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Construction of Low-Rise Concrete Masonry Buildings

November 2018

INTRODUCTION

The current trend of urban renewal and infill has sparked a high volume of new low-rise masonry residences. These structures come in many forms, but quite often they employ the use of load-bearing concrete masonry walls supporting a wood floor system. These new buildings are largely derivative of the historic load bearing masonry “brownstone” or “three flat” structures of old. This guide is intended to assist contractors and architects to give this building type a modern approach to detailing.

FLOOR SYSTEM CONNECTIONS

When designing low-rise loadbearing structures, the connection detail between the floor system and the wall system is critical for achieving a watertight structure. Much of this TEK will deal with which strategy should be utilized in connecting a wood floor system to a masonry load-bearing wall. Connection methods covered are joist hangers, beam pockets and ledger beam details. Other floor systems are used in low-rise construction that are not addressed here – see 05-07A for further information (ref. 2).

BRICK AND BLOCK COMPOSITE WALL DETAILS

Quite often, the front facade of these structures is composed of brick to give the building a more residential, more human scale. One way to construct a brick and block wall is to separate the two wythes with an airspace, creating a cavity wall. Another is to use a composite wall design. The composite wall consists of an exterior wythe of brick directly mortared or grouted and tied to an inner wythe of CMU. The collar joint between the two wythes should be 100% solid as it is the only defense against water penetration. Minimum tie requirements are one tie per 22/3ft2 of wall area for W1.7 (MW11)(9 gauge) wire or one tie per 41/2ft2 of wall area using W2.8 (MW19)(3/16 in.)wire (ref. 2). A W1.7 (MW11)(9 gauge) joint reinforcement @16 in. (406 mm) on center would meet this requirement and is often used. Details covered for this system are base flashing, window head and window sill details.

EXTERIOR CONCRETE MASONRY

The use of water repellent admixtures in concrete masonry and mortars can greatly reduce the amount of water entering the masonry. In addition, they inhibit any water that penetrates the face from wicking to the back of the wall.

Proper selection and application of integral water repellents and surface treatments can greatly enhance the water resistive properties of masonry, but they should not be considered as substitutes for good fundamental design including flashing details and crack control measures. See TEKs 19-01, 19-02A, and 19-04A (refs. 6, 3, & 5) for more information on water resistant concrete masonry construction.

Because a 4 in. (102 mm) concrete masonry veneer will shrink over time, a 4 in. (102 mm) hot-dipped galvanized ladder type joint reinforcement should be placed in bed joints spaced 16 in. (406 mm) vertically.

Compared to type N or O, type S mortar tends to be less workable in the field and should only be specified when dictated by structural requirements. Sills, copings and chimney caps of solid masonry units, reinforced concrete, stone, or corrosion resistant metal should be used. Copings, sills and chimney caps should project beyond the face of the wall at least 1 in. (25 mm) and should have functional flashing and weep holes.

In addition, all sills, copings and chimney caps should have a minimum slope of 1:4, be mechanically anchored to the wall, and should have properly sized, sealed, and located movement joints when necessary.

Flashing should be installed at locations shown on the plans and in strict accordance with the details and industry standard flashing procedures. Functional, unpunctured flashing and weep holes are to be used at the base of wall above grade, above openings, at shelf angles, lintels, wall-roofing intersections, chimneys, bay windows, and below sills and copings. The flashing should be extended past the face of the wall. The flashing should have end dams at discontinuous ends, and properly sealed splices at laps.

JOIST HANGER DETAILS

The use of a joist hanger system can greatly simplify the bearing detail. The floor system does not interrupt the continuity of the bearing wall. Installation is quicker and easier resulting in a more economical installation.

BEAM POCKET DETAILS

The traditional beam pocket detail still can be effective. Stepped flashing above the bearing line is critical to the performance of this system. Without the flashing, any water present in the wall has an unobstructed path inside the building and has the potential to deteriorate the floor structure.

LEDGER BEAM DETAILS

The use of a ledger beam which is bolted to a bond beam is also a good option for this bearing condition. Through wall flashing is still required to maintain a watertight wall. Any water that penetrates the block with run down the inner cores of the block until it hits the flashing. The flashing and weep holes will allow the water to exit without damaging the structure.

