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Concrete Masonry Basement Wall Construction

November 2018

Introduction

Basements allow a building owner to significantly increase usable living, working, or storage space at a relatively low cost. Old perceptions of basements have proven outdated by stateofthe-art waterproofing, improved drainage systems, and natural lighting features such as window wells. Other potential benefits of basements include room for expansion of usable space, increased resale value, and safe haven during storms.

Historically, plain (unreinforced) concrete masonry walls have been used to effectively resist soil loads. Currently, however, reinforced walls are becoming more popular as a way to use thinner walls to resist large backfill pressures. Regardless of whether the wall is plain or reinforced, successful performance of a basement wall relies on quality construction in accordance with the structural design and the project specifications.

Materials

Concrete Masonry Units: Concrete masonry units should comply with Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90 (ref. 8). Specific colors and textures may be specified to provide a finished interior to the basement. Drywall can also be installed on furring strips, if desired. A rule of thumb for estimating the number of concrete masonry units to order is 113 units for every 100 ft2 (9.3 m2) of wall area. This estimate assumes the use of 3/8 in. (9.5 mm) mortar joints.

Mortar: Mortar serves several important functions in a concrete masonry wall; it bonds the units together, seals joints against air and moisture penetration, and bonds to joint reinforcement, ties, and anchors so that all components perform as a structural element.

Mortar should comply with Standard Specification for Mortar for Unit Masonry, ASTM C 270 (ref. 9). In addition, most building codes require the use of Type M or S mortar for construction of basement walls (refs. 2, 4, 5, 9, 13), because Type M and S mortars provide higher compressive strengths. Table 1 lists mortar proportions.

Typical concrete masonry construction uses about 8.5 ft3 (0.24 m3) of mortar for every 100 ft2 (9.3 m2) of masonry wall area. This figure assumes 3/8 in. (9.5 mm) thick mortar joints, face shell mortar bedding, and a 10% allowance for waste.

Grout: In reinforced concrete masonry construction, grout is used to bond the reinforcement and the masonry together. Grout should conform to Standard Specification for Grout for Masonry, ASTM C 476 (ref. 10), with the proportions listed in Table 2. As an alternative to complying with the proportion requirements in Table 2, grout can be specified to have a minimum compressive strength of 2000 psi (13.8 MPa) at 28 days. Enough water should be added to the grout so that it will have a slump of 8 to 11 in. (203 to 279 mm). The high slump allows the grout to be fluid enough to flow around reinforcing bars and into small voids. This initially high water-to-cement ratio is reduced significantly as the masonry units absorb excess mix water. Thus, grout gains high strengths despite the initially high waterto-cement ratio.

Construction

Prior to laying the first course of masonry, the top of the footing must be cleaned of mud, dirt, ice or other materials which reduce the bond between the mortar and the footing. This can usually be accomplished using brushes or brooms, although excessive oil or dirt may require sand blasting.

Masons typically lay the corners of a basement first so that alignment is easily maintained. This also allows the mason to plan where cuts are necessary for window openings or to fit the building’s plan.

To make up for surface irregularities in the footing, the first course of masonry is set on a mortar bed joint which can range from 1/4 to 3/4 in. (6.4 to 19 mm) in thickness. This initial bed joint should fully bed the first course of masonry units, although mortar should not excessively protrude into cells that will be grouted.

All other mortar joints should be approximately 3/8 in. (9.5 mm) thick and, except for partially grouted masonry, need only provide face shell bedding for the masonry units. In partially grouted construction, webs adjacent to the grouted cells are mortared to restrict grout from flowing into ungrouted cores. Head joints must be filled solidly for a thickness equal to a face shell thickness of the units.

Tooled concave joints provide the greatest resistance to water penetration. On the exterior face of the wall, mortar joints may be cut flush if parging coats are to be applied.

When joint reinforcement is used, it should be placed directly on the block with mortar placed over the reinforcement in the usual method. A mortar cover of at least 5/8 in. (15.9 mm) should be provided between the exterior face of the wall and the joint reinforcement. A mortar cover of 1/2 in. (12.7 mm) is needed on the interior face of the wall. For added safety against corrosion, hot dipped galvanized joint reinforcement is recommended.

See Figures 1-4 for construction details.

Reinforced Masonry: For reinforced masonry construction, the reinforcing bars must be properly located to be fully functional. In most cases, vertical bars are positioned towards the interior face of basement walls to provide the greatest resistance to soil pressures. Bar positioners at the top and bottom of the wall prevent the bars from moving out of position during grouting. A space of at least 1/2 in. (12.7 mm) for coarse grout and 1/4 in. (6.4 mm) for fine grout should be maintained between the bar and the face shell of the block so that grout can flow completely around the reinforcing bars.

As mix water is absorbed by the units, voids can form in the grout. Accordingly, grout must be puddled or consolidated after placement to eliminate these voids and to increase the bond between the grout and the masonry units. Most codes permit puddling of grout when it is placed in lifts less than about 12 in. (305 mm). Lifts over 12 inches (305 mm) should be mechanically consolidated and then reconsolidated after about 3 to 10 minutes.

Surface Bonding: Another method of constructing concrete masonry walls is to dry stack units (without mortar) and then apply surface bonding mortar to both faces of the wall. The surface bonding mortar contains thousands of small glass fibers. When the mortar is applied properly to the required thickness, these fibers, along with the strength of the mortar itself, help produce walls of comparable strength to conventionally laid plain masonry walls. Surface bonded walls offer the benefits of excellent dampproof coatings on each face of the wall and ease of construction.

