ref. 3). 

 

STRUCTURAL DESIGN METHODS

Composite wall structural design requirements are contained in Building Code Requirements for Masonry Structures (ref. 4) and the International Building Code (ref. 5).

Allowable stress design of unreinforced composite walls is typically governed by the flexural tensile capacity of the masonry system (see Table 1), although compression and shear must also be checked. Shear stress in the plane of interface between wythes and collar joints is limited to 5 psi (34.5 kPa) for mortared collar joints; 10 psi (68.9 kPa) for grouted collar joints; and the square root of the unit compressive strength of the header (over the net area of the header) for headers.

Tables 2 through 13, for lateral loads with or without concentric axial loads (see Figure 1), are based on Chapter 2, Allowable Stress Design, of Building Code Requirements for Masonry Structures (ref. 4) and the following:

  1. specified compressive strength of masonry, f’m = 1500 psi (10.3 MPa),
  2. section modulus based on the minimum net area of the composite wall cross section,
  3. faceshell and web dimensions based on ASTM C 90 (ref. 6) minimum requirements for hollow units,
  4. loads include ⅓ increase in allowable stress for load combinations including wind or seismic (where ⅓ increase does not apply, multiply the Table values by ¾), and
  5. allowable tensile stress in masonry, Ft, for hollow ungrouted concrete masonry normal to the bed joints is as noted in Table footnotes.
Figure 1—Concentric Axial and Lateral Loading
Table 1—Allowable Flexural Tension, psi (kPa) (ref. 4)
Tables 2 through 13

CONSTRUCTION

Concrete brick walls and wythes of concrete brick should be laid with full head and bed mortar joints. For composite construction, the collar joint (the vertical longitudinal joint between wythes of masonry) is filled with grout or mortar to allow structural interaction between the wythes.

In composite walls, Building Code Requirements for Masonry Structures (ref. 4) requires that concrete brick be bonded to the backup wythe using either masonry headers or wall tie and grout or mortar. These minimum requirements, described below, help ensure that composite action is present between the wythes.

When bonded using masonry headers, the headers must make up at least 4 percent of the wall surface and extend at least 3 in. (76 mm) into the backing. The shear stress developed in the masonry header is limited to the square root of the unit compressive strength of the header (in psi (MPa) over the net area of the header).

Figure 2 illustrates wall tie spacing requirements for composite walls bonded with corrosion resistant ties or wire and collar joints filled with mortar or grout. Cross wires of joint reinforcement and rectangular ties are commonly used as wall ties for composite walls. Z-ties, however, are not permitted with ungrouted hollow masonry (ref. 7).

For cavity wall construction, the following construction recommendations apply:

  • keep cavity substantially clean to allow free water drainage,
  • install weep holes at 32 in. (813 mm) o. c.,
  • install granular fill, mesh or other mortar collection device in bottom of cavity to prevent mortar droppings from blocking weep holes, and
  • embed wall ties at least 1 ½ in. (38 mm) into the mortar bed of solid units.
Figure 2—Wall Tie Spacing for Composite Walls

REFERENCES

  1. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  2. Standard Specification for Concrete Brick, ASTM C 55-01. American Society for Testing and Materials, 2001.
  3. Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
  4. Building Code Requirements for Masonry Structures, ACI 530-99/ASCE 5-99/TMS 402-99. Reported by the Masonry Standards Joint Committee, 1999.
  5. International Building Code. International Code Council, 2000.
  6. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-01. American Society for Testing and Materials, 2001.
  7. Specification for Masonry Structures, ACI 530.1-99/ ASCE 6-99/TMS 602-99. Reported by the Masonry Standards Joint Committee, 1999.

Foam Plastic Insulation in Concrete Masonry Walls

  • November 2018

INTRODUCTION

Foam plastic insulation is often used in exterior concrete masonry construction to improve steady state thermal performance (R-values), and in some cases to improve air and moisture infiltration properties as well. Because of their potential flammability and smoke generation in case of fire, the International Building Code (IBC) (ref. 1) imposes additional requirements on these materials when they are used in exterior walls. These requirements are covered in IBC section 2603.

Foam plastic insulations include both rigid board (expanded polystyrene, extruded polystyrene, polyisocyanurate) as well as open cell and closed-cell spray-applied or foamed-in-place insulations. They may be used on the interior, exterior or in the cores (as either inserts or foamed-in-place) of single wythe masonry walls, and in the cavities of masonry cavity walls.

