Abstract

Concrete’s weak tensile strength renders it susceptible to cracking under prolonged loads, leading to reduced load-bearing capacity and reinforcing bar corrosion. This study investigates the effectiveness of microbial-based self-healing in high-strength concrete, focusing on two bacterial strains: Sporosarcina koreensis and Bacillus. Results demonstrate significant enhancements in micro- and macro-physical properties of high-strength bacterial concrete with Bacillus flexus and S. koreensis, surpassing the control. Bacillus flexus-infused concrete exhibits a remarkable 21.8% increase in compressive strength at 7 days and 11.7% at 56 days. Similarly, S. koreensis-treated concrete shows 12.2% and 7.4% increases at 7 and 56 days, respectively. Enhanced crack healing occurs due to calcite precipitation, confirmed by X-ray diffraction and scanning electron microscopy. Both bacterial strains achieve crack closure within 42 days, with widths of 259.7 µm and 288.7 µm, respectively. Moreover, bacterial concrete from these strains excels in durability against water, acid, and salt exposure, surpassing control concrete. These findings emphasize microbial-based self-healing’s potential in high-strength concrete, providing a practical strategy to enhance structural resilience and extend concrete infrastructure lifespan.

Introduction

High-strength concrete (HSC) has gained widespread recognition for its enhanced mechanical properties, making it suitable for demanding structural applications (Krishnapriya et al., 2015). However, like conventional concrete, HSC is susceptible to cracking under various stress conditions, which can compromise its durability and load-bearing capacity (Iheanyichukwu et al., 2018). The resulting cracks not only diminish load-bearing capacity but also trigger environmental and economic issues, including carbon emissions during repair (Rosewitz et al., 2021). Additional weakness of concrete structure is the presence of voids and fissures in its body matrix. The circumstance of such voids in the matrix of concrete plays substantial role in determining its mechanical properties and durability (Feng et al., 2021; Khaliq & Ehsan, 2016).

The concept of self-healing concrete, facilitated by microbial activity, has emerged as a promising avenue to address these challenges, aiming to autonomously fill and repair such voids and cracks via bio-mineralization. In this process, bacteria induce the conversion of calcium particles within the cement composite into calcium carbonate, calcite. Precipitation and deposition of calcite due to microbial activity of bacteria seals such voids and fissures in the concrete body matrix. This in turn advances the mechanical properties and durability of concrete as well as healing rate of crack formed due variously initiated stress (Rohini & Padmapriya, 2021; Singh & Gupta, 2020). These bacteria must exhibit robust urease activity, withstand high pH levels, and endure mechanical stress within the concrete matrix (Rohini & Padmapriya, 2021; Rosewitz et al., 2021).

Conclusions

This study investigated the impact of microbes on high-strength concrete properties, utilizing B. flexus and S. koreensis. The research encompassed their effect on mechanical strengths (compressive, flexural), ultrasonic pulse velocity, acid and salt resistance, and microbial healing. Key findings are presented as follows:

High-strength bacterial concrete, employing B. flexus and S. koreensis, exhibited improved macro-physical properties compared to conventional concrete. B. flexus concrete showed 21.8% and 11.7% higher compressive strength at 7 and 56 days, while S. koreensis concrete exhibited 12.2% and 7.4% increases. Flexural strength for B. flexus concrete was 6.0% and 9.0% higher at 14 and 28 days, and for S. koreensis concrete, it increased by 5.2% and 6.4%.

Microbial-induced calcite deposition significantly enhanced the healing capacity of cracked bacterial concrete. Within 42 days, B. flexus and S. koreensis concrete restored cracked sections by 259.7 µm and 288.7 µm crack widths, respectively.

Bacterial concrete exhibited greater strength restoration following 55% stress-level crack initiation. While normal concrete regained only 64.0% of its 28-day compressive strength, B. flexus and S. koreensis concrete achieved 96.1% and 94.1% restoration, respectively.

Enhanced durability against acid and salt attacks was observed in bacterial concrete, with reduced weight and strength loss compared to conventional concrete. B. flexus and S. koreensis concrete experienced lower strength losses (4.8% and 5.3% for sulfuric acid, 1.8% and 2.1% for magnesium sulfate) than conventional concrete.

Abstract

Durability property is a critical performance indicator of concrete in different environments and locations. However, durability also plays a key role in the sustainability and climate resiliency of concrete structures which is mostly ignored in the context of ways to improve the sustainability of concrete materials and structures. Hence, this paper aims to bring focus to this area by presenting an overview of the critical role of concrete properties especially durability on the sustainability and climate resiliency of concrete structures. This paper first presents a general overview of concrete materials followed by the connection of concrete to sustainability and climate resiliency. Discussions from this paper indicate that to improve concrete sustainability, there is a need to use materials with lower environmental impacts upfront in addition to producing concrete with improved durability to ensure long-term performance. In other words, merely reducing the upfront embodied carbon of materials is not sufficient to achieve sustainable concrete if the impact of such alternative low-carbon materials used to replace the traditional materials in concrete is not considered. On the other hand, the climate resiliency of concrete structures is mostly dependent on improved durability which would sustain their adaptation over time to the impacts of the changing climate conditions. Overall, to achieve sustainability and climate resilience of concrete structures; lower environmental impact, higher durability performance and resilience to applicable climatic conditions must be achieved.

Introduction

Concrete being the most used construction material in the world can be associated with its outstanding performance coupled with its lower cost and availability of raw materials locally in various parts of the world. Concrete has been used extensively to build various infrastructures ranging from transportation systems, health, residential and commercial buildings, water systems, etc. However, the high production and use of concrete have been found to yield negative environmental impacts on the environment due to its contribution to greenhouse gas (GHG) emissions and consumption of natural resources [1] , [2] .

