Reuse of Concrete Components in new Construction Projects

Assignment Task

Background

The reuse of concrete components in the construction process has the potential to revolutionize the architecture industry and make a meaningful contribution to a more sustainable future. Using recycled concrete from demolished buildings is much more efficient than at times blindly destroying our existing infrastructure and resources. Repurposing concrete components can reduce the amount of new raw materials needed for new construction projects and help to reduce a building's negative environmental impact. By focusing on sustainably sourced materials, architects and engineers can make an impact in communities big and small.

Reusable concrete components are an important part of sustainable construction projects, as they can be customized to meet the needs of a particular project while also limiting waste and maximizing the utilization of its resources. Reused concrete components have been used in a variety of sustainable building projects, such as for floors, walls, and even bridges (Hui et al., 2018). Reused concrete components can also be reused in different forms, such as for insulation or for fire protection, which helps to further reduce the use of primary materials (Shrirao et al., 2017). Additionally, the reuse of concrete components can also provide cost savings, as they do not require the acquisition of new materials (Buhl, 2012). Finally, reused concrete components can also provide important environmental benefits, including those of energy efficiency and durability (Chliya, 2016). By incorporating reusable concrete components into sustainable construction projects, architects and engineers can create structures that are both cost- and resource-efficient while also creating safe and comfortable living environments.

Throughout this section, there will be an introduction to compound concrete (CC) and concrete-filled steel tubes (CFST), where I will discuss the members and how they are made along with their environmental impacts. Further into this section I will also discuss the aims and objectives of my individual project where I explain the methodology to achieve these goals.

Compound concrete and concrete-filled steel tubes

Firstly, there are two types of recycled material from demolished structures. Recycled aggregate concrete (RAC) and compound concrete (CC). RAC is produced from a mixture of recycled concrete aggregates (RCA) less than 31.5mm (Jian and Wu, 2021) and fresh concrete (FC). However, compound concrete (CC) is a mixture of fresh concrete and demolished concrete lumps (DCL) with size greater than 40mm (Wu et al., 2015).

Concrete-filled steel tubes (CFST) can be used for several different structural applications such as piles, building columns and bridge piers (Wang et al., 2017) because they have high elastic stiffness and axial strength (Han et al., 2014). The steel tube used in CFST members serves as permanent formwork and reinforcement, cutting down on building costs and time. The concrete core confines the steel tube, and the steel tube is prevented from localized buckling inward by the concrete core, improving the mechanical performance of the member (Han et al., 2014). Similar in structural design to CFST members, compound concrete-filled steel tubular (CCFST) members use CC rather than regular concrete. As a result, CCFST members use less cement and natural aggregates than comparable CFST members, which is good for the environment.

The Dazhi Bridge in Kaohsuing, Taiwan

The Dazhi Bridge, located in Kaohsiung, Taiwan is an iconic example of a steel tube bridge. This bridge is composed of concrete filled steel tubes, providing structural support. The bridge is “formed from three box girder spans of varying lengths and a single length of cantilever” (Gu, et al., 2008). This bridge is also engineered with a compound concrete method that creates a tight “monolithic connection” between concrete and steel (Gu, et al., 2008). The tightness of the joint contributes to the bridges strength and longevity, making it a landmark in the Kaohsiung region.

This building is an interesting example of structural engineering: a steel tube bridge made up of compound concrete and compound concrete filled steel tubes (Shadman, 2007). The steel tubes provide the bridge with strength and rigidity, while the compound concrete acts as a stabilizer, adding support and even dampening out any potential vibration from traffic. This makes it incredibly strong yet also lightweight, which is especially important for a bridge – it must support heavy traffic and yet be able to be built quickly. This is just one example of how engineering is used to improve our way of life, creating more efficient and durable infrastructure.

