Sr. Principal Scientist, CSIR-Structural
Engineering Research Centre (SERC), Chennai
In the present, traditional construction materials are of high demand due to scarcity on availability of natural resources such as sand, aggregates, clay bricks, timber etc. India’s urban population is expected to be increased in many fold over the period and huge challenges are expected to meet the basic need especially affordable housing. The current demand of the hour is to build a house with quality, speedy and affordability. Considering the above all, large wall and roof panels are developed using expanded Polystyrene (EPS) as inner core material and Wythes with Self-Compacting Concrete (SCC). The total thickness of panels comes to 150 mm which consists of bottom and top Wythes of 25 mm and EPS of 100 mm thickness. There is two third reduction in weight is found as compared to conventional concrete wall panels. Parametric studies on panels of different configurations have been carried out under flexural and axial loading. The results indicate that suitable panels shall be designed for a given span by introducing shear beams and reinforcement in tensile zone. A G+1 story building is fabricated with large wall and roof panels using EPS and studied for its behaviour under seismic load of different earthquake intensity. Further, push over study also conducted on this structure and the outcome of the study is very encouraging. The overall study conducted on these panels indicate that materials of this nature is very good for mass housing and also it has got more resistant towards lateral load due its material property and light weight.
Details of Expanded Polystyrene Panels
Expanded polystyrene (EPS) is a light-weight polymeric material made by a temperature controlled chemical process. Two mesh sizes such as 50mm and 100mm are available having 2.0 mm dia. wires. The wire meshes are connected by truss-type continuous shear connectors that are oriented along the longitudinal (spanning) direction of the panel, and hence, the panel can effectively resist bending only in the spanning direction. The number of trusses (#13) are same for all panels. The shear connector wires are inclined at 45° and are connected to the wire meshes by weld. Expanded Polystyrene (EPS) is used as the core. The density of EPS ranging from 18-20 kg/m3 and the tensile strength of wires ranging from 615 – 715 N/mm2. Welding strength of steel mesh for single point determined by pull–out found to be 46.0 kg. Typical view of EPS with steel mesh is shown is Fig.1.
Development of Wall and Roof Panels
EPS panel of size 3000 × 1200× 100/150 mm (Length × Width × Thickness) is considered to develop Concrete Sandwich Panels (CPS) by providing 25 mm thick Self Compacting Concrete (SCC) of M40 grade on both top and bottom sides. Stiffening beams were also provided on both edges along the spanning directions (i.e.3000 mm) to increase the capacity for the requirement for the floor panel. Edge beams were provided at supporting ends for the wall panels to achieve full capacity of the panel under composite action. Figure 2(a) shows schematic sketch of Concrete Sandwich Panels (CSP), and Fig. 2(b) shows exploded view of panels showing details.
Casting of Concrete Sandwich Panels
The sequence of casting the panel is shown in Fig. 3. A steel mould of size 1200 x 3000mm was placed on a level surface and SCC was poured to a depth of 25mm to form bottom wythes. EPS with wire mesh was placed over the concrete. SCC was then poured on the EPS to form top Wythes of 25mm thickness. Stiffening concrete beams were provided along the supporting edges to avoid failure due to local crushing of concrete. The panels manufactured were cured for 28 days. The manufacturing methods reported in the literature involved either plastering on the EPS panel using cement mortar or placing normal concrete on the EPS panel and vibrating for achieving better compaction of concrete. These methods require skilled labour and sufficient time for casting and finishing the panel. The method of manufacturing adopted in this paper does not require highly skilled labour. Time taken for casting a panel is 30 minutes. This method of manufacturing light-weight concrete sandwich panel using ready-made EPS panel and SCC is expected to suit mass production of the panels.