Because these plastics are flammable, the IBC mandates that they be protected by fire-resistance-rated materials or assemblies in wall and roof assemblies, to prevent the plastic insulation from contributing to the spread of fire in a building.

This TEK describes the IBC requirements for assemblies containing foam plastic insulation and presents details of concrete masonry walls that comply with those requirements. Note that this TEK focuses on the requirements for masonry wall assemblies: there may be additional requirements for the insulation, such as flame spread index and labeling.

IBC REQUIREMENTS

IBC Section 2603 regulates the use of foam plastic insulation in all types of construction, both combustible and noncombustible, with the intent of limiting the spread of fire via these materials. For exterior walls, Section 2603 requires:

  • a thermal barrier between foam plastic insulation and the building interior, which can be satisfied with a 1 in. (25 mm) minimum thickness of concrete or masonry,
  • ignition testing for foam plastic insulations applied to wall exteriors, although assemblies protected with at least 1 in. (25 mm) of concrete or masonry on the exterior are exempt from testing, and
  • successful testing in accordance with NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components (ref. 2).

Note that there are two important exceptions to the requirement for NFPA 285 testing:

  1. Wall assemblies where the foam plastic insulation is covered on each face by a minimum 1 in. (25 mm) thickness of masonry or concrete and meeting one of the following:
    a) there is no air space between the insulation and the concrete or masonry (as occurs with foamed-in-place insulation); or
    b) the insulation has a flame spread index of 25 or less as determined by ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, or UL 723, Standard for Test for Surface Burning Characteristics of Building Materials, (refs. 3, 4) and the air space between the insulation and the concrete or masonry does not exceed 1 in. (25 mm).
  2. One-story buildings meeting the following conditions: foam plastic with a flame spread index of 25 or less and a smoke-developed index of 450 max can be placed in exterior walls without a thermal barrier where it is covered with aluminum (at least 0.032 in. (0.813 mm) thick) or corrosion-resistant steel (at least 0.0160 in. (0.406 mm) thick), provided that the insulation is not thicker than 4 in. (102 mm), and that the building is equipped with an automatic sprinkler system.

Wall assemblies meeting the requirements listed under number 1 above and buildings meeting the requirements listed under number 2 are deemed to comply with the Section 2603 requirements. Note that in cases where there is less than 1 in. (25 mm) of masonry over the insulation, there are insulations available that will meet the NFPA 285 requirements.

NFPA 285 REQUIREMENTS

NFPA 285 addresses the possibility of fire entering wall cavities through door or window openings, igniting foam plastic insulation and spreading vertically to upper stories.

The test evaluates exterior wall assemblies for buildings required to have exterior walls of noncombustible construction. The test provides a method of determining the flammability characteristics of exterior nonloadbearing wall assemblies. It is intended to evaluate combustible components included within wall assemblies required to be noncombustible, under conditions of a fire originating in the building interior.

NFPA 285 evaluates four conditions:

  • flame propagation over the exterior face;
  • flame propagation within combustible components from one story to the next;
  • vertical flame propagation on the interior wall surface from one story to the next; and
  • lateral flame propagation from one compartment to the next.

To evaluate these conditions, a two-story wall assembly with a window opening on the first floor is constructed in the test assembly. After a 30-minute fire exposure with the burner in the window opening, recorded temperatures are compared to the Standard’s conditions of acceptance to determine compliance. Note that the test evaluates wall assemblies, not specific materials.

SINGLE WYTHE CONCRETE MASONRY WALLS

Single wythe walls may incorporate foam insulation in the cores of the masonry units as either rigid foam inserts or foamed in-place insulation. As discussed above, IBC Chapter 26 essentially requires a minimum of 1 in. (25 mm) of concrete or masonry on the interior and exterior of the foam insulation, as well as protection at headers to prevent ignition of the insulation above door and window openings.

When placed in concrete masonry cores, the foam plastic insulation is protected on the interior and exterior by the concrete face shells. Minimum face shell thickness for concrete masonry units is governed by ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units, (ref. 5) as listed in Table 1. Table 1 shows that concrete masonry units of 6-in. (152 mm) thickness or greater provide the IBC-required 1 in. (25 mm) interior and exterior protection. Because of the small core size of 4-in. (102-mm) units, the cores of these units are rarely insulated. When insulation is placed in the cells of concrete masonry units and bond beams are provided at each story and lintels over each opening, the insulation is fully encapsulated. This meets the intent of the code to prevent the propagation of fire within wall cavities and no further isolation is necessary in this case.