Dry-stacked walls should be laid in an initial full mortar bed to level the first course. Level coursing is maintained by using a rubbing stone to smooth small protrusions on the block surfaces and by inserting shims every two to four courses.

Water Penetration Resistance: Protecting below grade walls from water entry involves installation of a barrier to water and water vapor. An impervious barrier on the exterior wall surface can prevent moisture entry.

The barrier is part of a comprehensive system to prevent water penetration, which includes proper wall construction and the installation of drains, gutters, and proper grading.

Building codes (refs. 2, 4 , 5, 9, 13) typically require that basement walls be dampproofed for conditions where hydrostatic pressure will not occur, and waterproofed where hydrostatic pressures may exist. Dampproofing is appropriate where groundwater drainage is good, for example where granular backfill and a subsoil drainage system are present. Hydrostatic pressure may exist due to a high water table, or due to poorly draining backfill, such as heavy clay soils. Materials used for waterproofing are generally elastic, allowing them to span small cracks and accommodate minor movements.

When choosing a waterproof or dampproof system, consideration should be given to the degree of resistance to hydrostatic head of water, absorption characteristics, elasticity, stability in moist soil, resistance to mildew and algae, impact or puncture resistance, and abrasion resistance. A complete discussion of waterproofing, dampproofing, and drainage systems is included in TEK 19-03A (ref. 6).

All dampproofing and waterproofing systems should be applied to walls that are clean and free from dirt, mud and other materials which may reduce bond between the coating and the concrete masonry wall.

Draining water away from basement walls significantly reduces the pressure the walls must resist and reduces the possibility of water infiltration into the basement if the waterproofing (or dampproofing) system fails. Perforated pipe has historically proven satisfactory when properly installed. When placed on the exterior side of basement walls, perforated pipes are usually laid in crushed stone to facilitate drainage. To prevent migration of fine soil into the drains, filter fabrics are often placed over the gravel.

Drainage pipes can also be placed beneath the slab and connected into a sump. Pipes through the footing or the wall drain water from the exterior side of the basement wall.

The drainage and waterproofing systems should always be inspected prior to backfilling to ensure they are adequately placed. Any questionable workmanship or materials should be repaired at this stage since repairs are difficult and expensive after backfilling.

Backfilling: One of the most crucial aspects of basement construction is how and when to properly backfill. Walls should be properly braced or have the first floor in place prior to backfilling. Otherwise, a wall which is designed to be supported at the top may crack or even fail from the large soil pressures. Figure 5 shows one bracing scheme which has been widely used for residential basement walls. More substantial bracing may be required for high walls or large backfill pressures.

The backfill material should be free-draining soil without large stones, construction debris, organic materials, and frozen earth. Saturated soils, especially saturated clays, should generally not be used as backfill materials since wet materials significantly increase the hydrostatic pressure on the walls.

Backfill materials should be placed in several lifts and each layer should be compacted with small mechanical tampers. Care should be taken when placing the backfill materials to avoid damaging the drainage, waterproofing or exterior insulation systems. Sliding boulders and soil down steep slopes should thus be avoided since the high impact loads generated can damage not only the drainage and waterproofing systems but the wall as well. Likewise, heavy equipment should not be operated within about 3 feet (0.9 m) of any basement wall system.

The top 4 to 8 in. (102 to 203 mm) of backfill materials should be low permeability soil so rain water is absorbed into the backfill slowly. Grade should be sloped away from the basement at least 6 in. (152 mm) within 10 feet (3.1 m) of the building. If the ground naturally slopes toward the building, a shallow swale can be installed to redirect runoff.

Construction Tolerances

Specifications for Masonry Structures (ref. 8) specifies tolerances for concrete masonry construction. These tolerances were developed to avoid structurally impairing a wall because of improper placement.

Dimension of elements in cross section or elevation
…………………………………….¼ in. (6.4 mm), +½ in. (12.7 mm)
Mortar joint thickness: bed………………………..+⅛ in. (3.2 mm)
head………………………………..-¼ in (6.4 mm), +⅜ in. (9.5 mm)
Elements
- Variation from level: bed joints……………………………………….
  ±¼ in. (6.4 mm) in 10 ft (3.1 m), ±½ in. (12.7 mm) max
  top surface of bearing walls……………………………………………..
  ±¼ in.(6.4 mm), +⅜ in.(9.5 mm), ±½ in.(12.7mm) max
- Variation from plumb………….±¼ in. (6.4 mm) 10 ft (3.1 m)
  ………………………………………±⅜ in. (9.5 mm) in 20 ft (6.1 m)
  ……………………………………………±½ in. (12.7 mm) maximum
- True to a line…………………..±¼ in. (6.4 mm) in 10 ft (3.1 m)
  ………………………………………±⅜ in. (9.5 mm) in 20 ft (6.1 m)
  ……………………………………………±½ in. (12.7 mm) maximum
- Alignment of columns and bearing walls (bottom versus top)
  ……………………………………………………………..±½ in (12.7 mm)
Location of elements
- Indicated in plan……………..±½ in (12.7 mm) in 20 ft (6.1 m)
  …………………………………………….±¾ in. (19.1 mm) maximum
- Indicated in elevation
  ……………………………………….±¼ in. (6.4 mm) in story height
  …………………………………………….±¾ in. (19.1 mm) maximum

Insulation: The thermal performance of a masonry wall depends on its R-value as well as the thermal mass of the wall. Rvalue describes the ability to resist heat flow; higher R-values give better insulating performance. The R-value is determined by the size and type of masonry unit, type and amount of insulation, and finish materials. Depending on the particular site conditions and owner’s preference, insulation may be placed on the outside of block walls, in the cores of hollow units, or on the interior of the walls.

