The future of construction: Micro reinforcement
Tuesday, 11 September 2007
M. Firoze P. E.
ONE of the most versatile and most widely used materials in the world is concrete; in fact, its use is so widespread and common that it is hardly noticed. Concrete is a mixture of cement, which is a factory manufactured product, with naturally occurring materials such as sand and stone chips. It is mixed with water at a construction site. In most common applications 3000 psi [20 Mpa] strength concrete is used, throughout the country. With additives and admixtures and proper machinery, it is possible to make concrete with compressive strengths of up to 5000 psi [35 Mpa] strength. Many construction firms, in the country regularly make this grade of concrete. In the construction of skyscrapers, special concrete with compressive strengths of 11,000 psi [75 Mpa] have been achieved.
However, concrete alone would be fairly useless if it was not reinforced with steel bars. To understand the importance of steel one must understand the types of forces on a building. Basically there are two types of forces acting on a building structure: "compression" and "tension". Compression forces are resisted fairly well with concrete, but concrete fails immediately in any tensile force. It is here that steel reinforcement plays the pivotal role. Concrete with steel reinforcement is known as reinforced concrete or RC, for short. It is RC construction which makes apartment buildings, shopping malls and bridges possible.
In Bangladesh, as in any part of the world cement, steel making and the construction industry is a major economic activity which employs millions of people. In recent years construction activity has become costlier and difficult as global steel prices have continuously surged higher. While the reasons behind the rise are varied and would require an article in itself to discuss and understand, a few salient causes are worth mentioning. Foremost, among them is the rise of China and India as economic power houses; for example China has overtaken the Western world and Japan to emerge as the world's largest producer and consumer of steel in just 10 years. China's crude steel production today stands at an astounding 375 million tons. India's steel production has risen from 35 million tons to 51 million tons in just three years. Massive spending in housing and infrastructure in the oil-rich OPEC nations and Russia has fuelled a construction boom which has sent the global price of steel to skyrocket. As all iron and steel products are imported in our country, the price of steel has more then doubled in a span of less then three years. This has placed unbearable pressure on all sections of society as prices of long products or bars used in government and costly private constructions as well as C.G.I. sheets used in rural construction have risen likewise.
Steel reinforcement is supplied in round bar form with deformations or lugs on its surface usually in diameters 8mm to 25mm. It is commonly called "iron rods" or "M.S. rods" by common people. Steel reinforcements in the country is available in two strength classifications known as Grade 40 which has a yield strength of 36,000 to 40,000 psi [250 Mpa to 275 Mpa] and Grade 60 with yield strength of 60,000 psi [420 Mpa]. The Bangladesh Standards Testing Institution, BSTI for short, recognizes fice strength categories for steel reinforcement: 250 Mpa, 275 Mpa, 300 Mpa, 400 Mpa, and 500 Mpa. The BSTI is the legal authority to administer and regulate the production and sale of all construction related materials.
Steel of Grade 500 strength and in sizes below 8mm were not available in the market, till BSRM introduced three smaller sizes -- 7.1 mm, 5.7mm and 4.5mm in the 500 Mpa strength categories -- late last year.
Extensive testing and evaluation of the micro reinforcements, of the above three sizes were carried out in BUET and in the House Building Research Institute laboratories, besides BSRM's own testing facility. The products 5.7mm and 7.1mm of Grade 500 Mpa [72,500 psi] yield strength have been successfully applied in the construction of residential slabs of apartments in more then 20 major sites in Dhaka and Chittagong. The savings in steel consumption are truly remarkable: 40% less steel then Grade 40 [275 Mpa] strength bar is required! A combination of high strength and lower bar diameters makes the savings possible. Besides, the larger number of small size reinforcement provides more uniform stress distribution to the structure, thus making the building safer. It is worth mentioning that the savings do not come at the expense of quality or safety in construction.
This is the first time high strength reinforcement bars in 500 Mpa strength categories were successfully used for construction in the country. Though the bars were initially used in Dhaka and Chittagong city-based construction, it was subsequently tried out very successfully in rural construction as a cost effective substitute for C.G.I. sheet in North Bengal.
As an example the cost of building a pitched C.G.I. sheet roof with wooden truss is Tk.95,000 for a 35 feet by 25 feet, single storey bricked wall construction. A light weight concrete slab construction using 5.7mm and 7.1mm Gr.500 strength reinforcement costs only Tk.70,000. If the life cycle cost of materials is factored, the savings are even more dramatic as the concrete slab will last for at least 15 to 20 years with minimum or no maintenance. The C.I. sheet roof does not last beyond three to five years.
Thus the direct cost savings in using light weight slab over C.I. sheet is 26%. The fact that in 15 years the C.I. sheet roof would have to be re-constructed thrice, while the reinforced concrete roof would remain untouched, the life cycle cost of the concrete roof is only 1/4 that of a C.G.I. sheet construction. Besides, the concrete roof provides a level of comfort and hygiene, not possible with C.G.I. sheet.
