by Robert M. Chamra, EIT and Beth Anne Feero, EIT
Single-wythe concrete masonry walls are popular because they are inexpensive to construct, and combine structural support and cladding in one system. However, they can be associated with leakage when the waterproofing design is simplistic. A single-wythe wall can, and should, have multiple waterproofing components.1
Concrete masonry units (CMUs) are characteristically porous building materials. When manufactured in accordance with the industry standard, ASTM C90, Standard Specification for Load-bearing Concrete Masonry Units, commonly used lightweight CMUs absorb up to 17 percent of their weight in water.
This porosity is due in part to their composition. The mix for the units contains the usual concrete components of water, cement, and aggregates, but that third component will be a smaller coarse aggregate (i.e. gravel) than cast-in-place concrete. The smaller aggregate decreases the workability of the mix if all other variables are held constant. In some cases, this decrease in workability is compensated by the addition of water to the mix. Similar to cast-in-place concrete, the higher the water-to-cement (w/c) ratio in the CMU mix, the higher the permeability of the units. However, even a good-quality mix will remain permeable (Figure 1).
Furthermore, the geographical location where the CMUs are manufactured affects permeability. The types of aggregate available in different regions varies, which results in mixes with identical proportions of components, but with much different absorption. For this reason, a prescriptive approach for waterproofing CMUs cannot be applied globally. The guidelines for methods of waterproofing remain the same, but the proportions of water repellents must be tailored for the available materials.
An additional factor affecting the porosity of CMUs is the unit-forming process. After the components have been combined, the mix is compacted and vibrated in molds. If properly compacted, a large volume of the interconnected pores within the unit is eliminated. If poorly compacted, the resulting interconnected pores can provide a path for water to migrate through the unit. Even if the overall unit is compacted, extremely porous localized pockets can remain, as demonstrated in the testing described in this article.
Similarly, a CMU containing cracks will be prone to moisture migration. The curing process CMUs undergo after forming will limit shrinkage cracking within the units, but it does not prevent all subsequent shrinkage—especially when CMUs are installed immediately after manufacturing (21 days of curing is recommended). In addition to drying shrinkage, creep (i.e. time-dependent deformation) can occur in concrete masonry walls after sustained loading.2 The resulting hairline cracks from these phenomena will provide routes for water through the unit.
In addition to the units themselves, the mortar joints can provide water sources into a concrete masonry wall assembly. If the mortar loses the water it needs to complete curing—due to wind, sun, or suction from the CMUs—shrinkage cracks and separations between units and mortar will develop. Similar to the CMUs, the mortar will also undergo creep after sustained loading—up to five times as much as the CMUs—since the mortar is less stiff than the concrete.3
For waterproofing, cracks within the mortar are worse than cracks within the units, since it is common to have mortar only at the inside and outside faces of the masonry (i.e. face shell bedding). Then, water only has to travel the thickness of the unit wall, approximately 32 mm (1 1/4 in.) to penetrate the assembly (Figure 2).
National Concrete Masonry Association (NCMA) publishes technical articles to provide recommendations for the design and construction of concrete masonry. TEK 19-2B, Design for Dry Single-wythe Concrete Masonry Walls, outlines waterproofing strategies for single-wythe concrete masonry walls at the surface, within the CMU, and at the drainage path. NCMA recommends redundancy to protect concrete masonry from water penetration, including surface repellents or coatings, integral repellents (admixtures), and adequate drainage systems.4
Surface repellents for concrete masonry—typically silicones, silanes, and siloxanes—provide waterproofing at the exterior of the wall assembly. They are applied by a roller or spray equipment after the mortar has had an opportunity to cure. The product is absorbed into the units and mortar and coats the pores. While some products can penetrate deeper, most surface repellents remain within 12.7 mm (1/2 in.) of the CMU surface. In addition to their ability to repel water, surface repellents provide other benefits, such as reducing dirt and staining on the wall’s surface.
Surface repellents typically allow water vapor to be transferred in and out of the wall, and drying when water does penetrate the assembly through cracks or other penetrations.5 These products have varying ultraviolet (UV) resistance, but most need to be reapplied at intervals recommended by their manufacturers.6
Integral water repellents are available to be incorporated into CMUs as admixtures during manufacturing and into mortar during site mixing to limit water migration through the wall assembly. Since the mortar is mixed onsite and not in the unit plant, it is crucial masons also provide proper admixture quantity and mixing practices for the mortar to avoid a waterproofing weakness within the wall assembly. Integral water repellents also improve efflorescence control. Despite concerns with changes to the concrete’s properties, research has shown integral water repellents do not interfere with the assembly’s bond strength.7
Although it may seem counterintuitive, it is better to use mortar of lower strength to limit cracking.8 High-strength mortars are stiffer; they crack at a lower strain compared to low-strength mortars. Movement related to thermal and moisture changes, as well as foundation shifting, can cause cracking in strong and stiff wall assemblies. These cracks may not impair the wall’s structural performance, but all cracks add opportunities for water’s entry into the assembly.
The mortar’s installation can be as important to the mortar joints’ performance as the materials used. Proper tooling practices help protect concrete masonry walls from unwanted moisture penetration. Choosing a concave or V-joint mortar joint profile will push the mortar against the CMUs to improve bond and provide drainage when the assembly is wet. Raked joints decrease the bond between the CMU and mortar, and provide an area to trap water.9
In addition to surface repellents or coatings and integral repellents, NCMA’s other primary recommendation is to provide adequate drainage systems for moisture penetrating the wall assembly. For ungrouted assemblies, through-wall flashing can be installed at bond-beams and floor slabs. Flashing is often eliminated in fully grouted walls to avoid severing the grout which makes it important to consider supplemental waterproofing measures.