Behaviour of Roof Panels under Flexural Loadings
The Concrete Sandwich Panels were tested under four-point bending. This type of loading was chosen because of constant bending moment being developed between the loading points. Displacement controlled loading was applied until the panels failed. One edge of the panel was supported by a hinge and the other was supported by a roller. It was ensured that the supports were provided on the stiffening beams. Linear Voltage Displacement Transducers (LVDTs) with 50mm range were used to measure the deflections of the panels. Strain gauges with gauge length of 2 mm and 30 mm were used to measure the strains on the wires and concrete surface, respectively. Photograph of a panel ready for testing is shown in Fig. 4.
The following observations are noted when the different configuration of floor panels tested under flexure. Presence of reinforced edge ribs oriented along the spanning direction and re-bars in bottom Wythe significantly affects the failure mode, failure load, load-deflection behavior and load-strain behavior of precast concrete sandwich panels. Panel thickness affects the failure load more than the grid size of the wire mesh. Presence of reinforced edge ribs preclude shear failure of concrete sandwich panels when re-bars are present in bottom Wythe together with wire mesh. The flexural behavior of the panels with reinforced edge ribs was similar to conventional RC one-way slab behavior. Presence of reinforced edge ribs and/or re-bars in bottom Wythe is required to increase the failure load of the panels. Spacing of flexural cracks is significantly affected by grid size of the wire mesh and cracking behavior is similar to ferrocement cracking behavior. Wires of the mesh and shear connectors are found to be effective until panel failure. Presence of re-bars in bottom Wythe or lowering grid size of wire mesh is required to increase the load at which the mesh wire reach its yield strain. Presence of re-bars in bottom Wythes and lowering grid size of wire mesh reduced strain in the shear connector.
Concrete Sandwich Panel under Axial Compression
The support condition of the panels simulated pin-ended at both top and bottom edges. Axial compressive load was applied through a 2000kN capacity hydraulic jack. Load was distributed uniformly by a rigid plate placed on top of the panel. Care was taken to avoid eccentricity in the applied loading. A load cell was placed between the jack and the distributor beam to measure the load transferred. As a safety measure to prevent sudden fall of the panel, angles were placed horizontally on either sides of the panel ensuring that these angles do not act as lateral restraints. For all the panels tested the load was gradually increased until they failed. Figure 5 shows a typical panel ready for testing.
Panels under axial compression for different configuration on presence/absence of stiffening concrete beam, thickness and reinforcement mesh size are summarized below. Providing stiffening concrete beams near the loading and supporting edges improves the composite action achieved by panels. Stiffening concrete beam precludes panel failure due to crushing of Wythes. Location of failure cross section of CSP under axial compression is dependent on the slenderness ratio of panel. Predictability of the ultimate load of these panels based on strength equations available for RC walls may be checked and suitable modification if required may be made so as to derive a semi-empirical design equation.
Development of Light Weight Large Concrete Sandwich Panels for G+1 Building
A typical room size of 3.60 x 3.60 m with the floor height of 3.0 m was considered to study the behaviour of concrete sandwich panels of large size in constructing G+1 story building. Openings were also provided in the panels for doors of size 0.90 x 2.10 m and windows of size 1.00 x 1.50 m. The large size wall and roof panels were fabricated by placing rib beams in between and around the panels. This large panels of 3.6 m x 3.0 m can be achieved by joining three EPS panels of 1.2 m width. Rib beams also provided on the periphery of the openings. Fig.6 Shows the large wall panel of size 3.60 m x 3.00 m casting with window opening. Fig.7 Shows Large wall panel with door opening and also openings left for joining panel to panel.
Seismic Performance Evaluation of Prefabricated Building Constructed using Light Weight Large Concrete Sandwich Panels
A (G+1) single bay pre-fabricated building, with plan area of 3600 mm x 3600 mm and storey height of 3000 mm, made with lightweight wall and roof panels was assembled and rigidly mounted on the 4m x 4m shake table at CSIR-SERC and subjected to realistic earthquake motions (Fig.8). Spectrum compatible time history was applied in a progressively increasing manner and the response of the structure was studied. The design spectra composed of a band of frequencies specified by the Indian code of practice IS-1893 for earthquake resistant design of structures founded on medium stiff soil is made use of to derive the input displacement time history. There is “no-damage” response of the structure and the absence of any cracks or permanent deformations of the structure was observed. This shows that the structure has withstood the maximum earthquake loads successfully. This study clearly indicates that the prefabricated light weight wall panel building can be effectively used as seismic resistant building. The strain observed in this building is well within linear limits and never reached the yield strain.
