Key Words: clay brick, compressive strength, hollow brick, reinforced masonry.

Hollow brick masonry is a natural development in brick masonry construction. Several reasons led to the production of hollow brick. These include the need for units that can be more easily reinforced and grouted, and more economically constructed with large size brick. More stringent structural requirements for buildings in high wind and seismic regions have required masonry units that can be grouted and reinforced. These requirements can not be achieved as easily with solid masonry units.

The development of hollow brick began with 4 by 4 by 12 in. (100 by 100 by 300 mm) oversized solid units followed by 6 by 4 by 12 in. (150 by 100 by 300 mm) and 8 by 8 by 16 in. (200 by 200 by 400 mm) hollow brick. These large hollow brick are often called through-the-wall (TTW) units because the wall is a single wythe of brick masonry. They offer a considerable advantage in both speed and economy of construction. Some brick manufacturers initially experienced production problems with large TTW units with void areas of 25 percent or less. Consequently, manufacturers changed their coring patterns to increase the void area. The coring patterns were redesigned to increase production recovery and also to reduce weight and permit reinforcing. A new classification of brick was established - the hollow brick, whose void area is greater than 25 percent of its gross area. These units are more economical to produce due to the greater percentage of recovery in production, less fuel used in production and less expensive to ship because of their lighter weight. Larger units are also more economical to place in the wall. For example, it takes over 13 modular size brick to equal the volume of three 8 by 4 by 12 in. (200 by 100 by 300 mm) units.

Hollow brick have void areas which are not less than 25 percent, but not more than 60 percent of the gross area. This compares to solid brick whose maximum void area is 25 percent. Hollow brick have different requirements from solid brick and clay tile with respect to sizes of hollow spaces, web thicknesses and face shell thicknesses. Hollow brick can be efficiently used in bearing walls, reinforced or prestressed masonry and walls requiring exposed brick. Typical applications of hollow brick include commercial, retail and residential buildings, hotels, schools, noise barrier walls and retaining walls. Figure 1 is an example of a hollow brick load-bearing structure.

This Technical Notes describes the classifications, properties and uses of hollow brick. Properties of hollow brick masonry related to structural design, water penetration resistance, fire resistance and sound resistance are provided. Proper material selection and construction procedures are discussed.

The materials used in hollow brick construction do not differ dramatically from that of other masonry construction. ASTM standards dictate many of the physical properties of hollow brick units. More information on material properties and classification can be found in Technical Notes 3A and Technical Notes 9A, respectively.

Hollow brick should be specified to comply with ASTM C 652 Specification for Hollow Brick (Hollow Masonry Units Made From Clay or Shale). When it was first issued in 1970, ASTM C 652 covered units with void areas up to 40 percent. The standard has since been modified to allow void areas up to 60 percent of the units gross area.

Grade. Two Grades exist in ASTM C 652: Grades SW and MW. The Grade establishes requirements to ensure adequate freeze/thaw resistance. Grade SW units provide high and uniform resistance to frost action while saturated with water. Grade MW units are intended for applications that are unlikely to be saturated with water when exposed to freezing temperatures. The physical property requirements are shown in Table 1. Two alternates exist in the standard to provide compliance with the durability requirements. These alternates include: a cold water absorption less than 8 percent and passing a 50 cycle freezing and thawing test. The freeze/thaw test only applies if the units do not meet the saturation coefficient and absorption requirements in Table 1.

Hollow Spaces - The thickness of face shells and webs are limited by ASTM C 652. Table 2 defines the nomenclature associated with hollow brick units and the minimum required thickness of face shells and cross webs. The dimensions of the unit and the configuration of its voids are critical for reinforced brick masonry. The cells intended to receive reinforcement must align so that reinforcing bars can be properly placed. Most Class H60V hollow brick contain two cells that are aligned when laid in running bond. Other bond patterns, such as one-third bond and bonds at corners may require different unit configurations to permit placement of reinforcement. It is advisable to check with the brick manufacturer to determine the coring patterns available.

Compressive strength of hollow brick can be reported on a basis of either the gross cross-sectional area or the net cross-sectional area, depending on how the value is to be used. The gross area compressive strength is used to determine compliance with ASTM C 652 for purposes of durability. The net area compressive strength is needed for structural computations.

A survey conducted in 1994 showed the range of compressive strength of hollow brick based on gross cross-sectional area is between 2,190 psi (15.1 MPa) and 12,795 psi (88.2 MPa), with an average compressive strength equal to 6,740 psi (46.5 MPa).

Many designers are familiar with the design, construction and performance of masonry built with solid units. Hollow brick masonry is similar in many instances. Some of the more important performance requirements for hollow brick masonry are discussed.

The structural design of hollow brick masonry is governed by the three model building codes and the ACI 530/ASCE 5/TMS 402 Building Code Requirements for Masonry Structures, also known as the Masonry Standards Joint Committee (MSJC) Code [1]. Hollow brick masonry can be designed by empirical requirements or by rational design procedures. Where appropriate, allowable stresses are different for hollow brick masonry and solid brick masonry. The following sections highlight some of the specific requirements for hollow brick units.