The concrete slab is more environmentally friendly as it does not contain lead which is commonly used in the galvanizing of C.I. sheets. The lead wears out in rainwater and carried in to the groundwater from where it gets trapped in the food cycle.
This is potentially deadly, as continuous buildup in lead concentrations in the soil and groundwater will have serious health consequences for humans and domestic animals as well.
The use of 500 Mpa strength 4.5mm diameter reinforcement has found immediate applications in the economic construction of slab on grade: industrial flooring, vehicle parking lots, and food grain storage ware-houses, in rural areas. This very low diameter bar has a unique property to form extremely high strength bonding with concrete due to its smaller diameter and closer spacing matrix. It is a very economic reinforcement for preventing shrinkage or temperature cracks in concrete.
The major application of 4.5mm diameter, 500 Mpa strength reinforcement will be in pre-cast light weight hollow cylindrical poles to replace bamboos in rural construction. This will be a boon for the environment as bamboos are essential forest plant and prevent soil erosion in river banks. Besides, the concrete hollow poles will have a much longer life as it will be immune to termite attack and rotting in rain water.
The welding of wire reinforcement in to ready made meshes is the next logical step in the development of wire reinforcement. Reinforcement in mesh form reduces site placement time by 90% and by extension the financing interest costs of the construction project. It is often said that Europe, especially Germany, recovered rapidly from the ruins of the Second World War due to welded wire mesh! It is a truism which could be repeated for an endemically poor country like ours, by reducing quantity and cost of construction materials and the time to complete it.
Cost effective and durable housing for the rural population is one of the crucial national challenges for the country.
At a national level we continue to endure immense waste, through overuse of steel. This is due to outdated grades of steel [Grade 250 and 275] used widely in the country but nowhere in the world. As example European steel standards recognizes one strength category of reinforcement only, Grade 500. With concrete of appropriate strength, this will translate in 40% reduction in mass of material used.
The country's annual construction steel consumption, which is entirely imported, is at around two million tons with an estimated value of U.S. $ 1.2 billion. Thus the potential savings is U.S. $ 480 million in hard foreign currency.
All the new micro reinforcements comply with the requirement of Bangladesh National Building Code (BNBC) and conforms to international standards such ISO 6935 and ASTM A-496. Micro reinforcements are globally used in all types of construction.
The writer is General Manager, Product Development, BSRM
ONE of the most versatile and most widely used materials in the world is concrete; in fact, its use is so widespread and common that it is hardly noticed. Concrete is a mixture of cement, which is a factory manufactured product, with naturally occurring materials such as sand and stone chips. It is mixed with water at a construction site. In most common applications 3000 psi [20 Mpa] strength concrete is used, throughout the country. With additives and admixtures and proper machinery, it is possible to make concrete with compressive strengths of up to 5000 psi [35 Mpa] strength. Many construction firms, in the country regularly make this grade of concrete. In the construction of skyscrapers, special concrete with compressive strengths of 11,000 psi [75 Mpa] have been achieved.
However, concrete alone would be fairly useless if it was not reinforced with steel bars. To understand the importance of steel one must understand the types of forces on a building. Basically there are two types of forces acting on a building structure: "compression" and "tension". Compression forces are resisted fairly well with concrete, but concrete fails immediately in any tensile force. It is here that steel reinforcement plays the pivotal role. Concrete with steel reinforcement is known as reinforced concrete or RC, for short. It is RC construction which makes apartment buildings, shopping malls and bridges possible.
In Bangladesh, as in any part of the world cement, steel making and the construction industry is a major economic activity which employs millions of people. In recent years construction activity has become costlier and difficult as global steel prices have continuously surged higher. While the reasons behind the rise are varied and would require an article in itself to discuss and understand, a few salient causes are worth mentioning. Foremost, among them is the rise of China and India as economic power houses; for example China has overtaken the Western world and Japan to emerge as the world's largest producer and consumer of steel in just 10 years. China's crude steel production today stands at an astounding 375 million tons. India's steel production has risen from 35 million tons to 51 million tons in just three years. Massive spending in housing and infrastructure in the oil-rich OPEC nations and Russia has fuelled a construction boom which has sent the global price of steel to skyrocket. As all iron and steel products are imported in our country, the price of steel has more then doubled in a span of less then three years. This has placed unbearable pressure on all sections of society as prices of long products or bars used in government and costly private constructions as well as C.G.I. sheets used in rural construction have risen likewise.
