Resources

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 Old perceptions of basements have proven outdated by stateofthe-art waterproofing, improved drainage systems, and natural lighting features such as window wells.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.

Preventing Water Penetration in Below-Grade Concrete Masonry Walls

August 2018

INTRODUCTION

Concrete masonry has traditionally been the material of choice for foundation wall construction. State-of-the-art waterproofing, dampproofing, and drainage systems applied to concrete masonry provide excellent protection from water penetration, ensuring protection for building contents and comfort for occupants.

Protecting below-grade walls from water entry involves installing a barrier to water and water vapor. Below grade moisture tends to migrate from the damp soil to the drier area inside the basement. 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.

WATERPROOFING AND DAMPPROOFING

Building codes (refs. 1, 2) 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, through granular backfill into a subsoil drainage system.

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 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, and impact, puncture and abrasion resistance.

WATERPROOF AND DAMPPROOF SYSTEMS

Waterproof and dampproof systems must be continuous to prevent water penetration. Similarly, the barrier is typically carried above the finished grade level to prevent water entry between the barrier and the foundation wall. Cracks exceeding ¼ in. (6 mm) should be repaired before applying a waterproof or dampproof barrier. Repair of hairline cracks is typically not required, as most barriers will either fill or span small openings. In addition, most waterproofing and dampproofing systems should be applied to clean, dry walls. In all cases, manufacturer’s directions should be carefully followed for proper installation.

Particular attention should be paid to wall penetrations and to re-entrant corners at garages, porches, and fireplaces. Because differential movement often occurs at these intersections, stretchable membranes are often used to span any potential cracks. Alternately, the main wall in some cases can be coated prior to constructing the cross wall provided that structural adequacy is maintained.

Coatings are sprayed, trowelled, or brushed onto below-grade walls, providing a continuous barrier to water entry. Coatings should be applied to clean, structurally sound walls. Walls should be brushed or washed to remove dirt, oil, efflorescence, or other materials which may reduce the bond between the coating and the wall.

Sheet membranes and panels are less dependent on workmanship and surface preparation than coatings. Many membrane systems are better able to remain intact in the event of settlement or other foundation wall movement. Seams, terminations, and penetrations must be properly sealed.

Prescriptive Systems

Both the International Building Code (IBC) (ref. 1) and the International Residential Code (IRC) (ref. 2) include prescriptive methods for waterproofing and dampproofing. Except where a damproofing material is approved for direct application to the masonry, masonry walls are required to have not less than ⅜ in. (9.5 mm) portland cement parging applied to the exterior of the wall before applying damproofing. The following materials are specified in the IBC as acceptable waterproofing and dampproofing materials:

two-ply hot-mopped felts;
6 mil (0.006 in.; 0.152 mm) or greater polyvinyl chloride;
40 mil (0.040 in.; 1.02 mm) polymer-modified asphalt;
6 mil (0.006 in.; 0.152 mm) polyethylene; or
other approved methods or materials capable of bridging nonstructural cracks.

In addition, the IRC includes the following materials for concrete and masonry foundation waterproofing:

55 pound (25 kg) roll roofing;
60 mil (1.5 mm) flexible polymer cement;
⅛ in. (3 mm) cement-based, fiber-reinforced, waterproofing coating; or
60 mil (1.5 mm) solvent-free liquid-applied synthetic rubber.

Both the IBC and IRC list the following materials as acceptable for dampproofing only (note—any of the waterproofing materials are acceptable for dampproofing):

bituminous material;
3 lb/yd² (16 N/m²) of acrylic modified cement;
⅛ in. (3.2 mm) coat of surface-bonding mortar complying with ASTM C887 (ref. 3); or
other approved methods or materials.

The following discusses details of some of the prescriptive code methods for waterproofing and dampproofing.

Rubberized Asphalt Systems

A wide variety of rubberized and other polymer-modified asphalt waterproofing systems are available. Most of these are applied as sheet membranes, although some are available as liquid coatings. These systems provide a continuous barrier to water with the ability to elastically span small holes or cracks.