PARAPETS AND WINDOW SILLS

Below are details for a parapet condition and a window sill condition. The parapet is reinforced with No. 4 bars at 48 in. (No.13M @1219 mm) on center or as required for wind resistance. If a metal cap is used, it should extend down the face of the wall at least 3 in. (76 mm) with continuous sealant at the joint on both sides of the wall. The sill detail shows the arrangement of flashing, end dam, weep holes and drip edge and how they all form a watertight

WINDOW HEAD DETAILS

These two window head details show the relationship between the steel lintel, drip edge, flashing, end dams, and weep holes. The first option shows the use of a concrete masonry lintel which is grouted solid and reinforced. The second detail shows two steel lintels used for spanning the opening.

CONTROL JOINT DETAILS

Control joints simply are weakened planes placed at approximately 20 ft. (6 m) on center in concrete masonry walls and at changes in wall elevation/thickness. Notice that the joint reinforcement is discontinuous at the joint. Cores are shown grouted adjacent to the joints as well to ensure structural stability in taller walls and/or high load situations.

COMPOSITE WALL BASE FLASHING DETAILS

Figure 14 shows a stair-stepped ﬂashing detail with the exposed drip edge and weep holes. Figure 15 shows a straight through wall ﬂashing detail. The ﬂashing must be set in mastic on top of the concrete foundation, or the ﬂashing must be self adhesive. The ﬂashing should be turned up on the inner side of the wall to direct water to the outside of the wall.

COMPOSITE WALL WINDOW DETAILS

Here steel lintels back-to-back create the above window span. Stepped ﬂashing turned up on the inside, and folded to form an end dam protects the head condition from moisture. The sill detail also uses ﬂashing, end dams and weep holes to keep moisture out of the wall. The use of a precast concrete or stone sill is highly suggested over using brick rowlock sills.

CONCRETE MASONRY VENEER DETAILING

Figure 18 shows the detailing of a 4 in. (102 mm) concrete masonry veneer used in conjunction with a 8 in. (205 mm) CMU backup wall.

Three types of joint reinforcement are shown including tri-rod, tab and adjustable types. It is imperative that the veneer have a continuous wire embedded in every other course to control movement. With the tri-rod system, the joint reinforcement satisﬁes this requirement. With the other two systems, an additional ladder type joint reinforcement is used to provide this movement control for the veneer.

REFERENCES

Building Code Requirements for Masonry Structures,
ACI 530-05/ASCE 6-05/TMS-402-05. Reported by the
Masonry Standards Joint Committee, 2005.
Floor and Roof Connections to Concrete Masonry
Walls, TEK 05-07A, Concrete Masonry & Hardscapes
Association, 2001.
Design for Dry Single-Wythe Concrete Masonry
Walls, TEK 19-02B, Concrete Masonry & Hardscapes
Association, 2004.
Flashing Details for Concrete Masonry Walls, TEK 19-05A,
Concrete Masonry & Hardscapes Association, 2004.
Flashing Strategies for Concrete Masonry Walls, TEK 19-
04A, Concrete Masonry & Hardscapes Association, 2003.
Water Repellents for Concrete Masonry Walls, TEK 19-01,
Concrete Masonry & Hardscapes Association, 2002.

Design for Dry Single-Wythe Concrete Masonry Walls

August 2018

INTRODUCTION

Single-wythe concrete masonry walls are cost competitive because they provide structural form as well as an attractive and durable architectural facade. However, because they do not have a continuous drainage cavity (as do cavity and veneered walls), they require special attention to moisture penetration.

The major objective in designing dry concrete masonry walls is to keep water from entering or penetrating the wall. In addition to precipitation, moisture can find its way into masonry walls from a number of different sources (see Figure 1). Dry concrete masonry walls are obtained when the design and construction addresses the movement of water into, through, and out of the wall. This includes detailing and protecting building elements including parapets, roofs, all wall penetrations (utility and fire protective openings, fenestration, doors, etc.), movement joints, sills and other features to resist water penetration at these locations. Annotated Design and Construction Details for Concrete Masonry (ref. 1) contains comprehensive details for reinforced and unreinforced concrete masonry walls. Further, condensation and air leakage must be controlled. See the Condensation Control section on page 7.

The primary components of moisture mitigation in concrete masonry walls are flashing and counter flashing, weeps, vents, water repellent admixtures, sealants (including movement joints), post-applied surface treatments, vapor retarders and appropriate crack control measures. For successful mitigation, all of these components should be considered to be used redundantly, however not all will be applicable to all wall systems. For example, flashing and weeps are not necessary in solidly grouted construction, and may not be appropriate in areas of high wind or seismic loading where compromise of masonry shear resistance may occur (see the Wall Drainage section on page 3 for more information). The determination on structural effect must be made by the structural engineer. As another example, the use of integral water repellents for surfaces to receive a stucco finish may not be appropriate. Successful design for moisture mitigation considers each of these components, and provides for redundancy of protection, also known as a “belt and suspenders” approach.