In single wythe construction, door and window headers are typically constructed using either a reinforced precast lintel or a reinforced concrete masonry lintel (shown in Figure 1). This detail provides concrete cover well over the 1 in. (25 mm) minimum required by Section 2603. The detail and level of protection would be similar with a precast concrete lintel. Refer to TEK 19-02B, Design for Dry Single Wythe Concrete Masonry Walls (ref. 6), for additional details on flashing single wythe walls.

MULTI-WYTHE WALLS

Multi-wythe concrete masonry construction is most commonly masonry cavity walls, which often incorporate foam plastic insulation in the cavity formed by the two masonry wythes. In this case, there is more than 1 in. (25 mm) of masonry on both the interior and exterior, so the focus for protecting the insulation is on the headers and jambs of window and door openings.

Per Building Code Requirements for Masonry Structures (ref. 8) concrete masonry veneer walls are to have a minimum specified 1 in. (25 mm) air space with special precautions to limit mortar overhangs inside the cavity to allow adequate drainage between the wythes. Exception b to NFPA 285 testing (see page 1) limits the air space between the insulation and the masonry to 1 in. (25 mm) maximum. Therefore, when exception b is being used, the designer should specify a 1 in. (25 mm) air space to meet both requirements.

Figure 3 shows a window top of opening detail in a concrete masonry cavity wall. In this case, 1 in. (25 mm) of mortar is slushed into the cavity below the insulation to provide the required level of protection. In addition, testing (refs. 7, 10) has shown that mineral wool fire safing covering insulation board exposed at openings in a masonry cavity wall is sufficient to pass NFPA 285 requirements. Note that mineral wool insulation cannot be exposed to the moisture in the drainage cavity. If used, it must be behind flashing or similarly protected.

The jambs of metal doors (see Figure 4) are typically filled with mortar as the wall is constructed, again providing adequate protection for the insulation.

For wood door jambs, several options are shown in Figures 5 and 6. Figure 5 shows a detail where the insulation is held 1 in. (25 mm) back from the jamb. An additional piece of insulation bridges the cavity and acts as a backer for a 1 in (25 mm) layer of mortar. Another option is shown in Figure 6, where the unit adjacent to the jamb is turned 90o, and the unit is cut so that part of the face shell extends across the cavity, between the jamb and the insulation. On the alternate courses, a piece of the cut face shell can be mortared across the cavity to provide the protection. Wood window jamb details are very similar, as shown in Figures 7 and 8.

REFERENCES

  1. International Building Code. International Code Council, 2015.
  2. Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components, NFPA 285. National Fire Protection Association, 2012.
  3. Standard Test Method for Surface Burning Characteristics of Building Materials, ASTM E84-13a. ASTM International, 2013.
  4. Standard for Test for Surface Burning Characteristics of Building Materials, UL 723. Underwriter’s Laboratories, 2008.
  5. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-13. ASTM International, 2013.
  6. Design for Dry Single-Wythe Concrete Masonry Walls, TEK 19 02B. Concrete Masonry & Hardscapes Association, 2012.
  7. NFPA 285-[06] Approved Wall Assemblies Using Foam Plastic Insulation From Dow, Tech Solutions 514.0. Dow Chemical Company, 2009.
  8. Building Code Requirements for Masonry Structures, TMS 402 11/ACI 530-11/ASCE 5-11. Reported by the Masonry Standards Joint Committee, 2011.
  9. Standard Test Method for Determining Ignitability of Exterior Wall Assemblies Using a Radiant Heat Energy Source, NFPA 268. National Fire Protection Association, 2012.
  10. Commercial Complete™ Wall System NFPA 285 Tested Wall Assemblies. Owens Corning Insulating Systems, LLC, 2012.

Concrete Masonry Veneers

  • August 2018

INTRODUCTION

In addition to its structural use or as the exterior wythe of composite and noncomposite walls, concrete brick and architectural facing units are also used as veneer over various backing surfaces. The variety of surface textures, colors, and patterns available makes concrete masonry a versatile and popular exterior facing material. Architectural units such as split-face, scored, fluted, ground face, and slump are available in a variety of colors and sizes to complement virtually any architectural style.

