Evaluation of Analysis and Design of Precast Concrete Panels - Report Example

3.0 Introduction Precast concrete panel construction involve casting the concrete in reusable moulds or forms followed by curing in controlled environment, then transported to the site of construction and then lifted in place. The production of precast concrete in a controlled environment usually referred to as precast plant, the precast concrete panels are given treatment that ensures optimum curing through close monitoring employees in the precast plant. The use of precast concrete system has several advantages over site casting. The process of production of precast concrete panels is usually done at ground level and this ensures safety in the process of production. The many advantages associated with concrete are also seen in precast panel. Thomas (1999a) note that the precast products are made in the factory and thus are independent of weather in the natural environment, working on the panel is 24 hours and there is economy of scale. 3.1Advantages of precast concrete There are various advantages which are associated with precast concrete in general but the outstanding are quality, speed and economy. 3.1.1 Speed Precast concrete is regarded as products that can used as onsite assembly components that are manufactured in factories. The use of precast concrete in building industry clean, faster and safe working environment at the site where the assembly is done and thus being beneficial both the contractors, their clients and others that are part of the process. The precast element may be used in this manner as long as there is pre-plan of design and detailed requirements have been given. It is perceived precast panels require a long lead time but the advice to designers is to get in contact with manufacturers well in time so as to have the expertise of the manufacturers to their advantage. The fast construction realized by use of precast concrete ensures quick realization of hand-over and thus the client being able to occupy the premises early. As a result saving is realized in terms of labour, overheads, the cost of financing and quick return on investment. In addition the design team is not required to find a length weather time for undertaking construction activities and thus they are not subjected to unnecessary pressure. The other advantage that has been noted is that use of precast concrete in construction require less skilled labour in comparison to traditional methods of construction. The de-skilled and dry process ensures that the construction is fast as trade interfaces are minimized thus fitting out activities commence early. The use of precast concrete in construction industry may involve shifting of trade’s people from working at the site to factory and thus leading to multi-skilled work personnel being established at the factory environment (Barry & Cronin, 1999). As a result of IT, interoperability and bar-coding technology on-site activities that are associated with precast concrete component assembly the speed at which site activities are undertaken is increased considerably. 3.1.2 Quality The controlled environment provided in the factory during the production of precast concrete provides a high level of control which is ensures high quality products are produced in comparison to what can be produced onsite. Working in a factory environment ensures that that there is no interference from the natural environment and the working environment is fully controlled. As a result makes it possible to easily achieve desired qualities such as strength, surface quality and consistency and other relevant design features in the products are easily achieved. The factory set up production of the precast concrete also is beneficial in terms of both material and dimensional properties. It is recognized by designers that factory production and prefabrication lead to production of products with high tolerance thus making it advantageous in terms of speedy and high accuracy site assembly of the components. The design of precast concrete can be in such away that some degree of flexibility is incorporated where circumstances allow. Once the precast concrete are at the site better ‘line and level’ is offered is comparison to what can be achieved in the traditional way of construction , thus being beneficial in the subsequent fit out activities. Precast concrete panel has high surface finish quality because in the factory they employ specialist mould-makers and the manufacturer invest in formworks of high quality this being profitable because the products are produced in large numbers. 3.2 Precast concrete panel The use of generic types of precast flat panel or single skin concrete construction has been increasing in commercial application such construction of students’ accommodation and hotels. The choice of the precast panels ensures that simple construction is done on site which is then followed by finishing trades. This advantage has caught the interest of many in the hotel industry where time and quality is of essence although this not the case with the less repetitive housing market where there are other criteria that are to be put into consideration. Currently wall panel use in housing is not common but there is high probability of such opportunities arising in multistory apartment where there can be use of precast concrete panel for walls and floors. Load bearing wall elements may find application in cross-wall or spine wall type of application with the most likely option for low-rise housing being cross-wall construction with party walls provided between the dwelling acting as the structural element. In single and two storey buildings, the use of a single concrete panel with a concrete thickness range of 90-300mm (with higher thickness for panels with insulation) for the case of external walls and 70-700mm thick precast panel for internal walls. Load bearing cross walls in combination with precast floors