Circular economy

The circular economy (CE) is a development strategy where everything is engineered to be continuously reused or recycled. It employs the "3 Rs" (reduce, reuse, and recycle) to lessen the negative effects of human activity (Li et al., 2010) such as rethinking the design, manufacturing, selling and consumer ownership to retain resources for as long as possible and extract maximum value. In the UK, building construction and maintenance are thought to be responsible for 50% of all CO2 emissions. Additionally, the UK wants to cut greenhouse gas emissions by 80% by 2050, which will call for new buildings to be more energy efficient (Borchers, 2010). Within a circular economy, structures need to include reusable and versatile components such as recycled aggregate concrete (RAC). Conversely, reinforced concrete (RC) members are expensive, cannot be reused and can only be recycled if the structure is demolished, which requires a lot of energy. According to preliminary tests, CCFST members have mechanical performance that is equivalent to or slightly below that of CFST members while providing greater environmental benefits (Zhao et al., 2016). However, the structural components made from recycled concrete are not entirely trusted by the building sector. Therefore, recycled concrete can only be used for non-structural purposes, such as road subbases (Wu et al., 2019)

Aims of the project

Construction and the built environment in general are substantial consumers of energy and natural resources. In addition, the construction industry is reportedly producing unacceptably high levels of waste (Ekanayake & Ofori 2000). The relevance of the environmental effects connected with waste generated during the demolition of old structures has recently come to the attention of the construction industry. Demolition waste volume increased, damaging the environment locally and globally. The goal of this project is understanding compound concrete-filled steel tubular (CCFST) members' compressive behaviour in greater detail. Reduced greenhouse gas emissions from concrete use and a slower pace of resource depletion are the driving forces behind this study (Tam et al., 2018). The UK plans to reach its target of 80% reduction in greenhouse gas emissions by 2050 (Borchers, 2010). To reach these aims, there will be 3 objectives:

• Investigating the axial compressive behaviour of concrete filled steel tubes
• Testing out the material properties of compound concrete
• The design proposals for the recycled concrete filled steel tubes

Research methodology

Firstly, I will construct a literature review explaining the types of concretes I will be testing such as compound concrete using demolished concrete lumps, compound concrete filled steel tubes and concrete filled steel tubes. Then, I will go into the concrete lab and conduct experiments such as the compressive test in order to achieve compressive behaviour of both compound concrete and fresh concrete. Furthermore, I will compare the differences in strengths, and this would prove using recycled concrete would be suitable in sustainable construction. After I would repeat this process for different recycled aggregates in order to remove any anomalies.

Literature review

Compound concrete and demolished concrete lumps

Throughout this section I will focus on the compressive behaviour of CCFST, CFST and CC. Compound concrete (CC) include DCLs greater than 40mm which results in a significant amount of mortar adhered to their surface, reducing the quantity of natural aggregates and cement which means it can be a direct concrete replacement (Li et al., 2016). The lower the DCL replacement ratio, the more recycled material there is.

Compound concrete is made by combining demolished concrete lumps (DCL) and fresh concrete. DCLs are composed of both smooth and rough surfaces, which come from the exterior and internal faces of the larger concrete members from which they are made. Equation (2.1) shows how to calculate the DCL replacement ratio (η) using the mass of the DCLs (m _DC ) and fresh concrete (m _FC ) (Jian and Wu., 2021). At a DCL replacement ratio of 0, compound concrete converts to ordinary concrete. As the DCL replacement ratio rises, so does the proportional need for cement and natural aggregates. Achieving a workable concrete mixture while maximizing environmental advantages, the ideal DCL replacement ratio is around 30% (Wu et al., 2018). If not carefully evaluated, the size and shape of DCLs may impact the interfacial binding strength between the DCLs and the mortar and the compaction of fresh concrete during casting.

