DELIVERING THE FUTURE: PRECAST CONCRETE AS AN APPROPRIATE CLADDING

FOR NEW ZEALAND BUILDINGS

MORTEN GJERDE, School of Architecture, Victoria University of Wellington

After a heroic period of development from the mid-1950s to the end of the 1980s, architectural concrete claddings appear now to be used in commercial developments less frequently. Following international fashion trends, lightweight cladding solutions, many employing imported and expensive materials, have been popular with specifiers for the past decade or more. However, as society comes to grips with climate change and the need to live within its means, it is timely to reconsider the many positive attributes of architectural concrete claddings in the New Zealand context. The paper provides comparative analysis of concrete cladding in a number of areas related to economic, environmental and social sustainability. Clear economic and environmental advantages of concrete claddings related to embodied energy and durability/maintenance are discussed in relation to a case study of the Clarendon Tower project in Christchurch. Aesthetic outcomes are presented as a key factor in achieving social sustainability and concrete claddings are revealed to be ideally suited to achieving strongly positive aesthetic responses. While the material and prefabrication processes provide the potential it is critical that designers recognise and exploit the potential to achieve success. Keywords: aesthetics, concrete cladding, durability Introduction

Concrete was the very first manmade, heterogeneous building material [1] and can trace its history back some 6,000 years. The modern history of concrete use in building, dating to the invention of Portland cement as we know it in 1824, roughly equals that of New Zealand’s history of colonial settlement and development, an interesting coincidence if we reflect on the enthusiasm with which it is used throughout the country. In part this can be attributed to its accessibility, both in supply and use, as a construction material. Concrete played a particularly important role during the heroic period of the country’s development following the Second World War, right up through the early 1990s. During this timeframe, New Zealand’s built environment expanded vigorously and substantial infrastructure was created in concrete. Influenced by work being done in other countries, architects played around with concrete as they never had before. Over a 30 year period the architectural profession grappled with how design could work in harmony with local culture and environmental conditions so as to express a New Zealand identity. At least some of the enthusiasm for concrete could be traced to it being considered an indigenous material.

In the mid-1990s other materials began to be used commercially with increasing frequency. As the world continued to become smaller during this period many of the restrictive influences that led to the development of local materials and methods of construction were erased. The financial cost of transporting materials into New Zealand is now considered to be relatively low. Restrictions on importation, long a feature of the political landscape in New Zealand, were set aside in favour of free trade principles. In addition to barriers being lowered, recent times have seen added pressures generated from within to import and use building materials and methods from elsewhere. The change in attitude that has seen other imported materials being used more frequently can be traced to changes in fashion and aesthetic sense as well as freedoms to use those materials. There has been increasing emphasis on completion times and concrete has naively came under scrutiny, being seen as more time consuming to work with. It is now widely acknowledged that man is at least contributing to, if not wholly responsible for, changes to the earth’s climate and environment. After transport, creation and operation of the built environment has the greatest effect on climate change. As knowledge of the effects we are causing

becomes more certain and widespread, there is increased incentive to develop more sensibly. Sustainability is a measure of the extent to which a society lives within its means, so as to not unbalance the ecosystem or to disadvantage future generations, and is considered to have environmental, economic and social/cultural dimensions. However, understanding the full meaning and extent of sustainable design is problematic as there is currently no commonly agreed definition [2]. And so we find ourselves in a dilemma of sorts, with knowledge that in a broad sense current practices are no longer considered sustainable but with little common ground to guide our future actions such that that they would clearly be more sustainable. Nevertheless, within each respective criterion the sustainable rating systems are conceptually underpinned by comparative analysis. Decisions leading to a final design are inevitably linked to analysis that compares alternatives. As such, comparative analysis is often used as a means for communication between various stakeholders; comparative values can be used to demonstrate immediate and long term advantages of improved ecological performance of buildings [3]. This paper adopts and adapts the idea of comparative analysis to consider precast concrete as a cladding solution for cladding medium to large commercial structures. The sustainability of using precast concrete is considered against other commonly used materials to clad commercial buildings in New Zealand.