VENEER—DESIGN CONSIDERATIONS

Veneer is a nonstructural facing of brick, stone, concrete masonry or other masonry material securely attached to a wall or backing. Veneers provide the exterior wall finish and transfer out-of-plane loads directly to the backing, but they are not considered to add to the load-resisting capacity of the wall system. Backing material may be masonry, concrete, wood studs or steel studs.

There are basically two types of veneer—anchored veneer and adhered veneer. They differ by the method used to attach the veneer to the backing, as illustrated in Figure 1. Unless otherwise noted, veneer requirements are those contained in the International Building Code (IBC) and Building Code Requirements for Masonry Structures (refs. 2, 3).

For the purposes of design, veneer is assumed to support no load other than its own weight. The backing must be designed to support the lateral and in some instances the vertical loads imposed by the veneer in addition to the design loads on the wall, since it is assumed the veneer does not add to the strength of the wall.

Masonry veneers are typically designed using prescriptive code requirements that have been developed based on judgement and successful performance. The prescriptive requirements relate to size and spacing of anchors and methods of attachment, and are described in the following sections. The assembly can be designed as a noncomposite cavity wall where the out-of-plane loads are distributed to the two wythes in proportion to their relative stiffness. Design criteria are provided in IBC Chapter 16 as well as in TEK 16-04A, Design of Concrete Masonry Noncomposite (Cavity) Walls, (ref. 4).

In addition to structural requirements, differential movement between the veneer and its supports must be accommodated. Movement may be caused by temperature changes, moisture-volume changes, or deflection. In concrete masonry, control joints and horizontal joint reinforcement effectively relieve stresses and accommodate small movements. For veneer, control joints should generally be placed in the veneer at the same locations as those in the backing, although recommended control joint spacing can be adjusted up or down based on local experience, the aesthetic requirements of the project, or as required to prevent excessive cracking. See CMU-TEC 009-23, Crack Control for Concrete Brick and Other Concrete Masonry Veneers (ref. 5), for further information.

For exterior veneer, water penetration into the cavity is anticipated. Therefore, the backing system must be designed and detailed to resist water penetration and prevent water from entering the building. Flashing and weeps in the veneer collect any water that penetrates the veneer and redirects it to the exterior. Partially open head joints are one preferred type of weep. They should be at least 1 in. (25 mm) high and spaced not more than 32 in. (813 mm) on center. If necessary, insects can be thwarted by inserting stainless steel wool into the opening or by using proprietary screens. For anchored veneer, open weeps can also serve as vents, allowing air circulation in the cavity to speed the rate of drying. Additional vents may be installed at the tops of walls to further increase air circulation. More detailed information is contained in TEK 05-01B, Concrete Masonry Veneer Details, TEK 19-04A, Flashing Strategies for Concrete Masonry Walls, and TEK 19-05A, Flashing Details for Concrete Masonry Walls (refs. 1, 6, 7).

ANCHORED VENEER

Anchored veneer is veneer which is supported laterally by the backing and supported vertically by the foundation or other structural elements. Anchors are used to secure the veneer and to transfer loads to the backing. Anchors and supports must be noncombustible and corrosion-resistant.

The following prescriptive criteria apply to anchored veneer in areas with velocity pressures, qz, up to 40 psf (1.92 kPa). Modified prescriptive criteria is available for areas with qz greater than 40 psf (1.92 kPa) but not exceeding 55 psf (2.63 kPa) with a building mean roof height up to 60 ft (18.3 m). These modified provisions are presented in the section High Wind Areas. In areas where qz exceeds 55 psf (2.63 kPa), the veneer must be designed using engineering philosophies, and the following prescriptive requirements may not be used.

In areas where seismic activity is a factor, anchored veneer and its attachments must meet additional requirements to assure adequate performance in the event of an earthquake. See the section Seismic Design Categories C and Higher for details.

Masonry units used for anchored veneer must be at least 2 ⅝ in. (67 mm) thick.

A 1 in. (25 mm) minimum air space must be maintained between the anchored veneer and backing to facilitate drainage. A 1 in. (25 mm) air space is considered appropriate if special precautions are taken to keep the air space clean (such as beveling the mortar bed away from the cavity). Otherwise, a 2 in. (51 mm) air space is preferred. As an alternative, proprietary insulating drainage products can be used.

