64 BUILD 127 December 2011/January 2012

RESEARCH

Brick veneer stands the testTwisted wire ties are one reason many brick veneer houses failed in past earthquakes, such as Napier in 1931. However, modern brick veneer houses perform much better, and BRANZ research has identified the critical practices needed for this. By Stuart Thurston, BRANZ Senior Structural Engineer

While many loadbearing brick buildings collapsed in the Canterbury earthquakes, giving brickwork a bad press, the

performance of brick veneer was satisfactory, with no deaths attributed to its failure.

Damage provides insight

Liquefaction and lateral spreading from the earthquakes caused large ground differential movements and the distortion of entire houses. Sometimes, cracks formed in the foundation slabs and through the walls (see Figure 1). However, the choice of cladding is unlikely to have influenced this outcome, and despite the gross movements, there was little veneer shedding, which shows that the modern veneer performed well.

During the 4 September 2010 7.1 magnitude earthquake, approximately 1.5 times design accelerations were experienced within 25 km of the epicentre. Most veneers suffered little or no damage due to this shaking. Three examples of veneer shedding were found within 15 km of the epicentre, but these were attributed to poor construction – in each case, brick ties had been omitted in the top half of the veneer.

In another case, the shedding was attributed to weak powdery mortar, probably from construction during hot, dry weather. Cracking in the mortar joints was common in older veneer construction where some settlement may have occurred.

Mainly older brick veneer damaged

The 22 February 2011 Christchurch earthquake had higher accelerations in the city and Port Hills/Lyttelton areas than the September earthquake. The most severe shake damage to houses not influenced by liquefaction occurred in the Port Hills areas during this event.

Significant brick veneer damage occurred in older houses in these suburbs, but with more modern homes, the veneer generally hung onto the frame. Although no ground motion recording stations were available on the Port Hills at the time, some had been installed before the severe 13 June 2011 shaking. An analysis of the combined data showed that the Port Hills houses experienced shaking approximately four times the design level shaking in February.

Consequently, the brick veneer damage observed was well within the design expectations for such extreme shaking. Although the period of intense shaking was short, the high vertical accelerations would also have affected the veneer’s performance.

Modern veneers stronger than historic

Modern brick veneer construction should be significantly stronger than historic construction. In older construction: ❚ fewer ties per unit area were used ❚ ties were often bent wire, which corroded significantly after installation so that veneer fell away in discrete panels during the earthquake as the ties broke

❚ staples or nail fixings were used that withdrew easily from the timber framing

❚ the vibration from nail-fixing ties to the light timber framing substantially weakened the bond between the tie and the mortar

❚ split-stone veneer, commonly used about 50 years ago, often had a dusty surface when laid, resulting in a poor bond to the mortar.

Modern construction: ❚ requires ties to be robustly screwed to the timber studs (since 1995)

❚ must meet minimum connection strength requirements between ties and veneer mortar from AS/NZS 2699.1 tests

❚ uses modern clay bricks with core holes, which are lighter (less seismic load), and the mortar penetrates core holes, interlocking the bricks.

BRANZ mortar elemental tests

Research over recent years by BRANZ measured the compressive and bond strengths of a range of brick veneer mortar mixes both on site and in the laboratory. A selection of mixes was used to construct brick veneer walls that were later tested under out-of-plane seismic shaking (see Figures 2 and 3). By examining the mortar properties that gave good wall performance, critical mortar properties for good seismic performance were determined.

Figure 1: Brick veneer house damaged in the Christchurch earthquake.

Figure 2: General set-up for the out-of-plane shake table tests.

BUILD 127 December 2011/January 2012 65

Because NZS 4210 states that the bond between mortar and brick is the single most important factor affecting veneer face-load strength, the elemental tests carried out focused on finding mortar characteristics that influenced the bond strength for clay-brick veneer construction.

Variables considered were admixture type, mix time, cement quantity and sand type. The effect of prewetting the bricks and the presence or absence of brick vertical core holes was also examined. Sand grading, mortar flow and air content were measured.

Mortar flow critical factor

The critical mortar property for good seismic brick veneer performance is the couplet bond strength – recommended to be at least 200 kPa for single-storey brick veneer and 500 kPa for 2-storey brick veneer. The main factor influencing this strength is the mortar flow – mortar should be as wet as practical for the tradesperson to use. Sand grading (provided the sand is clean), cement content, mix time and admixture used

(as long as it does not contain clay) have less influence on performance.

The minimum compressive strength of mortar for brick veneer is not clearly specified in NZS  4210. It is recommended that the mortar compressive strength of brick veneer be stipulated to be at least 6 MPa.

Dos and don’ts

The testing found practices that improve bond strength include: ❚ wetting bricks before they are laid ❚ pressing and tapping the bricks to firmly embed them in the mortar

❚ not dislodging the bricks once they are placed ❚ minimising the time between spreading mortar, placing the bricks and tooling the mortar joints

❚ adequately curing the freshly constructed veneer in hot, dry weather.

Well constructed modern brick veneer with the recommended mortar bond strengths should be able to resist at least twice the design earthquake for a given house location. This is

consistent with the performance of houses in the recent Canterbury earthquakes.

This work was funded by the Building Research Levy.

Figure 3: Walls collapsing under dynamic out-of-plane testing at BRANZ.