(Patent No. 223944)
Prof. Dr. S.P. Mukhopadhyay
Introduction
Multistoried buildings are nowadays built with R.C.C. frame structures. In this system of building technology the main frames are constructed first followed by brick or block masonry work. Such masonry elements, though non-load bearing type, are required to protect the built environment and configure the indoor spaces. In doing so they are required to withstand stresses caused by indoor activities and also due to changes in external weather conditions.
With the advancement of science and improved concrete technology the main frame structures are designed and constructed with much better care nowadays than ever before, while the masonry work is still being executed with days old practice. This poses a common problem while negotiating the gap between the last course of masonry work and the soffit of the corresponding R.C.C. element. Such gaps are some times as little as 10mm or less or as big as 50mm or more. The common practice is to fill the gaps with mortar packing horizontally from sides. Such mortar joints can not fill the gaps uniformly resulting into a weak union between R.C.C. member and the masonry work. When the building is subject to natural stresses like heat, sound etc. such weak unions develop cracks. This defect appears on the surface plaster as continuous or discontinuous horizontal hair cracks. This may occur in external walls as well as on internal partitions. The cracks on external walls cause two types of problems – aesthetic and weathering. The later is more nagging in a building.
In wet weather the water particles find their way through these cracks, accumulate within the gaps and cause dampness on interior surfaces.
In existing practice, a chicken-wire mesh is stretched across the joints between masonry and the R.C.C. elements before applying plaster. This is a kind of masking work and the study showed development of surface cracks even after applying this technique. This is because the weakness of the junction still remains. To address this problem a metallic devise, termed as Masonry Creep Bar, has been developed.
The Masonry Creep Bar ( Fig.1 and Fig. 2) is a four-fold (20mm, 20mm, 15mm and 4mm) Galvanized Iron (GI) sheet of suitable thickness as per area of use and with different lengths, preferably a meter long, for easy handling, packaging, transporting, stacking, positioning etc. However the MC Bars can be lap jointed at site to negotiate any length. The Masonry Creep Bars should run on the soffit of the entire lengths of the beams or slabs where the masonry work shall later terminate.
The first-fold of a Masonry Creep Bar is opened and placed on the bottom of the form work and held in position by cover blocks under the reinforcement bars. After casting and de-shuttering the first and the second folds of the Masonry Creep Bars remain in the corresponding R.C.C. element while the third and the fourth folds are opened and dipped in the mortar joints.
The Masonry Creep Bars act as blockers in the joints and also as anti slip elements to hold the two different materials, - concrete and the masonry, in position under stresses.
Although it is devised for buildings with R.C.C. frame structure, the Masonry creep bar with its anti-slip property can also be used as a shear-key in any construction work where similar weakness in union is envisaged.
Masonry Creep Bar
(Patent No. 223944, date of issue 24.09.2008, The Patent Office, Govt. of India)
Patent License available with Prof. (Dr) SP Mukhopadhyay. Interested people may contact Prof. (Dr) S.P. Mukhopadhyay.
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