Concrete filled steel tubes

Concrete filled steel tubes (CFST) are created by filling in steel hollow tubes with natural aggregate concrete. CFST uses 3 different steel hollow sections: square hollow sections, rectangular hollow section, and circular hollow section. These all act as reinforcement and permanent formwork as they can withstand several levels of construction before inserting the concrete (Han et al., 2014). As a result, using CFST members would decrease economic costs and construction times. Furthermore, the steel members are easily accessible to be inspected and treated with anti-corrosive coatings which increases the upkeep costs (He et al., 2019).

Compound concrete-filled steel tubes

Compound concrete-filled steel tubes (CCFST) members are usually casted on-site as they place a layer of 20mm fresh concrete and then insert demolished concrete lumps (DCLs) in the compound concrete (CC) core whilst the mixture is constantly vibrating (Zhao et al., 2016). Alternatively, you can insert steel tubes with pre-cast CC units inside of them, with cast-in-place concrete being used to cover any gaps (Wu et al., 2018b). Because fewer levels of FC and DCLs are required, the pre-cast CC units can be cast horizontally, which increases construction efficiency. Throughout this section I will discuss the compressive, flexural and creep behaviour of CCFST.

Compressive behaviour of CCFST

The use of DCLs with a 31.0 MPa compressive strength and FC with 31.2 MPa and 94.7 MPa compressive strengths by CCFST members has been studied (Wu et al., 2018e). The three steel configurations used in the CCFST members were 2.4 mm, 2 mm, and 2 mm steel tubes combined with 8 mm thick steel stirrups. The members with the steel stirrups and the thicker tube both had the same amount of steel. All these members had a DCL replacement ratio of 33%, and a reference specimen with a DCL replacement ratio of 0% was also evaluated using only thin steel tubes. In figure 2.4 it shows the structural arrangement of CCFST members which use steel stirrups.

Flexural behaviour of CCFST

It is anticipated that CCFST columns will experience mixed compressive and flexural loading. Due to the negative impact of the ITZs between the DCLs and mortar, the flexural strength of CC is always lower than that of FC and declines with increasing replacement ratio (Liu et al., 2017). However, with CFST members, the interaction of the steel tube and concrete core prevents localized buckling of the steel tube and the development of full-length cracks in the concrete. As a result, the CFST member's longitudinal plasticity can fully develop, increasing its flexural strength and ductility. This indicates that CCFST members are probably going to perform well under flexural loading (Wang et al., 2014).

Creep behaviour of CCFST

Creep is a long-term stressing of a material that typically becomes critical at high temperatures. Thin walled CCFST members that were evaluated over a 200-day period at standard room temperature have been the subject of experimental studies that look at the creep behaviour (Wu et al., 2018a). Given that they were not tested at high temperatures, the strain placed on these members during this time was minimal.

Experimental investigation

Methodology

Casting compound concrete members

Casting compound concrete members refers to the process of constructing structural elements or components of a structure using concrete. This can include beams, columns, walls and other elements that are made of concrete and used to support the load of a structure.

Saturated surface dry condition

Photos of the DCLs being soaked and dried overnight in preparation for casting are shown in Figure 3.3. By soaking demolished concrete lumps in water for a day before testing the compressive strength of the compound concrete, you can help to ensure that the concrete has reached its maximum strength and is able to withstand the load of the compressive strength test. This can give you a more accurate measure of the concrete's strength. DCLs before casting where they are being soaked overnight a day prior to casting and on the morning of casting separated into test groups.

Equations for the compressive strength of compound concrete

I will be using Equation 3.1 the predict the compressive strength of compound concrete where f _exp,CC is the compound concrete compressive strength, f _exp,FC is the fresh concrete compressive strength and η is the DCL replacement ratio. Equation 3.2 and 3.3 shows how to achieve ‘a’ and ‘b’ in equation 3.1 where ω is the normalized lateral dimension and ε is the normalized strength difference between FC and DCLs. Equation 3.4 and 3.4 show how to acquire (normalized lateral dimension) and in equations 3.2 and 3.3 where is the compressive strength of the demolished concrete lumps.

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