Coconut Shells and Hemp Fibers Tested as Concrete Armor Against Underwater Abrasion

Concrete is the workhorse of hydraulic engineering, but underwater structures face a relentless enemy: abrasion. Sediment-laden water swirling around bridge piers, dam spillways, and offshore platforms grinds away concrete surfaces particle by particle, a cumulative mechanical erosion that can compromise structural integrity over decades. Now, a team of researchers from the Vellore Institute of Technology in Chennai, India, and Universiti Tenaga Nasional in Malaysia has published a comparative study in Scientific Reports that tests how sustainable lightweight concretes made from agricultural waste aggregates and natural fibers stand up to this punishment under controlled rotational hydraulic conditions.

The study, led by A. Hemanth Kumar and K. Karthikeyan with collaborator G. Murali, addresses a significant gap in concrete durability research. While lightweight aggregate concrete has become increasingly popular in coastal, offshore, and hydraulic engineering applications, comparative data on the abrasion resistance of different agricultural waste aggregates under high-energy underwater conditions have been scarce. The researchers set out to compare conventional concrete with lightweight aggregate concretes incorporating coconut shell, expanded clay aggregate, oil palm shell, and areca nut shell aggregates, each reinforced with natural fibers including hemp, jute, and coir.

What makes this research particularly compelling is its experimental design. Rather than testing materials under static conditions, the team subjected concrete specimens to underwater rotational abrasion at two distinct speeds: 1200 rpm and 1600 rpm. This approach allowed them to investigate the combined effects of rotational speed, abrasion time, aggregate properties, and the interfacial transition zone between aggregate and cement matrix — factors that collectively determine how well a concrete mixture resists mechanical erosion in real hydraulic environments.

The findings revealed a clear pattern: abrasion loss increased linearly with exposure time, confirming that underwater wear is a cumulative process rather than a sudden failure event. After six hours of testing at 1600 rpm, weight loss increased from 461.93 grams for conventional concrete to a range of 549.44 to 698.91 grams for the lightweight concretes. This means that while all lightweight mixtures experienced greater material loss than conventional concrete, the spread within the lightweight group was substantial, suggesting that aggregate choice and fiber reinforcement significantly influence abrasion behavior.

Wear rate data told an even more nuanced story. At 1200 rpm, wear rates ranged from 23.35 to 38.80 grams per hour across the tested mixtures. When the rotational speed was increased to 1600 rpm, wear rates jumped dramatically to between 55.42 and 116.49 grams per hour. The weight loss ratio varied between 1.78 and 2.60, indicating that some lightweight mixtures were considerably more sensitive to speed increases than others. This speed sensitivity is a critical parameter for engineers designing structures that will face varying hydraulic energy conditions throughout their service life.

Among all the lightweight mixtures tested, the palm oil shell-hemp fiber combination emerged as the standout performer. This mixture exhibited the lowest speed sensitivity and the most stable abrasion performance under severe underwater conditions. The finding suggests that the interaction between oil palm shell aggregates and hemp fibers creates a microstructure that better resists the escalating mechanical energy delivered at higher rotational speeds, potentially through improved crack bridging and enhanced bonding at the interfacial transition zone.

The implications of this research extend far beyond the laboratory. Agricultural waste products like coconut shells, oil palm shells, and areca nut shells are generated in enormous quantities across tropical regions, often ending up in landfills or open burning. Converting these byproducts into concrete aggregates offers a dual benefit: reducing waste disposal problems while decreasing reliance on natural stone aggregates, whose extraction carries significant environmental costs. Natural fibers such as hemp, jute, and coir further enhance the sustainability profile of these mixtures, as they are renewable, biodegradable, and require minimal processing energy compared to synthetic alternatives.

For the construction industry, the results provide practical guidance for material selection in hydraulic and marine applications. Engineers can now weigh the trade-offs between conventional concrete, which offers superior abrasion resistance but higher density and greater environmental footprint, and lightweight alternatives that reduce structural dead loads while incorporating recycled materials. The identification of palm oil shell-hemp fiber as the most stable performer under severe conditions gives designers a specific, evidence-based option for structures facing high-energy underwater environments.

The research also highlights the importance of understanding the interfacial transition zone — the thin region between aggregate particles and the surrounding cement paste — as a key determinant of abrasion behavior. The properties of this zone influence how cracks initiate and propagate under mechanical loading, and the natural fibers likely play a role in bridging microcracks that form during abrasion, slowing the rate of material loss. This microstructural perspective opens avenues for future research into optimizing fiber content, aggregate surface treatment, and mixture proportions to further enhance durability.

As climate change intensifies flooding events and coastal infrastructure faces growing hydraulic stresses, the need for durable, sustainable construction materials has never been more urgent. This study demonstrates that agricultural waste aggregates combined with natural fibers can produce lightweight concretes with measurable, quantifiable abrasion performance under controlled underwater conditions. While lightweight mixtures still trail conventional concrete in raw abrasion resistance, the palm oil shell-hemp fiber combination offers a promising balance of sustainability, reduced weight, and stable performance that could reshape material choices for the next generation of hydraulic and marine structures.

Subject of Research: Energy-dependent underwater abrasion behavior of sustainable lightweight concrete incorporating agro-waste aggregates and natural fibers under rotational hydraulic conditions

Article Title: Energy-dependent underwater abrasion behavior of sustainable lightweight concrete incorporating agro-waste aggregates and natural fibers under rotational hydraulic conditions

Article References: Kumar, A. H., Karthikeyan, K., & Murali, G. (2026). Energy-dependent underwater abrasion behavior of sustainable lightweight concrete incorporating agro-waste aggregates and natural fibers under rotational hydraulic conditions. Scientific Reports. https://doi.org/10.1038/s41598-026-72514-x

Image Credits: AI Generated

DOI: 10.1038/s41598-026-72514-x

Keywords: Energy-dependent, underwater, abrasion, behavior, sustainable, lightweight, concrete, incorporating, agro-waste, aggregates, natural, fibers