Environmental and Economic Sustainability

Embodied Energy

Building activity consumes energy that in many circumstances is generated from non-renewable resources. Quantifying the energy embodied in a product of construction activity allows comparison between products, a useful decision-making tool. Embodied energy values for any one building material, component or completed building are unique to the country or region in which the building is built. These values are affected by the form of energy used and the transport distances involved as well as the energy used in manufacture. Given that concrete is produced throughout New Zealand and the materials are all locally available, the embodied energy of concrete compares very favourably to that of many other common building materials in New

Zealand. A very basic comparison of the embodied energy values for three common cladding systems is shown in Table 1. The comparison considers only the exterior cladding and any necessary support framework.

Table 1: Comparison of the embodied energy values for three common cladding materials in New Zealand

Cladding system Embodied Energy (Mega joules ) per

m2 wall area [

i]

150 mm steel reinforced concrete (30 MPa) with exposed aggregate finish

680.4 MJ

7.5 Fibre cement sheet on steel framing

719.2 MJ

High performance aluminium curtainwall

1,806.8 MJ

1. 1kWh = 3.6 MJ or 1MJ = 0.277kWh Source: Alcorn [4]

A comparison of embodied energy values for the external cladding of the Clarendon Tower project has been made. The project (figure 1) was built to the designs of Warren & Mahoney in Christchurch in 1985 during a buoyant period of commercial real estate development in New Zealand, and was, at 17 floors, the tallest building in the city at the time it was constructed. The building shell is largely concrete. The structure consists of a perimeter moment resistant frame with internal concrete columns and beams supporting a precast concrete floor system. The exterior cladding panels are also concrete and an important factor in the success of the project, both as a building project and as an ongoing investment. The architects developed the design with modular precast concrete panels of approximately 3.4 m height x 2.9 m width (figures 2 & 3). One of the prime requirements of the developer, who also acted as the builder, was to minimise the construction time. The floor to floor turnaround time for the structural frame was refined to 7 days. While the frame was being built on site, the concrete cladding panels were being constructed off site. Two basic panel designs were used for the building, one being flat and the other with a recessed window opening. A dark greywacke aggregate from one of the several braided rivers that cross the Canterbury Plains was used in the concrete mix, which was then

Figure 1: View of the Clarendon project from the northwest

Figure 3: One of the two main panel configurations. The concrete has an exposed aggregate surface finish

Figure 2: Plan detail of the concrete panel at a structural column

exposed on the surface. The design of the panels was further articulated with the introduction of a grid pattern of recessed grooves at a spacing module related to the panel dimensions in an effort to downplay the joints between panels. Each panel was then ‘dressed up’ with a single row of ceramic tiles below the windows. The upper floors were different in design to give the building a distinctive top. Although more open at these levels, the architects maintained the use precast concrete for the solid portions.

The design for construction was developed with experience gained by the architects on several earlier projects in which they experimented with precast concrete as a cladding. Precasting allowed the concrete work to be done in controlled conditions and the architects worked with the builder to maximise opportunities for a high quality result. Several prototypes of the cladding were manufactured and evaluated. The tile band was installed in the factory and the panels were delivered to the site complete with aluminium window frame. The window frame was installed with the inside face to the outside. This allowed all glazing work to be done from inside the building, reducing safety risks and the cost of working. Attaching the panels to the structure was also done from inside. Although it was not considered at the time, the design of the fixings will allow for the ease of removal of the complete panels at some point in the future, when the building reaches the end of its useful life and is a candidate for recycling. The building is of a moderate scale in the New Zealand context. Table 2 compares the embodied energy values for fibre cement sheet cladding and a high performance aluminium

curtainwall system against the concrete panels that were used. Refer to Table 1 for a description of the systems. The comparison highlights the significant reduction of embodied energy realised in the decision to use precast concrete claddings in this project. The net reduction is in the order of 4,263,054 MJ or 62%. In economic terms this equates to a reduction of NZ$ 256,000.00 at 2009 energy prices.

