Glass Fibre Reinforced Gypsum (GFRG) Panel Building System

          Glass Fibre Reinforced Gypsum (GFRG), is a new building panel product, also known as Rapidwall, introduced in India, and manufactured by Rashtriya Chemicals & Fertilizers (RCF), Mumbai and FACT Ltd, Kochi. It is made essentially of gypsum plaster, reinforced with glass fibers. This product, suitable for rapid mass-scale building construction, was originally developed and used since 1990 in Australia by Rapidwall Building Systems. GFRG is of particular relevance to India, where there is a tremendous need for cost-effective mass-scale and rapid housing, and where gypsum is abundantly available as an industrial by-product waste. The GFRG building system can be designed to provide affordable mass housing for a wide variety of applications – from single storey to medium rise (multi-storey) structures. It is necessary to plan meticulously all the details related to the design and construction, as well as issues related to transportation, storage and erection,

Gypsum plaster has been used extensively in the building industry worldwide to provide high quality architectural and decorative finishes, and also in the production of plasterboard for interior partition walls. Until recently, gypsum plaster was considered to be too weak to be used for load bearing walls. The Australian technological breakthrough of combining glass fibrestrands with gypsum plaster, produced in an energy-efficient fluidized bed calcining process, resulted in GFRG wall panels, which had the desired properties of strength and water resistance. In India, the GFRG panels are made from processed phospho-gypsum (recycled industrial waste from the fertilizer industry). The Panel is manufactured in semi-automatic plants using slurry of calcined gypsum plaster mixed with certain chemicals including water repellent emulsion and glass fibre rovings, cut, spread and imbedded uniformly into the slurry with the help of screen roller. The panels are dried at a temperature of 275 ^oC before shifting to storage area or the cutting table. The wall panels can be cut as per dimensions and requirements of the building planned for construction.

Typical Dimension of GFRG building panel are 12m long, 3m high and 124 mm thick with modular cavities. Each 1.0m segment of the panel contains four cells; thus each panel has 48 modular cavities of 230 mm x 94 mm x 3m dimension. The cellular cavities are formed between the two outer skins (15 mm thick) and the inter-connecting solid ribs (20 mm thick), spaced at 250 mm intervals (Fig.1). This structural arrangement makes the wall panels very light, being only 10-12% of the weight of comparable concrete or brick masonry.

Fig.1: GFRG Panels - (i) Overall dimensions and (ii) enlarged view of a typical cell

GFRG panels, on erection, may generally be used (i) as load Bearing Walling – With cavities filled with reinforced concrete is suitable for multi-storeyed structures; (ii) as partition walls in multi storeyed frame buildings;  and (iii) as horizontal floor/roof slabs with reinforced concrete micro beams and screed. This system can also be used in inclined configuration, such as staircase waist slab and pitched roofing (Fig.2).

 

Fig.2: Configuration of GFRG panels in building with pitched roof

The cavities in the panels, on erection, may be unfilled, partially filled in various combinations or fully filled, with reinforced concrete, as per structural requirement (Fig.3), to provide for additional strength and to improve ductility. The vertical and lateral load capability of GFRG wall Panel can be increased many fold by infill of concrete after placing reinforcement rods vertically.

Fig.3: Various combinations of cavity filling (plan section - grey shows concrete and the black dots show reinforcement, white shows unfilled cavities)

Experimental studies have shown that GFRG panels, suitably filled with reinforced concrete, possess substantial strength to act not only as load-bearing elements, but also as shear walls, capable of resisting lateral loads due to earthquake and wind. It is possible to design such buildings up to ten storeys in low seismic zones (and to lesser height in high seismic zones). The unfilled cavities in the GFRG floor panels can be effectively used to house and conceal various building services such as electrical conduits and piping. In fact the electrical / plumbing drawings should be such that most of the pipes go through the cavities (in order to facilitate minimum cutting of panel).

GFRG panel can also be used advantageously as non-load bearing walls in conventional RCC framed construction of multi-storey buildings without any restriction on number of stories.

Details of construction

            For construction, first each wall panel is cut at the factory with millimeter precision using an automated cutting saw. Door / window / ventilator, and other openings are cut out and panels for every floor is marked relating to its location in the building drawing.