In contrast to other construction materials, such as steel and glass; concrete has a lower unit embodied carbon as depicted in Fig. 1 which shows the carbon emissions per kilogram of each material. However, the production and use of concrete in large quantities have resulted in the concrete industry being a major contributor to the world’s human-induced GHG emissions. A critical part of concrete responsible for the high GHG emission is the production of portland cement (PC) which is the primary binder in concrete and other cementitious materials. Out of all cementitious materials (i.e. concrete, mortar, grout, etc.), concrete consumes PC the most with an estimated amount of 70% while the other 30% is used for other cementitious materials such as mortar, grout, etc.

Conclusion

This paper presents an overview of various properties, specifically the durability of concrete and how it influences concrete sustainability and climate resiliency. Discussions in this paper show that the durability of concrete is very critical to both concrete sustainability and climate resiliency and a holistic approach needs to be employed. Improving the durability of concrete would result in the reduction or elimination of raw material and energy use for rehabilitation and reconstruction resulting in lower environmental impact thereby enhancing sustainability. Similarly, enhanced durability of concrete would ensure that concrete structures can adapt to the changing climate changes thereby improving its climate resiliency.

Thus, to improve the sustainability and climate resiliency of concrete infrastructures; core attention must be placed on the embodied carbon of the materials and the durability performance rather than the former only. Finally, initiatives to improve concrete sustainability and climate resiliency should be specific geographical zone-based rather than a global approach as the availability of raw materials and climate conditions varies with location.

1- Introduction

2- A general approach for valuing firm’s financial flexibility under default risk and bankruptcy costs

3- A simulation analysis

4- Concluding remarks

References

Abstract

Purpose – The purpose of this paper is to present a model to value leveraged firms in the presence of default risk and bankruptcy costs under a flexible firm’s debt structure.
Design/methodology/approach – The authors assume that the total debt of the firm is a combination of two debt components. The first component is an active debt component which is assumed to be proportional to the firm’s value. The second one is a passive predetermined risk-free debt component. The combination of the two debt categories makes the firm’s capital structure more realistic and allows us to include flexibility into the firm’s debt structure management. The firm’s valuation is performed using the discounted cash flow technique based on the weighted average cost of capital (WACC) method.
Findings – The model can be used to define active debt management strategies that can induce the firm to deviate from its capital structure target in order to preserve debt capacity for future funding needs. The firm’s valuation is performed by using the WACC method and a closed form valuation formula is provided. Such a formula can be used to value costs and benefits of financial flexibility.
Research limitations/implications – The proposed approach provides a good compromise between mathematical complexity and model capability of interpreting the various economic and financial aspects involved in the firm’s debt structure puzzle.
Practical implications – This model offers a realistic approach to practical applications where real financing decisions are characterized by a simultaneous use of these two debt categories. By comparing costs and benefits deriving from using unused debt capacity for future funding needs, the model provides a quantitative support to investigate if financial flexibility can add value to firms.
Originality/value – To the authors knowledge, the approach the authors propose is the first attempt to build a valuation scheme that accounts for firm’s financial flexibility under default risky debt and bankruptcy costs. Including financial flexibility, this model fills an important gap in the literature on this topic.

Introduction

In a seminal paper, Modigliani and Miller (1959) showed that the use of debt financing can increase the market value of a firm as a consequence of the debt tax shield. However, the presence of debt in the capital structure can lead the firm to incur into default and distress costs (Warner, 1977; Altman, 1984; Opler and Titman, 1994). The valuation of distress costs is therefore an important task in financing decisions making (Oded and Michel, 2007; Lahmann et al., 2017). The impact of default costs on the firm’s value has been deeply investigated in the literature. Andrade and Kaplan (1998) estimate losses on the order of 10–23 percent of the predistress firm value. Glover (2016) confirms that the average cost among defaulted firms is about 25 percent of the predistress firm value.

Abstract

Financial flexibility is a firm’s ability to deploy its financial resources to meet future financing needs. Flexibility-building firms’ financial decisions are geared toward building up financial flexibility — maintain low leverage by issuing equity in order to raise cash. As firms progress and start making investments, they utilize their financial resources — flexibility-utilizing firms increase debt and use reserved cash in order to exercise investment options. As more cash flows are generated and investment opportunities diminish, firms recharge their financial flexibility by repaying debt and increasing cash using internal funds. The findings have important implications for previously documented empirical regularities.

Identifying and financing investment opportunities are crucial to any successful corporation. Financial flexibility reflects a firm’s ability to deploy financial resources in response to future investment opportunities. Not surprisingly, corporate managers are most concerned about financial flexibility in their financial decisions.1 Yet, the empirical literature has not yet systematically explored the relation between the financial flexibility demand and corporate behavior. This study examines how the demand for financial flexibility affects firms’ financial decisions. A recent theoretical development provides a framework for understanding how a firm’s demand for financial flexibility drives financial decisions. Gamba and Triantis (2008) suggest that by managing their cash and capital structure policies, firms alleviate the impact of external financing costs on investments. Moreover, they find that the effect of financial flexibility on firm value is quite significant when constrained firms have high growth opportunities and low internal cash flows. Acharya, Almeida, and Campello (2007) also suggest that firms with low correlation between cash flows and investment opportunities have greater incentives to transfer resources into future states. DeAngelo et al. (2011) develop a model in which firms preserve the ability ex ante to access the capital market ex post in the event of unexpected cash flow shortfalls or investment opportunities.