Internal Environment

Not only is the initial energy cost of the construction an aspect of consideration, but the ongoing energy costs as well as the health and comfort aspects related to thermal performance should also be considered when specifying the exterior envelope. The greatest thermal quality that concrete cladding systems offer is that of mass. Thermal mass is most effective in moderating temperature changes when it is directly exposed to the internal environment [5]. However, the most common methods of construction create a layer of insulation between the mass and the interior space. Many advantages derive from this construction method, including the opportunity to conceal fixings, space in which to run services and the opportunity to create a continuous insulation layer. Even when isolated in this manner the concrete does help to minimise temperature changes by the thermal inertia that derives from its mass. Most of the energy demand in large buildings is for cooling, mainly due to heat that is generated within by people and machines. Solar gain through the exterior envelope compounds the situation, a factor that has influenced the development of performance glazing systems. With lightweight construction, heat from the sun passes more

Table 2: Embodied Energy Comparison of three contemporary cladding methods, based on the Clarendon Tower project.

Cladding type Surface area1

Embodied energy / m

2

Total embodied energy

Precast concrete 3784 m2 680 MJ 2,574,634 MJ

Fibre cement sheet 3784 m2 719 MJ 2,721,453 MJ

Aluminium 3784 m2 1,807 MJ 6,837,688 MJ

1 Net area of cladding only. Windows and other elements common to all three systems not

measured

easily through, whereas the heat transfer is delayed through heavier weight materials such as concrete (figure 4). A similar delay due to thermal inertia can be seen in situations where heat travels out of the building. The effect is ideally suited to New Zealand conditions. The climate is highly variable and does not feature the extremes seen in continental areas. The thermal mass of concrete coupled with the relatively narrow temperature band and dependable diurnal swings can reduce heating and cooling demands within a building.

Of course the greatest benefit can be derived from mass within the building. The development of sandwich panel construction techniques (figure 5), where an insulation layer is bound between two layers of concrete, will see the full thermal benefits of concrete cladding realised. Increasing environmental noise leads to consequential increases of unwanted noise inside buildings. Wellington, like many other territorial authorities in New Zealand, has recognised the negative impact noise has on building occupants and recently legislated that the exterior envelopes of buildings used for residential purposes in the urban centre achieve a minimum sound insulation level. It is likely that this requirement will follow on to other building types as users become more sensitive to noise pollution. The legislation background notes the poor performance of lightweight building claddings and detailing around openings as areas of the greatest concern. 150 mm thick concrete panels provide a noise insulation value of STC 55, easily complying with the requirements. This compares with

STC ratings of 15 - 20 for common metal curtainwall systems, a performance level that requires additional mass, often by way of multiple layers of interior lining. Composite systems can be designed to effectively insulate against higher range frequencies, however in the lower frequencies the only effective method of insulating is by way of mass.

Of paramount importance in any system designed to provide effective acoustic performance is the detailing around openings and changes between materials. As concrete is a mass material there are fewer such junctions that are potential sources for sound leakage.

Durability and Maintenance

Development models have tended to be guided by the initial cost of the project, largely disregarding the ongoing costs. Often it is not the developer of a building who is liable for the ongoing costs, yet it is decisions made at the time of building that will have greatest influence on the full economic cost. Sustainable practice requires a project to be specified in consideration of the full costs, which include the initial cost but also the ongoing maintenance costs and value of a building at the end of its economic life. Concrete materials, especially those without applied architectural coatings, provide a durable and relatively maintenance free surfaces. Clarendon Tower remains a landmark in the skyline of Christchurch. After more than twenty years the appearance of the building

Fig. 4: Thermal mass slows transfer of heat Fig. 5: Sandwich panel construction

allows full thermal mass benefits to accrue

remains fresh. The cladding panels have performed very well and cost little to maintain. The majority of the building is unpainted, relying instead on the subtle colour variegation and textures of the exposed aggregate to add architectural interest. A nearby building of similar scale and constructed subsequent to the Clarendon Tower shows clear signs of fading and deterioration of the imported metal cladding. While there is no evidence that the deterioration is leading to leaks at this stage, the poor appearance has a negative effect on the image of the building. According to the building managers, virtually no costs have been incurred to maintain the exterior cladding. In the near future it is likely that the sealants between panels will require routine maintenance to be carried out. After 20 years the strong UV light in New Zealand may have cause the two part polysulphide material to deteriorate. All areas of sealant are readily accessible from the building maintenance unit. The building managers have reported that overall, the precast concrete cladding has not incurred any unforeseen additional costs and has instead led to overall cost savings when compared to other lightweight buildings. A further study has been planned to quantify the lifecycle cost differences in the case of this project.