            For GRFG buildings, a conventional foundation like strip footing, RCC column footing, raft, or pile foundation, depending on the soil condition and loading, is used. RCC plinth beam is provided where walls have to be erected. For erection of the panels as walls, 12 mm dia vertical reinforcement ( 0.75m long of which 0.45m protrudes up and remaining portion to be embedded with 0.15m angle) is placed into the RCC plinth beams before casting. These start-up rods are at 1m centre to centre.

            The panels brought from the factory are kept vertically in stillages placed at the construction site close to the foundation, for erection using vehicle mounted or other type of crane with enough boom length required for construction of low, medium and high rise building, as the case may be. The panels are erected over the RCC plinth beam. Protruded start-up rods go inside cavities (Fig.4a). 

Fig.4: Erection of walls - (i) Placement of panels over start up bars and (ii) props to support panels

            All the panels are erected as per the building plan by following the notation. Each panel is erected and supported by lateral props to keep the panel in level and plumb, and secured in position. Once wall panels are erected, door and window frames are fixed in position (Fig.4b). Embedded RCC lintels are to be provided wherever required. RCC sunshades too can be provided with required reinforcement, shuttering and support.

Concrete infill: After positioning the panels with vertical reinforcement rods inserted and clamps for wall corners in place to keep the wall panels in perfect position, concrete of 12 mm size aggregate is poured from top into the cavities using either hose (for directly pumping the concrete from ready mix concrete truck) or using a funnel (for small building constructions). Filling the panels with concrete is done in three layers of 1m height with an interval of 1 hr between each layer. There is no need to use vibrator because gravitational pressure acts to self compact the concrete inside the water tight cavities.

Joints: Wall to wall vertical angle joints of ‘L’, ‘T’, ‘+’ and vertical straight wall joints are made by cutting of inner or outer flanges or web appropriately and filling of concrete with vertical reinforcement with stirrups for anchorage (Fig.5). 

Fig.5: Various panel joints shown in plan section (L, T, + and straight)

            For floor/roof slabs the GFRG panels are cut to required size and marked with notation. First the wall joints and other cavities, and horizontal RCC tie beams are in-filled with concrete; finally roof panels are lifted by crane, floating the panel perfectly horizontal. Each roof panel is then placed over the wall in such a way with the ribs oriented along the shorter span, supported on GFRG wall panels. Each roof panel is positioned over the walls in such a way that there is a gap of 40 mm to enable vertical rods from the cavities of the bottom floor wall panels to proceed  continuously – from floor to floor – and provide a monolithic RCC frame within the GFRG panel wall. Reinforced concrete micro beams are provided at regular intervals (typically every third cavity) by cutting the top flange of the cavity wherever embedded micro-beams are to be provided (Fig.6b). Before cutting top flange, the roof panels are supported from underneath with ply wood suitably propped. Reinforcement suitably designed for micro-beams is placed in the cut open cavities and weld mesh as reinforcement for screed is positioned. An embedded RCC tie beam to floor slab is to be provided at each floor slab level, as an essential requirement of National Building Code against earthquakes. Side shuttering is provided for containing the concrete of RCC horizontal tie beams, micro-beams and screed floor, and concrete is poured. This results in the embedded RCC micro beams and 50 mm thickness screed concrete becoming a series of T-beams (Fig.6a)

Fig.6: Micro-beams in floor panel - (i) typical cross section and (ii) panels in position.

Erection of wall panel and floor slab for upper floor: After leaving the concreting of the floor slab for a day, erection of wall panels for the upper floor is arranged. Vertical reinforcement of floor below is provided with extra length so as to protrude to 0.45 m to serve as start up rods and lap length for upper floor (Fig.7).

Fig.7: Erection of upper floor panels

Once the wall panels are erected on the upper floor, vertical reinforcement rods are provided, door/window frames are fixed and RCC lintel cast. Then concrete is filled in wall cavities where required and in joints. The RCC tie beams and roof panel for upper floor is concreted. For every upper floor the same procedure is repeated.

Properties of GFRG

The following table provides some of the important properties of GFRG building panel (for both unfilled panels and panels filled with M20 concrete).

