EVALUATING DIRECTLY BONDED STUCCO

BRIAN LEE COPE, PE WISS, JANNEY, ELSTNER ASSOCIATES, INC. 2915 Premiere Parkway, Ste. 100, Duluth, GA 30097

Phone: 770-923-9822 • Fax: 770-232-9044 • E-mail: lcope@wje.com

JOHN FRACZEK, PHD, PE, SE WISS, JANNEY, ELSTNER ASSOCIATES, INC.

330 Pfingsten Road, Northbrook, IL 60062

Phone: 847-272-7400 • Fax: 847-291-9599 • E-mail: jfraczek@wje.com

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ABSTRACT

Wiss, Janney, Elstner Associates, Inc. (WJE) has evaluated several high-rise buildings clad with stucco applied directly to concrete or concrete masonry in coastal environments. Large areas of stucco delamination or poor bond between the stucco and the substrate were frequent concerns with these buildings. WJE observed several conditions that caused these issues, including substrate preparation, placement of stucco accessories, and installation techniques. The authors will discuss the significance of delaminated stucco and the condi-tions that affect the bond. They will also discuss field and laboratory test methods (destruc-tive and nondestructive) used to evaluate the conditions and present quantitative values for the bond strength for consideration.

SPEAKERS

BRIAN LEE COPE, PE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. ­ DULUTH, GA

LEE COPE, PE, is a licensed professional engineer who has extensive experience in detailing and proper installation of Portland cement plaster (stucco) façade systems, air bar-riers, window systems, waterproofing, and the interfaces of various envelope components. He has evaluated a variety of structures relating to the cause and/or distress of the build-ings’ exterior façade/curtain wall systems and building envelope systems. He is a voting member of ASTM Committee C11 on Gypsum and Related Building Materials and Systems.

JOHN FRACZEK, PHD, PE, SE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. ­ NORTHBROOK, IL

JOHN FRACZEK, PhD, PE, SE, is a licensed professional and structural engineer engaged in the resolution of design and construction-related structural and materials prob-lems. He has investigated low-strength concrete problems and various other concrete mate-rials issues. Dr. Fraczek is an established author and previously served as a principal speaker in a nationwide seminar series on concrete repair and restoration. He has present-ed numerous lectures on structural failures, repair techniques, materials performance, and nondestructive testing. His professional affiliations include the American Concrete Institute (ACI), the Precast/Prestressed Concrete Institute (PCI), and the Structural Engineers Association of Illinois (SEAOI).

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Why Stucco Can Be PotentiallyProblematic for High-Rise Construction

ACI Tolerances for Formed-ConcreteSurfaces

EVALUATING DIRECTLY BONDED STUCCO

INTRODUCTION There are hundreds of high-rise struc-

tures in Florida and other coastal regions that are clad with directly applied Portland cement plaster (stucco). From a brief obser-vation, the stucco façade appears to be in good condition. However, a close inspection of virtually any of these structures will like-ly reveal regions of delaminated stucco. Therefore, using directly applied stucco as the cladding of a high-rise structure can potentially be problematic and lead to cost-ly repairs.

In the last several years, Wiss, Janney, Elstner Associates, Inc. (WJE) has evaluat-ed several buildings in coastal environ-ments that are clad with stucco and has found large areas of delamination or poor bond between the stucco and the substrate. This paper discusses the significance of delaminated stucco and the conditions we have found that affect the bond strength, including substrate preparation, placement of stucco accessories, and installation tech-niques. Furthermore, WJE’s research into these bond failures has revealed a signifi-cant lack of quantitative requirements for the bond strength of directly applied stucco. ASTM C926, Standard Specification for Application of Portland Cement­Based Plaster, provides guidelines for the applica-tion of directly applied plaster to solid bases, including requirements to achieve an adequate bond between the plaster and substrate, but does not provide specific requirements to evaluate the bond nor the minimum bond strength necessary. Therefore, this paper also discusses field and laboratory test methods (both destruc-tive and nondestructive) that WJE has used to evaluate bond conditions and presents quantitative values for necessary bond strength for consideration.