This TEK provides a brief overview of the issues to consider when designing single wythe walls for water penetration resistance. The information presented is not meant to be comprehensive. Where appropriate, references to more detailed sources are provided.

SOURCES OF WATER IN WALLS

Driving Rain

Although concrete masonry units and mortar generally do not allow water to pass through quickly, rain can pass through if driven by a significant force. Cracks caused by building movements, or gaps between masonry and adjoining building elements are common points of water entry. If rain enters wall other than by way of the roof or at element interfaces (such as penetrations and window openings), it often can be traced to the masonry unit-mortar interface.

Capillary Action

Untreated masonry materials (without a compatible integral water repellent and/or post-applied surface treatment) typically take on water through absorption, adsorption and/or capillary forces. The amount of water depends on the characteristics of the masonry and mortar. Integral water repellents greatly reduce the absorption and adsorption characteristics of the units and mortar, but may not be able to prevent all moisture migration if there is a significant head pressure of approximately 2 in. water (51 mm) or more. Post-applied surface treatments reduce moisture penetration of masonry at the treated surface as well, but have little effect on the interior of the units.

Water Vapor

Water as vapor moves through a wall either via air leakage or by diffusion (from higher to lower: relative humidity, pressure and/or temperature). As air cools, it becomes more saturated, and when it reaches the dew point temperature the water vapor will condense into liquid form. See the Condensation Control section on page 7 for more information.

Ground Water

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 (location of finished grade as well as grade sloping away from the building). Landscaping can also contribute to water ponding adjacent to the foundation wall and/or to insufficient drainage. IBC Section 1805 contains requirements for dampproofing and water proofing foundations. More detailed information for concrete masonry foundation walls can be found in Preventing Water Penetration in Below Grade CM Walls, TEK 19-03B (ref. 2).

DESIGN CONSIDERATIONS

When designing for moisture mitigation in walls, three levels of defense should be considered: surface protection (properly constructed mortar joints, surface water repellents, surface coatings), internal protection (integral water repellents), and drainage/drying (flashing, weeps, vents). The most successful designs often provide redundancy among these three levels. This redundant design approach helps ensure that the wall remains free of moisture problems even if one of the defense mechanisms is breached. Flashing and weeps, for example, provide a backup in case surface coatings are not reapplied as needed or leaks develop around windows or other openings. The following sections discuss the individual mechanisms in more detail.

Physical Characteristics of the Units

Open-textured concrete masonry units possessing large voids tend to be more permeable than closed-textured units. The texture can be affected by aggregate gradation, water content of the concrete mix, amount of cement in the mix, other materials in the mix such as admixtures, and the degree of compaction achieved during molding. These factors can also affect capillary action and vapor diffusion characteristics. Units should be aged at least 21 days if possible before installation to reduce the chance of shrinkage cracks at the mortar-unit interface.

Smooth-faced units facilitate mortar joint tooling, so will generally result in a more water resistant wall, as opposed to fluted units which are more difficult to tool and therefore the most susceptible to leakage. Horizontal effects such as corbels and ledges that may hold water are more prone to water penetration.

Integral Water Repellents

The use of integral water repellents in the manufacture of concrete masonry units can greatly reduce the wall’s absorption characteristics. When using units with an integral water repellent, the same manufacturer’s water repellent for mortar must be incorporated in the field for compatibility and similar reduced capillary action characteristics.

Integral water repellents make masonry materials hydrophobic, significantly decreasing their water absorption and wicking characteristics. While these admixtures can limit the amount of water that can pass through units and mortar, they have little impact on moisture entering through cracks and voids in the wall. In addition, when using an integral water repellent, any water that does penetrate can not exit as easily. Therefore, even with the incorporation of integral water repellents, flashing and weeps, as well as proper detailing of control joints and quality workmanship are still essential. See Water Repellents for Concrete Masonry Walls, TEK 19-01 (ref. 3), and Characteristics of CMU with Integral Water Repellent, TEK 19-07 (ref. 4), for more complete information on integral water repellents for concrete masonry walls.