Thermal mass describes the ability of materials like concrete masonry to store heat. Masonry walls remain warm or cool long after the heat or air-conditioning has shut off, keeping the interior comfortable. Thermal mass is most effective when insulation is placed on the exterior or in the cores of the block, where the masonry is in direct contact with the interior conditioned air.

Exterior insulated masonry walls typically use rigid board insulation adhered to the soil side of the wall. The insulation requires a protective finish where it is exposed above grade to maintain durability, integrity, and effectiveness.

Concrete masonry cores may be insulated with molded polystyrene inserts, expanded perlite or vermiculite granular fills, or foamed-in-place insulation. Inserts may be placed in the cores of conventional masonry units, or they may be used in block specifically designed to provide higher R-values.

Interior insulation typically consists of insulation installed between furring strips, finished with gypsum wall board or panelling. The insulation may be fibrous batt, rigid board, or fibrous blown-in insulation.

Design Features

Interior Finishes: Split faced, scored, burnished, and fluted block give owners and designers added options to standard block surfaces. Colored units can be used in the entire wall or in sections to achieve specific patterns.

Although construction with staggered vertical mortar joints (running bond) is standard for basement construction, the appearance of continuous vertical mortar joints (stacked bond pattern) can be achieved by using of scored units or reinforced masonry construction.

Natural Lighting: Because of the modular nature of concrete masonry, windows and window wells of a variety of shapes and sizes can be easily accommodated, giving basements warm, natural lighting. For additional protection and privacy, glass blocks can be incorporated in lieu of traditional glass windows.

References

Basement Manual-Design and Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.
BOCA National Building Code. Country Club Hills, IL: Building Officials and Code Administrators International, Inc. (BOCA), 1999.
Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
International Residential Code. Falls Church, VA: International Code Council, 2000.
International Building Code. Falls Church, VA: International Code Council, 2000.
Preventing Water Penetration in Below-Grade Concrete Masonry Walls, TEK 19-03A. Concrete Masonry & Hardscapes Association, 2001.
Seismic Design Provisions for Masonry Structures, TEK 14-18B, Concrete Masonry & Hardscapes Association, 2009.
Specifications for Masonry Structures, ACI 530.1-02/ASCE 6-99/TMS 602-02. Reported by the Masonry Standards Joint Committee, 2002.
Standard Building Code. Birmingham, AL: Southern Building Code Congress International, Inc. (SBCCI), 1999.
Standard Specification for Grout for Masonry, ASTM C 476-01. American Society for Testing and Materials, 2001.
Standard Specification for Load-Bearing Concrete Masonry Units, ASTM C 90-01. American Society for Testing and Materials, 2001.
Standard Specification for Mortar for Unit Masonry, ASTM C 270-00. American Society for Testing and Materials, 2000.
Uniform Building Code. Whittier, CA: International Conference of Building Officials (ICBO), 1997.

Design and Construction of Dry-Stack Masonry Walls

August 2018

INTRODUCTION

Construction of masonry wall systems is possible without the use of mortar. The use of standard CMU units laid dry and subsequently surface bonded with fiber reinforced surfaced bonding cement has been well documented in the past. (ref. 16) With the use of specially fabricated concrete masonry units known as “dry-stack units,” construction of these mortarless systems is simple, easy and cost effective. This TEK describes the construction and engineering design of such mortarless wall systems.

The provisions of this TEK apply to both specialty units manufactured specifically for dry-stack construction and conventional concrete masonry units with the following system types:

Grouted, partially grouted or surface bonded
Unreinforced, reinforced, or prestressed

Note that dry-stacked prestressed systems are available that do not contain grout or surface bonding. The provisions of this TEK do not apply to such systems due to a difference in design section properties (ref 8).

Specially designed units for dry-stack construction are available in many different configurations as shown in Figure 1. The latest and most sophisticated designs incorporate face shell alignment features that make units easier and faster to stack plumb and level. Other units are fabricated with a combination of keys, tabs or slots along both horizontal and vertical faces as shown in Figure 1 so that they may interlock easily when placed. Physical tolerances of dry-stack concrete units are limited to ±¹/₁₆ in. (1.58 mm.) which precludes the need for mortaring, grinding of face shell surfaces or shimming to even out courses during construction. Interlocking units placed in running bond resist flexural and shear stresses resulting from out-of-plane loads as a result of the keying action: (a) at the top of a web with the recess in the web of the unit above, (b) at two levels of bearing surface along each face shell at the bed joint, and (c) between adjacent blocks along the head joint. The first of these two interlocking mechanisms also ensures vertical alignment of blocks.

The interlocking features of dry-stack units improve alignment and leveling, reduce the need for skilled labor and reduce construction time. Floor and roof systems can be supported by mortarless walls with a bond beam at the top of the wall which expedites the construction process.