Rubberized asphalt sheet membranes are applied over a primer, which is used to increase adhesion of the sheet. The membrane is adhesive on one side and protected by a polyethylene film on the other. Adjacent pieces of membrane must be lapped, and the top and bottom edges sealed with mastic to provide continuous protection from water entry. After the membrane is placed on the wall, the surface is rolled with sufficient pressure to ensure adequate adhesion.

Rubberized asphalt is also available in a form that can be melted at the jobsite, then spread to completely cover foundation walls. Liquid coatings can be applied by airless spray, roller, or brush. Both the liquid-applied and sheets are covered with a protection board, which protects from construction traffic and during backfilling.

Cementitious Coating Systems

Cement-based coatings are typically trowelled onto concrete masonry walls or brushed on using a coarse-fibered brush. The coatings sufficiently fill block pores, small cracks, and irregularities. Some cementitious coatings are modified with various polymers to increase elasticity and water penetration resistance.

Elastomeric Systems

Elastomeric materials are acrylic-based products which provide a flexible barrier to water penetration for below grade walls. Elastomerics are available as liquid coatings and as sheet membranes. The sheets are attached with adhesive, and may be reinforced with fabric to further increase tensile strength and resistance to tears and punctures. Liquid coatings can be applied by airless spray, roller, or brush.

Other Waterproofing and Dampproofing Systems

The systems listed above (and within the building codes) are only some of the materials and systems available; several others are discussed below. See Basement Manual—Design & Construction Using Concrete Masonry (ref. 4) for more detailed information.

Parging and Bituminous Coating Systems

Where drainage is good, a dampproof coating of parging with a permanent bituminous coating has proven to be satisfactory. A portland cement and sand mix (1:3.5 by volume), or Type M or S mortar may be used for the parge coat. The parge coat should be beveled at the top to form a wash, and thickened at the bottom to form a cove between the wall base and top of footing, as shown in Figure 1.

To further increase water penetration resistance, a bituminous coating is applied over the parging. Coal tar or asphalt based bitumens are available in solvent for hot application, or in emulsions for application at ambient temperatures. These coatings can be sprayed, brushed, or trowelled onto the finish coat of parging.

Bentonite Panel Systems

Bentonite is a mineral that swells to many times its original size when wet. Waterproofing panels incorporate dry bentonite encased in kraft paper or fabric. After installation, the bentonite swells up the first time it is exposed to water, expanding between the foundation wall and the backfill, and forming an impervious barrier. The swelling seals small cracks in the foundation wall or punctures in the panels themselves.

To prevent premature hydration bentonite panels must be protected from moisture until they are properly installed and the foundation wall has been backfilled.

Other Systems

There are several systems for which Acceptance Criteria, developed by the ICC Evaluation Service, exist. Cold, liquid-applied, below-grade exterior dampproofing and waterproofing materials should demonstrate compliance with ICC ES AC29 (ref. 5). For rigid, polyethylene, below-grade dampproofing and waterproofing materials, compliance should be shown to ICC-ES AC114 (ref. 6).

Some systems fulfill the requirements of both waterproofing/dampproofing and wall insulation. These systems, however, may not be specified directly in the building code or have an Acceptance Criteria. In these cases, materials should be evaluated both for general waterproofing (or dampproofing) characteristics (such as resistance to hydrostatic pressure, etc.) as well as for criteria specific to the material or system. The Acceptance Criteria listed above can be used as a baseline for a material, although not all requirements may apply to all materials. An engineering evaluation of the product testing results can demonstrate acceptable performance for use as dampproofing or waterproofing.

DRAINAGE

Draining water away from basement walls significantly reduces the pressure the basement wall must resist. This reduces both the potential for cracking and the possibility of water penetration into the basement if there is a failure in the waterproof or dampproof system.

Perforated pipe or drain tiles laid with open joints have proven to be effective at collecting and transporting water away from foundation walls. The invert of drain pipes should be below the top of the floor slab elevation, as shown in Figure 1. The backfill drain should be connected to solid piping to carry the water to natural drainage, a storm sewer, or a sump. For adequate drainage, drains should slope at least ⅜ in. in 10 ft (10 mm in 3 m).