Most high-rise structures that are clad with directly applied stucco consist of a cast-in-place concrete frame with concrete masonry unit (CMU) infill walls. Table 1 in ASTM C926 specifies the total nominal

thickness for two-coat and three-coat sys-tems over solid substrates. For a three-coat system, the total nominal stucco thickness over unit masonry or cast-in-place concrete is 5/8 in. For a two-coat system, the total nominal stucco thickness over unit mason-ry is 1/2 in., and 3/8 in. over cast-in-place concrete.

Since the stucco is directly applied to the substrate, there is no weather barrier behind the stucco. Therefore, for these structures, the stucco itself is intended to function as a water barrier. For it to func-tion as a water barrier, it is critical that the stucco be properly placed and bonded to its substrate. However, if proper bond is not achieved, directly applied stucco can be problematic, and delamination and crack-ing will likely occur. The delaminations can occur between the stucco and its substrate and/or between the various layers (lifts) in the stucco.

When delaminations occur in the stuc-co, the stucco becomes unsupported within the delaminated regions. When the stucco becomes unsupported, it cannot transfer thermal or wind loads to its substrate and cracking typically occurs in the stucco. The cracking can allow moisture to migrate through the stucco and possibly into the building. From performing a number of investigations over the last several years, WJE has observed several conditions that affect the bond between the various layers in the stucco or between the stucco and the substrate. These conditions are as follows:

• American Concrete Institute (ACI) tolerances for formed-concrete sur-faces do not correspond to the toler-ances for solid substrates to receive stucco provided in ASTM C926.

• Poor surface preparation • Poor placement of stucco • Build-out materials • Stucco accessories • Incompatible substrates

One of the main conditions that affects stucco installation on a high-rise structure is access to the exterior façade. For high-rise construction, access to the façade is

typically obtained by using swing stages, mast climbers, or a combination thereof. Therefore, access is limited and can con-tribute to the conditions that cause delami-nation in stucco. The limited access likely results in poor supervision between the general contractor and the stucco subcon-tractor and likely contributes to the condi-tions related to poor surface preparation, poor placement, and poor curing.

The following discuss each of these con-ditions in detail and summarize their effects on the bond between the stucco and the substrate and between the various stucco layers.

ACI tolerance for formed-concrete sur-faces are not consistent with the tolerances for solid substrates to receive stucco pro-vided in ASTM C926. When constructing a high-rise structure that consists of a cast-in-place concrete frame, the tolerances for plumbness and horizontal variances are controlled by ACI 117, Specification for Tolerances for Concrete Construction and Materials. Section 4.1 of ACI 117 provides plumbness requirements for cast-in-place concrete building heights less than or equal to 83 ft., 4 in. The permissible out-of-plumb dimension is the lesser of 0.3% of the total height or 1 in. Therefore, for a building with a 10-ft. floor-to-floor height, the allowed out-of-plumbness in a building surface between the top and bottom of the floor is 0.36 in., or approximately 3/8 in. Similarly, the maximum allowed tolerance over two stories is 0.72 in., or approximately 3/4 in., and 1 in. for three or more stories.

Section 4.2 of ACI 117 provides toler-ances for horizontal variances in the vertical edge of an opening in a floor, wall, or beam. The permissible out-of-plumb dimension for floor edges, beams, or walls is +/-1 in. Therefore, for a typical building with multi-ple bays, the first bay is allowed to be out-of-variance by +1 in. compared to the spec-ified plan dimension, and the adjacent bay is allowed to be out-of-variance by -1 in., which equates to an offset in the framed

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Poor Surface Preparation

floor edges of 2 in. across the two bays. When placing stucco on a high-rise

structure that consists of a cast-in-place concrete frame with CMU infill walls, the columns, edges of floors, and block walls are to be within ¼ in. of the same vertical plane. This tolerance is based on Section 5.2 of ASTM C926, Standard Specification for Application of Portland Cement­Based Plaster, which requires the substrate to be “straight and true within 1/4 inch in 10 feet.” Since the stucco is intended to be rel-atively thin (3/8 to 5/8 in., depending on the substrate), if the concrete frame is out of plumb or if its horizontal variances exceed ¼ in. in 10 ft., it is almost guaran-teed that the thickness of the stucco will exceed the nominal thicknesses specified in ASTM C926.

