Precast Ferrocement Roofing Units

P.C.Sharma, Retd, Head Material Sciences S.E.R.C (G), Chairman Indian Concrete Instt. Western U.P. Ghaziabad Centre, Chief Editor, New Building Materials & Construction World, New Delhi.

Ferrocement is one of the most suitable construction materials for roofing systems is Precast or cast in situ form. It offers high strength, resistance against ingress/seepage of water, high crack resistance and can offer solution with reduced weight & cost. Casting with or without mould is possible. Many shapes and types of roofing systems have been developed such as trough shaped, segmental shell elements, domes, bamboo F.C. domes, F.C roofing has been used at large scale for housing, schools, hospital buildings, godowns, sentry posts, toilets etc. Single precast roofing units can be used to cover small structures. Roofing system with in between support elements can span larger roofs.

Precast Ferrocement Roofing Units
Photo 1: Making of mould 2.5m x 2.0m

This paper presents a very simple structurally sound F.C. roof which can be precast and used for smaller, medium span structures in rural areas and in urban slum development projects.

The whole process has been presented through five photo figures. The mould for such units can be prepared using a masonry, steel or wood. Masonry mould is the cheapest and easiest to make. When only few units are to be produced then a soil deposit mould with cement plaster lining at top is good enough. Photo 1 shows making a peripheral brick on edge lining and depositing of soil in shape for mould. Soil is compacted well to form the shape if mould and the top surface is plastered using 1:4 cement sand, mortar.

Precast Ferrocement Roofing Units Precast Ferrocement Roofing Units
Photo 2: Mould surface plastered and covered with plastic sheet, reinforcement being fabricated Photo 3: Wire mesh reinforcement being fixed

Photograph 2&3 show a thin plastic sheet laid over the mould and making a skeletal cage reinforcement for the roof. Two layers of hot dip galvanized woven wire mesh of 20 g x ½" x ½" were provided as mesh reinforcement one at top and 2nd at bottom of 5 cage. The casting matrix used, photo Figure 4, was designed to obtain 250 kg/cm2 strength at 28 days and mix used was one part of ordinary Portland cement and 2.5 parts of good quality graded sand. Water-cement ratio was maintained at 0.40 .Pidiproof L.W, a waterproofing chemical from Pidilite was used for improving water resistance and workability of mortar. The size of roof shown was 2.5 m x 2.0 m and the photographs belong to a trg programme organized by author for National Drinking Water Mission (when he was with SERC) at Agratalla for PHED Engineers.

An orbital vibrator developed at SERC (R&G) was used for compacting the unit. The thickness at top was kept as 2.5 cm and near the edge as 3.0 cm. Produced units were used for assembling roofs of 2.5 x 4.0m, Figure 1.

Precast Ferrocement Roofing Units
Figure 1: Top plan for twin unit

The Large areas can be covered by providing precast support beams in between. Larger units can be produced if erection facilities are available. A single unit for a one room low cost house designed as a composite unit having toilet and kitchen, can also be produced. Such a system can reduce the construction time drastically. The only disadvantage is its use as upper most roof or as single story construction.

The cost of such roofs come to almost at par with G.I sheet roof when life cycle cost is considered. Several other type of roofs have also been developed at SERC (R) and extensively used in field. Photo 5 shows a 2.5 m x 1.6 m extensively used for making army sentry posts.

Precast Ferrocement Roofing Units Precast Ferrocement Roofing Units
Photo 4: Casting of F.C. Roofing unit at Agartala (Tripura) Photo 5: Precast F.C. Roof for sentry post for BEG Roorkee. (1976) these units used at large scale by indian army

A do it yourself book on F.C. roof, written by author for international Ferrocement information centre AIT Bangkok, Thailand is in wide circulation since 1978. This publication provides a total package on Trough Shaped F.C. roofing unit up to 6 m span. We can produce such booklets for indian condition.

Large number of training programmes have been conducted by author for IFIC, Unicef, NDWM (Min of R.D Govt. of India), 35 point programme, CAPART etc in which F.C. roofing has been covered.

References:

  • Ferrocement roofing panels for single and multiple use - technical report 7/76, SERC Roorkee.
  • Rainwater harvesting and ferrocement structures for north eastern states in India - report & Trg manual, National Drinking water mission project structural Engg Research Centre Ghaziabad - 1998.
  • Ferrocement roof do it yourself manual - P.C.Sharma V.S. Gopalaratnam International Ferrocement Information Centre (IFIC) A.I.T. Bangkok (Thailand) 1978.
NBM&CW May 2011
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