with hollow cores are capable of providing internal spans of up to 12m. When precast concrete panels are used as internal walls there is additional value of good sound insulation, integrity of the structure and direct finish where it is specified. The use of precast concrete panels has some complication because there is need for both thermal insulation external appearances to be put into consideration. There may be need for the precast concrete panel to be insulated externally and to be faced with masonry or any other cladding material, dry lining with insulation and providing internal surface finish not unless it is used as an insulated sandwich type of panel. According to Smith (1999), even though use of precast panels offers both speed and performance benefit the possibility that there maybe need for over cladding may make one question the need to use them in construction of a build. Manufacturers have come up with methods of producing panels which are twin-skinned which are appropriate for direct application of decorative finishes through use of fiber reinforced concrete as a substitute of steel reinforcing bars. In some recent versions it has been possible to produce fiber reinforced panels with each leaf of the wall having a thickness of only 35mm and being separated by 70mm lattice joist. (Long, 1999). The benefit of this system include reduction of material volume, reduction of crane capacity as well as weight for transportation at a greater net area is achieved. The production lines of this type also offer variation in styles of panels, insulation levels, electrical trunking, doors, windows and panel connection details thus enabling better response to customers needs. The level of take up of the solutions is dependant the thermal and acoustic insulation requirements as per the building regulations of each country. 3.3 Double Wall Precast (sandwich) Concrete Sandwich Panels While the single wythe precast concrete panel have been in use for a long time the double layer panels came in use much later. In European countries double wall panels have been in much longer use in comparison to countries like Canada and Australia which the technology has spread to much later. The panels are made of two wythes of concrete which are manufactured in factories with an insulation layer placed between the two panels. The architectural finish and weather resistant cladding is provided by the exterior wythe. The exterior wythe, interior whythe together with the insulation when combined results into provision of all the necessary performance required for an exterior wall. The 8 inches double panel is believed to the most common with other panels being 10 and 12 inches. For a typical 8-inch double panel each of the wythe is 2-3/8 inch in thickness with the a high R-value of about 3-1/4 being sandwiched between the two panels. Steel trusses and composite fiberglass connectors are used for holding the two interior and exterior concrete layers in position. Use of steel trusses in connection of the panels makes the panels to be inferior in comparison to where composite fiberglass is used. This is due to the fact that steel acts a thermal bridge between the two walls resulting into a significant reduction in the isolative performance of panels with overall reduction of the ability of the building to make use of its thermal mass in energy efficiency. Another risk with steel is that its expansion coefficient is different from that of concrete and due to heating and cooling of the walls the expansion of the steel and concrete will expands at different rates, a situation which is likely to cause cracking and spalling. The use of fiberglass specifically designed for this type of application reduces the problem significantly. The double wall panels have a continuous insulation between the exterior and the interior walls with the overall R-value of the panel being over R-22. The panels have a height of up to 12 feet with the reference of many being 9 foot for quality look and feel given to the building(Dawson,1999). In manufacturing double wall panels the walls can be made to have smooth surfaces on both sides as dictated by the unique manufacturing processes. The desired color or surface texture of the walls is achieved through painting or staining. Depending on ones desire the manufacturing process makes it possible for the exterior walls to have a wide variety of stone, brick, wood or other appearances (formed or patterned) which involve the use of formliners that are reusable and removable. The treatment required for double wall panels is similar to that of the conventional wall as the walls have dry wall quality appearance as they are released from plant. 3.4 Design Fabrication and assembly of panels The architect has the responsibility of defining the appearance of the exterior finish and should be well versed with the limitations paused by the moulds and the casting materials. There is need for the design details to address exterior water management and moisture, air and heat control across the panels that may pass through the joints and junctions with adjacent systems. The design team is required to specify both loads and structural requirements with the broad code compliance being part of the tender documents. The panel manufacturer is usually responsible for the production of structural design of precast panels and also the shop drawings. The location and tolerance of panel anchors, interface tolerances and connection details with elements like doors windows being other important issues that need to be addressed. The accuracy with which shop drawings are done together with their coordinated review is important in ensuring all design concepts are incorporated in final product. Both the panel manufacturer and the general contractor are normally involved site access and lifting capacity review. Fabrication of double wall precast concrete involves several processes that are to be followed in order to