Post-Applied Surface Treatments

For integrally colored architectural masonry, a clear surface treatment should be post-applied whether or not integral water repellent admixtures are used. Most post-applied coatings and surface treatments are compatible with integral water repellents although this should be verified with the product manufacturers before applying. When using standard units for single-wythe walls, application of a clear treatment, portland cement plaster (stucco), paint, or opaque elastomeric coating improves the water resistance of the wall. Coatings containing elastomerics have the advantage of being able to bridge small gaps and TEK 19-02B 3 CONCRETE MASONRY & HARDSCAPES ASSOCIATION masonryandhardscapes.org cracks. See Water Repellents for Concrete Masonry Walls, TEK 19-01 (ref. 3) for more detailed information.

Wall Drainage

In areas with high seismic loads, masonry walls tend to be heavily reinforced and it is often more economical to fully grout the masonry. In fully grouted masonry, flashing is not necessary. In these cases, the wall is designed as a barrier wall, rather than as a drainage wall.

When flashing is used, the importance of proper detailing cannot be over-emphasized. Traditionally, through-wall flashing has been used to direct water away from the inside wall face and toward weep holes for drainage. Figure 2 shows one example of flashing that spans completely across the width of the wall. In this example, the termination angle prevents any water that collects on the flashing from penetrating to the interior, and the weeps and drip edge drain water to the exterior.

Where it is necessary to retain some shear and flexural resistance capabilities, there are several options. One is to terminate the flashing within the inside face shell of the wall, as shown in Figure 3. In reinforced walls, some shear is provided through doweling action of the reinforcement, and by design the reinforcement takes all tension (refs. 5, 6). Proper grouting effectively seals around where the vertical reinforcement penetrates the flashing. The absence of reinforcement to provide doweling in plain masonry may be more of a concern, but loads tend to be relatively low in these applications. If structural adequacy is in doubt, a short reinforcing bar through the flashing with cells grouted directly above and below can be provided as shown in Figure 3c.

A better option to maintain shear at the level of the flashing is to use a product that maintains some bond in both face shells, such as that shown in Figure 4.

Ensuring that a buildup of mortar droppings does not clog the cells or weep holes is critical. Traditionally, a cavity filter consisting of washed pea stone or filter paper immediately above the flashing was provided to facilitate drainage, as shown in Figure 3. This should be accompanied by a means of intercepting or dispersing mortar droppings, as an accumulation can be sufficient to completely fill and block a cell at the bottom. As an alternative, mortar interception or isolation devices that provide pathways for the water to migrate through the layer of mortar droppings, or filling the cells with loose fill insulation a few courses at a time as the wall is laid up, can disperse the droppings enough to prevent clogging. Examples of polyester mesh drainage mats are shown in Figures 4 and 5. Another alternative is to leave out facing block at regular intervals just above the flashing until the wall is built to serve as cleanouts. The units left out can be mortared in later. See Flashing Strategies for Concrete Masonry Walls, TEK 19-04A and Flashing Details for Concrete Masonry Walls, TEK 19-05A, (refs. 7, 8) for an in-depth discussion and additional details regarding flashing.

In addition to conventional flashing systems, proprietary flashing systems are available that direct the water away from the inside face of the wall to weep holes without compromising the bond at mortar joints in the face shells. See Figure 4 for one example. These are not intended to be comprehensive, but rather to provide examples of some types of available systems. Specialty units that facilitate drainage are also available from some manufacturers.

Solid grouted single-wythe walls do not require flashing because they are not as susceptible to moisture penetration, since voids and cavities where moisture can collect are absent. However, fully cured units and adequate crack control measures are especially important to minimize cracks. In some regions of the country, the bottom of the wall is recessed about 1 in. (25 mm) below the floor level to ensure drainage to the exterior.

Figure 2—Three-Piece Through-Wall Flashing

Figure 3—Flashing Details to Maintain Structural Continuity

Figure 4—Pan Flashing with Integral Weeps

Figure 5—Stainless Steel Strip with Integral Weeps

Crack Control

Because cracks provide an entry point for rainwater and moist air, crack control provisions are very important in producing dry walls. There are various sources of potential wall cracking. A detailed list, as well as an overview of crack control strategies, can be found in Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23 (ref. 9).

Control joints and/or horizontal reinforcement should be located and detailed on the plans to alleviate cracking due to thermal and shrinkage movements of the building. Specifying a quality sealant for the control joints and proper installation is a must to maintain the weather-tightness of the joint. Joint Sealants for Concrete Masonry Walls, TEK 19-06A (ref. 10) contains more comprehensive information on this topic. See Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23 (ref. 11) for detailed information on control joint placement and construction.