In-situ Experimental Building with EPS Panels
Based on the Seismic Performance Evaluation of Prefabricated Building with EPS Panel, it is evident that these panels can be designed suitably for construction of seismic resistant building. In order to bring confident amongst the user, an experimental building was constructed with EPS panel at CSIR-CLRI Campus, Chennai during 2012. The execution of experimental building was carried out by construction RCC frame such as columns and roof beams. The EPS panels were placed over the roof beam for casting roof slab. Each EPS panel was reinforced at 750 mm intervals. The panels were joined by overlapping mesh as well as by tying rod laterally at every 400 mm intervals. The EPS panels were properly tied in roof beam with dowel bars. Concrete of 40 mm thick was placed over the EPS panels placed on the roof beam. For walls, the EPS panels were placed in between the RCC framed structure and the EPS panels are properly tied to RCC members through dowel bars. The construction details using EPS panels are shown in Fig.9.
Demo Building with Precast Light Weight Large Concrete Sandwich Panels
Prefab pre-cast large light weight wall and roof panels using EPS were used to construct a demo building in CSIR-SERC campus during 2013. Prefab large wall panels of size 3.60 m x 3.0 m with door 2 Nos. and without door openings 2 Nos., Wall Panels of size 3.25 m x 3.0 m with windows opening 4 Nos. and roof Panels of size 3.60 m x 3.60 m 2 Nos. All the panels were cast with an overall thickness of 150 mm. Fig.10 Shows the demo building with precast light weight large concrete sandwich wall and roof panels.
School Building Construction with EPS Panel
Construction of additional four class rooms for KV CLRI has been taken up using the CSIR-SERC. There are six technologies were adopted in the construction namely Self-Compacting Concrete (SSC), Prefab Light weight large wall panel using Expanded Polystyrene (EPS), Fly ash concrete, Geo-polymer concrete blocks, Fly ash bricks, Light weight foam concrete blocks. The total construction area is about 2600 square feet. Each classrooms having an area of 600 sq.ft. The cost of construction works out to be `1250 per sq.ft. The total period of project completion is about three months. The project was aimed to demonstrate the CSIR-SERC technologies as a showcase amongst the construction agencies.
Development of Fast, Durable and Energy Efficient Mass Housing Scheme, CSIR-SERC, Chennai
Structures with precast light-weight components is thus expected to have combined advantages of precast technology and light-weight structures. Precast light-weight structural components have advantages of easy handling and transportation systems. Considering the importance, at CSIR-Structural Engineering Research Centre (SERC), Chennai, and research work has been taken up to develop precast lightweight large wall and roof panels for mass housing. Nearly two-third of the panel thickness is replaced by EPS which reduce about 67% of weight of the structure without compromising structural efficiency. The connections and joints in the prefabricated building is a very vital factor in affecting the structural behavior. The main structural difference between precast buildings and cast-in-situ buildings lies in their structural continuity. The structural continuity of conventional cast-in-situ buildings is inherent and will automatically follow as the construction proceeds. For precast structures, there must be deliberate efforts to ensure structural continuity when jointing of precast components such as slabs and walls are made. The connections are the bridging links between the structural components. Structural considerations for stability and safety becomes necessary at every stage, as the structural elements in precast building will only form a stable structural system. A Complete package is planned to provide a solution for mass housing scheme.
Identifiable and significant benefits/advantages of using Light weight Concrete Sandwich Panels
- The entire structure consisting of is light in weight would carry gravity and lateral loads. Efficiently connected together to form rooms and contribute to the transfer of loads to the foundation.
- The reinforcement provided in uni-directionally contribute to structural strength of the system. The well detailed joints further make the system more ductile to resist seismic loads.