get the desired products. First, exterior wythe cast in the mould that has architectural profile of the panel, then a layer of rigid insulation is then placed over the exterior wythe with a drainage layer being incorporate between the concrete and the insulation layer. Provision of a drainage layer is achievable through insulation with vertical grooves either on a separate material or on the exterior. Where separate material is to be used, the material needs to have the ability to drain or be able to be removed after casting so that a drainage cavity is left. The final stage involves casting the interior wythe on the insulation thus completing the whole process. In order to ensure that the panel has the required tensile and shear capacity that will ensure that the panels are kept together in the process of fabrication, during erection and when in use, both connecting and reinforcing ties are cast into the wythes. It is important for support anchors to be cast into the structural wythes so that the panel can be attached to the structure. The design should ensure that panel anchors are able to provide the required level of strength to withstand seismic load, gravity and wind loads and be able to transfer the loads to the building structure. The anchors are supposed to allow for both horizontal and vertical adjustment that will take care of construction tolerance and final alignment of the panels in the erection process. Each panel need to have two gravity anchors located at floor line next to columns in multi-level building frames. Gravity anchors have seismic design incorporation with one of the gravity anchor allowing for thermal movement. Each panel has two or four lateral anchors the number being dependant on the size and shape of the panel; all lateral anchors facilitate thermal and structural movements. Design of the double wall joints is a very important component of the design. In designing joints between double wall panels it is important to ensure that continuity is maintained in air, thermal and moisture control functions so that continuity can be provided for the wall. A two stage joint is the solution where air seal is located at the line of interior wythes with a weather seal being located on the outside face of exterior withes. A vent is provided between the weather seal and the air seal then drained to the exterior at the point of intersection of vertical joints and horizontal joints and the base of the wall. The joint seals in a double wall panel comprise of a combination of sealants and backer rods. The provision of weather and air seals from the exterior is a deliberate step of avoiding interference from floors and columns. The panels are also joined to other components and their proper detailing is of great importance. Foundation design details are important in provision of drainage of the wall cavity and in directing exterior water that originate from the external wall away from the foundation. The interface with components like doors and windows are to be designed in away that allow continuous air, thermals and moisture seals. The roof to precast detail has been noted as being one of the most frequent areas of failure of precast concrete panel application (Southworth, 1998). This failure is attributed to temperature changes, building frame movement, wind and moisture changes not being put into consideration. The live load resulting from snow or rain water is of major importance when addressing design failure in precast panels. The roof and wall junction are the critical location which separates the difference in moisture and pressure between the interior and the exterior. Just like any other cladding, in the precast panels there is need of coordinating the details so that the integrity of the envelope is ensured when used together with other materials and finish systems. Combination of double wall panels with masonry, cut stone, metal glass curtain wall, concrete shear wall are common in current building industry. The difference in planes of air, thermal and moisture barriers are likely to be sources of problems. It is important for the designers to have thorough understanding of the difference in physical characteristics of the systems so that provision is made in designing the interface and ensuring performance of the wall is achieved. Performance problem are likely to be experience as a result of volume moisture changes, difference in thermal behavior, effect of shrinking and creep deflection being experienced between adjacent systems. 3.5 Prevalence of precast concrete building products in different countries While the use of precast concrete panels is common in most of the developed countries its level varies greatly in each of the countries. In UK precast concrete is a quarter of all the cement products in the market (Construction markets, 1998). The precast concrete in this context include paving, structural frames, bridges, architectural cladding and cast stone. The biggest output of tonnage sold per annum is taken by suspended floor. There is high level of precast concrete use with taking 60% share. There is still a lot of market as traditional walling is used in 90% housing projects. Just as in Australia the precast concrete market is well established in Germany and Netherland in comparison to UK with fierce internal competition being experienced. In the two countries various factors such as technical, social and economic, influence the specification of material use in building industries. With greater experience in precast concrete a lot of advancements have been seen in production and materials which has resulted into greater use of precast building components such as precast concrete panels. The high volumes of production demanded has made building of plants dedicated to production of concrete panels to be an economical venture. Use of precast concrete is popular as it ensures permanent housing structures are erected within a very short time as low as