Mortar and Mortar Joints

The type of mortar and type of mortar joint can also impact a wall’s watertightness. A good rule of thumb is to select the lowest strength mortar required for structural and durability considerations. Lower strength mortars exhibit better workability and can yield a better weather-resistant seal at the mortar/unit interface. See Mortars for Concrete Masonry, TEK 09-01A (ref. 12), for a more complete discussion.

Unless otherwise specified, mortar joints should be tooled to a concave profile when the mortar is thumbprint hard (refs. 5, 13), as shown in Figure 6. For walls exposed to weather, concave joints improve water penetration resistance by directing water away from the wall surface. In addition, because of the shape of the tool, the mortar is compacted against the concrete masonry unit to seal the joint. V-shaped joints result in sharper shadow lines than concave joints. Raked, flush, struck, beaded, grapevine, squeezed or extruded joints are not recommended in exposed exterior walls as they do not compact the mortar and/or they create ledges that intercept water running down the face of the wall.

Head and bed joints should be the full thickness of the face shells for optimum water resistance. Head joints are particularly vulnerable to inadequate thickness (see Figure 7).

Figure 7—Mortar Joints Should Be the Full Thickness of the Face Shells

Condensation Control

Condensation is a potential moisture source in building assemblies. Because condensation potential varies with environmental conditions, seasonal climate changes, the construction assembly, building type and building usage, condensation control strategies vary as well. For a full discussion, see Condensation Control in Concrete Masonry Walls, TEK 06-17B, and Control of Air Leakage in Concrete Masonry Walls, TEK 06-14A (refs. 14, 15).

Note that the location and vapor permeability of insulation can influence the condensation potential of a wall. The following references provide more detailed information. Insulating Concrete Masonry Walls, TEK 06-11A (ref. 16), discusses various insulation strategies and the advantages and disadvantages of each. R-Values and U-Values for Single Wythe Concrete Masonry Walls, TEK 06-02C, and Thermal Catalog of Concrete Masonry Assemblies (refs. 17, 18) provide calculated thermal values of various walls and insulation types. Details for Half-High Concrete Masonry Units, TEK 05-15 (ref. 19), contains comprehensive details of various single wythe walls.

Cleaning

Concrete masonry cleaning methods can generally be divided into four categories: hand cleaning, water cleaning, abrasive cleaning and chemical cleaning. In general, the least aggressive method that will adequately clean the wall should be used, as overzealous cleaning can damage the water repellent characteristics of the wall. Keeping the masonry wall clean as the construction progresses using a brush and water minimizes cleaning efforts after the mortar has hardened. See Cleaning Concrete Masonry, TEK 08-04A (ref. 20) for more detailed information.

SPECIFICATIONS

Well-worded specifications are essential to ensure the design details are properly constructed. Items to address in the contract documents in addition to those previously mentioned include:

All work to be in accordance with the International Building Code and Specification for Masonry Structures (refs. 5, 13).
Require a qualified mason by documentation of experience with similar type projects.
Require sample panels to assure an understanding of the level of workmanship expected and to be used as a standard of reference until the project is completed.
Proper storage of all masonry materials (including sand) at the job site to protect from contaminants such as dirt, rain and snow.
The tops of unfinished walls shall be covered at the end of each work day. The cover should extend 2 ft (610 mm) down each side of the masonry and be held securely in place.

REFERENCES

Annotated Design and Construction Details for Concrete Masonry, TR 90. National Concrete Masonry Association, 2002.
Preventing Water Penetration in Below-Grade CM Walls, TEK 19-03B, Concrete Masonry & Hardscapes Association, 2012.
Water Repellents for Concrete Masonry Walls, TEK 19-01, Concrete Masonry & Hardscapes Association, 2006.
Characteristics of CMU with Integral Water Repellent, TEK 19-07, Concrete Masonry & Hardscapes Association, 2008.
International Building Code. International Code Council, 2012.
Building Code Requirements for Masonry Structures, TMS 402-11/ACI 530-11/ASCE 5-11, reported by the Masonry Standards Joint Committee, 2011.
Flashing Strategies for Concrete Masonry Walls, TEK 1904A, Concrete Masonry & Hardscapes Association, 2008.
Flashing Details for Concrete Masonry Walls, TEK 19-05A, Concrete Masonry & Hardscapes Association, 2008.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Joint Sealants for Concrete Masonry Walls, TEK 19-06A, Concrete Masonry & Hardscapes Association, 2014.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Mortars for Concrete Masonry, TEK 09-01A, Concrete Masonry & Hardscapes Association, 2004.
Specification for Masonry Structures, TMS 602-11/ACI 530.1-11/ASCE 6-11, reported by the Masonry Standards Joint Committee, 2011.
Condensation Control in Concrete Masonry Walls, TEK 06-17B, Concrete Masonry & Hardscapes Association, 2011.
Control of Air Leakage in Concrete Masonry Walls, TEK 06-14A, Concrete Masonry & Hardscapes Association, 2011.
Insulating Concrete Masonry Walls, TEK 06-11A, Concrete Masonry & Hardscapes Association, 2010.
R-Values and U-Values for Single Wythe Concrete Masonry Walls, TEK 06-2C, Concrete Masonry & Hardscapes Association, 2012.
Thermal Catalog of Concrete Masonry Assemblies, CMU-MAN-004-12, Concrete Masonry & Hardscapes Association, 2012.
Details for Half-High Concrete Masonry Units, TEK 05-15, Concrete Masonry & Hardscapes Association, 2010.
Cleaning Concrete Masonry, TEK 08-04A, Concrete Masonry & Hardscapes Association, 2005.

