SB13 Dubai

SB13 Dubai Paper - 174

STRENGTH AND ENERGY AUDIT OF REINFORCED CONCRETE SANDWICHED PANELS (RCSPs)

Nouman Khattak1,a, Asim Abbas1,b, Mohammad Adil1,c, Naveed Saeed2,d, Bashir Ahmad2,e, Pervez Saeed2f, Asad Mahmood3g 1

Civil Engineering Department, University of Engineering and Technology Peshawar, Pakistan NORWEST GROUP (Pvt) Ltd., Islamabad, Pakistan. 3 ENERCON, Government of Pakistan, Islamabad, Pakistan. 2

a

[email protected],[email protected],[email protected], [email protected],[email protected],[email protected],

d

g

[email protected]

ABSTRACT This paper addresses the strength and energy audit of reinforced concrete sandwiched panels (RCSPs) A Reinforced Concrete Sandwiched Panel (RCSP) is composed of an EPS (Expanded Polystyrene) foam core surrounded by spray-on reinforced concrete skins on both sides. The Reinforced Concrete Sandwiched Panel (RCSP) building is more than 50 years old technology, which was not well studied until very recently due to the demand of energy efficient and earthquake resistant structural requirements have emerged as one of the basic needs of modern buildings. Based on these needs RCSP Panels were subjected to different loading conditions including flexure, Axial and shear loading tests. The results of these tests were quite satisfactory. It was found that these panels can be used to construct buildings that can behave efficiently in case of earthquakes. Also there was a need to evaluate the energy efficiency of these panels to evaluate that how much energy these panels can save in the form of saving heating and air-conditioning costs. Although, the thermal conductivity of composites can be calculated mathematically, in order to evaluate RCSPs for application in a variety of weather conditions a device called “Hot-Box” will be produced locally to measure thermal conductivity of RCSP panels experimentally. Based on the tests performed on individual elements of RCSP composites by previous researchers it has been concluded that RCSP panels are energy efficient both hot and cold weather. KEYWORDS: concrete sandwich panel, thermal efficiency, energy efficient

1. INTRODUCTION TO RCSP A reinforced concrete sandwich panel (RCSP) is composed of an EPS (Expanded Polystyrene) foam core surrounded by spray-on reinforced concrete skins on both sides. A schematic of a typical RCSP is given in Figure 1a.The Reinforced Concrete Sandwiched Panel (RCSP) building, also called Sandwich Concrete Insulated panel (SCIP), ThreeDee Panel and other proprietary names, is more than 50 years old technology (PCI committee report, 1997), which was not well studied until very recently. Due to the current hype of demand of energy efficient and earthquake resistant structural requirements, RCSPs have emerged as one of the must have technologies to be adopted in modern buildings. Also to cope with global warming, the reduction of carbon-footprint demands a control on building industry,


which is the largest contributor to it. The materials used to build RCSP buildings are sustainable and non-CFC producing.

a. RCSP Panel (Three Dee Panel Section, 2013)

b. Single storey Poultry farm in Hattar Industries, Pakistan

Figure 1.(a) A typical RCSP panel and (b) industrial level building One of the things that makes the onsite sprayed RCSP buildings different from the traditional brick masonry and framed (reinforced concrete) buildings is that all the structural elements (beams, columns, floor slabs, walls etc.) of the building constructed are continuous with each other, without any construction joint (Adil, 2010). This is achieved by a special method of construction where first EPS foam panels encased in steel wire mesh are erected on site. These panels have been manufactured in the factory and continuity is provided by connecting them with wire meshes. After fixing the utility service elements (pipes and conduits etc.) the walls are then sprayed with concrete. An underconstruction poultry farm RCSP building in Hattar Industrial State, Hattar, Pakistan is shown in Figure 1b. Along with evaluating the energy efficiency of RCSPs it is also important to access out the structural integrity of these panels which is main application of these panels. This has been discussed in the next section. 2. SANDWICH STRUCTURAL COMPOSITE Precast lightweight concrete sandwich panel systems can be designed to be composite or noncomposite structural members. In non-composite walls, one layer is counted on to resist the entire applied loading, and the second layer is considered to be non-structural as shown in Figure: 2(a). In composite construction, the two concrete layer share in the load resistance through the connectors that is capable of resisting the interface shear force resulting from composite action. Precast lightweight concrete sandwich panel systems can be classified into three major categories (Maximos et. al, 2007; Johnson, 2004). i. Non-composite panels ii. Fully composite panels iii. Partially composite panels When the joining layers can easily slide over each other, no horizontal shear is transferred between them, and the resulting element has non-composite action, Figure 2(a). When positive connection between adjacent layers is provided by using some shear transferring mechanism (e.g. shear


connectors or adhesive) so that the joining layers cannot slide over each other, then all the horizontal shear is transferred between the adjacent layers. This phenomenon of transferring of shear between the layers is called composite action and such a composite element is addressed as fully-composite element, Figure 2(b). When the shear connection between the adjacent layers is not sufficient to fully transfer the horizontal shear, the joining layers slightly slide relative to each other. Such composite elements are called partially-composite elements, Figure 2(c). (Adil, 2010).

