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Concrete is a ubiquitous building material, and it is often cited as the most consumed commodity on Earth, second only to potable water. Concrete and reinforced concrete have been used in construction since the mid-19th century, and specialists regularly improve the properties of these materials. With the rapid consumption of natural resources, there is a growing concern about sustainable development. More than 6 billion tons of concrete are produced annually for various construction purposes with limited life expectancy. In addition, increasing cost of construction and demolition of concrete structures in densely populated areas is a great concern for the future.
As this inherited concrete infrastructure continues to age, maintaining and repairing concrete is of increasing strategic importance to both defense and civilian infrastructure. One of the best solutions to tackle these challenges is to increase the life expectancy of the structures. Residential structures and important civil structures are typically designed for a life span of 50 and 100 years respectively. However, the life expectancy of structures can be increased to several hundred years with careful planning and proper design.
The DoD relies on steel-reinforced concrete structures such as missile silos and naval piers that are many decades old, not easily replaced, and subject to cracking and corrosive deterioration. Much of the DoD’s shoreside infrastructure and hardened terrestrial infrastructure, such as Minuteman missile silos, date from the ‘40s and ‘50s. For the Navy, shoreside infrastructure alone contributes hundreds of millions of dollars in annual sustainment costs.
The DoD also relies on concrete airfield pavements in expeditionary settings, which are vulnerable to damage from overuse or attack and require rapid repair under logistically challenged circumstances. Airfields are key targets for an adversary; following an attack, pavement repairs must be made on rapid, tactical timelines to minimize down time, avoid further
vulnerabilities, and maintain operational tempo. Rapid patching of craters is the current repair
strategy to restore runway surfaces after an attack. However, the logistical requirements of
transporting bulky materials used in more standard airfield damage repair solutions present a
significant vulnerability.
To address this challenge, the DoD is developing an Expedient and Expeditionary Airfield Damage Repair (E-ADR) capability suited to austere settings. E-ADR minimizes the time to restore airfield operations with a minimal logistic footprint by sacrificing the durability of the patch repairs (e.g., these repairs are only rated to support 500 passes of a fighter aircraft). This tradeoff results in a quick restoration of operations; however, the potential for rapid deterioration and failure of a repair patch is likely to lead to subsequent periods of inoperability.
Unfortunately, state-of-the-art approaches to maintaining concrete are one-time surface interventions (e.g., mortar coatings, compressive sleeves, and more recently, biomineralizing sprays). These approaches are limited to surface treatments that are short-lived and do not address the underlying causes of decay. These one-time interventions, limited to remediation of defects at or near the surface, typically necessitate downtime for critical assets.
However, cracking often arises in the interior of thick concrete components before propagating to the surface, so surface-based repairs cannot mitigate cracks at their site of origin. Particularly in corrosive environments (e.g., marine structures) or when architecture limits access (e.g., buried, hardened structures often have steel liners covering concrete), new approaches are needed to improve durability by healing and preventing cracks when and where they emerge.
No current technology provides ongoing crack repair and prevention for defects deep inside existing aged concrete or prolonged repair of damaged airfield pavements. New research suggests, however, that cross-disciplinary technologies can be used to impart aged concrete with self-healing capabilities.
Several emerging technologies seek to address the challenge of self-sustainable concrete construction, including new classes of Engineered Living Materials (ELMs) and self-healing concretes. While these approaches show promise to repair cracks, including those emerging at depths within a material beneath the immediate surface, neither ELMs nor self-healing concretes address challenges posed by extant concrete structures or rapid runway repairs performed in austere settings with limited resources.
To explore this possibility, DARPA launch a new program called Bio-inspired Restoration of Aged Concrete Edifices, or BRACE program in March 2022. The goal of BRACE is to develop technologies that impart long-lasting, self-healing capability to concrete at depths that address cracks early, to repair them, prevent their propagation, and extend the serviceability of critical infrastructure. Inspired by the vascular systems that support continuous repair in multicellular organisms and ecosystems, BRACE will develop approaches to integrate a healing “vasculature” for prolonged damage repair and prevention.
A major challenge to imbue concrete with ongoing repair of deep defects is the need to transport
substances for crack healing and prevention throughout the threedimensional bulk of material. Many biological systems solve this problem via vascular networks that are typically composed of
filamentous structures. Large and complex multicellular organisms, including many species of animals and plants, possess vascular systems that transport nutrients and metabolites across macroscopic distances (> 10 meters) to support functions such as injury repair. Ecosystems also include vascular approaches for nutrient and signal translocation across large distances,
and in some ecosystems, soil-dwelling filamentous fungi subserve this function by forming
symbiotic relationships with plants and other species across areas spanning >1000 acres.
Inspired by these biological systems, BRACE technology will develop strategies to impart aged structures and E-ADR patch repairs with a healing vasculature that can be applied rapidly to integrate deep within concrete and provide prolonged functionality to repair cracks and restore the material’s durability.
Extending the service life of existing concrete structures is critical for maintaining an asymmetric
advantage over potential adversary nations, some of whom are building new military facilities at
a rapid pace. These new structures will likely incur far lower maintenance costs than the DoD’s
older facilities in the coming decades.
“Today’s DoD has inherited, and relies upon, a significant amount of concrete infrastructure from the 1940s and 1950s that cannot be easily replaced,” noted Dr. Matthew J. Pava, BRACE program manager. “The BRACE program, if successful, will prevent new damage, shorten repair time, and reduce maintenance costs, allowing for extended infrastructure service life.”
BRACE Program
The 4.5-year research effort will include two Technical Areas (TAs) focused on developing long-lasting systems for transport of healing substances throughout concrete, as well as practical tools for applying, maintaining, and predicting the long-term function and performance of these systems.
TA1 will address the challenges of engineering bio-inspired approaches for establishing long-acting vascular structures deep within concrete both to repair cracks and to provide self-diagnostic signals that let the user know they are still functioning after years or decades.
In TA2, performers will develop methods for applying and maintaining TA1 systems in concrete, rapid aging testbeds for vascularized concrete, and models that predict the system’s effectiveness in averting the need for future repairs. Performers will respond to both TAs, and BRACE will refine these capabilities in two tracks aligned to repairs on long-term (e.g. steel-reinforced marine or buried infrastructure) and rapid (e.g. expeditionary airfield runway repair) use case timelines.
Although the program will target these use cases, technologies developed in BRACE may eventually be adapted for civilian infrastructure.
“The United States ranks 13th worldwide when it comes to the overall quality of infrastructure,” added Pava. “While BRACE is focused on DoD applications, our hope is that the technologies generated will have potential civilian benefits as well.”
BRACE performers will engage with U.S. government and DoD stakeholders, as well as appropriate regulatory authorities. Safety is paramount, and all research will be subject to regular review by both an independent laboratory and regulatory agencies to ensure that BRACE technologies do not pose a threat to human or structural health. In addition, teams are further required to collaborate with ethical, legal, and societal implications (ELSI) experts and ensure the research addresses any related concerns.