Drain tile and perforated pipes are typically laid in crushed stone to facilitate drainage. At least 2 in. (51 mm) of washed gravel or free-draining backfill (containing not more than 10% material finer than a No. 4 sieve) should be placed beneath perforated pipes. Drain tiles laid with open joints are more effective when laid on the undisturbed soil where the water begins to accumulate. At least 6 to 12 in. (152 to 305 mm) of the same stone should cover the drain and should extend 12 in. (305 mm) or more beyond the edge of the footing. To prevent migration of fine soils into the drains, filter fabrics are often placed over the gravel.

Drainage pipes may also be placed beneath the slab and connected to a sump. In some cases, pipes are cast in or placed on top of concrete footings at 6 to 8 ft (1.8 to 2.4 m) o.c. to help drain water from the exterior side of the foundation wall.

The backfill material itself also significantly affects water drainage around the wall. The backfill material should be well-draining soil free of large stones, construction debris, organic materials, and frozen earth. Saturated soils, especially saturated clays, should generally not be used for backfill, since wet materials significantly increase the hydrostatic pressure on foundation walls. The top 4 to 8 in. (102 to 203 mm) of backfill should be low permeability soil so rain water is absorbed into the backfill slowly.

The finished grade should be sloped away from the foundation at least 6 in. within 10 ft (152 mm in 3 m) from the building, as shown in Figure 2. If the ground naturally slopes toward the building, a shallow trench or swale can be installed to direct water runoff away from the building.

Finally, gutters and downspouts should be installed to minimize water accumulation near the foundation. Water exiting downspouts should be directed away from foundation walls using plastic drainage tubing or splash blocks. Roof overhangs, balconies, and porches also shield the soil from direct exposure to rainfall.

Figure 2—Landscape Elements for Draining Water Away From Foundation Walls

CONSTRUCTION

Methods of construction can also impact the watertightness of foundation walls. Properly tooled mortar joints help prevent cracks from forming, and contribute to the watertightness of the finished work. Concave-shaped mortar joints are most effective for resisting water entry. Tooling the mortar compresses the surface to make it more watertight, and also reduces leakage by filling small holes and other imperfections. On the exterior face of the wall, mortar joints may be struck flush if parging will be applied.

The drainage and waterproof or dampproof system should be inspected prior to backfilling to ensure they are properly placed. Any questionable workmanship or materials should be repaired at this point, because repair is difficult and expensive after backfilling.

Backfilling methods are important, since improper backfilling can damage foundation walls or the dampproof or waterproof system. Foundation walls should either be properly braced or should have the first floor in place prior to backfilling so the wall is supported against the soil load.

Final grade should be 6 to 12 in. (152 to 305 mm) below the top of the waterproof or dampproof membrane, and should slope away from the foundation wall. In no case should the backfill be placed higher than the design grade line.

These topics are covered in more detail in ref. 7.

LANDSCAPING

Landscaping directly adjacent to the building impacts the amount of water absorbed by the foundation backfill. Particular care should be taken when automatic sprinklers are installed adjacent to foundation walls. Whenever possible, large-rooting shrubs and trees should be placed 10 to 15 ft (3 to 4.6 m) away from foundation walls. Smaller shrubs should be kept at least 2 to 3 ft (0.6 to 0.9 m) from walls. Ground covers help prevent erosion and can extend to the foundation. These elements are illustrated in Figure 2.

Asphalt and concrete parking lots, sidewalks, building aprons, stoops and driveways prevent direct absorption of water into soil adjacent to the foundation, and should be installed to slope away from the building.

