Ferrocast Structural Elements


Er.Arun N Purandare is a Graduate in Civil Engineering from VJTI Mumbai and a Post Graduate from Imeprial College Londan. He has been in practice from 1965 and has more than 1500 large projects to his credit including the D.Y.Patil Cricket Stadium Navi Mumbai and MCA Stadium, at Gahunje,Pune. He is also an Environmental Consultant dealing with the issue of solid waste management and disposal. He is a member of several professional organisations. He has read and writted papers nationally and internationally on Earthquake engineering and Landfill Design. He has designed and patented Ferrocast Precasting system and is carrying our research in this field.
Building industry is passing through a very difficult time. The quantum of construction is increasing very fast specially in residential and commercial projects. This increase is not matched by equal increase in the availability of trained labour force as well as supervisory personal. This has resulted in totally untrained and persons with little skills working in all trades. The obvious outcome is the serious drop in quality of executed work. One such trade is carpenters for shuttering. There is a serious shortage of carpenters and those doing this work are of very doubtful quality, leading to bad concreting, bad line and level and no dimensional accuracy. Similar problem is experienced with respect to masons for brick work and plastering. The only other remedy to improve quality is to precast the structural elements in specially set up casting yards and erect them at site.
Precasting industry, though in great need , has not taken root due to its heavy capital costs in plant and machinery. A system had to be devised which can be started with substantially less capital cost. All this is possible with Ferrocast Structural elements. The dead load of individual ferrocast member is about 1/3 of the full section R.C. precast member making it easy to handle in the casting yard. The subsequent transportation and erection becomes easier due to lighter dead loads. In case of residential buildings, the individual elements will not weigh more than 600/700 Kg, thus making use of 2/3 T.  Hydra – mobile crane more than adequate to carry out all lifting and shifting jobs in the casting yard. This reduced capital cost makes it very attractive even for “B” and “C” class towns well as on site casting yard.
The present system of making ferrocement element consists of hand plastering stiff mortar into weld mesh / chicken mesh layers fixed on welded reinforcement forming a required shape. Hand applied plaster on weld mesh forms has several inadequacies. The complete section formed with hand plaster will not have expected strength throughout the cross section and length. This is a major problem leading to Codes not offering a value for tensile as well as compressive capacity. The plastered section does not have dimensional accuracy. The final sectional thickness also cannot be ensured. This is a detriment to proper structural design of the member. To get over this difficulty, members in this system cast in moulds, with self compacting mortar. This ensures correctly cast section with correct section dimensions. This method has been developed for column, beam and slab elements.
As ACI does not give any design values for tensile, shear or compression capacity for ferrocement, this material was not used for the normal structural elements used in building construction. The present method of ferrocement construction, with variable and doubtful quality, must have led to this reluctance on the part of the persons drafting the recommendations. The method suggested was like R.C. design without taking into consideration the high tensile qualities of ferrocement. This quality problem is fully eliminated with the use of ferrocasting the structural members.
In order to get correct design stresses, research was started. Concrete was encased in a ferrocast ring with variable layers of weld mesh. The confinement provided by the ferrocast ring enhanced the crushing strength of M-30 concrete beyond. 60 MPa. This was double the concrete mix strength. This clearly indicated that the confinement offered by ferrocast ring is vastly superior to that possible with reinforcement used as binders and ties. The enhancement in crushing strength indicated large ductility for core concrete. This is great step forward. The cracking tensile stress in beam tests showed elastic behavior upto 25 MPa. This tensile capacity can be used for the slab and beam design for tension. Both these experiments are pointing to enhanced compressive stresses in column design and tension capacity in the bottom flange of beams. All this will lead to substantial savings in the construction of the building frame.
The new system consists of casting ferrcast tubes of 25mm wall thickness. Concrete is poured into the annular space to form the column. The beam sections are “U” forms resting on columns. Slab units are placed on the two sides of the beam. Negative steel for continuity of slabs and beams is tied over supports and topping concrete is poured. A perspective view of a proposed construction with all structural elements in place is shown. The advantages of this system are,
1.            Light Weight, leading to saving in concrete and steel
2.            Advantage in Seismic Design                              
3.            Smaller foundation
4.            Factory made product with assured strength
5.            Form finished, does not need plastering
6.            Slab every 7/8 days
7.            Cheaper than R.C. Frame