Concrete Block Density102.8 pcf (lightweight)
Concrete Block Coarse AggregatePumice, Scoria, and Expanded Shale
Mortar Type (per ASTM C270)Type S
Mortar Cement TypePortland Cement
Mortar Batching MethodPre-Blended, Performance Specification

Since ACI Committee 531published “Building Code Requirements for Concrete Masonry” in 1978, describing working stress structural design of masonry, relatively slender reinforced concrete block walls have emerged as a popular, economic, and reliable exterior wall system (Drysdale and Hamid 2008).

Single-wythe concrete block construction has become a competitor to tilt-up and precast concrete wall panels, especially in warehouse and light industrial construction where an effort is usually made to minimize exterior envelope cost. In recent years, single-wythe concrete block has become a relatively common system in retail and big box construction, especially as integrally colored, split faced block units have become more common.

With the use of single-wythe construction in retail applications, the practice of applying interior finishes such as insulation and gypsum wallboard to the interior surface of the single-wythe block assembly has become more common. With limitations on public funding and often overwhelming demands on public space, using single-wythe block construction has even extended into institutional construction, again often incorporating interior finishes.

The broader use of single-wythe concrete block has taken advantage of the economy and aesthetic qualities of modern concrete block. However, the long-term weather resistance of the single-wythe system in these new applications does not have the established track record of masonry cavity wall veneer systems and similar redundant exterior envelope systems.

Cases have emerged where interior wallboard finishes have been damaged by moisture infiltrating through exterior single-wythe concrete block walls or where exposed interior block surfaces have shown dampness and efflorescence associated with moisture infiltration.

While limited moisture infiltration through single-wythe concrete block systems is not new or unexpected, the consequences of moisture infiltration have changed as the system has moved into retail and institutional settings. What was hardly noticed in a warehouse may no longer be acceptable in a library or courthouse.

So can single-wythe construction be safely used in these applications? What about using lightweight block with porous aggregate? If so, what must be done to ensure acceptable weather resistance? While far from a comprehensive study of single-wythe block weather resistance, the following begins to address these questions.

Purpose of Testing

Testing was sponsored by the Rocky Mountain Masonry Institute (RMMI), Basalite Concrete Products, LLC., Boral Best Block, JVS Masonry Inc., and Texas Industries Inc. (TXI). The purpose was to determine whether or not a single-wythe concrete block wall could provide acceptable weather resistance for retail and institutional applications using common materials and practices. The objective also was to evaluate performance of a system that does not forsake the essential benefits of the modern single-wythe block system: economy and aesthetics.

Specifically, the test included lightweight concrete block, which is the most common and economical type of block in Colorado. The test was intended to represent the types of materials and construction methods that would commonly be used for applications in the local market.

A summary of the masonry material properties used in the study is provided in Table 1. Local aggregate, materials, and methods will vary significantly in different regions, so we encourage similar testing using other regional materials. In fact, as the practice of mockup construction and water testing becomes more prevalent for window, door, balcony, and flashing construction, it is not unreasonable to consider preconstruction water testing of single wythe wall systems on a project-by-project basis where moisture infiltration at the single wythe system is important, due to inherent variability in units, geometry, construction practice, integral water repellants, and surface-applied penetrating water repellants.

Table 1. Masonry material properties for tested wall.

Test Setup

The test panel was constructed using ungrouted standard modular (8x8x16-inch nominal) lightweight split faced concrete block. The block aggregate was specifically chosen as a worst case scenario for moisture penetration, including expanded shale, scoria, and pumice lightweight aggregate. Both the block and mortar contained an integral water repellant (IWR). The panel was constructed and tested in accordance with ASTM E514, “Standard Test Method for Water Penetration and Leakage Through Masonry.”

A photograph of the subject wall is shown in Figure 2 and the test setup is shown in Figure 3. This test involves wetting a 4x3-foot area while pressurizing the outside surface using a chamber to simulate wind pressure. The test is very severe, simulating a 62.5 mph wind and sheeting rain (3.4 gal/ft2/h) for at least four hours.

The ASTM E514 test is designed to determine two different rates of moisture infiltration: moisture that reaches the interior face of the wall (Ri) and moisture that is collected by the hollow CMU cells but doesn’t reach the interior wall surface (Rc). We also recorded the rate of all moisture infiltration past the surface of the wall, including both of the items above plus moisture absorbed by the masonry units (Rs). The Rs value can be compared to results of the field adaptation of this test, ASTM C1601, if both are performed using the same parameters and water penetrating past the face of the block is recorded in the laboratory test.

Testing of the panel was performed at three times during construction:

1. About 28 days after the block was laid, before cleaning of the wall.
2. After the wall had been cleaned using aggressive power washing with a high pressure nozzle with the tip within 1 foot of the masonry surface.

