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shear key

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The purpose of the shear key is to improve the bonding between concrete surfaces. The shear key is usually made at the construction joint location to enhance the adhesion between the old and new casted concrete. Construction joint is considered a weak point because the concrete is not casted at one time, in large structures where the concrete can not be casted for some members at one time, such as a slab. We usually divide the slab and cast it at many stages. The large quantities of concrete will make it impossible to cast the slab at one time. Therefore, the casting will be done in several stages. The location of the construction joint should be selected carefully. Construction joint shall be made at the location of minimum shear and moment. 

The bonding between old and new concrete can be improved by roughening the old concrete and forming a shear key on the concrete surface while the concrete is still plastic. The shear key can be formed by pressing timber into the concrete surface to create a depression in the concrete surface, as shown in figure no:1. The timber should be soaked in water before use to ensure that it will not absorb the concrete moisture. The roughening of the concrete surface and creating shear keys will increase concrete surface area, which ensures a good bonding between old and new concrete pour.


Shear keys are not limited to cast-in-situ concrete. It is also constructed in precast concrete. In segmental bridges, shear keys are formed in the precast segment to ensure good bonding between bridge segments, as shown in figure no:2. Shear keys can significantly enhance the bonding between old and new concrete, and it can increase the resistance of lateral forces at the construction joint's location. 


Figure 2


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My name is Mohammad Hamdan, and I am a Civil/bridge Engineer with extensive professional background and experience in the field of construction. I have 10+ years of experience in the construction field. During my professional career, my emphasis has been placed on structural projects, mainly bridges, infrastructure and roads. Reading and practicing the design of structures is my passion.i was lucky to be a part of a big team that executed major highway interchanges. I believe in spreading the knowledge, thus, i compose these courses to share my experience and knowledge. My course focuses on the practical side of construction field. On other word, i am sharing some of what new engineers need to know about construction field. Beside working on construction field, i love writing some articles on my website ( curious civil engineer) Moreover, i wrote some engineering papers, you can check them  by searching my name on Google (Mohammad mamon Hamdan)