Wall strength and stability are greatly enhanced with grouting which provides the necessary integrity to resist forces applied parallel, and transverse to, the wall plane. Vertical alignment of webs ensures a continuous grout column even when the adjacent cell is left ungrouted. Grouting is necessary to develop flexural tensile stress normal to the bed joints, which is resisted through unit-mortar bond for traditional masonry construction. Strength of grouted dry-stack walls may also be enhanced by traditional reinforcement, prestressing, post-tensioning or with external fiber-reinforced surface coatings (surface bonding) as described in the next section.

Typical applications for mortarless concrete masonry include basement walls, foundation walls, retaining walls, exterior above-grade walls, internal bearing walls and partitions. Dry-stack masonry construction can prove to be a cost-effective solution for residential and low-rise commercial applications because of it’s speed and ease of construction, strength and stability even in zones of moderate and high seismicity. More information on design and construction of dry-stack masonry can be found in Reference 5.

CONSTRUCTION

Dry-stack concrete masonry units can be used to construct walls that are grouted or partially grouted; unreinforced, reinforced or prestressed; or surface bonded. With each construction type, walls are built by first stacking concrete masonry units.

For unreinforced construction as shown in Figure 2a, grouting provides flexural and shear strength to a wall system. Flexural tensile stresses due to out-of-plane bending are resisted by the grout cores. Grout cores also interlace units placed in running bond and thus provide resistance to in-plane shear forces beyond that provided by friction developed along horizontal joints. Grout cores can also be reinforced to increase flexural strength.

Reinforcement can be placed vertically, in which case only those cells containing reinforcement may be grouted as shown in Figure 2b, as well as horizontally, in which case the masonry must be fully grouted. Another version is to place vertical prestressing tendons in place of reinforcement. Vertical axial compressive stress, applied via the tendons, increases flexural and shear capacity. Tendons may be bonded to grout, or unbonded, based upon the design. Placement of grout may be optional. Horizontally reinforced bond beam lintels can be created using a grout stop beneath the unit to contain grout.

As an alternative to reinforcing or prestressing, wall surfaces may be parged (coated) with a fiber-reinforced surface bonding cement/stucco per ASTM C887(ref. 14) as illustrated in Figure 2c. This surface treatment, applied to both faces of a wall, bonds concrete units together without the need for grout or internal reinforcement. The parging material bridges the units and fills the joints between units to provide additional bonding of the coating to the units through keying action. The compressive strength of the parging material should be equal to or greater than that of the masonry units.

Figure 2–– Basic Dry-Stack Masonry Wall Types

Laying of Units

The first course of dry-stack block should be placed on a smooth, level bearing surface of proper size and strength to ensure a plumb and stable wall. Minor roughness and variations in level can be corrected by setting the first course in mortar. Blocks should be laid in running bond such that cells will be aligned vertically.

Grout and Reinforcement

Grout and grouting procedures should be the same as used in conventional masonry construction (ref. 1, 10) except that the grout must have a compressive strength of at least 2600 psi (190 MPa) at 28 days when tested in accordance with ASTM C 1019 (ref.12). Placement of grout can be accomplished in one lift for single-story height walls less than 8 ft (2.43 m). Grout lifts must be consolidated with an internal vibrator with a head size less than 1 in. (25 mm).

Vertical Reinforcing

As for conventional reinforced masonry construction, good construction practice should include placement of reinforcing bars around door and window openings, at the ends, top and bottom of a wall, and between intersecting walls. Well detailed reinforcement such as this can help enhance nonlinear deformation capacity, or ductility, of masonry walls in building systems subjected to earthquake loadings – even for walls designed as unreinforced elements. Additional information on conventional grouting and reinforced masonry wall can be found in TEK 09-04A and TEK 03-03B (refs. 9 & 6).

Pre-stressed Walls

Mortarless walls can also be prestressed by placing vertical tendons through the cores. Tendons can be anchored within the concrete foundation at the base of a wall or in a bottom bond beam and are tensioned from the top of a wall.

Surface Bonded Walls

For walls strengthened with a surface bonding, a thin layer of portland cement surface bonding material should be troweled or sprayed on to a wall surface. The thickness of the surface coating should be at least ⅛ in. (3.2 mm.) or as required by the material supplier.

ENGINEERING PROPERTIES

Walls constructed with mortarless masonry can be engineered using conventional engineering principles. Existing building code recommendations such as that produced by the building code (ref. 1) can serve as reference documents, but at the time of this printing it does not address mortarless masonry directly. It is thus considered an alternate engineered construction type. The International Building Code (ref. 7) does list allowable stresses based on gross-cross-sectional area for dry-stacked, surface-bonded concrete masonry walls. These values are the same as presented in TEK 03-05A (ref. 16). Suggested limits on wall or building height are given in Table 1.

Test data (refs. 2, 3 and 4) have shown that the strength of drystack walls exceeds the strength requirements of conventional masonry, and thus the recommended allowable stress design practices of the code can be used in most cases. When designing unreinforced, grouted masonry wall sections, it is important to deduct the thickness of the tension side face shell when determining the section properties for flexural resistance.

Table 1 –– Summary of Wall Heights for 8” (203 mm) Dry-stacked Units (ref. 5)

* Laterally supported at each floor

Unit and Masonry Compressive Strength

Units used for mortarless masonry construction are made of the same concrete mixes as used for conventional masonry units. Thus, compressive strength of typical units could vary between 2000 psi (13.79MPa) and 4000 psi. (27.58 MPa) Standard Methods of Sampling and Testing Concrete Masonry Units (ref. 11) can be referred to for determining strength of dry-stack units.

