ABSTRACT
The improper disposal of residual mortar from overmixed concrete presents environmental and logistical challenges for ready-mix concrete producers. This study explores the viability of reusing such mortar as a partial replacement material in the production of paving bricks intended for pedestrian and light traffic applications. Residual mortar was collected from a local concrete batching plant and used to replace conventional cement and sand in varying proportions: 10%, 20%, 30%, 40%, and 50%. Bricks were fabricated using a 1:2:4 mix ratio and tested for compressive strength after a 14-day curing period. Results showed that bricks with 10%, 20%, and 50% residual mortar achieved interpolated 28-day compressive strengths exceeding the ASTM C902 minimum requirement of 2,500 psi, while 30% and 40% mixes fell below the threshold. These findings suggest that residual mortar, when used in controlled proportions, can be repurposed effectively in non-structural applications, offering a sustainable and economical alternative for the disposal of concrete waste.
Keywords: residual mortar, overmixed concrete, paving bricks, reuse, compressive strength, sustainability, ASTM C902
INTRODUCTION
Concrete is the most widely used construction material worldwide, with over 20 billion tons produced annually for various structural and non-structural applications (Santa Monica Daily Press, n.d.). The rise of ready-mix concrete (RMC) plants has contributed to faster and more efficient construction practices. However, improper batching, delays in delivery, and prolonged mixing often result in overmixed concrete—a type that has exceeded its maximum workable time and is considered unsuitable for structural applications (ASTM C94, 2023). Once past the allowable discharge period of 90 minutes, such mixes experience a decline in workability and strength, prompting engineers to reject them from use on-site.
As a result, ready-mix producers frequently face issues concerning the disposal of compromised or excess concrete. Traditional disposal practices—such as dumping in vacant lots or near riverbanks—are increasingly discouraged due to their environmental impact (Santa Muñoz, 2024). At Transmix Builders & Construction, Inc., located in Dasmariñas City, Cavite, this challenge became more prominent when the Department of Environment and Natural Resources (DENR) issued a warning against improper disposal of concrete waste in riverbanks, citing its potential harm to vegetation and nearby ecosystems.
To address this, the company implemented a silting pan system to reclaim aggregates from leftover concrete. However, this process still produces large quantities of residual mortar—a slurry of cement and fine particles that cannot be reused in structural concrete due to its degraded bonding properties. Retempering such mortar by adding water is often discouraged or restricted due to the potential reduction in strength and durability (Mane et al., 2022; Sobhani, Najimi, & Pourkhorshidi, 2012).
Despite these challenges, residual mortar still contains binders and fine aggregates that may offer potential in non-structural applications. Literature suggests that when applied appropriately, waste-based binders can be used to produce masonry or paving units without compromising performance under light-duty conditions (Brick & Tile, 2000). One such opportunity lies in the manufacturing of paving bricks for pedestrian and light traffic areas, which typically require lower compressive strength thresholds compared to structural components.
This study, therefore, investigates the potential reuse of residual mortar from overmixed concrete as a partial substitute for cement and sand in the production of paving bricks. Specifically, the objectives of this study are: (1) to produce bricks with varying replacement levels of residual mortar (from 0% to 50% at 10% increments); (2) to evaluate the compressive strength of the resulting bricks after a 14-day curing period; and (3) to determine the optimal replacement percentage that meets the minimum ASTM C902 standard for pedestrian and light traffic paving applications. By converting a previously unusable waste material into a functional product, the study aims to support more sustainable and environmentally responsible construction practices.
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