- The perfect bond between the inner and outer core makes the system integrated for composite action which further enhances the performance.
- Yet another unique feature of the system is that the joint that cast in-situ with a quick setting high strength material makes the jointing system simple and structurally adequate to resist the estimated loads.
- The entire system is thus a unique combination of strength, light weight, ductile and durable. Light weight / Alternate innovative material for building construction / Precast and Prefab material / affordable material.
- The panels perform excellently well as and are a unique combination of strength, lightweight, ductility and durability.
- A further contribution of the study includes innovative method of connecting individual core of panel units of size 3000 mm long x 1200 mm wide x150 mm thick to make room size elements.
- Room size wall and floor units which are produced off-site can be assembled and jointed efficiently on-site to form robust disaster resistant structures.
- The system provides satisfactory thermal and acoustic insulation for the constructed facility. Besides structural soundness the use of waste material in large quantities is also involved which adds to the reduction in cost and ecofriendliness of the system.
The CSIR-Structural Engineering Research Centre (SERC), Chennai has been actively engaged in the past and present to develop precast light weight large wall and roof panels using EPS as inner core material which reduces the panel weight by two third weight of the conventional RCC panels. The structural performance of these light weight panels have been evaluated for flexural, axial and seismic actions. The developed technology was demonstrated by construction a model, Creche building and school building. Technology was transferred to three agencies. Presently, CSIRSERC is currently working towards the development precast light weight large wall and roof panels including connections and joints using EPS for fast, durable and energy efficient Mass Housing Scheme.
a) PCI Committee, ‘ Precast Concrete Sandwich Wall Panels: State of the Art of Precast/ Pre-stressed Sandwich Wall Panels”, PCI Journal-Pre-cast Pre-stressed Concrete Institute , Vol.42, Issue 2,pp. 91-133, March/April 1997.
b) Flores-Johnson E.A., and Li Q.M, ‘Structural Behaviour of Composite Sandwich Panels with Plain and Fiber Reinforced Foamed Concrete Cores and Corrugated Steel Faces’ , Composite Structures, Vol. 94, Issue 5, pp.1555-1563, April 2012.
c) Amir F., and Sharaf T., (2010) ‘ Flexural Performance of Sandwich Panels comprising Polyurethane Core and GFRP skin and ribs of various configurations’, Composite Structures, Vol.92, pp.2927-2935, November 2010.
d) Stephen Pessiki, Alexander Mlynarczyk, ‘Experimental Evaluation of the Composite Behaviour of Precast Sandwich Wall Panels’, PCI Journal , Vol.48, Issue 2, pp.54-71, March/April 2003.
e) Thomas D.Bush and Zhiqi Wu, (1998) ‘Flexural Analysis of Pre-stressed Concrete Sandwich Panels with Truss Connectors, PCI Journal, Vol.43, Issue 5, pp.76-86, September/October 1998.
f) Benayoune, A., Abdul Samad, A.A., Trikha, D.N., Abang Ali, A.A and Ellina, S.H.M ‘Flexural Behaviour of Precast Concrete Sandwich Composite Panel-Experimental and Theoretical Investigations, Construction and Building Materials, Vol.22, No.4, pp.580-592, April 2008.
g) Salmon David C., Amin Einea, Maher K. Tadros and Todd D. Culp, ‘Full Scale Testing of Precast Concrete Sandwich Panels, ACI Structural Journal, V. 94, No.4, pp. 354-362, 2008.
h) IS 1893 (Pert I) 2002, ‘Indian standard criteria for earthquake resistant design of structures’ General Provisions and Buildings (Fifth Revision), Bureau of Indian Standards, New Delhi.
i) Annie Peter J, J.Prabakar and Nagesh R Iyer., ‘Precast Lightweight Large Panel Wall and Roof Elements for Seismic Resistant Building’, Proceeding of the International UKIERI Concrete Congress , Jalandhar, India, 5-8 March 2013., pp.612-624.