Compressive Strength. The compressive strength of hollow brick walls depends on unit strength, mortar type, mortar bedding area, grouting and thicknesses of face shells and webs. The compressive strength of the masonry can be verified by testing prisms (prism test method) or from tabulated values based on brick strength and mortar type (unit strength method). Prism tests give more accurate values for compressive strength. Ungrouted prisms exhibit failure in compression by a splitting of the unit through the cross webs, as shown in Fig. 3. The splitting effect is due to the lateral expansion of the mortar. Filling the cells of hollow brick with grout will generally increase the masonry's capacity; however, the result is a decrease in the net area compressive strength. The strength of grouted hollow prisms is affected more by the tensile strength of the unit and a change in mortar strength [8].

The compressive strength of hollow brick masonry is based on the minimum net cross-sectional area. This is normally the net mortar bedded area (face shell bedding) and is used in structural calculations. When determining compliance of the specified compressive strength, the MSJC Code requires the units to be fully bedded in mortar. Fig. 4 shows various cross-sectional areas for hollow brick masonry.

Values obtained from prism tests must be corrected based on the height-to-thickness (h/t) ratio of the prism. The h/t ratio provides a uniform basis for the determination of compressive strength. Hollow units are less sensitive to the h/t effects of slenderness than solid units. Codes stipulate the correction factors to use for hollow brick prisms. Past research has used an h/t of 5 as a basis, as do most current codes. However, an h/t of 2 is becoming more recognized, especially for larger hollow units.

Research shows typical values of ungrouted hollow brick masonry compressive strength based on net area ranging from 3,470 psi (23.9 MPa) to 6,620 psi (45. 6 MPa). Figure 5 shows typical values of the net area compressive strength of grouted and ungrouted hollow brick masonry prisms from the research [3,8].

Reinforcement is grouted into hollow brick walls to increase the flexural strength, provide ductility and carry tension forces. The flexural strength of a hollow brick wall depends primarily on the amount of vertical reinforcement because the compressive strength is rarely the limiting factor. The reinforcement resists the flexural tension and the brickwork resists the flexural compression. Building codes may dictate a minimum amount of reinforcement for improved ductility in seismic regions. In reinforced masonry, the steel takes all of the tension forces and any tension in the masonry is neglected.

The water penetration resistance of hollow brick masonry depends upon the materials and construction used. Most hollow brick walls are single-wythe walls and many are designed to act as barrier walls. Hollow brick walls are not impervious to water, so a secondary barrier to water must be used. Although the cells of the units may act as drainage spaces, they will not provide the excellent type of protection that a continuous drainage cavity provides. Hollow brick are usually face shell bedded; that is, only the face shells of the unit are mortared. This may not provide adequate resistance to wind-driven rain.

Increased water penetration resistance may be obtained by requiring full head joints. In many cases the walls are grouted solid which helps the wall resist water penetration. Other methods used to increase the resistance to water include the incorporation of an internal drainage space in the wall or the application of a colorless coating.

In cases where water penetration resistance is critical, a drainage space may be provided on the interior of the wall assembly, as shown in Fig. 6. The interior may be furred out with insulation and gypsum board attached. Flashing and weep holes are used to drain the space. Another precaution may be the use of a water-resistant membrane placed on the inside face of the wall. Waterproof membranes or polyethylene sheet have been used to resist water that has penetrated the hollow brick wall. Any puncture in the membrane must be properly sealed.

Flashing should be provided at the wall base, below and above all wall openings and at the tops of walls. Flashing and weep holes will collect water that enters the wall and direct it back to the exterior. Flashing is a bond break so the tensile strength of the wall at that location is zero. Also the shear stress will be reduced. The structural design of the buildings should address this issue appropriately. Flashing is necessary in all areas except where there is negligible rainfall.

Another alternative to increase the water penetration resistance of a wall is the application of a clear water repellent on the exterior face of the wall. Water repellents will help deter moisture absorption. However, water repellents must be used with caution since they may cause problems in climates where freezing occurs. Other considerations include limited lifetime, trapping of efflorescence or stains behind the coating, reapplication and ineffectiveness on cracks in the wall. See Technical Notes 6A for more information on the appropriate use of clear water repellent coatings.

The excellent fire-resistant qualities of brick masonry are well known. However, there have been relatively few full-scale fire tests of hollow brick masonry walls. This is due in part to the acknowledgment that brick masonry is inherently fire resistant. Results from actual wall tests in accordance with ASTM E 119 are listed in Table 4 [5]. When a 5/8 in. (16 mm) thick layer of plaster is added to these walls, the fire rating may be increased by 1 hour.

