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Chapter: Civil : Housing Planning and Management : Construction Types And Cost Effective Materials

Cost Effective Technologies adopted by Building Centre

1) Rat Trap Bond 2) Stabilised Mud Block Walling 3) Stabilized Quarry Dust Solid / Hollow Block Walling 4) Filler-Slab Roofing 5) Prestressed Concrete 6) Precast Concrete

Cost Effective Technologies adopted by Building Centre


1) Rat Trap Bond


Rat trap bond is a technique by Lawrie Baker in which bricks are placed on edge leaving gaps within the wall. The strength of such a wall is the same as the traditional wall but the savings in quantity of bricks and cement mortar is upto 25% and for this bond no plastering is required.


A. Introduction To Rat Trap Bond Masonry (RTB)


A 'Rat-Trap Bond' is a type of wall brick masonry bond in which bricks are laid on edge (i.e. the height of each course in case of a brick size 230x110x75 mm, will be 110 mm plus mortar thickness) such that the shiner and rowlock are visible on the face of masonry as shown below.

This gives the wall with an internal cavity bridged by the rowlock. This is the major reason where virgin materials like brick clay and cement can be considerably saved. This adds this technology to the list of Green building technologies and sustainability for an appropriate option as against conventional solid brick wall masonry.


This cavity adds an added advantage as it adds a Green building feature of help maintain improved thermal comfort and keep the interiors colder than outside and vice versa.


The Rat trap bond construction is a modular type of masonry construction. Due care must be taken while designing the wall lengths and heights for a structure. The openings and wall dimensions to be in multiples of the module. Also the course below sill and lintel to be a solid course by placing bricks on edge. The masonry on the sides of the openings also to be solid as will help in fixing of the opening frame.


2) Stabilised Mud Block Walling


Here instead of bricks, locally available mud is compressed into blocks by a hand operated machine along with 5% cement by volume for stabilization and cured with water for 7 days. These wall made up of such blocks required no plastering. The cost saving is more due to 95% mud mortar used for bonding with each other. Some of the multi storied buildings have been built using mud.


3) Stabilized Quarry Dust Solid / Hollow Block Walling


The quarry dust which is available from stone crushers can be used to make cement stabilized compressed block. The hollow block this gives lesser quantity of material for a given size without compromising on the strength. The cost of construction is very much reduced by this method than the cost of traditional cement and   brick  walls.          The  airspace in    the          hollow  blocks      keeps  the   building  cool  in summer        and  warm  in       winter.   The rat        trap   bonded  walls       are  called as  thermal insulators.  The other variance of stabilized blocks include rubble stones compressed together as solid blocks.


4) Filler-Slab Roofing


This concept can be employed in reinforced cement concrete (RCC) roofs of buildings. Here bottom half of the RCC roof slab can have filler material such as old


clay (Mangalore) tiles, country bricks, water bottles or even coconut shells, instead of cement concrete. While laying the roof such filler material is placed between steel rods used for reinforcement and then concrete is spread over it. The strength of such a roof is no way affected by the presence of such filler material which is a cheaper substitute for costly cement. And there is no danger of the filler material falling on the head though it is always an option to plaster the roof from inside at additional direct solar radiation, due to the air trapped between the two tiles, so that such filler slab roof buildings are more comfortable to live in.



8) Prestressed Concrete


Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors, or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete.


Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.


Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction method using pre-stressing is the stressed ribbon bridge design.


The advantages of this system over unbonded post-tensioning are:


Large reduction in traditional reinforcement requirements as tendons cannot distress in accidents.


Tendons can be easily 'woven' allowing a more efficient design approach.


Higher  ultimate  strength  due  to  bond  generated  between  the  strand  and concrete.


No long term issues with maintaining the integrity of the anchor/dead end.


9) Precast Concrete


Precast concrete is a construction product produced by casting concrete in a reusable mold or ' form' Which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and cured on site. Precast stone is distinguished from precast concrete by using a fine aggregate in the mixture, so the final product approaches the appearance of naturally occurring rock or stone.


By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure  and  be closely  monitored  by  plant  employees. Utilizing  a precast    Concrete system offers many  potential   advantages   over   iste casting   of concrete. The production process for Precast Concrete is performed on ground level, which helps with safety throughout a project. There is greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced, which allow cost of formwork per unit to be lower than for site-production.


Many states across the United States require a precast plant to be certified by the Architectural precast Association (APA), National Precast Concrete Association (NPCA) or Precast Prestressed Concrete Institute (PCI) for a precast producer to supply their product to a construction site sponsored by State and Federal DOTs.


There are many different types of precast concrete, forming systems for architectural applications, differing in size, function, and cost. Precast architectural panels are also used to clad all or part of a building façade free-standing walls used for landscaping, soundproofing, and security walls, and some can be Prestressed concrete structural elements. Storm water drainage, water and sewage pipes, and tunnels make use of precast concrete units.



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