Water Repellents for Concrete Masonry Walls

August 2018

INTRODUCTION

Water repellents are used on exterior walls to provide resistance to wind-driven rain. In addition, water repellents can also reduce the potential for efflorescence and staining from environmental pollutants, and enhance the color or texture of a wall.

When applied in accordance with manufacturer’s recommendations, water repellents effectively control water penetration. Water repellents are generally recommended for use on single wythe concrete masonry walls exposed to the weather. The choice of water repellent will depend on the surface to be protected, the exposure conditions, and on aesthetics. A wide variety of water repellents is available, offering many choices of color, surface texture, glossiness, and application procedures.

WATER RESISTANCE

Water penetration resistance of concrete masonry walls is dependent on wall design, design for differential movement, workmanship, wall maintenance, and the application of water repellents. This TEK focuses on water repellent products for above grade walls. The other factors are discussed in CMUTEC-009-23, TEKs 19-04A and 19-05A (refs 3, 5, and 4).

The effectiveness of water repellents can be evaluated in several ways. In the laboratory, Standard Test Method for Water Penetration and Leakage Through Masonry, ASTM E 514 (ref. 9), is currently the only standard test method for water penetration. The test simulates 51/2 in. (140 mm) of rain per hour with a 62.5 mph (101 km/h) wind for a duration of 4 hours. This test is often used to evaluate water penetration before and after application of a water repellent, or to judge the relative performance of several water repellent systems.

TYPES OF WATER REPELLENTS

There are two general types of water repellents: surface treatment repellents and integral water repellents. Surface treatment repellents are applied to the weather-exposed side of the wall after the wall is constructed. In addition to water repellency, surface treatment repellents also improve the stain resistance of the wall, by preventing dirt and soot from penetrating the surface, causing deep stains.

When used on new construction, choose water repellents that are able to resist the alkalinity of the fresh mortar. As an alternative, an alkali-resistant fill coat can be applied to the wall first, or the wall can be allowed to weather for about six months until the alkalinity is reduced.

In general, surface treatment repellents should allow for vapor transmission to ensure that interior humidity within the wall and structure can escape. Treatments which are impermeable to water vapor tend to fail by blistering and peeling when moisture builds up behind the exterior surface.

When choosing a surface treatment repellent, manufacturer’s guidelines should be consulted regarding appropriate substrates and applications for a particular product.

Regardless of the type of surface treatment chosen, it should be applied to a sample panel or on an inconspicuous part of the building to determine the appearance, application method, application rate, and compatibility with the masonry surface. Surface treatment repellents will require reapplication after a period of years to ensure continuous water repellency.

Integral water repellents are added to the masonry materials before the wall is constructed. The water repellent admixture is incorporated into the concrete mix at the block plant. This way, each block has water repellent throughout the concrete in the unit. For mortar, the water repellent is added to the mix on the jobsite. It is critical when using integral water repellents that the repellent is incorporated into both the block and the mortar to ensure proper performance of the wall.

The following sections describe in more detail the characteristics of various generic surface treatment repellents and integral water repellents.

SURFACE TREATMENT REPELLENTS

Cementitious coatings:

Coatings such as stucco or surface bonding mortar can be used to increase the water resistance of a wall, as well as to significantly change the texture of the finished wall surface. Consideration should be given to differential movement which may transmit stress into the coating. Further information on stucco is found in TEK 09-03A (ref. 8).

Paints:

Paints are colored opaque coatings, used when color uniformity of the wall is important for aesthetic reasons. Paints are a mixture of pigment, which hides the surface, and resin, which binds the pigment together. The proportion of pigment to resin, and the type of resin will affect the fluidity, gloss, and durability of the paint.