These suggestions, along with other considerations found in TEK 19-2B, are given to help ensure moisture will not penetrate the masonry. Although CMUs are characteristically permeable, they can be used successfully in single-wythe walls by following NCMA’s recommendations. Since water penetration can come from various sources, the need for a careful and comprehensive waterproofing approach is essential to providing dry and durable concrete masonry construction.
Absorption testing of 24 lightweight CMUs was performed by the authors. Half the units contained an integral water repellent. An informal droplet test was performed initially on selected CMUs from each group; then, all the CMUs underwent a RILEM tube test.10 For additional information about these test methods, see “Field Testing Methods of Water Repellency.”
The units tested were smooth-faced CMUs. Split-face blocks, with their more aesthetically appealing surfaces, would likely be even more porous because of the fracturing that creates the appearance (Figure 3).
To comply with ASTM C90, CMUs must meet maximum absorption requirements dependent on the units—the denser the unit, the less absorption the standard allows. ASTM C140, Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, outlines the absorption testing procedures to comply with ASTM C90. Each CMU in this study underwent ASTM C140 absorption testing (Figure 4).
The addition of integral water repellent to the CMUs resulted in a 34 percent reduction in absorption (and nearly 50 percent less than allowed by ASTM C90). However, these low absorption values do not correlate with water penetration through the units; the low-absorption CMUs still allowed water to penetrate during water-spray testing. The authors believe this disconnect is a leading reason for leakage in single-wythe concrete masonry walls—the industry standards for the components address absorption, rather than water penetration.
The CMUs without integral water repellent had droplet test results classified as ‘totally absorbed’—immediately after placing the droplet on the unit, the water was absorbed, but the surface remained slightly damp. For the units with the integral water repellent, the classification was ‘partially absorbed.’ Once the water was placed on the unit, some of the water was absorbed, but there was still partial beading and standing water remaining on the unit. After a five-minute period, most of the beaded water had absorbed into the units with integral water repellent and appeared the same as units without integral water repellent.
These observations show an integral water repellent can aid in preventing water from penetrating into the unit. However, the integral water repellent was not impenetrable—some water made its way into the units during the droplet tests. More importantly, there was an extreme range of absorptions on the surface of individual CMUs, which indicates porous pockets of less consolidated concrete were present as described earlier (Figure 5).
RILEM tube testing
The second procedure conducted on the concrete masonry units was RILEM tube testing. When tested using a standard 5-ml (0.16-oz) tube, all 24 specimens failed. However, units containing an integral water repellent were able to hold the water column of a short RILEM tube test for more than 20 minutes with little to no reduction in the water level, thus passing the less-severe testing method.
The units without integral water repellent quickly failed even when tested with a short RILEM tube. In a matter of one to two seconds, the entire water column had been depleted, and significant water penetration could be seen in the unit surrounding the RILEM tube and putty. These results clearly indicate the necessity for CMUs to have deliberate waterproofing components to avoid catastrophic leakage.
Medium- or normal-weight CMUs would be expected to perform better than their lightweight counterparts because research indicates water repellents’ effectiveness correlates with concrete density. This is another reason for water ingress in single-wythe concrete masonry walls—the repellents most commonly employed are least effective on lightweight CMUs. In some regions, lightweight units dominate the market despite their poor water penetration performance. This point alone indicates the benefit of using redundant waterproofing components.
Concrete masonry units are porous structural elements that need to be properly installed with appropriate components to prevent water infiltration in single-wythe exterior walls. High-quality CMUs and mortar (complying with ASTM standards), integral water repellents, and good design and construction practices (following NCMA recommendations) are important steps. However, these measures may not suffice.
Redundant waterproofing components are required because of the likelihood of cracks, mortar joint separations, and variable absorption characteristics in a single-wythe concrete masonry wall (Figure 6). The variability of available materials in a given region supports the need for tailoring the design to achieve the desired performance. Field testing during the construction phase is recommended to confirm performance. Even adding a surface-applied repellent will not stop water from migrating through cracks. An elastomeric wall coating should be considered for crack-bridging ability.11
1 The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at The University of Texas at Austin. Also, the authors thank Featherlite Building Products for donating concrete masonry units for lab testing. (back to top)
2 For more, see Failure Mechanisms in Building Construction, edited by David H. Nicastro, PE (ASCE Press, 1994). (back to top)
3 See Note 2. (back to top)
4 See NCMA’s TEK 19-2B, Design for Dry Single-wythe Concrete Masonry Walls. (back to top)
5 See NCMA’s TEK 19-1, Water Repellents for Concrete Masonry Walls. (back to top)
6 See the article, “Testing the Test: Water Absorption with RILEM Tubes,” by Adrian Gerard Saldanha and Doris E. Eichburg in the August 2013 issue of The Construction Specifier. (back to top)
7 See NCMA TEK 19-7, Characteristics of Concrete Masonry Units with Integral Water Repellent. (back to top)
8 See Note 4. (back to top)
9 See Note 4. (back to top)
Robert M. Chamra, EIT, is a project engineer with Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and monitoring the construction of the remedies. He participates in the research being performed at The Durability Lab—a testing center established by Building Diagnostics at The University of Texas at Austin (UT). He can be reached by e-mail at firstname.lastname@example.org.
Beth Anne Feero, EIT, is completing her master’s degree in architectural engineering at UT. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be reached via e-mail at email@example.com.