The maximum distance between the inside face of the veneer and the outside face of the backing is limited to 4 ½ in. (114 mm), except for corrugated anchors used with wood backing, where the maximum distance is 1 in. (25 mm).

When anchored veneer is used as an interior finish supported on wood framing, the veneer weight is limited to 40 lb/ft2 (195 kg/m2).

Deflection Criteria

Deflection of the backing should be considered when using masonry veneer, in order to control crack width in the veneer and provide veneer stability. This is primarily a concern when masonry veneer is used over a wood or steel stud backing. Building Code Requirements for Masonry Structures, however, does not prescribe a deflection limit for the backing. Rather, the commentary presents various recommendations for deflection limits.

For anchored veneer, Chapter 16 of the International Building Code requires a deflection limit of l/240 for exterior walls and interior partitions with masonry veneer.

Support of Anchored Veneer

The height and length of the veneered area is typically not limited by building code requirements. The exception is when anchored veneer is applied over frame construction. For wood stud backup, veneer height is limited to 30 ft (9.14 m) (height at plate) or 38 ft (11.58 m) (height at gable). Similarly, masonry veneer over steel stud backing must be supported by steel shelf angles or other noncombustible construction for each story above the first 30 ft (9.14 m) (height at plate) or 38 ft (11.58 m) (height at gable). This support does not necessarily have to occur at the floor height, for example it can be provided at a window head or other convenient location.

Exterior anchored veneer is permitted to be supported on wood construction under the following conditions:

  • the veneer has an installed weight of 40 psf (195 kg/m2) or less,
  • the veneer has a maximum height of 12 ft (3.7 m),
  • a vertical movement joint in the veneer is used to isolate the veneer supported on wood construction from that supported by the foundation,
  • masonry is designed and constructed so that the masonry is not in direct contact with the wood, and
  • the horizontally spanning member supporting the masonry veneer is designed to limit deflection due to unfactored dead plus live loads to l/600 or 0.3 in. (7.5 mm).

Over openings, the veneer must be supported by non- combustible lintels or supports attached to noncombustible framing, as shown in Figure 2.

The following requirements assume that the veneer is laid in running bond. When other bond patterns are used, the veneer is required to have joint reinforcement spaced no more than 18 in. (457 mm) on center vertically. The joint reinforcement need only be one wire, with a minimum size of W1.7 (MW11).

Figure 2—Steel Lintel Flashing Detail (ref. 8)

(backing not shown for clarity)

Anchors

Veneers may generally be anchored to the backing using sheet metal anchors, wire anchors, joint reinforcement or adjustable anchors, although building codes may restrict the use of some anchors. Corrugated sheet metal anchors are permitted with masonry veneer attached to wood backing only. Requirements for the most common anchor types are summarized in Figures 3 through 5 and Table 1. As an alternative, adjustable anchors of equivalent strength and stiffness may be used. Cavity drips are not permitted. See TEK 12-01B, Anchors and Ties for Masonry, (ref. 9) for detailed information on anchor materials and requirements.

Attachment to Backing

When masonry veneer is anchored to wood backing, each anchor is attached to the backing with a corrosion- resistant 8d common nail, or a fastener with equivalent or greater pullout strength. For proper fastening of corrugated sheet metal anchors, the nail or fastener must be located within ½ in. (13 mm) of the 90° bend in the anchor. The exterior sheathing must be either water resistant with taped joints or be protected with a water- resistant membrane, such as building paper ship-lapped a minimum of 6 in. (152 mm) at seams, to protect the backing from any water which may penetrate the veneer.

When masonry veneer is anchored to steel backing, adjustable anchors must be used to attach the veneer. Each anchor is attached with corrosion-resistant screws that have a minimum nominal shank diameter of 0.19 in. (4.8 mm), or an anchor with equivalent pullout strength. Cold-formed steel framing must be corrosion resistant and should have a minimum base metal thickness of 0.043 in. (1.1 mm). Sheathing requirements are the same as those for wood stud backing.

Masonry veneer anchored to masonry backing may be attached using wire anchors, adjustable anchors or joint reinforcement. Veneer anchored to a concrete backing must be attached with adjustable anchors.