The pigment volume concentration (PVC) compares the amount of pigment in a paint to the amount of binder. As the PVC increases, the paint has more pigment and less binder. High PVC coatings are used where limited penetration is desired, such as for fill coats on porous materials. High PVC paints generally brush on easier, have greater hiding power, and usually cost less than low PVC paints. Low PVC paints are generally more flexible, durable, washable, and are glossier.

Fill Coats:

Fill coats, also called primer-sealers or fillers, are sometimes used to smooth out surface irregularities or fill small voids before application of a finish coat. Common fill coats include latex coatings and portland cement. In addition, acrylic latex or polyvinyl acetate is sometimes combined with portland cement for use as a fill coat. Fill coats should be scrubbed vigorously into the masonry surface using a relatively short stiff fiber brush.

Cement-Based Paints:

Cement-based paints contain portland cement as the binder, which creates a strong bond to the masonry and is not subject to deterioration from alkalis. Cement-based paints effectively fill small voids so that large amounts of water are repelled. Durability is excellent.

Cement-based paints are sold either premixed, or in dry form and mixed with water just before use. They should be applied to a damp surface using a stiff brush, and kept damp for 48 to 72 hours, until the cement cures. If the cement-based paint is modified with latex, however, wet curing is not necessary. White and light colors tend to be the most satisfactory.

Latex Paints:

Latex paints are water-based, with any one of several binder types. They are inherently resistant to alkalis, have good hiding characteristics, and are durable and breathable, making them a good choice for concrete masonry walls. Butadiene-styrene paints and polyvinyl acetate emulsion paint are both categorized as latex paints. Latex paints can be applied to either damp or dry surfaces, and dry quickly, usually within 1 to 1 ½ hours. They are generally inexpensive and easy to apply by brush, roller, or spray.

Alkyd Paints:

Alkyd paints are durable, flexible, have good gloss retention, are low in cost, but have low alkali resistance. They should be sprayed on, since they tend to be difficult to brush apply. They dry quickly once applied.

Clear Surface Treatment Repellents:

Clear treatments are used to add water resistance to a wall without altering the appearance. These treatments are classified by the resin type, such as silicone or acrylic.

Clear treatments can be classified as either films or penetrant repellents. Penetrant repellents are absorbed into the face of the masonry, lining the pores. They adhere by forming a chemical bond with the masonry. Penetrant repellents do not bridge cracks or voids, so these should be repaired prior to applying the treatment. Silanes and siloxanes are penetrant repellents. Films, such as acrylics, form a continuous surface over the masonry, bridging very small cracks and voids. Because of this, films can also reduce the vapor transmission of a concrete masonry wall. Films tend to add a glossier finish to the wall surface, and may intensify the substrate color.

Silicones: Silicones can be further subdivided into silicone resins, silanes, and siloxanes. These treatments change the contact angle between the water and the pores in the face of the masonry, so that the masonry repels water rather than absorbing it. Silicones have been found to reduce the occurrence of efflorescence on concrete masonry walls.

Silicone resins: These are the most widely used silicone-based water repellents for masonry. They can penetrate the surface of masonry very easily, providing excellent water repellency. Silicone resins should be applied to air dry surfaces, and are usually fully dry after 4 to 5 hours.

Silanes: Like silicone resins, silanes have good penetration characteristics. Although volatility of silane has been a concern, the absorption of silane by masonry generally occurs at a much faster rate than evaporation of the silane. Silanes, unlike silicone resins, can be applied to slightly damp surfaces.

Siloxanes: Siloxanes have the benefits of silanes, i.e., good penetration and ability for application on damp surfaces. Siloxanes are effective on a wider variety of surfaces than silanes, and dry relatively quickly. Costs are comparable to silanes, and are slightly higher than silicone resins.

Acrylics: Acrylics form an elastic film over the surface of masonry to provide an effective barrier to water. Acrylics dry quickly and have excellent chalk resistance. Acrylics should be applied to air-dry masonry surfaces. Costs tend to comparable to silicone resins.

OTHER TREATMENTS

Epoxy, Rubber, and Oil-Based Paints:

These paints form impervious moisture barriers on concrete masonry surfaces. This makes for an excellent water barrier, but does not allow the wall to breathe. As such, these paints are generally not considered water repellents. These treatments are better limited to interior walls, since they can blister and peel when used on exterior walls.