An alternative way of determining the fire resistance of a wall assembly is by the equivalent thickness method. This approach has been approved by the model building codes to determine fire ratings of walls not physically tested by ASTM E 119. The fire rating of hollow brick masonry is determined by its equivalent solid thickness. The equivalent thickness is calculated by subtracting the volume of core or cell spaces from the total gross volume of a brick unit and dividing by the exposed face area of the unit. Technical Notes 16B explains the procedures for determining a hollow bricks equivalent thickness and the calculated fire resistance of various hollow brick masonry walls.

Because sound resistance improves with increasing wall weight, hollow brick masonry provides very good sound penetration resistance. The Sound Transmission Class (STC) rating is used to determine the sound insulation of walls. Tests on units similar to hollow clay brick (structural clay tile) ranged from an STC of 39 for a 4 in. (100 mm) structural clay tile wall, to an STC of 45 for an 8 in. (200 mm) structural clay tile wall. Assumed values of STC ratings can be determined from an equation based on existing test data. The following equation could be used to estimate an STC rating when existing tests are not available [6].

STC = 0.17W + 40

The STC rating is a function of the weight of the wall, w, which is expressed in pcf. This equation is a best fit to a curve based on the average of the data.

Since hollow brick have large voids, units are laid with face shell bedding. Face shell bedding consists of mortar coverage on the inner and outer face shells of the unit. Cross webs or end webs of the unit may require mortar bedding when grout must be confined within certain cells of partially grouted masonry or on the first course of brickwork.

Grout is a highly fluid mixture used to fill the cells of units. Grout should conform to ASTM C 476 Specification for Grout for Masonry. The proportion specification is recommended for grout in hollow brick. The use of fine grout and coarse grout is limited by the size of the space to be grouted. See Table 5. Spaces smaller than these listed in Table 5 will not be fully filled due to congestion in the space.

Walls can be ungrouted, fully grouted or partially grouted. This depends on the design of the wall. In some cases, only those cells containing reinforcement are grouted. However, as the spacing of the reinforcement becomes less than about 30 in. (760 mm), it becomes more economical to grout the entire wall.

Brick masonry absorbs water from the grout, resulting in considerable shrinkage of the grout. Although consolidation of the grout is required, it may not compensate for all of the shrinkage. Therefore, a non-shrink grout admixture is recommended [7].

Prior to grouting, the cells of the hollow units should be cleared of mortar protrusions and blockage to allow uninterrupted flow of the grout. This may be done during construction with a sponge or after construction with a rod to knock down hardened mortar protrusions. A clear grout space allows the required clearances and intimate contact between grout, masonry units and steel reinforcement. The height to which grout is placed in one operation is limited by the size of the cells to be grouted, as shown in Table 5. Grouting after the wall has been constructed, often referred to as high-lift grouting, is more difficult the smaller the cells are in size. After grout is placed, and before it loses its plasticity, the grout should be consolidated to completely fill all spaces designed to receive grout. Grout pour heights greater than 12 in. (300 mm) are typically reconsolidated by mechanical vibration after initial water loss and settlement has occurred.

Cleanouts are required when grout pours exceed 5 ft (1.5 m) in height. Cleanouts are typically made by removing face shells of units in the bottom course of spaces to be grouted. These cleanouts allow removal of mortar droppings that have collected at the bottom of a cell. Once the cleanouts have been cleared of debris, the face shell of the unit can be replaced. The face shell can be cut to form a wedge so that the grout pressure will hold it in place. The replaced face shell may require additional bracing against the hydrostatic pressure of the grout to avoid blowing out the face shell.

Although reinforcement is not always used in hollow brick masonry, the large cells allow the units to be easily reinforced and grouted. The reinforcing must be embedded in grout, not mortar. The maximum bar size permitted by code for masonry is a No. 11 bar. The size of the reinforcement is often limited by the size of the space to be grouted and the need for adequate cover. A recommended rule-of-thumb is that the maximum bar size should equal the nominal unit thickness. For example, the maximum bar size for nominal 6 in. (150 mm) hollow brick is a No. 6 bar.

Reinforcement is placed prior to grouting. The reinforcement must be accurately positioned in the wall as designed. Reinforcing is most often positioned in the center of the wall, but may be placed to one side to maximize the distance from the compression face. The reinforcement should be spaced to coincide with the spacing of the cells of the hollow brick. Spacing is typically in increments of 4 in. (100 mm) or 6 in. (150 mm). Reinforcement is sometimes held in place by the use of positioners or chairs. Positioners are typically placed horizontally, no more than 200 bar diameters apart.

Splices in reinforcement must be in accordance with building code requirements. Splices are most often achieved by lapping the discontinuous ends of the bars by a certain minimum number of bar diameters. Mechanical devices and butt welding of reinforcing is also permitted to provide continuity.

This Technical Notes provides a discussion of the classification and use of hollow brick. Various properties of hollow brick masonry are described. Design, material and construction requirements are provided as a basis for good masonry performance.

The information and suggestions contained in this Technical Notes are based on the available data and the experience of the engineering staff of the Brick Institute of America. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of brick masonry. Final decisions on the use of the information contained in this Technical Notes are not within the purview of the Brick Institute of America and must rest with the project architect, engineer and owner.