Masonry compressive strength f’_m can conservatively be based on the unit-strength method of the building code (ref . 15), or be determined by testing prisms in accordance with ASTM C1314 (ref. 4). Test prisms can be either grouted or ungrouted depending on the type of wall construction specified.

Solid Grouted, Unreinforced Construction

Out-of-Plane & In-Plane Allowable Flexural Strength

Because no mortar is used to resist flexural tension as for conventional masonry construction, flexural strength of mortarless masonry is developed through the grout, reinforcement or surface coating. For out-of-plane bending of solid grouted walls allowable flexural strength can be estimated based on flexural tensile strength of the grout per Equation 1.

Consideration should be given to the reduction in wall thickness at the bed joints when estimating geometrical properties of the net effective section.

Correspondingly, flexural strength based on masonry compressive stress should be checked, particularly for walls resisting significant gravity loads, using the unity equation as given below.

Buckling should also be checked. (Ref. 8)

In-Plane Shear Strength

Shear strength for out-of-plane bending is usually not a concern since flexural strength governs design for this case. For resistance to horizontal forces applied parallel to the plane of a wall, Equation 3 may be used to estimate allowable shear strength.

F_v is the allowable shear strength by the lesser of the three values given in Equation 4.

Grouted, Reinforced Construction

Mortarless masonry that is grouted and reinforced behaves much the same as for conventional reinforced and mortared construction. Because masonry tensile strength is neglected for mortared, reinforced construction, flexural mechanisms are essentially the same with or without the bed joints being mortared provided that the units subjected to compressive stress are in good contact. Thus, allowable stress design values can be determined using the same assumptions and requirements of the MSJC code. (ref.1)

Out-of-Plane & In-Plane Allowable Flexural Strength

Axial and flexural tensile stresses are assumed to be resisted entirely by the reinforcement. Strains in reinforcement and masonry compressive strains are assumed to vary linearly with their distance from the neutral axis. Stresses in reinforcement and masonry compressive stresses are assumed to vary linearly with strains. For purposes of estimating allowable flexural strengths, full bonding of reinforcement to grout are assumed such that strains in reinforcement are identical to those in the adjacent grout.

For out-of-plane loading where a single layer of vertical reinforcement is placed, allowable flexural strength can be estimated using the equations for conventional reinforcement with the lower value given by Equations 5 or 6.

In-Plane Shear Strength

Though the MSJC code recognizes reinforced masonry shear walls with no shear, or horizontal reinforcement, it is recommended that mortarless walls be rein- forced with both vertical and horizontal bars. In such case, allowable shear strength can be determined based on shear reinforcement provisions (ref. 1) with Equations 7, 8 and 9.

Where F_v is the masonry allowable shear stress per Equations 8 or 9.

Solid Grouted, Prestressed Construction

Mortarless masonry walls that are grouted and pre- stressed can be designed as unreinforced walls with the prestressing force acting to increase the vertical compres- sive stress. Grout can be used to increase the effective area of the wall. Flexural strength will be increased because of the increase in the f_a term in Equation 1. Shear strength will be increased by the N_v term in Equation 4.

Because the prestressing force is a sustained force, creep effects must be considered in the masonry. Research on the long-term behavior of dry-stacked masonry by Marzahn and Konig (ref. 8) has shown that creep effects may be accentuated for mortarless masonry as a result of stress concentrations at the contact points of adjacent courses. Due to the roughness of the unit surfaces, high stress concentrations can result which can lead to higher non-proportional creep deformations. Thus, the creep coefficient was found to be dependent on the degree of roughness along bed-joint surfaces and the level of applied stress. As a result, larger losses in prestressing force is probable for dry-stack masonry.

Surface-Bonded Construction

Dry-stack walls with surface bonding develop their strength through the tensile strength of small fiberglass fibers in the 1/8” (3.8mm) thick troweled or surface bonded cement-plaster coating ASTM C-887(Ref. 14). Because no grouting is necessary, flexural tension and shear strength are developed through tensile resistance of fiberglass fibers applied to both surfaces of a wall. Test data has shown that surface bonding can result in a net flexural tension strength on the order of 300 psi.(2.07 MPa) Flexural capacity, based on this value, exceeds that for conventional, unreinforced mortared masonry construction, therefore it is considered conservative to apply the desired values of the code (ref. 1) for allowable flexural capacity for portland cement / lime type M for the full thickness of the face shell.

Out-of-Plane and In-Plane Flexural Strength

Surface-bonded walls can be considered as unreinforced and ungrouted walls with a net allowable flexural tensile strength based on the strength of the fiber-reinforcement. Flexural strength is developed by the face shells bonded by the mesh. Allowable flexural strength can be determined using Equation 1 with an F_t value determined on the basis of tests provided by the surface bonding cement supplier. Axial and flexural compressive stresses must also be checked per Equation 2 considering again only the face shells to resist stress.

Surface Bonded In-Plane Shear Strength

In-plane shear strength of surface-bonded walls is attributable to friction developed along the bed joints resulting from vertical compressive stress in addition to the diagonal tension strength of the fiber coating. If the enhancement in shear strength given by the fiber reinforced surface parging is equal to or greater than that provided by the mortar-unit bond in conventional masonry construction, then allowable shear strength values per the MSJC code (ref. 1) may be used. In such case, section properties used in Equation 3 should be based on the cross-section of the face shells.

