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Abstract
Ferrocement is ideally suited for thin wall structures as the uniform distribution and
dispersion of reinforcement provide better cracking resistance, higher tensile strengthto-weight
ratio, ductility and impact resistance. By adapting available mechanised
production methods and proper choice of reinforcements it can be cost competitive in
industrialised countries. Research and development works of ferrocement, at the
National University of Singapore, since early 1970's, has resulted in several
applications such as sunscreens, secondary roofing slabs, water tanks, and repair
material in the building industries. The salient features of the design, construction, and
performance of some of these applications of ferrocement structural elements are
highlighted in this paper.
Introduction
In the early 1970's, labour intensive ferrocement construction was viewed as particularly suitable for
rural applications in developing countries. In urban environment like Singapore and other developed
countries, the applications of ferrocement must be viewed from a different perspective due to the
competitiveness in the construction industry and the increase in labour cost coupled with shortage of
skilled construction workers. In order to alleviate these problems, mechanised production and proper
choice of reinforcements must be pursued to ensure the cost competitiveness and speed of construction.
The National University of Singapore has since early 1970's made effort to popularise ferrocement as
a construction material through research and development. Extensive investigations were carried out on
its mechanical properties and several prototypes structural elements were built to demonstrate
construction technique and to evaluate their performance in service [1-10]. From the experiences gained
in these studies, considerable progress has been made in the use of ferrocement in public housing in
Singapore as well as neighbouring countries. Ferrocement structural elements have gained gradual
acceptance by the building authorities through research and development even though ferrocement
design has not been regulated by a formal code of practice. ACI publications [11,12] also provide useful
guidelines and information of technical know-how.
Ferrocement has a very high tensile strength-to-weight ratio and superior cracking behaviour in
comparison to reinforced concrete. This means that ferrocement structures can be relatively thin, light
and water-tight. Hence it is an ideally suited material for thin wall structures. A team of researchers at the
Department of Civil Engineering, National University of Singapore, has collaborated with the local housing
authorities and precast industries to introduce precast ferrocement structural elements in public housing.
Several case studies involving the adaptation and successful implementation of the research results into economical and beneficial applications of ferrocement. The salient features of the design, construction
and performance of these ferrocement structural elements are discussed briefly in this paper.
2. Sunscreens
In Singapore, the housing developments consist of multistorey apartment blocks. All the west-facing
blocks were, in general, provided with cast-in-situ reinforced concrete sunscreens to prevent direct
exposure to sunlight in the living rooms. In one occasion, three building estates were completed without
sunscreens in some of the blocks. The existing design of reinforced concrete sunscreens was too bulky
and heavy for long spans more than 3 m and also cumbersome connection details for the precast
construction. A number of alternative designs using light weight materials such as glass fibre
reinforcement concrete, aluminum and ferrocement were carefully assessed and compared with
conventional reinforced concrete. Considerations in terms of the ease of handling and erection,
architectural requirements, durability and overall cost led to the choice of ferrocement as the most suitable
alternative material in this application.
An inverted L-shape sunscreen module of length 2.7 m were proposed with bolted connections. In
the design of these sunscreens, due considerations were given to the aesthetic and functional
requirements. The top face of the flange was provided with a backward slope to flush out, by rain, the
accumulation dust without staining the front face to reduce the cost of maintenance. The design service
load consisted of a concentrated live load of 5 kN applied either vertically or horizontally at mid-span and a
wind load of 0.6 Pa in addition to self-weight. A thickness of 25 mm was found to be sufficient for the
design loadings. The reinforcement consisted of two layers of fine welded galvanised wire mesh, 1.2 mm
in diameter with a 12.5 mm square grid, separated by a layer of coarser welded wire mesh of diameter of
3.3 mm and a square grid of 150 mm (Fig. 1). For the mortar matrix, the mix proportions of
cementsand:water by weight was 1 :2:0.5. The sunscreens were cast in steel moulds in a precast factory.
After the necessary curing, they were painted and transported to the site. A special lifting device was
used during erection.
Three stainless steel bolts were used to connect the sunscreens to the existing structures at each
support; one 16 mm in diameter at the rear and two 12 mm in diameter at the front. A total of 500
sunscreens were installed on the 11-storey apartment blocks in three different estates. A typical block
after installation is shown in Fig. 2. It can be seen that the slender design achieved by using a
ferrocement imparts a graceful appearance to the buildings.