REFERENCES

International Building Code. International Codes Council, 2012.
International Residential Code for One- and Two-Family Dwellings. International Code Council, 2012.
Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C887-05(2010) . ASTM International, Inc., 2010.
Basement Manual—Design & Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.
Acceptance Criteria for Cold, Liquid-Applied, Below-Grade, Exterior Damproofing and Waterproofing Materials, ICC ES AC29. International Code Council, 2011.
Acceptance Criteria for Rigid, Polyethylene, Below-Grade, Damproofing and Wall Waterproofing Material, ICC-ES AC114. International Code Council, 2011.
Concrete Masonry Basement Wall Construction, TEK 03-11, Concrete Masonry & Hardscapes Association, 2001.

Strength Design of Reinforced Concrete Masonry Foundation Walls

August 2018

INTRODUCTION

Although concrete masonry foundation walls can be constructed without reinforcing steel, reinforcement may be required for walls supporting large soil backfill loads. The strength design provisions found in Chapter 3 of Building Code Requirements for Masonry Structures (ref. 1) typically provides increased economy over the allowable stress design method, as thinner walls or larger reinforcing bar spacings often result from a strength design analysis. Strength design criteria are presented in detail in TEK 14-04B, Strength Design Provisions for Concrete Masonry (ref. 2).

DESIGN LOADS

Soil imparts lateral loads on foundation walls. The load is assumed to increase linearly with depth, resulting in a triangular load distribution on the wall. This lateral soil load is expressed as an equivalent fluid pressure, with units of pounds per square foot per foot of depth (kN/m²/m). For strength design analysis, this lateral soil pressure is increased by multiplying by a load factor, which provides a factor of safety against overload conditions. The maximum moment on the wall depends on the total wall height, the soil backfill height, the wall support conditions, the factored soil load, the existence of any surcharges on the soil and the presence of saturated soils.

Foundation walls also provide support for the structure above the foundation, transferring vertical loads to the footing. Vertical compression counteracts flexural tension, increasing the wall’s resistance to flexure. In low-rise construction, these vertical loads are typically small in relation to the compressive strength of the concrete masonry. Vertical load effects are not addressed in this TEK.

DESIGN TABLES

Tables 1 through 4 present reinforcement schedules for 6, 8, 10 and 12-in. (152, 203, 254 and 305-mm) walls, respectively. Additional reinforcement alternatives may be appropriate, and can be verified with an engineering analysis. Walls from 8 to 16 ft (2.4 to 4.9 m) high and soil pressures of 30, 45 and 60 psf/ft (4.7, 7.0, and 9.4 kN/m²/m) are included.

The effective reinforcement depth, d, assumed for the analyses are practical values, taking into account variations in face shell thickness, a range of reinforcing bar sizes, minimum required grout cover and construction tolerances for placing the reinforcement.

The following assumptions also apply to the values in Tables 1 through 4:

there are no surcharges on the soil adjacent to the wall,
there are negligible axial loads on the wall,
the wall is simply supported at top and bottom,
the wall is grouted at cells containing reinforcement (although solid grouting is acceptable),
section properties are based on minimum face shell and web thickness requirements of ASTM C 90 (ref. 3),
the specified compressive strength of masonry, f’m, is 1500 psi (10.3 MPa),
Grade 60 (413 MPa) reinforcement,
reinforcement requirements listed account for a soil load factor of 1.6 (ref. 6),
the maximum width of the compression zone is limited to six times the wall thickness, or a 72 in. (1,829 mm) vertical bar spacing, whichever is smaller,
reinforcing steel is placed toward the tension (interior) face of the wall (as shown in Figure 1), and
the soil is well drained to preclude the presence of saturated soil.

Table 1-Reinforcement for 6-inch (152-mm) Concrete Masonry Foundation Walls

Table 2-Reinforcement for 8-inch (203-mm) Concrete Masonry Foundation Walls

Table 3-Reinforcement for 10-inch (254-mm) Concrete Masonry Foundation Walls

Table 4-Reinforcement for 12-inch (305-mm) Concrete Masonry Foundation Walls

DESIGN EXAMPLE

Wall: 12-in. (305 mm) thick concrete masonry foundation wall, 12 ft (3.66 m) high

Soil: equivalent fluid pressure is 45 psf/ft (7.0 kN/m²/m) (excluding soil load factors), 10 ft (3.05 m) backfill height

Using Table 4, the wall can be adequately reinforced using No. 9 bars at 72 in. o.c. (M# 29 at 1,829 mm).