five days. The buildings made of precast concrete are also associated with low energy consumption. In Netherlands precast concrete use in building industry has 10 percent share with a lot of competition coming from brickwork and masonry construction. The success in Netherlands is attributed 30% cost saving when standardized components are used, flexibility in manufacturing processes, streamlining the contracting process and use of industrialized building techniques. Having a flexible system concrete components has contributed a lot towards the success (Belton/Bevlon,1997). In Scandinavian countries prefabricated methods take a high proportion of the building market share with large quantities of steel, timber and concrete being consumed. In Finland there is 54% use of concrete in new housing with precast concrete being used in 42% of the new homes (Gann, 1999). Sekisui which one of the large house building contracting company in the country builds over 100,000 housing units in a single year. The company has been in the housing business since 1950s with 1.25 million houses at its credit. Due to its popularity, the Japan prefabricated market attracted a delegation of experts from UK in 1996. The delegation established that the type of housing was not similar to that in UK even though very popular in Japan (Bolton et al, 1996) Some of the facts that were established by the delegation were: Housing being considered a product on its own right The housing market in Japan was dominated by new house units There is a clear government and industry framework for innovation Use of electronic modeling is in common use There being willingness of idea exchange In USA prefabricated housing has 30% share of the housing. Even though most of the low-rise building in the country is made of timber the use of concrete is on increase more in areas that are prone environmental hazards such as tornados and hurricanes. According to PCA (Portland Cement Association) precast concrete panels are commonly used in low-rise housing in a number of states (VanderWerf& Munsell, 1995). Even with several advantages such as befits of quality, reduction in site time and low labour requirements, there was initial reluctance in adopting to the technology due to the panels previously having a plain appearance, panels being prone to water penetration and the difficulty paused in insulation installation. This has recently been changed because of improvement in moulds, new innovations in concrete mixing, rigid foams being introduced and availability of different type of surface finishes. 3.6 Issues associated with pre-cast concrete In as much as designers and other building industry participants are well aware of benefits of use of precast concrete some issues such as service delivery issues, appearance of precast concrete and inflexibility all of which discourage its full acceptance as a good building material. Some of specific issues that are of concern in acceptance of precast concrete material like precast panels include: cost and value perception; image of precast panels, structure and flexibility. 3.6.1 Cost and value perception Cost has been cited as being the most difficult aspect of the factory produced concrete product when it come s to assessment. While mass production and simplicity in assembly contributes greatly towards saving on cost but the cost of transportation, and the operation cost of the production plant must be factored in. lack of detailed publication on the costs associated with precast concrete products discourages the would be customers. The cost of new concrete products being introduced in the market is likely to be very high for the manufacturer and the best alternative to revert this situation is through adapting existing facilities and the manufacturer partnering with house builders thus making it feasible to undertake small scale which have minor cost penalty, usually through negotiation. Precast concrete panels require a lot of labor in the production stage at the factory and minor labour requirement at the site unlike the case of components where the reverse is true. Even though cost associated with labour may prove difficult to quantify, the conditions in a factory set up are far much better compared to site conditions. According to research findings in New Zealand use of precast concrete may incur only an extra cost of 2% in comparison to timber construction. The difference is minor and the extra cost may easily be recovered through life-cycle benefits such as energy consumption saving. Housing association are likely to find publication of whole life or part-life cost being beneficial as their interest is in the long term benefits derived from precast concrete housing. The potential benefits that are likely to be realized at the site when using precast concrete include: financial costs where money is borrowed for a shorter time; preliminaries like staff, access, power which are time related and the additional overheads like rend paid for storing equipment (Gray & Green, 1999). 3.6.2 Precast concrete image Research has revealed that the structural form of a house is not a major concern of house occupiers provided the house appearance is solid and durable and thus no special preference to concrete or masonry type of housing. However, concern will usually arise where there is use of facing materials which are non-traditional a good example being the case of external elevations where there is special preference for traditional masonry. Contrary to this general rule there are exceptional situations where the use of some visible non-traditional precast concrete elements is widely accepted, with the non-traditional products associated with addition of value to the property. It is from this background that it is obvious that preconceived notions on the use of nontraditional materials such as precast concrete panels need to be challenged. The claim by the house builders that the public do not prefer precast concrete products is attributed to lack of clear understanding of the product for both the house builders and the public. The historical association of precast concrete material with system building is a clear pointer that for this material to be able to compete in the housing market effectively re-invention is inevitable. 