Do raked joints affect the fire resistance of concrete masonry?

by Hannah Keelson and Ehab Zalok, P.Eng.

Photo ©
Photo ©

Masonry is employed for a variety of factors such as esthetics, water penetration, and durability. Mortar joints play a significant role in masonry walls as they primarily hold unreinforced masonry walls together and transfer structural/thermal loads. Different forms of mortar joints—concave, flush, raked, beaded, struck, and weathered—can be employed in the construction of masonry walls.

Objective-based National Fire Code of Canada (NFC), as compared to prescriptive codes, attempts to provide a clearer guidance about taking into consideration all functional objectives affecting the fire safety of a structure. These are usually guided by set objectives, and accordingly, met by the designer. In the design of nonloadbearing walls, Canadian Standards Association (CAN/CSA) A371-04, Masonry Construction for Buildings, specifies raked mortar joints for interior-exposed and non-exposed walls. This allows for raked joints to be used once the joints are set just enough to remove excess water. Further, the standard specifies raked joints must be of uniform depth as indicated on drawings. CAN/CSA A371-04 has a “non-mandatory” note that, unless otherwise specified, the joint should be concave for masonry exposed to precipitation. Otherwise, there is no specific mention of mortar joints, where the issue usually arises from an architect’s specifications. The architect may decide to opt for a concave joint just for fire protection reasons, adding to cost, or may allow a struck joint. However, if the joint is not perfectly full, which can happen with a struck joint, the architect will require it to be re-pointed. The lack of any mention of joints in standards or codes had led to the issue. This is probably because until now, there was no test data, so designers default to the more conservative approach.

Architects and clients often prefer the use of raked joints because of their esthetics. However, they have been thought to reduce the fire-resistance properties of concrete masonry. In fact, new research suggests raked and full mortar joints create potential lines of thermal bridging. This would indicate the possibility of having a higher heat transfer around the mortar than the surrounding materials (concrete masonry block), thereby resulting in an overall reduction of the thermal insulation of the wall. In the event of an anticipated fire, the rate of heat transfer could be significantly high, and therefore cause the wall to fail earlier than designed for based on the insulation and integrity criteria per Underwriters Laboratories of Canada (CAN/ULC) S101-14, Standard Methods of Fire Endurance Tests of Building Construction and Materials.

The fire resistance of walls can be determined by performing full-scale tests as per the requirements of objective-based codes. A fire temperature is based on standard fires. Professor Zalok of Carleton University in Ottawa (one of authors of this article) and his research team conducted from 2015 to 2018 full-scale fire tests on nonloadbearing masonry walls constructed with raked and standard concave joints. This was subject to CAN/ULC S101. Experimental fire tests results were used to build a 3D numerical model using finite element software to simulate the process of heat transfer under elevated temperatures.

Figure1: Test setup and a 3D-illustration of the full test up. Images © Hannah Keelson and Ehab Zalok
Figure1: Test setup and a 3D-illustration of the full test up.
Images © Hannah Keelson and Ehab Zalok

Building elements and assemblies are normally tested and rated based on their ability to perform intended functions under exposure to standard fires, commonly referred to as “fire-resistance ratings.” This is mostly quantified as the time for the element or assembly to perform its “fire barrier and separation” and/or loadbearing function in the building (This is referenced from Structural Design for Fire Safety by A.H. Buchanan, published in 2002 by John Wiley & Sons.). The experimental work being modelled is part of an ongoing fire-masonry research at Carleton University. A series of walls were constructed and tested as per CAN/ULC S101. Walls were 2.8 m (26 ft) wide and 3.2 m (10.5 ft) tall (seven blocks wide and 16 courses high). Each of the walls was built within a frame of reinforced concrete masonry columns and beams comprising 200 mm (8 in.) normal weight concrete (NWC) hollow block mortared with Type S mortar (Figure 1).

Figure 2: Thermocouple locations on the full wall.
Figure 2: Thermocouple locations on the full wall.

Type S mortar is suitable for general and below-grade uses and, particularly, when high lateral strength is required (Read Masonry Structures Behaviour and Design by R.G. Drysdale and A.A. Hamid, published in 2005 by the Canada Masonry Design Centre (CMDC), for more information.). The strength of the masonry block was 15 MPa (2175 psi). Masonry blocks employed were hollow. Since grout and reinforcement bars were not employed in the construction of the walls, the cells of the block were left hollow. The walls were then subjected to a standard fire temperature curve as prescribed in CAN/ULC S101. The modes of heat transfer on the masonry wall were through convection, radiation, and conduction. Eighteen thermocouples—nine on each side—were employed to measure the temperature of the exposed and unexposed sides of the wall (Figure 2).

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