CONSTRUCTION ISSUES

This section discusses those issues which directly relate to structural design assumptions. See TEK 03-11, Concrete Masonry Basement Wall Construction and TEK 05-03A, Concrete Masonry Foundation Wall Details (refs. 4, 5) for more complete information on building concrete masonry foundation walls.

Figure 1 illustrates wall support conditions, drainage and protection from water. Before backfilling, the floor diaphragm must be in place, or the wall must be properly braced to resist the soil load. Ideally, the backfill should be free-draining granular material, free from expansive soils or other deleterious materials.

The assumption that there are no surcharges on the soil means that heavy equipment should not be operated directly adjacent to any basement wall system. In addition, the backfill materials should be placed and compacted in several lifts. Care should be taken when placing backfill materials to prevent damaging the drainage, waterproofing or exterior insulation systems.

Figure 1—Typical Reinforced Basement Wall

REFERENCES

Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
Strength Design Provisions for Concrete Masonry, TEK 14-04B, Concrete Masonry & Hardscapes Association, 2008.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-03. ASTM International, 2003.
Concrete Masonry Basement Wall Construction, TEK 0311, Concrete Masonry & Hardscapes Association, 2001.
Concrete Masonry Foundation Wall Details, TEK 05-03A, Concrete Masonry & Hardscapes Association, 2003.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-02. American Society of Civil Engineers, 2002.

Allowable Stress Design of Concrete Masonry Foundation Walls

August 2018

INTRODUCTION

Basements provide: economical living, working and storage areas; convenient spaces for mechanical equipment; safe havens during tornadoes and other violent storms; and easy access to plumbing and ductwork. Concrete masonry is well suited to basement and foundation wall construction due to its inherent durability, compressive strength, economy, and resistance to fire, termites, and noise.

Traditionally, residential basement walls have been constructed of plain (unreinforced) concrete masonry, often designed empirically. Walls over 8 ft (2.4 m) high or with larger soil loads are typically designed using reinforced concrete masonry or using design tables included in building codes such as the International Building Code (ref. 4).

DESIGN LOADS

Soil imparts a lateral load on foundation walls. For design, the load is traditionally assumed to increase linearly with depth resulting in a triangular load distribution. This lateral soil load is expressed as an equivalent fluid pressure, with units of pounds per square foot per foot of depth (kPa/m). The maximum force on the wall depends on the total wall height, soil backfill height, wall support conditions, soil type, and the existence of any soil surcharges. For design, foundation walls are typically assumed to act as simple vertical beams laterally supported at the top and bottom.

Foundation walls also provide support for the structure above, transferring vertical loads to the footing. When foundations span vertically, this vertical compression counteracts flexural tension, increasing the wall’s resistance to flexure. In low-rise construction, these vertical loads are typically small in relation to the compressive strength of concrete masonry. Further, if the wall spans horizontally, vertical compression does not offset the flexural tension. Vertical load effects are not included in the tables and design example presented in this TEK (references 2 and 3 include vertical load effects).

EMPIRICAL DESIGN

The empirical design method uses historical experience to proportion and size masonry elements. Empirical design is often used to design concrete masonry foundation walls due to its simplicity and history of successful performance.

Table 1 lists the allowable backfill heights for 8, 10 and 12-inch (203, 254 and 305 mm) concrete masonry foundation walls. Table 1 may be used for foundation walls up to 8 feet (2.4 m) high under the following conditions (ref. 1):

terrain surrounding the foundation wall is graded to drain surface water away from foundation walls,
backfill is drained to remove ground water away from foundation walls,
tops of foundation walls are laterally supported prior to backfilling,
the length of foundation walls between perpendicular masonry walls or pilasters is a maximum of 3 times the foundation wall height,
the backfill is granular and soil conditions in the area are non-expansive,
masonry is laid in running bond using Type M or S mortar, and
units meet the requirements of ASTM C 90 (ref. 6).