3.6.3 Structure Design for joints is similar in both precast concrete and in-situ reinforced concrete is the same where in both cases the force analysis in joints is done through internal structs and ties. However there is a requirement of dealing with tolerances both design type and those arising from dimensional variations. Even though manufacturers always boost of their ability to designing and producing products of fine tolerance, it is worth while noting the difficult that comes with co-ordination accurately made components together with other site construction activities. The popular view is that using structural precast concrete makes the resulting members being oversized as a result of lack of continuity. Similar situation is not experienced in the case of in-situ concrete construction because of the continuity that exists between elements. In the case of housing project such a problem is unlikely to occur owing to the fact that the elements being dealt with are of modest size. 3.6.4 Flexibility and services Flexibility and services is another issue that cannot be evaded in any discussion concerning precast concrete. In a survey undertaken in Oxford Brookes University involving house builders, the revelation was that there was no total agreement on precast concrete being a flexible housing method with a lot of criticism on large precast concrete. It was said that large precast components were too regimented and it required some convincing for them to be accepted as of being of required quality and cost effective. Services provision takes the centre stage with regards flexibility, here the major issue being the degree to which integration and incorporation of services into precast concrete should be done. A system like Termodeck has both the structure and the heating system as being part of the precast concrete product, but the sentiments of some people is that flexibility in housing in near future would dictate the need of servicing being readily adaptable. In the case where services are not concealed in the concrete, there will be need for them to be accommodated in trenches, trunking, raised access floors, skirting or ducting. 3.7 Generic benefits of concrete construction In both structural and non-structural use of concrete in buildings, concrete has several benefits by virtue of its characteristics as a material. These benefit act as a complement to those that are offered by precast concrete panels by virtue of their design. The advantages which are worth mentioning with regard to housing are sound insulation, fire resistance and thermal performance. 3.7.1 Fire resistance Fire resistance in concrete is well documented with protection surface spread requirements of flame for the case of low rise buildings being well within concrete’s capability. A typical example is where a 150mm thick concrete is able to fire resistance of over 90 minutes which is far beyond the minimum requirement in most housing. When concrete is exposed its surface spread rating is class 0 and with the relatively modest nature of structural elements in housing is an indication that it is quite straight forward to comply with stability requirement. This gives concrete an inherent advantage over metal and timber with regard to fire performance. 3.7.2 Sound insulation, durability and robustness Concrete is a better material in comparison to timber and steel as it makes it feasible to have housing that is intrinsically solid with good acoustic performance. The mass law sound reduction is applicable there is sound reduction index of 4dB in doubling of the mass. Concrete can be relied upon in providing good sound reduction if detailing of joints and openings are properly done. A typical example of sound reduction is where a concrete wall which is 150mm thick is able to reduce sound transmission by 50dB between adjacent rooms, between buildings and from traffic in the neighborhood. In addition precast concrete houses have less joints or voids in comparison to traditional masonry construction and thus having improved sound reduction quality. Durability and robustness are also inherent in concrete. Concrete is able to provide a dwelling whose construction is solid and durable which is resistant damage in addition to being easy to maintain this being on condition of the normal standards being adhered to. 3.7.3 Concrete thermal performance Concrete with normal density can contribute a great deal to thermal comfort in a building. The thermal capacity of concrete allows it to absorb and store heat in the building structure and this is reradiated in cooler times. This characteristic of concrete may eliminate the need of having air conditioning in the building suppose this incorporates solar control and ventilation (Glass et al., 1999). Currently in the housing industry there is growth of interest in provision what is referred to as comfort cooling as a selling point. The impact resulting from this system is obvious and thus concrete can be used as a more sustainable alternative to HVAC systems thus resulting into tangible benefit to house builders. With time fabric thermal storage is able to provide a substantial payback benefits to the building occupier. In tests which were undertaken in New Zealand where comparison was done between timber and concrete homes it was found that concrete had the ability to contribute a cooling effect of 3-4o in the period of summer (Thomas, 1999a; Thomas, 1999b). Further more concrete has the ability of providing stability of internal temperature that can not be attained by lightweight construction. 