Steel reinforcement is supplied in round bar form with deformations or lugs on its surface usually in diameters 8mm to 25mm. It is commonly called "iron rods" or "M.S. rods" by common people. Steel reinforcements in the country is available in two strength classifications known as Grade 40 which has a yield strength of 36,000 to 40,000 psi [250 Mpa to 275 Mpa] and Grade 60 with yield strength of 60,000 psi [420 Mpa]. The Bangladesh Standards Testing Institution, BSTI for short, recognizes fice strength categories for steel reinforcement: 250 Mpa, 275 Mpa, 300 Mpa, 400 Mpa, and 500 Mpa. The BSTI is the legal authority to administer and regulate the production and sale of all construction related materials.
Steel of Grade 500 strength and in sizes below 8mm were not available in the market, till BSRM introduced three smaller sizes -- 7.1 mm, 5.7mm and 4.5mm in the 500 Mpa strength categories -- late last year.
Extensive testing and evaluation of the micro reinforcements, of the above three sizes were carried out in BUET and in the House Building Research Institute laboratories, besides BSRM's own testing facility. The products 5.7mm and 7.1mm of Grade 500 Mpa [72,500 psi] yield strength have been successfully applied in the construction of residential slabs of apartments in more then 20 major sites in Dhaka and Chittagong. The savings in steel consumption are truly remarkable: 40% less steel then Grade 40 [275 Mpa] strength bar is required! A combination of high strength and lower bar diameters makes the savings possible. Besides, the larger number of small size reinforcement provides more uniform stress distribution to the structure, thus making the building safer. It is worth mentioning that the savings do not come at the expense of quality or safety in construction.
This is the first time high strength reinforcement bars in 500 Mpa strength categories were successfully used for construction in the country. Though the bars were initially used in Dhaka and Chittagong city-based construction, it was subsequently tried out very successfully in rural construction as a cost effective substitute for C.G.I. sheet in North Bengal.
As an example the cost of building a pitched C.G.I. sheet roof with wooden truss is Tk.95,000 for a 35 feet by 25 feet, single storey bricked wall construction. A light weight concrete slab construction using 5.7mm and 7.1mm Gr.500 strength reinforcement costs only Tk.70,000. If the life cycle cost of materials is factored, the savings are even more dramatic as the concrete slab will last for at least 15 to 20 years with minimum or no maintenance. The C.I. sheet roof does not last beyond three to five years.
Thus the direct cost savings in using light weight slab over C.I. sheet is 26%. The fact that in 15 years the C.I. sheet roof would have to be re-constructed thrice, while the reinforced concrete roof would remain untouched, the life cycle cost of the concrete roof is only 1/4 that of a C.G.I. sheet construction. Besides, the concrete roof provides a level of comfort and hygiene, not possible with C.G.I. sheet.
The concrete slab is more environmentally friendly as it does not contain lead which is commonly used in the galvanizing of C.I. sheets. The lead wears out in rainwater and carried in to the groundwater from where it gets trapped in the food cycle.
This is potentially deadly, as continuous buildup in lead concentrations in the soil and groundwater will have serious health consequences for humans and domestic animals as well.
The use of 500 Mpa strength 4.5mm diameter reinforcement has found immediate applications in the economic construction of slab on grade: industrial flooring, vehicle parking lots, and food grain storage ware-houses, in rural areas. This very low diameter bar has a unique property to form extremely high strength bonding with concrete due to its smaller diameter and closer spacing matrix. It is a very economic reinforcement for preventing shrinkage or temperature cracks in concrete.
The major application of 4.5mm diameter, 500 Mpa strength reinforcement will be in pre-cast light weight hollow cylindrical poles to replace bamboos in rural construction. This will be a boon for the environment as bamboos are essential forest plant and prevent soil erosion in river banks. Besides, the concrete hollow poles will have a much longer life as it will be immune to termite attack and rotting in rain water.
The welding of wire reinforcement in to ready made meshes is the next logical step in the development of wire reinforcement. Reinforcement in mesh form reduces site placement time by 90% and by extension the financing interest costs of the construction project. It is often said that Europe, especially Germany, recovered rapidly from the ruins of the Second World War due to welded wire mesh! It is a truism which could be repeated for an endemically poor country like ours, by reducing quantity and cost of construction materials and the time to complete it.
Cost effective and durable housing for the rural population is one of the crucial national challenges for the country.
At a national level we continue to endure immense waste, through overuse of steel. This is due to outdated grades of steel [Grade 250 and 275] used widely in the country but nowhere in the world. As example European steel standards recognizes one strength category of reinforcement only, Grade 500. With concrete of appropriate strength, this will translate in 40% reduction in mass of material used.
The country's annual construction steel consumption, which is entirely imported, is at around two million tons with an estimated value of U.S. $ 1.2 billion. Thus the potential savings is U.S. $ 480 million in hard foreign currency.
All the new micro reinforcements comply with the requirement of Bangladesh National Building Code (BNBC) and conforms to international standards such ISO 6935 and ASTM A-496. Micro reinforcements are globally used in all types of construction.
The writer is General Manager, Product Development, BSRM