Where these conditions cannot be met, the wall must be engineered using either an allowable stress design (see following section) or strength design procedure (see ref. 5).

WALL DESIGN

Tables 2 through 4 of this TEK have been rationally designed in accordance with the allowable stress design provisions of Building Code Requirements for Masonry Structures (ref. 1) and therefore meet the requirements of the International Building Code even though the latter limits reinforcment spacing to 72 in. (1829 mm) when using their tables. Additional reinforcement alternatives may be appropriate and can be verified with an engineering analysis.

Tables 2, 3 and 4 list reinforcement options for 8, 10 and 12-in. (203, 254 and 305-mm) thick walls, respectively. The effective depths of reinforcement, d, (see Table notes) used are practical values, taking into account variations in face shell thickness, a range of bar sizes, minimum required grout cover, and construction tolerances for placing the reinforcing bars.

Tables 2 through 4 are based on the following:

no surcharges on the soil adjacent to the wall and no hydrostatic pressure,
negligible axial loads on the wall,
wall is simply supported at top and bottom,
wall is grouted only at reinforced cells,
section properties are based on minimum face shell and web thicknesses in ASTM C 90 (ref. 6),
specified compressive strength of masonry, f’_m, is 1,500 psi (10.3 MPa),
reinforcement yield strength, f_y, is 60,000 psi (414 MPa),
modulus of elasticity of masonry, E_m, is 1,350,000 psi (9,308 MPa),
modulus of elasticity of steel, E_s, is 29,000,000 psi (200,000 MPa),
maximum width of compression zone is six times the wall thickness (where reinforcement spacing exceeds this distance, the ability of the plain masonry outside the compression zone to distribute loads horizontally to the reinforced section was verified assuming two-way plate action),
allowable tensile stress in reinforcement, F_s, is 24,000 psi (165 MPa),
allowable compressive stress in masonry, F_b, is ⅓f’_m (500 psi, 3.4 MPa),
grout complies with ASTM C 476 (2,000 psi (14 MPa) if property spec is used) (ref. 7), and
masonry is laid in running bond using Type M or S mortar and face shell mortar bedding.

Table 2—Vertical Reinforcement for 8 in. (203 mm) Concrete Masonry Foundation Walls

Table 3—Vertical Reinforcement for 10 in. (254 mm) Concrete Masonry Foundation Walls

Table 4—Vertical Reinforcement for 12 in. (305 mm) Concrete Masonry Foundation Walls

DESIGN EXAMPLE

Wall: 12-inch (305 mm) thick, 12 feet (3.7 m) high.

Loads: equivalent fluid pressure of soil is 45 pcf (7.07 kPa/ m), 10 foot (3.1 m) backfill height. No axial, seismic, or other loads.

Using Table 4, #8 bars at 40 in. (M 25 at 1016 mm) o.c. are sufficient.

CONSTRUCTION ISSUES

This section is not a complete construction guide, but rather discusses those issues directly related to structural design assumptions. Figures 1 and 2 illustrate typical wall support conditions, drainage, and water protection.

Before backfilling, the floor diaphragm must be in place or the wall must be properly braced to resist the soil load. In addition to the absence of additional dead or live loads following construction, the assumption that there are no surcharges on the soil also means that heavy equipment should not be operated close to basement wall systems that are not designed to carry the additional load. In addition, the backfill materials should be placed and compacted in several lifts, taking care to prevent wall damage. Care should also be taken to prevent damaging the drainage, waterproofing, or exterior insulation systems, if present.

Figure 1—Typical Base of Foundation Wall

REFERENCES

Building Code Requirements for Masonry Structures, ACI 530-99/ASCE 5-99/TMS 402-99. Reported by the Masonry Standards Joint Committee, 1999.
International Building Code. International Code Council, 2000.
Strength Design of Reinforced CM Foundation Walls, TEK 15-02B, Concrete Masonry & Hardscapes Association, 2004.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-01. American Society for Testing and Materials, 2001.
Standard Specification for Grout Masonry, ASTM C476- 01. American Society for Testing and Materials, 2001.