Research exploring methods of mortarless construction has led to improvements in the structural performance of the system, as a whole. For example, experimental tests of pre-stressed reinforcement elements and surface bonding have been conducted to investigate the effect of these methods on structural strength; however, numerical modeling has not been widely applied in the current literature to enhance the understanding of these systems. This section discusses the previous work conducted using these alternative methods.
2.5.1 Mortarless Pre-Stressed Masonry Systems
One method to improve the structural performance of mortarless masonry is pre- stressed reinforcement elements within the walls. Pre-stressing can be applied with non- interlocking and interlocking units. Biggs’ (2002) early research conducted shear tests on the walls using FlexLock units (see Fig. 2-17). The results of the tests indicated a lower
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shear capacity than mortared systems due to the low levels of pre-stressing and the reduction of the coefficient of friction in the contact between units. Biggs and Forsberg (2006) conducted further research on walls using the FlexLock system, focusing on its constructability as applied to a full-scale house. In their assessment, these researchers concluded that using the FlexLock system instead of a mortared system would reduce the cost of construction by approximately 25%, in addition to increasing worker productivity by almost 120%. This research did not yet include Bolt-A-Blok systems to compare the flexural capacity of the walls.
Fig. 2-17. Stretcher FlexLock unit [Source: Biggs 2002, with permission].
Research on pre-stressed mortarless masonry studies was extended to include Bolt- A-Blok systems to determine the displacement ductility (Ota 2011). These walls were used under in-plane and out-of-plane loads to mathematically analyze the behavior of the system and determine the adjustments needing to be made to existing design equations in the MSJC 2008. Based on these results, the flexural capacity of these walls was found to be comparable to reinforced mortared masonry walls. In addition, these mortarless walls exhibited significantly higher displacement ductility. This research was nuanced by the
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2007 Yamaguchi et al. research that focused more strictly on the reinforced unbonded elements in masonry walls.
Yamaguchi et al. (2007) focused on the friction-resistant stress-transfer mechanism, a system containing reinforced unbonded elements in masonry walls, thus allowing for easy reuse of the materials (see Fig. 2-18). This system was used in full-scale houses that were subsequently subjected to a magnitude seven earthquake approximately one hundred days after construction. They showed no signs of structural damage. Furthermore, approximately 1200 days later, these structures still showed only a limited pre-stress loss of 20%. The research conducted by Yamaguchi et al. (2007) did not, however, focus on the importance of variations in grout level.
Fig. 2-18. Pre-stressed system for dry-stack masonry walls [Source: Yamaguchi et al.
2007, with permission].
Pre-stressed mortarless masonry has also been studied in grouted and ungrouted walls using both AZAR Block and Spar-lock units to assess their behavior under out-of- plane loads (Sokairge et al. 2017). Based on the results of both units, grouted pre-stressed walls resisted twice the failure load of ungrouted pre-stressed walls. Additionally, these grouted walls had a cracking load three times greater than that of reinforced masonry walls
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with grout. Walls tested using either the AZAR or Spar-lock units were found to resist lateral out-of-plane loading both with and without grouting. However, their results indicated that the ungrouted pre-stressed masonry wall exhibited shear failure under out- of-plane loads, implying that the equation’s design in the current MSJC code requires revision for pre-stressed dry-stack masonry walls. The current research on pre-stressed mortarless walls lacks numerical models to predict their response. In addition, it is important to note that all studies on these walls recommended further experiments to validate the results. Pre-stressed dry-stack systems are a relatively new construction method, requiring further investigation to be incorporated fully into the MSJC code.
2.5.2 Mortarless Masonry Using Surface Bonding
A method for enhancing the structural performance of dry-stack masonry is by using surface bonding. For example, Ferozkhan (2005) conducted compressive tests on walls using QuickBlock units that included fiber-reinforced cement composite (FRCC) on the faces of the wall. Ferozkhan found that this surface bonding increased the compressive strength of mortarless masonry walls compared to mortared walls. Based on these tests, Ferozkhan reported that walls with FRCC surface bonding exhibited less damage, including a significant reduction in cracking. In addition, these walls exhibited approximately 20% greater compressive strength than walls without surface bonding. This can be furthered by research on the compressive and shear capacities of dry-stack masonry walls.
The compressive and shear capacities of dry-stack masonry walls with surface bonding have been studied with a specific focus on ENDURA units (see Fig. 2-19). For
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example, Murray (2007) conducted compressive tests on mortarless walls. When the walls reached the failure load, the researchers removed the surface bonding from the walls to document the damage to the structure. Some units displayed significant concentrations of cracking and crushing along the top and bottom units of the walls. However, Murray found inconsistencies between the compressive capacities of these walls with and without a thin layer of mortar along their bed joints. Further tests related to the compressive behavior of the walls are recommended. Beyond this, Eixenberger and Fonseca’s (2016) research furthered this because it looked at how grouting, reinforcement, and shear capacity seem to be affected by each other.
More recently, Eixenberger and Fonseca (2016) conducted shear tests that showed that as the percentage of grouting and reinforcement increased, the shear capacity increased as well. Subsequently, comparing their results with equations from the MSJC and the International Code Council (ICC) equations, the researchers concluded that the MSJC equations overestimated and the ICC underestimated the shear capacity of the walls. In a later study, Eixenberger (2017) concluded that ENDURA units provided limited shear resistance; however, grout within the cells and surface bonding played important roles in determining the shear strength of the walls.
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Fig. 2-19. Concrete units used in dry-stack system [Source: Murray 2007, recreated with
permission].
These new masonry technologies and construction methods focus on pre-stressed and surface bonding mortarless masonry to reduce the need for highly skilled masons, allowing for faster construction and increasing structural performance.