a. Non-composite

b. Fully- Composite

c. Partially-Composite Figure 2.Types of Structural Composite (Salmon,et. Al, 1997) 2. STRENGTH OF RCSP (A Review) One of the earliest studies on precast concrete Sandwich wall panels (SWPs) was carried out by Pfeifer and Hanson (1964).They tested fifty reinforced SWPs for flexure under uniform loading. They used a variety of layer connectors for these tests. Test results showed that welded truss shaped steel connectors were more efficient in transferring shear than steel connectors without diagonal members. Pantelides et al., (2003) tested nine precast concrete wall panels with Concrete fiber reinforced polymers (CFRP) connectors. Test results showed that failure of the CFRP composite connection was non-ductile, similar to that of the steel connection. The CFRP composite connection resisted three times the lateral load then the steel connection. Lian(1999), carried out test on 4 reinforced concrete sandwich panels to study the behavior of reinforced concrete sandwich panel under axial and eccentric loads. The ultimate load capacity for pure axial loaded panels was computed using expressions applicable to solid walls could not be directly applied to sandwich panel. Itwas noted that the


slenderness ratio (H/t) is an important factor influencing the load bearing capacity of axial loaded panels. Abdelfattah, (1999) tested six precast concrete sandwich panels. The panels were 140 mm thick, 2.4m long and 1.2 m wide with different reinforced concrete ribs shear connector layouts (2 identical specimens for each connectors layout) with vertical and inclined ribs at 450 and 67.50, respectively. Each specimen was subjected to three types of lateral loading within elastic range, axial loading within elastic range and combined axial and lateral loading till failure. They were then theoretically evaluated by using STAAD III finite element software to simulate the physical tests to the elastic phase. Based on the theoretical investigations, it was found that the contribution of the shear connectors in carrying the axial load was very small. It was noted that the concrete layer carry most of the axial loads. Tarek and Sami (2010), performed the analysis and design of precast, pre-stressed concrete, composite load-bearing sandwich wall panels reinforced with CFRP grid, investigated three different precast concrete sandwich wall panels, reinforced with carbon-fiber-reinforced-polymer shear grid and constructed using two different types of foam, expanded polystyrene (EPS) and extruded polystyrene (XPS), were selected from the literature to validate the proposed approach. The results of the analysis indicated that the proposed approach is consistent with the actual behavior of the panels because the predicted strains compared well with the measured values at all load levels for the different panels. Besides that, the approach is beneficial to determine the degree of the composite interaction at different load levels for different panels at any given curvature. Bernard et al. (2011) investigated six precast, pre-stressed concrete sandwich wall panels to evaluate their flexural response under combined vertical and lateral loads. The study included panels fabricated with two different insulation types: expanded polystyrene (EPS) insulation and extruded polystyrene (XPS) insulation. According to the manufacturer, the selected EPS insulation had a nominal density of 1 lb/ft3 (16 kg/m3) and a nominal compressive strength of 13 psi (90 kPa). The selected XPS insulation had a nominal density of 1.8 lb/ft3 (29 kg/m3) and a nominal compressive strength of 25 psi (170 kPa). The panels were 20 ft tall × 12 ft wide (6.1 m × 3.7 m) and all panels were 8 in. (200 mm) thick and consisted of three layers. The flexural behaviors of six full-scale insulated precast, pre-stressed concrete sandwich wall panels were investigated. The panels were subjected to monotonic axial and reverse-cyclic lateral loading to simulate gravity and wind pressure loads, respectively. Based on the findings of this study, it was found that panel stiffness and deflections are significantly affected by the type and configuration of the shear transfer mechanism. Panel stiffness is also affected by the type of foam. 3. REVIEW ON THERMAL EFFICIENCY OF INSULATED PRECAST PANELS The Energy Efficiency (EE) by definition means using less energy to achieve same or better output compared to pre-implementation of the energy efficiency project (PEC ENERCON presentation, 2013). Thermal insulation is one of the most effective energy conservation measures for cooling and heating in buildings. The thermal conductivity (k) of insulation is a measure of the effectiveness of the material in conducting heat (Mohammad 2005). Two other properties related to the thermal performance of insulation are the thermal resistance (R) and the thermal transmittance (U). The thermal resistance (R) of a material can be defined as the measure of the resistance to heat flow as a result of suppressing conduction, convection and radiation. It is a function of material thermal conductivity, density and thickness. The thermal transmittance (U) is defined as the rate of heat flow through a unit area of a component with unit temperature difference between the surfaces of the two sides of the material. It is the reciprocal of the sum of the resistances of all layers composing that material plus the inside and outside air film resistances. Precast insulated wall panels have been identified to be one of the most structural efficient systems in terms of low material consumption and highly thermal efficient systems (Bush et al, 1994). The use of insulated precast wall panels can increase the thermal efficiency of concrete sandwich panels nearly 30