Figure 3 – A Mortarless Garden Wall Application

Figure 4 – A Residential, Mortarless, Single-Family Basement – Part of a 520 Home Development

REFERENCES

Building Code Requirements for Masonry Structures), ACI 530-02/ ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Drysdale, R.G., Properties of Dry-Stack Block, Windsor, Ontario, July 1999.
Drysdale, R.G., Properties of Surface-Bonded Dry-Stack Block Construction, Windsor, Ontario, January 2000.
Drysdale, R.G., Racking Tests of Dry-Stack Block, Windsor, Ontario, October 2000.
Drysdale, R.G., Design and Construction Guide for Azar Dry-Stack Block Construction, JNE Consulting, Ltd., February 2001.
Grout for Concrete Masonry, TEK 09-04A, Concrete Masonry & Hardscapes Association, 2002.
2000 International Building Code, Falls Church, VA. International Code Council, 2000.
Marzahn, G. and G. Konig, Experimental Investigation of Long-Term Behavior of Dry-Stacked Masonry, Journal of The Masonry Society, December 2002, pp. 9-21.
Hybrid Concrete Masonry Construction Details, TEK 0303B. Concrete Masonry & Hardscapes Association, 2009.
Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/ TMS 602-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140-02a, ASTM International, Inc. , Philadelphia, 2002.
Standard Method of Sampling and Testing Grout, ASTM C1019-02, ASTM International, Inc., Philadelphia, 2002.
Standard Specification for Grout for Masonry, ASTM C 476-02. ASTM International, Inc., 2002
Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C 887-79a (2001). ASTM International, Inc., 2001.
Standard Test Method for Compressive Strength of Masonry Assem blages, ASTM C1314-02a, ASTM International, Inc., Philadelphia, 2002.
Surface Bonded Concrete Masonry Construction, TEK 03-05A. Concrete Masonry & Hardscapes Association, 1998.

NOTATION

A_n net cross-sectional area of masonry, in² (mm²)
A_s effective cross-sectional area of reinforcement, in2 (mm2)
b width of section, in. (mm)
d distance from extreme compression fiber centroid of tension reinforcement, in. (mm)
F_a allowable compressive stress due to axial load only, psi (MPa)
F_b allowable compressive stress due to ß exure only, psi (MPa)
F_s allowable tensile or compressive stress in reinforcement, psi (MPa)
F_t flexural tensile strength of the grout, psi(MPa)
F_v allowable shear stress in masonry psi (MPa)
f_a calculated vertical compressive stress due to axial load, psi (MPa)
f_b calculated compressive stress in masonry due to ß exure only, psi (MPa)
f’ specified compressive strength of masonry, psi (MPa)
I moment of inertia in.⁴ (mm⁴)
j ratio of distance between centroid of flexural compressive forces and centroid of tensile forces to depth, d
k ratio of the distance between compression face of the wall and neu tral axis to the effective depth d
M maximum moment at the section under consideration, in.-lb (N-mm)
N_v compressive force acting normal to the shear surface, lb (N)
Q first moment about the neutral axis of a section of that portion of the cross section lying between the neutral axis and extreme fiber in.³ (mm³)
S_g section modulus of uncracked net section in.³ (mm³)
V shear force, lb (N)

Surface Bonded Concrete Masonry Construction

August 2018

INTRODUCTION

Surface bonding is an economical construction technique which was first introduced in the late sixties by the U. S. Department of Agriculture for use in low cost housing. In surface bonded construction, concrete masonry units are laid dry and stacked, without mortar, to form walls. Walls are constructed with units that have been precision ground or honed to achieve a uniform bearing surface, or with shims placed periodically to maintain a level and plumb condition. Both sides of the wall are then coated with a thin layer of reinforced surface bonding mortar. The synthetic fibers which reinforce the surface bonding mortar impart a tensile strength of about 1500 psi (10.3 MPa), producing a strong wall despite the relatively thin thickness of material on each side. The surface coating on each side of the wall bonds the concrete masonry units together in a strong composite construction, and serves as a protective water resistant shield.

Surface bonded concrete masonry has a number of advantages:

Less time and skill are required for wall construction. In a 1972 study of mason productivity sponsored by the U. S. Department of Housing and Urban Development and other interested organizations, it was found that surface bonded concrete masonry construction resulted in 70 percent greater productivity than that achievable with conventional construction.
The surface bonding mortar provides excellent resistance to water penetration in addition to its function of holding the units together. Tests of surface bonded walls have repeatedly shown their resistance to wind driven rain to be “excellent” even with wind velocities as great as 100 mph (161 km/h), and over test periods of 8 hours.
Colored pigment can be incorporated into the surface bonding mortar to produce a finished surface without the need to paint.

Surface bonded concrete masonry construction offers all of the benefits and advantages of conventional concrete masonry construction, such as:

fire safety
acoustic insulation
energy efficiency
lasting durability and beauty

DESIGN STRENGTH

Many structural and nonstructural tests have been performed on surface bonded walls to establish design parameters for the system.

The nonstructural properties, such as sound transmission class, fire resistance period, and energy efficiency, of surface bonded concrete masonry can be considered equivalent to a conventional mortared concrete masonry wall.

There are a few differences between the structural properties of the two types of construction. These differences are discussed in the following paragraphs, and are illustrated in Figure 1 for ungrouted, unreinforced walls. Although national building codes, such as the BOCA National Building Code and the Standard Building Code (refs. 1, 3) do not specifically address reinforced or grouted surface bonded walls, manufacturers of surface bonding mortars may have code-approved criteria for their products.