Another type of ferrocement sunscreens were installed in several partially completed apartment
blocks. The design should be flexible enough for the long spans ranging from 3 m to 5 m, with the ends
supported on two 200 mm thick short cantilever beams of depth 600 mm attached through the facade to
reinforced concrete walls. The sunscreens comprised a flat panel 0.6 m x 4.0 m with a thickness of 40
mm except for a 90 mm x 90 mm edge beam at the top. The design loads were the same as in previous
case. Two layers of galvanised fine wire mesh of 12.5 mm square grid and 1.2 mm wire diameter
separated by a layer of skeletal steel of 100 mm square grid and 6 mm diameter were used as
reinforcements. The mortar strength was 35 MPa and the mix proportions of the cement, sand and water
in the ratio of 1:1.5:0.4 by weight was used. The cracking strengths of the ferrocement composite in
flexure and direct tension were 7.9 MPa and 3.1 MPa, respectively. These strengths were checked against the stresses due to dead load, wind load and accidental live loads with the appropriate factors of
safety. The panels were also checked for deflection and natural frequency because of its slenderness.
The front panel of the sunscreens was cast in steel moulds in a precast factory as shown in Fig. 3
with the projecting steel bars for continuity of reinforcements with the supporting cantilever. After the
necessary curing, the sunscreens were transported to the site. Prefabricated steel trusses were attached
onto the reinforced concrete walls and bolted to the facade by a proprietary bolting system. The
sunscreens were lifted into position and the continuity steel bars spot-welded onto the steel trusses.
Additional galvanised fine wire meshes were assembled around the steel trusses and the supporting
beams cast-in-situ against precast permanent formwork.
The ferrocement sunscreens were much thinner and significantly lighter than the existing reinforced
concrete ones of similar design. The slender design of the ferrocement sunscreens give them a more
elegant and aesthetically appealing appearance. A typical block of apartment with the sunscreens
installed is shown in Fig.4. The building authorities have now accepted without any reservation the use of
ferrocement in sunscreens and facades.
3. Secondary roofing slabs
In tropical countries, secondary roofing slabs are installed on the roof top of the buildings to insulate
against intense heat. In Singapore these slabs consist typically of 1500 mm x 600 mm x 50 mm precast
cellular concrete slabs containing a centrally placed layer of galvanised welded wire mesh of 50 mm
square grid and 3.25 mm diameter. The slabs were assembled side by side, each being supported on
150 mm x 150 mm x 225 mm precast hollow blocks placed on the top of the structural roof to provide as
air gap of 225 mm. The cellular concrete mix has a sand:cement ratio of 2.2 with a density of about 1500
kg/m3. These slabs pose a problem of severe cracking even before they are transported and erected in
place. Although the presence of cracks may not be critical with respect to strength requirements, they are
undesirable from a durabil of view. there is a need to such slabs at least once every 10 years.
A study was carried out at National University of Singapore to examine the current design with the
intention of improving durability of the slabs. A ferrocement design of 30 mm thickness with two layers of
galvanised fine wire mesh of 25 mm square grid and 1.6 mm wire diameter separated by a layer of
skeletal steel of galvanised welded wire mesh of 150 mm square grid and 3.3 mm diameter, as shown in
Fig. 5, was found to be adequate. Because of the reduced thickness, the dead weight of the ferrocement
slabs remains approximately the same as that of the cellular concrete slabs.
The functionality of such slabs was investigated by carrying out flexural tests, patch load tests and
shrinkage measurements on specimens. It was found that the slabs could be subjected to deSign service
load of 1.8 kN patch load, two days after casting without cracking. The slabs also registered low long term
shrinkage of about 400 microns.
The effects of weathering and thermal fluctuations were also studied. Slabs subjected to alternate
wetting and drying test do not show any deterioration in first crack or ultimate strengths. Cyclic
compression test to simulate the effect of thermal stresses due to heating in the day and cooling at night
did not affect the strength significantly. Comparison in terms of production costs shows the ferrocement
slabs to be slightly more expensive than the cellular slabs. However, it is expected that with ferrocement
slabs the frequency of replacements will be reduced. The cost can be reduced through increasing
productivity by demoulding them in the shortest possible time, minimising the controlled curing period and
installation on site at the earliest time with less number of spoils during transportation and erection. The
recent experimental study using reliability analysis [13] shows that the ferrocement slabs can develop up
to 87 and 90 percent of their mean 28-day first crack and ultimate strengths, respectively, three days after
casting. The reliability study indicates that the ferrocement slabs used were safe against ultimate failure
one day after casting when subjected to both dead and live loads and in the case of first cracks with
respect to dead load alone. In another study [14] the durability of the ferrocement secondary roofing slabs
was investigated with respect to service life-cycle in relation to the actual load range that a typical slab
would experience. The results show that the slabs have good fatigue properties within the stress range
considered.