Deconstruction and Recycling

At the end of the economic life of any building it is important to capture and use the embodied energy and resources in the building rather allow these to go to waste. To that end, recycling and deconstruction of buildings is seen as an important phase in the life cycle of a structure. Unfortunately, the majority of buildings that are now and will in the foreseeable future be at the end of their economic lives have not been designed to facilitate deconstruction. The result is that buildings are more often demolished and put to landfill in New Zealand rather than recycled. Precast concrete claddings are in most circumstances recyclable. The construction process requires that cladding components be modular, easy to handle and readily transportable. In moderate to large scale buildings it is not common for the concrete cladding components to be designed as load bearing and in these circumstances it is likely that these are accessible and reversible. Precast concrete components are therefore readily recyclable. Consequently, the investment of material and energy resources does not have to be lost when a building or the

cladding reaches the end of its economic life in one format. In this light, the use of precast concrete claddings can be seen to be truly sustainable

AESTHETICS AND SOCIAL SUSTAINABILITY

While environmental and economic aspects of the built environment lend themselves to quantifiable measures thereby enabling clear comparisons, social sustainability indicators are much less so. Accordingly, social aspects of development have a much lower profile with researchers and stakeholders in the quest to create improvement in the built environment is created and managed. Physical aspects that influence social and cultural outcomes vary widely, from the manner in which a setting or environment accommodates people’s various needs to how a building might express the identity of a place. Witnessed by the increasing number of publications and other media interest, environmental aesthetics is of interest to the public as well as those who might be involved in the production of built form. Moreover, the appearance of the built environment has been linked to mental and physical health. Aesthetically pleasing places generate, celebrate and sustain life; they ‘heal in the sense of making us more whole’[6]. In the context of this paper, aesthetic outcomes are an aspect of social sustainability that is clearly influenced by a building’s cladding. This paper considers some of the principle ways by which concrete claddings can be seen to contribute to positive aesthetic outcomes. Indeed, the properties of precast concrete provide this format of construction with great aesthetic potential when used as a cladding for buildings. The two most important formal factors affecting aesthetic judgement are perceptual order and perceptual interest tending toward ambiguity and complexity [7]. A primary feature of a positive aesthetic experience is that the object being considered is able to be visually ordered. Humans appear to have intuitive capacity for aesthetic experience that is universal and which underlies the different manifestations each may take in accordance with cultural differences. It seems all can appreciate sense of pattern, sense of rhythm, balance and harmonic relationships [8]. Components of a cladding system that can be used to order an object or scene include setout of openings and expressed surface patterns. The more regular the patterns are, the higher the perceptions of redundancy or pattern will be to the viewer. The modular nature of precast concrete

claddings can be developed expressively in the façade set out to enhance aesthetic perceptions.

Figure 6: SIMU building in Christchurch provides a vivid example of how architectural concrete cladding can be used to generate rich rhythmic patterns.

The SIMU Building (now AMI House) in Christchurch’s Latimer Square provides clear illustration of the rhythmic effect of a modular pattern (figure 6). Patterns can be identified by viewers at a number of levels beginning with the strong expression of floor levels, reinforced by the junction of the full floor height precast concrete panels. Secondly, and perhaps most clearly, are the regular rhythms of the projecting vertical blades in the design of the cladding panels. The potential for precast concrete cladding panels to generate aesthetically pleasing patterns is inherent in the production processes. Standardisation of panel dimensions and setout, so as to reduce the number of variations, is utilised to minimise costs. Provided designers recognise the potentials, this should be seen as an opportunity to generate patterns that are known to be pleasing to the senses [9]. Scenes are also evaluated for their visual interest by viewers during an aesthetic experience. On its own, order can lead to