Compressive Loads

Resistance to vertical compressive loads depends primarily on the compressive strength of the concrete block used in the wall construction. Stronger units make stronger walls. With mortared construction, a rule of thumb is that the wall strength will generally be about seventy percent of the unit strength. In comparison, surface bonded walls built with unground concrete masonry units develop approximately thirty percent of the strength of the individual block. This reduced wall strength is depicted in Figure 1 for walls constructed with unground concrete masonry units.

The lower value obtained with the unground units is due to a lack of solid bearing contact between units, due to the natural roughness of the concrete units. The mortar bed used in conventional construction compensates for this roughness and provides a uniform bearing between units. If the masonry unit bearing surfaces are ground flat and smooth before the wall is erected, results similar to those for a mortared wall can be expected. In Figure 1, note that surface bonded walls built with precision ground concrete masonry units are equally as strong in compression as the conventional construction.

Flexural Resistance

The flexural strength of a surface bonded wall is about the same as that of a conventional mortared wall, as shown in Figure 1. When walls are tested in the vertical span (i.e., a horizontal force, such as wind, is applied to a wall that is supported at the top and bottom) surfaced bonded walls and mortared walls have about the same average strength; failure occurs in the surface bonded coating due to tensile stress at or near one of the horizontal joints. With mortared construction, failure occurs at a horizontal joint with bond failure between the mortar and the masonry units. The data from numerous tests on surface bonded constructions led to an allowable stress of 18 psi (0.12 MPa) based on the gross area.

When walls are laid in a running bond pattern, either with mortar joints or with surface bonding, and tested in the horizontal span, (i.e., a wall supported at each end is subjected to a horizontal wind force) the strength in bending depends primarily on the strength of the units. This is due to the interlocking of the masonry units laid when in a running bond configuration. In such tests in the horizontal span, the wall strength of the surface bonded wall is exactly the same as the conventional construction. In Table 1, an allowable flexural stress of 30 psi (0.21 MPa) is recommended for horizontal span when the units have been laid in running bond.

Shear Strength

The shear resistance of surface bonded construction is the same as that of conventional walls. With face shell mortar bedding, conventional concrete masonry walls averaged 42 psi (0.29 MPa) shear resistance, based on gross area. Nine surface bonded walls, 8 in. (203 mm) in thickness, had an average shear resistance of 39 psi (0.27 MPa), and three 6 in. (152 mm) thick surface bonded walls averaged 40 psi (0.28 MPa). These data are compared in Figure 1, and led to a recommended allowable shear stress of 10 psi (0.07 MPa) on the gross area (see Table 1).

Compression:

45 psi (0.31 MPa)

Shear:

10 psi (0.07 MPa)

Flexural Tension:

Horizontal span:	30 psi (0.21 MPa)
Vertical span:	18 psi (0.12 MPa)

_a References 1 & 3

Table 1—Allowable Stress, Gross Cross-Sectional Area, Dry-Stacked, Surface-Bonded Concrete Masonry Walls

Figure 1—Surface Bonded and Mortared Concrete Masonry Wall Strengths

CONSTRUCTION

The construction procedure for surface bonded walls is similar to that of conventional, except that mortar is not placed between the masonry units. Standard Practice for Construction of Dry-Stacked, Surface-Bonded Walls, ASTM C946 (ref. 4), governs the construction methods. Care should be taken to ensure uncoated walls are adequately braced.

Because the walls are constructed without mortar joints, surface bonded wall dimensions do not conform to the standard 4 in. (102 mm) design module. Wall and opening dimensions should be based on actual unit dimensions, which are typically 7 ⅝ in. high by 15 ⅝ in. long (194 by 397 mm).

Materials

Surface bonding mortar should comply with Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C887 (ref. 6), which governs flexural and compressive strength, sampling, and testing. ASTM C946 requires Type I, moisture-controlled, concrete masonry units be used for surface bonded construction. Type I units must be in a dry condition when delivered to the job site. Walls laid using dry units will undergo less drying shrinkage after construction, hence minimizing cracks. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90 (ref. 5) governs these requirements.

As for mortared masonry construction, materials should be properly stored on site to prevent contamination by rain, ground water, mud, and other materials likely to cause staining or to have other deleterious effects.

If the bearing surfaces of the concrete masonry units are unground, metal or plastic shims or mortar may occasionally be required between units to maintain the wall level and plumb. Shims must have a minimum compressive strength of 2,000 psi (13.8 MPa) to ensure their long term durability after the wall is loaded. Metal shims, if used, should be corrosion resistant to reduce the possibility that they will corrode and bleed through the finished masonry at a later time.

Leveling

Because the footing is not typically level enough to lay up the dry units without additional leveling, the first course of masonry units is laid in a mortar bed or set in the fresh footing concrete to obtain a level base for the remainder of the wall. Vertical head joints should not be mortared, even when the first course is mortar bedded, since mortar in the head joints will misalign the coursing along the wall length.

When required, additional leveling courses are constructed in the wall. Leveling courses should be placed when:

the wall is out of level by more than ½ in. (13 mm) in 10 ft,
at each floor level, and
at a horizontal change in wall thickness (see Figure 2).

After the first course of masonry units is laid level in a mortar bed, dry stacking proceeds with the remaining courses beginning with the corners, and followed by stacking, in running bond, between the corners. As they are dry stacked, the ends of the concrete masonry units should be butted together tightly. Small burrs should be removed prior to placement.

