Evaluation of Sealed Joint Performance for the Selection of Sealants Suitable for Use in Autoclaved Lightweight Concrete Panels

ABSTRACT: The strength of autoclaved lightweight concrete (ALC) is evi­dently lower than that of normal concrete. Therefore, when movement occurs at a sealed joint between ALC panels, the sealant is required to deform and remain intact without damaging the ALC substrate. However, there is cur­rently not sufficient information to permit evaluation of the expected perform­ance of sealants applied to ALC substrates. In this study, static and dynamic tests were carried out in order to obtain an index that could be used to select the modulus of a sealant that can be expected to provide long-term perform­ance when applied to an ALC substrate. To develop this index, an initial study was carried out in order to clarify actual joint movement between ALC panels of buildings; the expansion and contraction at the joint were measured, and shear joint movement was calculated based on the expected story-to-story drift of an external wall due to earthquake loads. Thereafter, in a subsequent stage of the study, five types of two-component polyurethane sealant

products, of different elastic modulus, were subjected to tensile and shear tests from which the relationship between stress and the type of joint fracture was determined. The results from these tests revealed that when the stress is greater than 0.6 to 0.7 N/mm2, the ALC substrate is more easily fractured than the sealant. In a final stage of the study, the cyclic fatigue resistance of the same two-component sealants was evaluated using tensile and shear fa­tigue tests. Results from the fatigue tests indicated that the high modulus sealants lost adhesion from the ALC substrate at an early stage in the test.

As well, the fatigue resistance of test specimens with joints having three­sided adhesion was lower than that of specimens having normally configured joints with adhesion on two sides of the sealant. Therefore, on the basis of results derived from all the studies, it was determined that a suitable sealant for use on ALC substrates is a sealant having a low modulus that is applied in the normal fashion as a two-sided joint.

KEYWORDS: sealant, autoclaved lightweight concrete, wall panel, modulus, fracture

Introduction

Autoclaved lightweight aerated concrete (ALC) (also referred to as autoclaved aerated concrete or autoclaved cellular concrete according to the Portland Cement Association) has exceptional qualities with respect to fire resistance, heat resistance, and thermal insulating properties. ALC is commonly used as the primary material for building envelope roof and wall components and is in­stalled in various types of buildings ranging from super high-rise buildings to residential homes. ALC panels are factory-produced materials having lasting quality and adequate durability.

In Japan, ALC panels are typically manufactured according to specifications regarding strength and modulus given in JIS A 5416-2007 [1] (compressive strength > 3 N/mm2; Young’s modulus > 1710 MPa). ALC panels manufactured for use as cladding components are, following casting, cured, removed from their bulk casting forms, roughly cut to size as might be required, and thereafter further cured in an autoclave. Hence their surfaces are essentially free of any form-release agents used to ease the removal from casting forms. Following the autoclave process, these panels may then be cut to exact sizes, with edges formed to accommodate the paneling requirements.

When ALC panels are installed as building envelope components, the joints between panels are filled with sealant to ensure the water – and airtightness of the enclosure. Typically a primer is applied to the joint surfaces to ensure a last­ing bond should the porous substrate absorb moisture. With respect to the long-term performance (durability) of the air – and watertightness of the enve­lope, sealed joints are a vulnerable component of the assembly. Consequently, the following issues might be raised when ALC panels are used for exterior wall cladding.

In the first instance it should be recognized that the tensile strength of the ALC panel is relatively low as compared to standard concrete materials. When

Copyright by AsTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014

Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

there is movement at sealed joints due to dilation of the panel from surface tem­perature effects, the ALC panels might rupture before the sealant accommo­dates the expected deformation. It is for this reason that in Japan the Architectural Institute of Japan has provided the “Recommendation for Design of Joints and Jointing for Control of Water and Air Penetration in External Walls" [2], in which it is specified that sealants used should have a reduced mod­ulus of 0.2 N/mm2 or less at the 50 % tensile stress level (i. e., referring to 50 % modulus). This provision is only for sealants specified for use in ALC panels. However, given that over time the sealant will age, it is possible that the 50 % tensile modulus for an aged deteriorated sealant can eventually exceed 0.2 N/mm2. The relationship between the deformation of such aged sealants and ALC panel strength has yet to be thoroughly studied. This is of particular significance with the use of acrylic sealants, as this type of product has been commonly used on ALC panel joints for a long time and is known to be suscepti­ble to hardening and reduced flexibility over time [3]. The importance of consid­ering the effects of the aging and deterioration of sealants on the long-term performance of the joint should not be ignored.

The next issue is the form of sealed joint. A standard exterior wall joint is typically applied as a two-sided adhesion joint system in which the sealant is applied to the surfaces of each adjacent side of the ALC panel and to specified width-to-depth ratios as provided, for example, in Fig. 1. However, in Japan, the use of a three-sided adhesion joint system that bonds joint surfaces at the base of the joint has, for ALC panel cladding, been accepted for a considerable time,

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

and the long-term adhesion of such joints with respect to fatigue resistance remains nonetheless undefined.

Another issue is the degree of movement of a sealed joint and fatigue resist­ance. The thermal expansion coefficient of ALC panels is small; given such, rather than on the expansion and contraction of panels due to temperature change, the focus should be on the shear movement of joints caused by an earthquake, with the degree of movement being estimated on the basis of the relative story displacement of panels.

Considerable research has been done on the tensile and shear deformation performance, as well as on the fatigue resistance, of sealed joints for metal, con­crete, and glass panels. Enomoto et al. [4,5] conducted a quantitative assess­ment of dynamic fatigue using cyclic movement devices that created continuous movement in expansion and contraction. Takeuchi et al. [6] con­ducted dynamic loading tests in order to understand the dynamic characteris­tics of sealant placed between glass panels. In addition, ongoing research has also been done on the adhesive properties of sealant under various conditions of the panel surface. Kenney and Kenney [7] assessed the adhesive properties of sealant, with the moisture condition of the substrate as a variable. Ma et al. [8] studied the adhesive properties of the sealant after solvent cleaning, using vari­ous types of glass panels.

It can be observed from these case studies that in every instance the panel substrate was stronger than the sealant; the expectation, then, is that the panel substrate will not fracture. In other words, the focus of existing research on the deformation performance of sealed joints is the tensile strength of the sealant and the adhesive strength between the sealant and the panel substrate. How­ever, in the case of ALC panels with a low tensile strength, the panel itself can fail before the sealant reaches its maximum level of performance in tensile de­formation. Consequently, this can yield a significant effect on the performance in the deformation of the sealed joint as a whole.

Given this background information, a research study was developed in which a sealant was formulated to have different values of modulus and was applied to ALC panel joints in order to elucidate the relationship between tensile stress and fracture mode. The variations in the sealant modulus were intended to mimic the effects brought on by the aging and deterioration of the sealant. Panel joint movement was also measured and calculated, and a fatigue test was conducted in order to assess the fatigue resistance of sealed joints against the continuous deformation of ALC panel joints.