percent over that of a stud wall system. These thermally efficient systems can save nearly 20 percent in energy cost compared to framed walls (Gleich, 2007). Insulated concrete sandwich panels with polystyrene cores can exhibit R-values up to a value of 30 in comparison to a stud wall system with an R-value of 5 to 10 (Christian et al., 1999). The presence of steel or concrete thermal bridges can reduce the R-value up to 40 percent resulting in R-values from 12 to 16. Lee and Pessiki (2004) investigated the thermal efficiency of three layer wall panels in comparison to two layer panels in an attempt to enhance the R-value while still providing structurally efficient concrete solid zones. Lee and Pessiki (2004) showed that: a. Three layer panels have greater thermal efficiency than a two layer panel b. Concrete layer thickness does not affect the R-values c. Three layer panels produces higher thermal savings in comparison to a two layer panel with more core insulation. 4. ENERGY EFFICIENCY AUDIT OF RCSP 4.1 NEED OF THE DAY The world will experience an ever increasing demand for energy savings in various areas. As buildings constitute a substantial part of the total energy consumption, savings within the building sector will be important, both for existing and new buildings. One of the key fields will be the thermal building insulation materials and solutions. In harsh climatic conditions, a substantial share of energy goes to the air-conditioning of buildings. This air-conditioning load can be reduced through many means; notable among them is the proper design and selection of building envelope and its components. The use of thermal insulation in building walls and roof does not only contribute in reducing the required air-conditioning system size but also in reducing the annual energy cost. Additionally, it helps in extending the periods of thermal comfort without reliance on mechanical air-conditioning especially during inter-seasons periods. Therefore, proper use of thermal insulation in buildings enhances thermal comfort at less operating cost. However, the magnitude of energy savings as a result of using thermal insulation vary according to the building type, the climatic conditions at which the building is located as well as the type, thickness, and location of the insulating material used. The question now is no longer should insulation be used but rather which type and how much. 4.2ENERGY AUDIT An energy audit can be simply defined as a process to evaluate where a building uses energy and identify opportunities to reduce consumption (Thurman et al., 2011). An energy audit results in an adequate knowledge of the existing energy consumption profile of a building or group of buildings, of an industrial operation and/or installation or of a private or public service, identify and quantify cost effective energy savings opportunities, and report the findings. In practical sense, it can be defined as “The verification, monitoring and analysis of use of energy including submission of technical report containing recommendations for improving energy efficiency with cost benefit analysis and an action plan to reduce energy consumption”.(PEC ENERCON presentation, 2013) 4.3Western Scenario Buildings have a significant impact on energy use and the environment. Commercial and residential buildings use almost 40% of the primary energy and approximately 70% of the electricity in the United States (EIA, 2005). The energy used by the building sector continues to increase, primarily because new buildings are constructed faster than old ones are retired. Electricity consumption in the commercial building sector doubled between 1980 and 2000, and is expected to increase another 50%


by 2025. Energy consumption in the commercial building sector will continue to increase until buildings can be designed to produce enough energy to offset the growing energy demand of these buildings. Toward this end, the U.S. Department of Energy (DOE) has established an aggressive goal to create the technology and knowledge base for cost-effective zero-energy commercial buildings (ZEBs) by 2025. 4.4Scenario of Pakistan Pakistan is amongst the most rapidly urbanizing countries in Asia. Over 40 million additional people are expected to live in towns and cities in Pakistan by 2025. This represents a large scale and fast moving change from rural to urban settlements, from rural to urban housing and from traditional construction to new technologies. The majority of the country experiences extremely hot summers with temperatures over 40 degrees for several months. The northern half of the country has cold winters with freezing temperatures at night. In previous rural lifestyles people would often sleep outside their homes during the hot summer nights. Urban life is not as conducive to this option and it is more important that buildings are comfortable during day and night time as people spend more time indoors.The residential sector is the most significant for electricity consumption. Residential consumption accounted for almost 50% of electricity consumption in FY10, shown in Figure 3.