Section 5.2.3 of ASTM C926 states that “where total plaster thickness will exceed the total thickness specified in Table 1 for types of solid bases, furred or self-furring metal plaster base shall be installed in accordance with Specification C 1063”. However, when a furred or self-furring metal plaster base (lath) is used, the total nominal thickness in Table 1 (ASTM C 926) is 7/8 in. However, whether or not a metal plaster base is used, depending on the plumbness and horizontal variances of the concrete frame, the total thickness of the stucco can easily exceed 7/8 in. and typi-cally does. In our experience, when investi-gating façades that consist of directly applied stucco, the stucco is typically always thicker than the nominal thickness provided by ASTM C926. In most cases, we have measured the stucco to be at least ¾-in. thick, with some measurements extend-ing up to 3 in. In these cases, the stucco is placed in several thin layers, which increas-es the possibility of a delamination occur-ring between the various coats. Figure 1 illustrates an example in which the thick-ness of the stucco on a project was mea-sured to be 2 in.

Therefore, for high-rise construction, due to less stringent tolerances for the con-crete frame, it is very likely that the stucco will be thicker than the required nominal thicknesses specified in ASTM C926, and sometimes substantially so. This condition in itself can be problematic. However, excessively thick stucco combined with poor surface preparation, poor placement of the stucco, improper build-out materials, improperly installed stucco accessories, and incompatible substrates will exacerbate

the potential for poor bond in the stucco and can result in very costly repairs.

As mentioned above, for directly applied stucco to perform as a proper cladding sys-tem for high-rise construction, the stucco must be properly bonded to its substrate. To help ensure there is proper bond between the stucco and the cast-in-place concrete frame and CMU infill walls, Section 5 of ASTM C926 provides require-ments for solid bases such as masonry, stone, cast-in-place, or precast concrete to receive stucco. These requirements discuss substrate preparation. Section 5.2 states that solid surfaces to receive plaster “shall be free of...oil or other elements which would interfere with bonding.” Section 5.2.1 states that “solid surfaces shall have the suction (ability to absorb water) or have surface roughness, or both, to provide the bond required for the plaster.”

Luckily, CMU provides an adequate substrate for proper bond. Therefore, CMU infill walls typically have the ability to absorb water and have a surface roughness that requires little surface preparation. However, since the cast-in-place concrete frame consists of a smooth, nonabsorbent surface, proper surface preparation is required to help ensure there is proper bond between the stucco and the concrete

frame. Section 5.2.2 provides guidelines on how to prepare the concrete surface. It states that “smooth or nonabsorbent solid surfaces, such as cast-in-place or precast concrete, shall be prepared to receive Portland cement plaster by one of the fol-lowing methods:”

• Section 5.2.2.1, “Sandblasting, wire brushing, acid etching, or chipping, or a combination thereof,”

• Section 5.2.2.2, “Application of a dash bond coat applied forcefully against the surface, left untroweled, undisturbed, and moist-cured for at least 24 hours,” or

• Section 5.2.2.3, “Application of a bonding compound suitable for exte-rior- or interior-exposure solid sur-faces in accordance with manufac-turer’s written directions.”

In addition to these methods, Section 5.2.3 states that where bond cannot be obtained over the entire surface to receive plaster by one of the methods discussed above, “furred or self-furring metal plaster base shall be installed in accordance with Specification C 1063.”

While ASTM C926 is clear on how to prepare a solid substrate, we typically find that the concrete frame has not been prop-erly prepared, and the requirements in ASTM C926 are often ignored (Figures 2 and

Figure 1 – View of stucco that is approximately 2 inches thick. Note: arrow indicates void between stucco and substrate.

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Poor Placement of Stucco

”

’

Figure 3 – An exploratory obtained over the entire surface to opening in the stucco façade receive plaster. revealing a smooth cast-in-While WJE typical-place concrete substrate. (Note ly does not find that the stained area is “liquid furred or self-fur-nails used to attach the ring metal plaster corner bead and not the bases used in coastal environ-ments, we have

remnants of a bonding agent.)