After every fourth course, the wall should be checked for plumb and level.

Crack Control

Temperature and moisture movements have the potential to cause small vertical cracks in a masonry wall. These cracks are an aesthetic, rather than a structural, concern. In exposed concrete masonry, where shrinkage cracks may be objectionable, horizontal joint reinforcement, control joints, or bond beams are used to control cracking. The absence of a mortar bed joint in surface bonded walls means that there is no space in the wall for joint reinforcement, so control joints or bond beams are used for crack control.

Control joints should be placed:

at wall openings and at changes in wall height and thickness
at wall intersections, at pilasters, chases, and recesses
in walls without openings, at intervals of 20 ft (6.1 m) when there are no bond beams in the construction, and at intervals of 60 ft (18.3 m) when bond beams are incorporated every 4 ft (1.2 m) vertically.

Control joints for surface bonded walls are similar to those for mortared concrete masonry. At the control joint location, the surface bonding mortar should be raked out and the joint caulked.

Placing Accessories & Utilities

The absence of a mortar bed joint in the construction also requires that the face shell and/or the cross web of the concrete masonry units be notched or depressed whenever wall ties or anchors must be embedded in the wall. A coarse rasp is typically used to make small notches, while deeper notches are cut with a masonry saw. Cores containing anchors or wall ties should be grouted, or other adequate anchorage should be provided.

Electrical lines and plumbing are often located in the cores of concrete masonry units. These lines should be placed before the surface bonding mortar is applied, so that the masonry units are visible.

Applying Surface Bonding Mortar

Manufacturer’s recommendations should be followed for job site mixing of the premixed surface bonding mortar and application to the dry stacked concrete masonry wall.

As with mortared masonry construction, clean water and mixing equipment should be used to prevent foreign materials from being introduced into the mortar. Batches should be mixed in full bag multiples only, to compensate for any segregation of materials within a bag.

All materials should be mixed for 1 to 3 minutes, until the mixture is creamy, smooth, and easy to apply. Note that mixing time should be kept to a minimum, as overmixing can damage the reinforcing fibers.

The stacked concrete masonry units should be clean and free of any foreign matter which would inhibit bonding of the plaster. Contrary to recommended practice with conventional mortared walls, the dry stacked concrete masonry units should be damp when the surface bonding plaster is applied to prevent water loss from the mortar due to suction of the units. Care should be taken to avoid saturating the units.

It is very important that the surface bonding mortar be applied to both sides of the dry stacked wall since the wall strength and stability depend entirely on this coating.

Premixed surface bonding mortars are smooth textured and easily applied by hand with a trowel. The workability is due to the short ½ in. (13 mm) glass fibers which reinforce the mixture. The mortar should be troweled on smoothly with a minimum thickness of ⅛ in. (3 mm).

Surface bonding mortar can also be sprayed on. On large projects, use of a power sprayer greatly increases the coverage rate of the mortar and further reduces wall costs. As applied, the “sprayed-on” surface bonding mortar usually has a rougher surface texture than a troweled finish, and possesses slightly less tensile strength due to the lack of fiber orientation in the plane of the mortar coating. This can be overcome by troweling, hand or mechanical, following spray application of the mortar. Hand or mechanical troweling of the sprayed coating also assures that all gaps and crevices are filled.

When a second coat of surface bonding mortar is applied, either by trowel or spray, it should be applied after the first coat is set, but before it is completely hardened or dried out. The second coat may be textured to achieve a variety of finishes.

Joints in surface bonding mortar are weaker than a continuous mortar surface, and, for this reason, should not align with joints between masonry units. If application of the surface bonding mortar is discontinued for more than one hour, the first application should be stopped at least 1 ¼ in. (32 mm) from the horizontal edge of the concrete masonry unit. At the foundation, the surface bonding mortar should either form a cove between the wall and the footer or, for a slab on grade, should extend below the masonry onto the slab edge, as shown in Figure 3. These details help prevent water penetration at the wall/footer interface.

Curing

After surface bonding application, the wall must be properly cured by providing sufficient water for full hydration of the mortar, to ensure full strength development. The wall should be dampened with a water mist between 8 and 24 hours after surface bonding mortar application. In addition, the wall should be fog sprayed twice within the first 24 hours, although with pigmented mortar, this may be extended to 48 hours.

The recommendations above may need to be modified for either cold or hot weather conditions. For example, dry, warm, windy weather accelerates the water evaporation from the mortarrequiring more frequent fog spraying.

At the end of the day, tops of walls should be covered to prevent moisture from entering the wall until the top is permanently protected. Typically, a tarp is placed over the wall, extending at least 2 ft (0.6 m) down both sides of the wall, and weighted down with lumber or masonry units.

REFERENCES

BOCA National Building Code. Country Club Hills, IL: Building Officials and Code Administrators International, Inc. (BOCA), 1996.
Building Code Requirements for Masonry Structures, ACI 530-95/ASCE 5-95/TMS 402-95. Reported by the Masonry Standards Joint Committee, 1995.
Standard Building Code. Birmingham, AL: Southern Building Code Congress International, Inc. (SBCCI), 1997.
Standard Practice for Construction for Dry-Stacked, Surface-Bonded Walls, ASTM C946-91 (1996)^e1. American Society for Testing and Materials, 1996.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-97. American Society for Testing and Materials, 1997.
Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C887-79a (1996)^e1 American Society for Testing and Materials, 1996.