monotony and it is therefore necessary to ensure that claddings stimulate visual interest. Again, concrete cladding panels offer inherent potential to generate visual interest, several of which are discussed below. Three dimensional modelling of contemporary cladding surfaces has all but disappeared in favour of engineered, layered, lightweight cladding solutions. Instead, the emphasis seems to be on transparency and lightness. As a consequence many contemporary buildings appear flat, to the detriment of aesthetic outcomes. Architectural concrete claddings have potential to be modelled three dimensionally with no penalty to the integrity of the cladding. In contrast with layered, lightweight claddings, where every change of plane introduces another potential junction for a leak to develop, concrete claddings are monolithic and 3D modelling does not increase weathertightness risks. Three dimensional modelling mainly works with daylight to create visual interest through shade and shadows that animate the surface over the course of a day. This potential is illustrated to great effect in the General Accident Building (figures 7 & 8). Surface texture can also be used to create interest. This is seen in all three example projects, where the surface finish is exposed natural aggregate. Illuminated by the sharp light conditions right around New Zealand, surfaces such as these can appear to vibrate and shimmer, ensuring the viewer’s interest remains piqued. New developments in the use of surface retardants and photo imaging have been used to print textures and patterns across the surface of concrete claddings.

Finally, the highest level of an aesthetic experience derives through interpretation and relating perceptions to schemas of meaning. In essence, meaning is central to one’s evaluation of a setting or object, and better associational values can lead to richer aesthetic experiences. Precast concrete cladding provides several opportunities for strong associational meanings to form. Craftsmanship is perceived positively in part because of associations with care, attention, skill and control. Comprising of prefabricated components that are generally made to exacting tolerances and crisply detailed, concrete claddings can elicit positive responses arising from attention to quality. The standard of craft is likely to rise on the back of recent developments such as controlled permeability formwork and self compacting concrete, both of which can lead to crisper detailing and surface finish. Natural materials are perceived more favourably than those that are obviously synthetic or ambiguous. As concrete comprises of naturally occurring materials, developing claddings in a manner that expresses these can also enrich the aesthetic experience.

CONCLUSIONS

Changing global and local circumstances have affected the use of cladding materials for New Zealand buildings. While precast concrete enjoyed popularity during the 1970s and 1980s, over the past several decades its use has diminished in favour of lightweight systems. However, with growing interest in sustainability it is opportune to once again consider precast concrete claddings. The paper has evaluated precast concrete claddings in several contexts. A precast concrete cladding system embodies considerably less energy than a lightweight cladding system based on aluminium components. A study has shown that the decision to use precast concrete cladding on the Clarendon Tower project in Christchurch resulted in a saving of some 4.2 million mega joules of energy or 62% of the energy that would have been used had a performance aluminium cladding been specified. In economic terms, this represents a reduction in the order of NZ$256,000.00 at 2009 prices.

Figure 7: The General Accident Building employs vigorously modelled architectural concrete cladding such that visual interest is sustained.

Figure 8: Detail of the cladding shows also the surface texture that comes with an exposed aggregate finish.

Precast concrete cladding systems have inherently better thermal performance in New Zealand conditions over lightweight systems due to the thermal inertia created by the mass of the material. This has the effect of slowing temperature change inside a building as outside temperatures change. Concrete provides superior sound insulation qualities over lightweight systems as a consequence of its mass. Even though composite systems can be detailed to provide similar insulation ratings in the high frequencies, concrete performs much better in the low frequency range. This is a particularly important consideration for buildings in urban settings, where environmental noise is ever increasing. Precast concrete cladding components are readily able to be recycled at the end of the economic life of a building. Concrete components are durable and likely to be able to be relocated into a new setting. The construction of precast concrete systems makes them ideally suited to disassembly. While social outcomes are not quantifiable in the manner that many economic and environmental measures of sustainability are, they are no less important. Previous research has identified that aesthetic experiences relate to perceptions of order, visual interest and the degree to which primary perceptions foster positive associational meanings. It has been argued that concrete claddings are inherently suited to generate positive aesthetic experiences. However, it is the role of the designer and other stakeholders to ensure that concrete used as a cladding reaches the potential of the material and the prefabrication process.

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