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CONSTRUCTION AND MAINTENANCE OF HIGH-PERFORMING MASONRY STRUCTURES

10/10/2018, by MeredithW, in News & Media, 0 comments

The Colosseum, the Taj Mahal, the Great Pyramids of Giza and the Great Wall of China…the fact that these structures continue to survive are a testament to the strength and durability of masonry materials used in construction. One of the oldest building materials used by man, masonry, still to this day, is the material of choice for building cladding throughout the architectural world.

Masonry systems give owners a better return on their investment due to their energy savings and excellent resistance to fire and weather. However, there are common building envelope design and construction mistakes when using masonry materials, that can lead to ongoing performance issues during the building’s life cycle. Among the many lessons learned in the field, Zero/Six has gained further insight into the proper requirements, techniques, and effective cleaning and maintenance methods that create higher performing masonry structures.

DESIGNING COMMERCIAL MASONRY VENEER SYSTEMS

As technology advances, methods of design and construction advance with it, increasing the demand on architects to understand and integrate their designs with a wide range of complex building materials, components, and systems. When designing commercial masonry veneer systems there are several factors that affect the cost, quality, and functionality of the finished product, including wall thickness, movement, and cleaning or maintenance. The American
Concrete Institute (ACI), the Masonry Society, the American Society of Civil Engineers (ASCE), and Structural Engineering Institute (SEI) are great resources to assist with design and implementation of masonry. These groups partnered to form the Masonry Standards
Joint Committee and created the MSJC code, TMS 402/ACI 530/ ASCE 5 as the reference standard for masonry requirements. While the subject has been studied for many years, there are still lessons to be learned from practical experience.

WALL TIES IN BRICK MASONRY

The use of wall ties in brick masonry cladding can be traced back to around 1850 in England. The purpose of which is to connect the internal and external walls (or wythes) when constructed of bricks or stone blocks. Traditionally made of galvanized steel, the metal rod (tie) is attached to the inner wall in the cavity wall during construction and spans the cavity so that the ends of the tie lock into the cement joints of the exterior cladding. Corrosion is the typical mode of failure with wall ties, causing expansion that forces apart cement joints. Wall ties have replaced masonry bonded double-wythe walls over time to allow the use of different substrates such as wood, steel, and concrete which all help to reduce cost and ease construction. This creation of a cavity using ties also allows for a more complex wall section, creating a drainage plane and allowing thermal insulation to be placed outside the sheathing.

THE ROLE OF WALL ANCHORS

Pintle Ties_Masonry Structures

Once pintle ties are modified they no longer have predicable structural characteristics

A wall anchor is a steel strap used to attach a facade cladding to a back-up structure. Wall anchors must be installed by qualified professionals or major structural failures in the masonry veneer can occur. Wire anchors are most commonly used for brick construction and should never be field-modified if the wall cavity is not the proper depth for the wire ties on-hand.

The wall anchor must keep its shape to ensure structural performance and prevent movement, while embedding at least 1 ½” into the brick veneer. It also must leave greater than 5/8” cover on the outside surface to allow for mortar. Using ties not designed for the specific depth of the wall cavity can cause potentially catastrophic failures by reducing the masonry’s ability to resist lateral loads. Changes in cavity width must be taken into consideration to ensure installers have the right length anchor for the job.

USING AIR CAVITIES TO ACCOMMODATE TOLERANCES

A minimum of 1” airspace is required by building code (ACI 530), however, we recommend designing walls with more than the one-inch air space to accommodate tolerances in the slab edge. Another method to assist with drainage behind masonry cladding is to allow ventilation at the top of the wall, so the additional airflow in the cavity can help weep water away from the drainage plane. Slab edges may be offset from the wall and still be within an acceptable range of tolerance for the concrete. The masonry veneer must stay plumb, without modifying the insulation in the cavity these slab edge offsets could make the airspace less than the required 1”.

If an airspace deficiency occurs, moisture buildup in the cavity can cause failures regarding waterproofing and insulation, as well as discoloration on the exterior of the masonry. If this occurs, there are few options available to rectify the problem, such as eliminating insulation thickness; chiseling away the slab to the proper plane, or incorporating a rain screen drainage mat between the veneer and insulation. Removing insulation thickness affects the R-values and code requirements, and chiseling the slab is time and labor intensive.

The last option – a rain screen drainage mat – creates a continuous drainage area to manage moisture in the cavity even with less than an inch. While long-term analysis has not been performed on this final option, it seems to be the most time and cost-effective solution.
A polymer mesh similar to a mortar net may be used, or if a more rigid and durable drainage mat is required, high-density polyethylene with three-dimensional dimples and grooves is also an option. This additional mat can improve drainage and ventilation to reduce the problems caused by the excess mortar in a small airspace.

DESIGNING FOR ANTICIPATED MOVEMENT

Excessive cracking across the brick or stone can cause structural failure, not to mention cosmetic issues; however, cracking commonly occurs at the mortar joints, causing separation of the brick and mortar which will require re-pointing.

Movement can be caused by internal wall forces that are quite complex and vary from structure to structure. While the facade is designed to resist wind, gravity, and possibly earthquake loads, internal facade forces are more difficult to quantify and design for. Internal forces may include restrained thermal expansion, contraction of the facade, moisture expansion in clay brick masonry, and shrinkage in concrete masonry screen walls.

The common way to avoid failure from movement is to anticipate and control the movement with flexible expansion joints. An expansion joint (sometimes called a control joint) is an assembly designed to safely absorb heat-induced expansion and contraction of construction materials. It may also absorb vibration and allow for movement due to ground settlement or earthquakes.

Typically, builders do not have the required information to make an informed decision, so expansion joints are placed in the contract documents by the designer. While mortar joints require re-pointing around 20 to 30 years after installation, expansion joints using
sealant will require more regular replacement at seven to 20 year intervals depending on the sealant and climate. Proper placement and sealant decisions with larger stone will make a considerable cost difference over the life of the building.