Oil-based paints adhere well to masonry, but are not particularly resistant to alkalis, abrasion, or chemicals. Rubber and epoxy paints offer high resistance to chemicals and corrosive gases, and are generally used in industrial applications.

APPLICATION OF SURFACE TREATMENT REPELLENTS

This section contains some general guidelines for application of surface treatments. In all cases, refer to manufacturers’ literature for final recommendations and procedures. Surface treatments should typically be applied to clean, dry walls. Wall surfaces should be cleaned in accordance with manufacturer’s instructions to ensure good adhesion and penetration. The wall should be allowed to dry for 3 to 5 days between cleaning or rain and application of the repellent. All cracks and large voids should be repaired prior to applying the repellent. If caulk is used in the repair, the caulk should be compatible with the surface treatment repellent and fully cured before treatment application.

Weather can have a significant effect on the application and curing of water repellents. It is usually recommended that the repellent be applied when temperatures are expected to remain above 40°F (4 °C) during the two to four days after application. There should be little or no wind during sprayon applications, to avoid an uneven coating and drift of the treatment onto other materials. Adjacent landscaping should be protected during application, and, depending on the surface treatment, it may also be necessary to protect other building materials, such as aluminum or glass.

Most manufacturers recommend applying clear surface treatments using a saturating flood coat, with a 6 to 8 in. (152 to 203 mm) rundown below the contact point of the spray. It is sometimes recommended that a second coat be applied when the first is still wet. Coverage rates vary from 75 to 200 ft²/gallon (1841 to 4908 m²/m³) depending on the surface treatment repellent used and the type and condition of the masonry.

When applying a water repellent over a previously treated wall, ensure that the new treatment is compatible with the old. With some surface treatments, masonry should be uncoated for proper adhesion. In these cases, the old treatment can be allowed to weather off, or, if time does not permit this, a pressurized wash followed by high pressure water rinse can remove previous surface treatments from masonry.

The durability of a coating is a function of the type of coating, the application procedure, the rate of application, the surface preparation, and the exposure conditions. For this reason, it is difficult to predict how the various surface treatment repellents will perform under field conditions.

INTEGRAL WATER REPELLENTS

Integral water repellents are usually polymeric products incorporated into the masonry products prior to construction. Because integral water repellents are evenly distributed throughout the wall, they do not change the finished appearance. In addition, integral water repellents are effective at reducing efflorescence, since water migration throughout the block is reduced.

As stated earlier, it is essential that an integral water repellent admixture be incorporated into the mortar at the jobsite, as well as into the block and any other masonry wall components, such as precast lintels. The same brand of water repellent admixture should be used in the mortar as was used in the block, to ensure compatibility and bond.

Questions often arise regarding the effect of integral water repellents on mortar bond strength, due to the decreased water absorption. Research has shown that bond strength is primarily influenced by the mechanical interlock of mortar to the small voids in the block.

When walls containing integral water repellents are grouted, the grout produces a hydrostatic pressure which forces water into the surrounding masonry unit, allowing proper curing of the grout.

Generally, the use of other admixtures in conjunction with integral water repellents is not recommended. Some other admixtures, especially accelerators, have been shown to reduce the effectiveness of integral water repellents.

Some integral water repellents are soluble when immersed in water for long periods of time. Conditions which allow standing water on any part of the wall should be avoided. For this reason, mortar joints should be tooled, rather than raked. In addition, walls incorporating integral water repellents should not be cleaned with a high-pressure water wash.

REFERENCES

Clark, E. J., Campbell, P. G., and Frohnsdorff, G., Waterproofing Materials for Masonry. National Bureau of Standards Technical Note 883. U. S. Department of Commerce, 1975.
Clear Water Repellents for Above Grade Masonry, Sealant, Waterproofing, and Restoration Institute, 1990.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Flashing Strategies for Concrete Masonry Walls, TEK 1904A, Concrete Masonry & Hardscapes Association, 2008.
Flashing Details for Concrete Masonry Walls, TEK 19-05A, Concrete Masonry & Hardscapes Association, 2008.
Fornoville, L., Water Repellent Treatment of Masonry, Proceedings of the Fourth Canadian Masonry Symposium, University of New Brunswick, Canada, 1986.
McGettigan, E., Application Mechanisms of Silane Waterproofers, Concrete International, October 1990.
Plaster and Stucco For Concrete Masonry, TEK 09-03A. Concrete Masonry & Hardscapes Association, 2002.
Standard Test Method for Water Penetration and Leakage Through Masonry, ASTM E 514-05a. ASTM International, 2005.