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Masonry Anchorage and Reinforcing

by admin on July 27th, 2010

Masonry anchorage devices and reinforcements both act as ways to stabilize the structure and add stability to the entire unit. These anchoring and reinforcement devices along with the masonry itself and frictional properties of the interface are the main components that provide stability to the entire unit. Masonry anchors are grouped into three distinct types – mechanical, powder-actuated and chemical. Chemical anchors and powder-actuated anchors are more complex, used in larger projects, and often require permits and certificates. Mechanical anchors are most common for smaller projects. Reinforced masonry serves the same purpose; however, the reinforcements are embedded into the material or masonry unit. These reinforcements can be in the form of bars, wires or strands.

Masonry anchors work in one of two ways – by expanding against the sides of a hole and gripping the concrete, or by friction against the sides of the hole. The strength and power of both reinforcement devices and anchor devices is dependent upon the actual property of the masonry unit and the placement and hold of the device. An important outside factor which will play into the decision of which type of device to use is that of load factor. Different types of loads will require different levels of support. Static loads put less stress on devices, and have little to no movement. Dynamic or vibrating loads, however, constantly change and require better reinforcement and anchorage. The common recommendation is that the device should be four times the weight it will carry if it bears static load and eight times the weight it carries if it carries a dynamic load.

Environmental considerations of masonry anchorage and reinforcing are similar to masonry mortar in that the application of mortar and these devices are often done at similar times, in similar conditions. For this reason, hot and cold weather conditions are a factor. it should also be noted that with steel anchors, corrosion is sometimes a concern. These anchors are often galvanized, or zinc-coated, to combat this effect.

Anchors and reinforcement units are designed to suit the specific needs of the project and to survive in the climate.  Anchor types are varied – concrete screws, hammer anchors, one-piece expansion anchors, two-step expansion anchors (sleeve anchors and wedge anchors). Sleeve and wedge anchors cannot be removed once installed. Hammer anchors are best for lightweight applications. Concrete screws are useful because they can be removed and used again, though they should always be used on smaller, lightweight applications. Steel reinforcing bars are used in the cells of hollow brick or concrete block to stabilize and strengthen the unit. These bars are commonly found in grouted walls, in both vertical and horizontal forms. In this instance, the bar does not touch the masonry unit, but is instead completely surrounded by grout. Bar positioners, or holders, are sometimes used to properly position the steel reinforcement bars.

Anchors are best placed when fastened into fresh mortar. Sketch or design a comprehensive layout of the structure, and where devices are planned to be placed. It should be noted that two anchors should never be placed closer than 10 diameters from one another, and an anchor should never be placed closer than 5 diameters from the edge of the masonry unit. The use of reinforcing devices should take into consideration the use of positioners and the grout to fill in hollow cavities.

Anchorage installation varies by the type of device used. However, a hammer drill with a masonry drill bit should always be used over a standard electric drill, as it is not as accurate and it has the potential to ream the hole. Each installation will add to the intended application: holding and stabilizing the masonry unit safely into place.

Adhesives acting as anchorage units should generally be avoided, as they often have short spans of bonding and can be affected by changes in moisture levels or temperature.