Figure 3—Adjustable Anchors
Figure 4—Corrugated Sheet Metal Anchor Requirements
Anchor Placement

When typical ties and anchors are properly embedded in mortar or grout, mortar pullout or pushout will not usually be the controlling mode of failure. For this reason, connectors must be embedded at least 1 ½ in. (38 mm) into a mortar bed of solid units, and the mortar bed joint must be at least twice the thickness of the embedded anchor. The required embedment of unit ties in hollow masonry is such that the tie must extend completely across the hollow units (Figure 6). Proper embedment can be easily attained with the use of prefabricated assemblies of joint reinforcement and unit ties. Because of the magnitude of loads on anchors, it is recommended that they be embedded in filled cores of hollow units. To save mortar, screens can be placed under the anchor and 1 to 2 in. (25 to 51 mm) of mortar can be built up into the core of the block above the anchor (Figure 7).

Figure 5—Requirements for Joint Reinforcement Used to Anchor Veneer
Figure 6—Minimum Embedment of Joint Reinforcement and Wire Ties
Figure 7—Use of Mesh to Facilitate Embedment of Anchor in Mortar
High Wind Areas

In areas with qz greater than 40 psf (1.92 kPa) but not exceeding 55 psf (2.63 kPa) with a building mean roof height up to 60 ft (18.3 m), the following modified prescriptive provisions may be used.

The modified prescriptive provisions are:

  • the maximum wall area supported by each anchor must be reduced to 70% of the value listed in Table 1,
  • anchor spacing is reduced to a maximum of 18 in. (457 mm), both vertically and horizontally, and
  • around openings larger than 16 in. (406 mm) in either direction, anchors must be placed within 12 in. (305 mm) of the opening and spaced at 24 in. (610 mm) on center or less.

In areas where qz exceeds 55 psf (2.63 kPa), the veneer must be designed using engineering philosophies.

Seismic Design Categories C and Higher

To improve veneer performance under seismic loading in Seismic Design Category (SDC) C, the sides and top of the veneer must be isolated from the structure, so that vertical and lateral seismic forces are not transferred to the veneer. This reduces accidental loading and allows more building deflection without causing damage to the veneer.

In SDC D, in addition to this isolation, the maximum wall area supported by each anchor must be reduced to 75% of the value listed in Table 1, although the maximum spacings are unchanged. In addition, when the veneer is anchored to wood backing, the veneer anchor must be attached to the wood using a corrosion-resistant 8d ring-shank nail, a No. 10 corrosion- resistant screw with a minimum nominal shank diameter of 0.190 in. (4.8 mm), or with a fastener having equivalent or greater pullout strength.

In SDC E and F, the requirements listed above for SDC C and D must be met, as well as the additional requirements listed here. The weight of each story of anchored veneer must be supported independently of other stories to help limit the size of potentially damaged areas. In addition, to improve veneer ductility the veneer must have continuous W1.7 (MW11) single wire joint reinforcement at 18 in. (457 mm) o.c. or less vertically, with a mechanical attachment to the anchors, such as clips or hooks.

ADHERED VENEER

Conventional adhered veneer is veneer secured and supported through adhesion with a bonding material applied over a backing that both meets required deflection limits and provides for necessary adhesion. When applied to a masonry or concrete backing, the veneer may be applied directly to the backing substrate using layers of neat cement paste and Type S mortar, as illustrated in Figure 1. When applied over steel or wood framing, the adhered masonry veneer is applied to a metal lath and portland cement plaster backing placed against the sheathing element and attached to the stud framing members.

Alternative design of adhered veneer is permitted under the International Building Code when in compliance with Building Code Requirements for Masonry Structures (MSJC), where the requirements of unit adhesion (shear stress > 50 psi, 345 kPa) are met, out-of-plane curvature of the backing is limited to prevent the veneer from separating from the backing, and freeze thaw cycling, water penetration, and air and water vapor transmission are considered. Although the MSJC does not stipulate a deflection limit to control out-of-plane curvature, the Tile Council of America limits the deflection of backing supporting ceramic tiles to l/360 (ref. 11). Similarly, IBC Chapter 16 (for engineered design) requires a deflection limit of l/360 for exterior walls and interior partitions with plaster or stucco, which would be similar to an adhered veneer application.

Proprietary polymer-fortified adhesive mortars exist that meet the adhesion requirements and are used as a mortar setting bed to adhere the masonry veneers directly to a masonry or concrete backing, or to a lath and plaster backing system over wood or steel studs.