Time of TestRate of Moisture Penetration Past Wall Surface, Rs (gal/h/ft2)Rate of Moisture Penetration Past Wall Surface, Rs (l/min)
Prior to Cleaning.007.005
After Cleaning.025.019
After PWR Application.003 .002

3. After applying an RTV silicone penetrating water repellant.

It should be noted that the tested panel did not have significant visible debonding or cracking at mortar joints. Early drying shrinkage of mortar resulting in separation of the mortar from the adjacent unit can dramatically increase moisture penetration through single-wythe block walls. An attempt to understand the causes of this type of debonding is the subject of a separate research project in cooperation with Dr. Jennifer Tanner at the University of Wyoming.

Test Results

The water resistance of the wall assembly during all three tests was excellent. The amount of water collected from the block cells and the amount of water collected from the interior wall face were not enough to measure (i.e. Ri and Rc ˜ 0). No damp spots or only small damp patches were visible on the interior face of the wall after four hours of testing in every case (Figure 4).

There were some changes in the rate of moisture penetration past the wall surface (Rs) for the three tests. The total volume of water penetrating past the wall surface over time for all three tests is shown in Figure 5. The rates of moisture infiltration past the wall surface are summarized in Table 2:

Table 2. Summary of moisture penetration rates.

While all of the rates are very low, moisture penetration past the wall surface increased slightly after aggressive power washing and decreased to a level below the pre-cleaned condition after installation of the penetrating water repellant.


In evaluating the results, the question, how much is too much, emerges That is, what rates of moisture penetration through the surface, at the cores, and at the interior surface, are acceptable? While this appears to be a relatively simple question, there is little published guidance to help provide an answer.

The current version of both ASTM E514 and C1601 provide no guidance regarding acceptable levels of moisture penetration. The 1974 version of ASTM E514 was the last version of the test method that provided a grade or rank based on the moisture penetration rates.

However, these rates were not specifically intended for use in single-wythe concrete block walls, and even if single-wythe block walls were considered in the performance classifications, the application of single-wythe concrete block has changed dramatically since 1974. The highest grade (E) in this standard requires that no dampness be visible at the interior face of the wall after 24 hours of continuous testing.

The relatively new standard for testing cavity drainage, ASTM C1715, lists an expected moisture penetration rate (Ri) of 0.11 gal/h/ft2 that appears to be based on clay brick masonry veneer construction. However, this penetration rate is clearly associated with a different type of construction (cavity wall).

We have anecdotally observed single-wythe concrete block walls with interior finish damage with surface penetration (Rs) rates ranging from 0.24 to 2.40 gal/h/ft2. We do not have a sufficient range of test conditions to establish an appropriate maximum or range of maximum penetration values based on our experience alone. However, the rates of infiltration on the tested single-wythe wall are roughly two orders of magnitude less than problematic conditions that we have encountered. This strongly indicates that the moisture penetration of the tested single-wythe wall would not cause distress.

The principals set forth in the 1974 version of the E514 standard are the most appropriate way to evaluate the water resistance performance of a single-wythe wall. Dampness and moisture at the interior surface of the wall are the most likely conditions to cause damage or affect appearance.

Therefore, observing and measuring this moisture is most critical. The interior face of the completed single-wythe test wall did not display any damp spots within the test area at the interior face after four hours of testing. Considering the severity of the ASTM E514 test and the high levels of interior moisture we have encountered when testing problematic walls, it is reasonable to conclude that the water penetration through the tested single-wythe wall is negligible. Further, it is reasonable to conclude that a wall of this construction in service would not experience moisture-related distress.

One of the important aspects of this single-wythe test program was determining whether weather-resistant performance was feasible using common lightweight aggregates in the block mix. The RMMI recommended practice for moisture-resistant lightweight walls before this required the block to be Medium Weight or Normal Weight per ASTM C90.

Masons do not prefer these heavier block and they are not common in Colorado.. The study indicated that lightweight block, even block with relatively porous lightweight aggregate, can be effective in resisting moisture penetration. Using integral water repellant in the masonry and surface-applied penetrating water repellant made the lightweight block very moisture-resistant, even under the very severe E514 test conditions.

The E514 test does not include testing of waterproofing details associated with window and door openings, base and bond beam flashing, beam pocket flashing, penetrations, and other potential sources of moisture infiltration. However, this testing illustrates that successful, moisture-resistant single-wythe concrete block walls can be constructed without significant risk of damage to interior finishes. Based on the results of this testing, RMMI updated their recommendations for exterior single-wythe concrete block construction as shown in Error! Reference source not found..

The author is associate vice president at Atkinson-Noland & Associates Inc., Boulder, Colo. E-mail Visit