3.8 Summary of literature review In this literature an insight on the development in the precast concrete panel use as a building construction material has been given. The advantages of concrete panel use have been discussed at length. Apart from the general design aspect of concrete panels a separate section has been dedicated to double wall concrete panel. Precast concrete panels being part of other precast concrete components, there was need to look at the trend of use the precast concrete products in selected countries. The inherent property of concrete as a material has also been looked into in the literature. References Barry, S. & Cronin, S. (1999) Death of trades in housing, Construction News, Belton/Bevlon (1997) Prefab Beton in de Woningbouw, Belton/Bevlon, Woerden, Netherlands. Bolton, G. et al. (1999) Housing: an exercise in industrial design, Arup Journal, Autumn. Construction Task Force, The (1998) Rethinking construction, DETR and HMSO, London, UK. Dawson, S. (1999) Working details: A precast concrete deck, Architects Journal, Gann, et at (1999) Flexibility and choice in housing, The Policy Press, Bristol, UK. Gray, C. & Green, L. (1999) The cost of time, Reading Production Engineering Group, University of Reading, UK. Long, K. (1999a) Change of gear for house design, Building Design, Smith, K. (1999) Building the homes of tomorrow today, Construction News, Southworth, G. (1998) Wall panels emerging as solution of choice, Ascent (USA), Autumn. Thomas, G. (1999a) Concrete construction - counting the real cost, New Zealand Concrete, September, pp.10-13. Thomas, G. (1999b) Thermal performance - putting the heat on residential housing standards, New Zealand Concrete, September, pp.14-19. VanderWerf, P.A. & Munsell, W.K. (1995) The Portland Cement Association’s guide to concrete homebuilding systems, McGraw-Hill, USA. Read More

Working in a factory environment ensures that that there is no interference from the natural environment and the working environment is fully controlled. As a result makes it possible to easily achieve desired qualities such as strength, surface quality and consistency and other relevant design features in the products are easily achieved. The factory set up production of the precast concrete also is beneficial in terms of both material and dimensional properties. It is recognized by designers that factory production and prefabrication lead to production of products with high tolerance thus making it advantageous in terms of speedy and high accuracy site assembly of the components.

The design of precast concrete can be in such away that some degree of flexibility is incorporated where circumstances allow. Once the precast concrete are at the site better ‘line and level’ is offered is comparison to what can be achieved in the traditional way of construction , thus being beneficial in the subsequent fit out activities. Precast concrete panel has high surface finish quality because in the factory they employ specialist mould-makers and the manufacturer invest in formworks of high quality this being profitable because the products are produced in large numbers. 3.2 Precast concrete panel The use of generic types of precast flat panel or single skin concrete construction has been increasing in commercial application such construction of students’ accommodation and hotels.

The choice of the precast panels ensures that simple construction is done on site which is then followed by finishing trades. This advantage has caught the interest of many in the hotel industry where time and quality is of essence although this not the case with the less repetitive housing market where there are other criteria that are to be put into consideration. Currently wall panel use in housing is not common but there is high probability of such opportunities arising in multistory apartment where there can be use of precast concrete panel for walls and floors.

Load bearing wall elements may find application in cross-wall or spine wall type of application with the most likely option for low-rise housing being cross-wall construction with party walls provided between the dwelling acting as the structural element. In single and two storey buildings, the use of a single concrete panel with a concrete thickness range of 90-300mm (with higher thickness for panels with insulation) for the case of external walls and 70-700mm thick precast panel for internal walls.

Load bearing cross walls in combination with precast floors with hollow cores are capable of providing internal spans of up to 12m. When precast concrete panels are used as internal walls there is additional value of good sound insulation, integrity of the structure and direct finish where it is specified. The use of precast concrete panels has some complication because there is need for both thermal insulation external appearances to be put into consideration. There may be need for the precast concrete panel to be insulated externally and to be faced with masonry or any other cladding material, dry lining with insulation and providing internal surface finish not unless it is used as an insulated sandwich type of panel.

According to Smith (1999), even though use of precast panels offers both speed and performance benefit the possibility that there maybe need for over cladding may make one question the need to use them in construction of a build. Manufacturers have come up with methods of producing panels which are twin-skinned which are appropriate for direct application of decorative finishes through use of fiber reinforced concrete as a substitute of steel reinforcing bars. In some recent versions it has been possible to produce fiber reinforced panels with each leaf of the wall having a thickness of only 35mm and being separated by 70mm lattice joist.

(Long, 1999). The benefit of this system include reduction of material volume, reduction of crane capacity as well as weight for transportation at a greater net area is achieved.

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