In addition, several proprietary systems are available to aid in placement of adhered masonry veneer on suitable exterior or interior substrates. These typically take the form of galvanized steel support panels that are mechanically anchored to a masonry or concrete backing, or placed against the sheathing element and attached to stud framing members. These products essentially take the place of the metal lath in the adhered veneer application. The metal panels contain support tabs and other features to facilitate the veneer application, carry the dead load of the veneer, and improve bonding of the veneer to the panel. In some cases, metal panel systems provide drainage or air flow channels as well. In lieu of mortar, construction adhesives having a shear bond strength greater than 50 psi (345 kPa) are used to bond the masonry veneer to the panel and masonry pointing mortar is used to fill the joint space between the masonry units. Installation using these products should follow manufacturer’s instructions.

Masonry units used in this application are limited to 2 ⅝ in. (67 mm) thickness, 36 in. (914 mm) in any face dimension, 5 ft2 (0.46 m2) in total face area and 15 lb/ft2 (73 kg/m2 ) weight. In addition, the International Building Code (ref. 4) stipulates: a minimum thickness of 0.25 in. (6.3 mm) for weather-exposed adhered masonry veneer; and, for adhered masonry veneers 2 used on interior walls, a maximum weight of 20 lb/ft2 (97 kg/ m2).

When an interior adhered veneer is supported by wood construction, the wood supporting member must be designed for a maximum deflection of 1/600 of its span.

Adhered veneer and its backing must also be designed to either:

  • have sufficient bond to withstand a shearing stress of 50 psi (345 kPa) based on the gross unit surface area when tested in accordance with ASTM C482, Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste (ref. 10), or
  • be installed according to the following:
    • A paste of neat portland cement is brushed on the backing and on the back of the veneer unit immediately prior to applying the mortar coat. This neat cement coating provides a good bonding surface for the mortar.
    • Type S mortar is then applied to the backing and to each veneer unit in a layer slightly thicker than ⅜ in. (9.5 mm). Sufficient mortar should be used so that a slight excess is forced out the edges of the units.
    • The units are then tapped into place to eliminate voids between the units and the backing which could reduce bond. The resulting thickness of mortar between the backing and veneer must be between ⅜ and ¼ in. (9.5 and 32 mm).
    • Mortar joints are tooled with a round jointer when the mortar is thumbprint hard.

When applied to exterior stud walls, the IBC requires adhered masonry veneer to include a screed or flashing at the foundation. In addition, minimum clearances must be maintained between the bottom of the adhered veneer and paved areas, adjacent walking surfaces and the earth.

Backing materials for adhered veneer must be continuous and moisture-resistant (wood or steel frame backing with adhered veneer must be backed with a solid water repellent sheathing). Backing may be masonry, concrete, metal lath and portland cement plaster applied to masonry, concrete, steel framing or wood framing. Note that care must be taken to limit deflection of the backing, which can cause veneer cracking or loss of adhesion, when adhered masonry veneer is used on steel frame or wood frame backing. The surface of the backing material must be capable of securing and supporting the imposed loads of the veneer. Materials that affect bond, such as dirt, grease, oil, or paint (except portland cement paint) need to be cleaned off the backing surface prior to adhering the veneer.

NOTATIONS:

l = clear span between supports, in. (mm)

qz = velocity pressure evaluated at height z above ground, in.-lb/ft2 (kPa)

REFERENCES

  1. Concrete Masonry Veneer Details, TEK 05-01B. Concrete Masonry & Hardscapes Association, 2003.
  2. 2012 International Building Code. International Code Council, 2012.
  3. Building Code Requirements for Masonry Structures, ACI 530-11/ASCE 5-11/TMS 402-11. Reported by the Masonry Standards Joint Committee, 2011.
  4. Design of Concrete Masonry Noncomposite (Cavity) Walls, TEK 16-04A. Concrete Masonry & Hardscapes Association, 2004.
  5. Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
  6. Flashing Strategies for Concrete Masonry Walls, TEK 1904A. Concrete Masonry & Hardscapes Association, 2008.
  7. Flashing Details for Concrete Masonry Walls, TEK 19-05A. Concrete Masonry & Hardscapes Association, 2008. 
  8. McMican, Donald G. Is Flashing Dangerous Without a Drip? The Aberdeen Group, 1999.
  9. Anchors and Ties for Masonry, TEK 12-01B. Concrete Masonry & Hardscapes Association, 2011.
  10. Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste, ASTM C482-02(2009). ASTM International, 2009.
  11. Handbook for Ceramic Tile Installation. Tile Council of America, 1996.