Figure 2 – View of an observed usage of exploratory opening in the PVC lath or nylon stucco façade revealing a scrim, typically smooth cast-in-place over floor lines. At concrete substrate. these locations, the lath or scrim spans the joint between the CMU infill walls and the concrete floor slab

WJE has seen delaminations at the bond in an attempt to control cracking between line between the various coats in the stuc- the two different substrates. From inspect-co. WJE attributes the poor bond between ing the stucco at these areas, it often the various coats to poor workmanship sounds hollow when tapped with a sound-(improper keying of the various stucco lay- ing hammer, indicating the presence of a ers together and poor curing) and lazy possible delamination. However, on creating

3). From creating openings in the façade after locating delaminations, we typically find that the concrete frame is generally smooth (nonabsorbent) and free of bonding agents. Therefore, poor surface preparation is a condition that can result in delamina-tion between the stucco and its substrate.

Another source of delamination in directly applied stucco is poor application of the stucco. As previously mentioned, direct-ly applied stucco is intended to be 3/8- to 5/8-in. thick, depending on its substrate and whether it is a two-coat or a three-coat system. The stucco should also be fully compressed and bonded throughout its thickness. In addition to a poor bond due to an improperly prepared substrate,

installation (poor compression during the stucco placement). This condition is exacer-bated when the thickness of the stucco exceeds the specified nominal thicknesses provided by ASTM C926 and when there are multiple (more than three) coats present. It is also exacerbated by delays between the applications of the various coats.

As noted above, ASTM C926 states that a furred or self-furring metal plaster base is to be used when the stucco thickness exceeds the nominal thicknesses provided in ASTM C926 or when bond cannot be

Figure 5 – Void between backside of stucco and cast-in-place

substrate due to lath s not being fully encapsulated in the stucco.

Figure 4 – View of poor key between the PVC lath and stucco.

exploratory openings to verify the delamina-tion, the apparent delamination is often related to a void that is created between the back of the PVC lath and the cast-in-place concrete substrate (Figures 4 and 5). When lath is used, the lath should be fully sub-merged in the base or scratch coat. Section 7.2.1 of ASTM C926 states that the “first coat shall be applied with sufficient materi-al and pressure to form full keys through, and to embed the metal base, and with suf-ficient thickness of material over the metal to allow for scoring the surface.”

Another problem that is typically uncov-ered with the use of PVC lath is improper fasteners. From creating exploratory open-ings, we often observe the PVC lath to be anchored to the concrete substrate with stub nails that are 3/8-in. long (Figure 6).

Figure 6 – View of 3/8-in.-long stub nails used to attach PVC lath.

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Build-Out Materials

Stucco Accessories

Incompatible Substrates

Section 6.7.1.1 of ASTM C1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement­Based Plaster, states that “nails for attaching metal plaster bases to solid substrates shall be not less than 3/4-in. long.” Therefore, to help ensure that the lath is properly anchored, the nails should

Figure 7 – Improper build-out material after the delaminated stucco was removed.

Figure 8 – Close-up view of improper build-out

materials with excessive cracking associated with

expansion due to moisture.

be 3/4-in. long in lieu of 3/8-in., even though PVC lath is used in lieu of metal lath.

Rather than use stucco, contractors often apply build-out material to the con-crete frame and CMU infill walls in order to create a straight and plumb substrate for the stucco. This is typically done when the

Figure 9 – View of vinyl corner bead that is poorly encapsulated in the stucco.

Figure 10 – Close-up view of 3-in. corner bead with

virtually no encapsulation.

concrete and CMU variances are so great that very excessive (1½ in. or more) stucco thicknesses will result. However, from our experience, the build-out material is typi-cally applied directly to the concrete or CMU substrate, and relatively thick sections are not properly anchored or reinforced.

Build-out materials are typically propri-etary, prepackaged concrete repair mortars

intended for vertical and overhead trowel application. As such, they contain addi-tives and/or admixtures to provide a nonsag consistency and relatively rapid set. We have found some of these materi-als to be volumetrically unstable in moist environments, resulting in delaminations at the concrete/CMU or the stucco inter-face (Figures 7 and 8).