Figure 3: Pakistan Electricity Consumption by Sectors (Source: Pakistan Energy yearbook 2010) 4.4 Equipment Used for Building Energy Audit The following basic equipment is used for the energy audit of the buildings: 1. Thermal Imager 2. Thermometer 3. Infrared temperature gun 4. Energy Analyzer and 5. Other accessories 5.0 CASE STUDY In order to find the thermal efficiency of RCSP buildings, energy audit of at least two existing buildings will be performed and selected at the same location in Islamabad, Pakistan. One building made of RCSPs and other of ordinary Brick masonry. The two buildings will be chosen so that they


will be in the close neighborhood of each other and have similar sunlight and wind conditions. The table 1 displays average monthly temperature in Islamabad based on 8 years of historical weather readings. The characteristics, features and properties of the two buildings will be assessed and compared thoroughly. Besides the energy audit of these buildings, thermal sensors will be installed for 72 hours on both the buildings simultaneously and their internal and external temperature will be compared without any air conditioning in winter and summer for comparing the energy efficiency of their building materials. Table 1.Temperature Data (°C) for the last 8 years of Islamabad, Pakistan (Source: http://www.climate-zone.com/climate/pakistan/celsius/islamabad.htm) Month

Jan

Feb

Mar

April

May

June

July

Aug

Sep

Oct

Nov

Dec

9

12

17

22

28

31

29

28

26

21

15

10

17

20

24

30

36

37

33

32

33

30

25

19

3

6

10

15

20

23

23

23

21

15

8

4

Avg. Temperature Avg. Max. Temperature Avg. Min. Temperature

The insulation thermal efficiency of RCSP panels can be calculated in the form of U-Value (Thermal transmittance) of the panel. Table 2 below shows a comparison of the U-Values of RCSP panels and typical brick masonry construction (9” thick wall) in Pakistan. Table 2. Comparison of U-Values of RCSP and Typical Brick Masonry in Pakistan. The K- values are adopted from The Engineering Box (2013). Wall type RCSP

Material Dense concrete EPS foam Dense concrete

K value (W/(m.K)) 1.4 0.03 1.4

Thickness (m) 0.03 0.1 0.03

R-Value (K.m2/W)

U-Value (W/K.m2)

0.0214 3.3333 0.0214 0.2962

Typical brick masonry

Cement Plaster Common Brick Masonry Cement Plaster

1.73 0.8 1.73

0.0127 0.2032 0.0127

0.0073 0.2540 0.0073 3.7219

Due to the facts that the mix used to cast concrete leaves of RCSP is denser and also contains poly propylene fibers; the EPS foam has a specified density and the steel connectors are used that pass through the EPS foam core, it is required that the thermal conductivity of the RCSP panels should be evaluated for the above specifications. Therefore, a Hot-box apparatus will be used as per ASTM C1363-97 for measuring the thermal conductivity of RCSP constituents. The primary purpose of a Hot-box (see Figure 4) is to measure the thermal properties of a particular material or a composition of materials (Martina et. al, 2012). This is achieved by trying to quantify the total heat flow through the material. After obtaining this information, it can be converted into a heat transfer coefficient or a U-value for that material (Curcija D., 1992). Hotbox will be produced, calibrated and then used to measure thermal properties of typical building walls and RCSPs for comparison.


Figure 4: Calibrated Hot Box (Courtesy Martina et. al. 2012)

7. DISCUSSION AND CONCLUSIONS RCSPs have already been proven to be energy efficient, yet it is required to assess and optimize the type of RCSP that is produced in Pakistan. Its design is needed to be specified for different geographic conditions in Pakistan. Also the general public and building authorities are required to get aware of its practicality in a variety of environments in Pakistan. Therefore, both strength and energy efficiency of these panels is required to be evaluated. This requirement has been identified and future strategies are presented in this research. RCSP buildings are theoretically 12 times more energy efficient compared to typical construction technologies used in Pakistan. Pakistan, being affected by energy crises, has started production of RCSP panels locally and currently only few companies are manufacturing these panels. In context of Pakistan, still there is a big room of research in testing the thermal, acoustic and structural performance of these panels since these panels have already been used to construct buildings in different weather sites of Pakistan. In near future, comparative study of the thermal efficiency of two buildings, one of masonry and one of RCSP system will be performed. The hot box apparatus will be used to experimentally find out the thermal conductivity of these panels. The hot box results will be used to assess the conductivity of these panels with variation of thickness of its different elements. The interview from the owners who have built their houses from RCSP system said that after adopting this technology the electricity bills have reduced dramatically because of reduction in use of airconditioners and electric/gas heaters. REFERENCES

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