Property	Value
Unit weight	0.433 kN/m2
Uni-axial compressive strength - Unfilled; Filled	160 kN/m ; 1310 kN/m
Uni-axial tensile strength	35 kN/m
Water absorption	< 5%

For computer modeling for analysis, the following values may be adapted

Property	Value
Modulus of elasticity, E	7500 N/mm2
Out-of-plane flexural rigidity, EI, Rib parallel to span - Unfilled	3.5 x 1011 Nmm2/m
Out-of-plane flexural rigidity, EI, Rib perpendicular to span - Unfilled	1.7x1011 Nmm2/m

and for design, the following values may be considered

Property	Value
Design shear strength – Unfilled; Filled; Partially filled	14.4 kN/m 40 kN/m  14.4+25.4r  kN/m
Out-of-plane moment capacity, Rib parallel to span - Unfilled	1.4 kNm/m
Out-of-plane moment capacity, Rib perpendicular to span – Unfilled	0.59 kNm/m

Note: r = no. of filled cavities / total no. of cavities 

Design of GFRG elements 

Design Philosophy: The design capacities are based on limit states design procedures, considering the ultimate limit state for strength design. It should also satisfy serviceability requirements. as per IS 456: 2000. Earthquake resistant design is carried out in compliance with the requirements of IS 1893 (Part 1) : 2002, with  R values as 3.0 for seismic load calculations.

Axial Load Capacity: While assessing the axial load capacity of GFRG building panel used as load bearing wall, the possible eccentricities in loading are considered when arriving at capacity. According to IS 456: 2000 (Cl.32.2.2), the design of a reinforced concrete wall should take into account the actual eccentricity of the vertical force subjected to a minimum value of 0.05t (6.2 mm for panel thickness t = 124 mm).

In-plane Bending Capacity: GFRG panels can be used not only as load bearing walls, but also as walls transferring lateral loads, resisting axial force (P), lateral in-plane shear force (V) and in-plane bending moment (M).

As already mentioned, as GFRG panels with ribs aligned in direction of bending possess flexure, such panels can be used as flexural slab, whose strength can be significantly enhanced by embedding ‘micro beams’, filled with reinforced concrete and topped with RCC screed (provided with suitable welded wire fabric), together acting as T-beams. Unfilled GFRG panels can be used as pitched roofs for single storey small span buildings. The arrangement of reinforcements in the microbeams and screed has already been described earlier. The shorter span of slab (floor / flat roof) should be restricted to 5 m. For convenience in design, the contribution of GFRG towards the flexural strength can be ignored and the GFRG is treated as lost formwork. One way slab action may be assumed for strength and deflection check, considering T beam action of the embedded micro beams. The design of reinforcement in the micro beams should conform to the requirements of IS 456: 2000.

References and examples

For reference to design and construct GFRG Buildings, Building Materials & Technology Promotion Council (BMTPC), Government of India, has published a GFRG Structural Design Manual (prepared by IIT Madras), and FACT Cohin Division has published a Construction Manual for GFRG based constructions.

With the idea of demonstrating this building system using the technology developed, a two-storey GFRG demonstration building was constructed at the IIT Madras campus, on a built-up area of 1981 sq.ft (Fig.8a). It is a model housing apartment, combining four flats, which can be replicated for mass housing, vertically and horizontally. It also demonstrated the speed of construction of the entire superstructure and render it fit for occupation, with completion within 30 days from the laying of foundation.

IIT Madras has also designed an 8-storeyed GFRG building for 32 flats for construction at the RCF campus at Mumbai (Fig.8b). Comparison of this building with a conventional RC framed building revealed significant savings. For the same carpet area, the GFRG building needs 12% less built-up area and 75% less time for construction. The weight of the structure reduces by close to 60%, also reducing the foundation costs. The savings in terms of steel and concrete consumption are 24% and 63% respectively in the superstructure. Additionally, there is a tremendous saving in plastering.

Fig.8: Examples of GFRG buildings - (a) the 2-storey building at IIT Madras campus and (b) The designed 8-storeyed building for RCF campus, Mumbai

Conclusion

            GFRG Panel provides a new method of building construction in fast track, fully utilising the benefits of  GFRG panels and time tested, conventional cast-in-situ constructional use of concrete with steel reinforcement. By this process, man power, cost and time of construction is reduced. The use of scarce natural resources like river sand, aggregates and water is also significantly reduced.

-- Mar 2016, Rahul Leslie, Assistant Director, Buildings Design, DRIQ, Kerala PWD, Trivandrum, India

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