In coastal environments, the stucco accessories (control joints, corner beads, etc.) typically consist of vinyl. While rela-tively small accessories (e.g., 1-in. corner beads) have relatively large openings that permit encapsulating the accessory in stuc-co, relatively large accessories (e.g., 3-in. corner beads) are, for the most part, solid, with relatively small holes to provide suffi-cient stiffness for the accessory to remain in position during application of the stucco. This makes it very difficult for a contractor to fully encapsulate the accessory in the stucco (Figures 9 and 10). Therefore, corner beads and other accessories are often incompletely filled with stucco, resulting in paths for moisture within the stucco cladding and flexing of the accessory due to temperature change. The flexing typically results in cracking of the stucco.

A bonding agent can be applied to help improve the bond between the stucco and its substrate. However, if not properly installed and if the stucco is not placed in a timely manner, the bonding agent can set prematurely and act as a bond breaker instead of a bonding agent (Figure 11). This is particularly true of epoxy-based bonding agents. Bonding agents based on polyvinyl acetate may reemulsify in a moist, high-pH environment and serve to delaminate the stucco from its substrate if moisture pene-trates to the substrate.

Figure 11 – View of stucco that was poorly bonded due to a bonding agent

that set prematurely.

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ASTM C926 provides methods for sub-strate surface preparation and stucco appli-cation to achieve proper bond. However, it does not define proper bond. In addition, there is currently no standard available that defines the proper testing of the bond between stucco and its substrate. To help ensure that stucco is properly bonded to its substrate, it is our opinion that a standard should be developed and acceptable bond strength levels should be defined.

To measure the bond strength between directly applied stucco and its substrate, WJE has performed direct tension tests on several projects where bond was a concern. The tension test we typically employ utilizes a reaction frame, a load cell, and a 3- by 3-in. square plate (Figures 12 and 13). The stucco can be cut with a diamond blade on a grinder to minimize stresses at the bond line associated with drilling cores. In addi-tion, the plate must be sufficiently stiff to resist flexing during the test. In performing these tests, we have measured various bond strengths ranging from 0 to 5 psi, to well over 100 psi. It is our opinion that bond strengths of 50 psi or more represent good bond. It is also our opinion that bond strengths of less than 30 psi are likely indicative of poor surface preparation

and/or poor stucco installation. Petrographic examination is another

method WJE has used to evaluate the quality of stucco bond. This method can assist in determining locations of poor bond between the various layers in the stucco and can help determine the over-all quality of the stucco (Figure 14).

CONCLUSIONS If poor bond strength exists between

directly applied stucco and its substrate, the performance of the stucco in service can be problematic and lead to costly repairs. To help ensure adequate bond between the stucco and its substrate, we recommend that the construction team (general con-tractor, concrete forming subcontractor, stucco subcontractor, architect, and struc-tural engineer) be qualified and aware of the potential problems that can occur during the application of directly applied stucco. Therefore, the team should

• Be aware of the potential downfalls with regard to building access, par-ticularly as it affects substrate preparation and stucco application and curing.

• Be aware of the varying tolerance between ACI and ASTM C926 for concrete construction.

• Ensure that the surface of the sub-

strate is properly prepared. • Ensure that the stucco is properly

installed and cured. • If build-out material is used, ensure

that the build-out material is suit-able, and ensure that it is properly anchored and reinforced.

• If bonding agents are used, ensure that they are of the appropriate type and installed properly.

• Ensure that stucco accessories are properly installed and that the accessory is properly encapsulated in the stucco.

We also recommend that the bond strength of the stucco be tested during the mock-up phase and during construction, as a method of quality control. The testing could include direct tension-bond testing and petrographic examination.

Figure 12 – View of direct tension test apparatus.

Figure 13 – View of stucco sample after direct tension test.

Figure 14 – View of stucco sample that has been subjected to petrographic examination, identifying various lifts, voids, and potential poor bond between layers.

What Is Proper Bond?

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