Plastic concrete is a specialized type of concrete designed to be flexible and watertight rather than strong. It contains bentonite clay mixed with cement, water, and aggregates, producing a material soft enough to deform with the surrounding soil instead of cracking under pressure. Engineers use it primarily inside dams and levees to block water from seeping through the ground. The term can also refer to conventional concrete made with recycled plastic as a partial replacement for stone or sand, though that’s a separate concept with different goals.
How Plastic Concrete Differs From Regular Concrete
Standard concrete is engineered to be as strong and rigid as possible. Plastic concrete takes the opposite approach. Its 28-day compressive strength tops out at about 1.0 MPa, which is roughly 100 times weaker than typical structural concrete. That weakness is intentional. The goal is a material with low stiffness that can flex and compress alongside the earth around it, rather than fracturing when the ground shifts.
Bentonite clay is the key ingredient that makes this possible. Bentonite swells dramatically when it absorbs water, filling tiny voids in the mix and creating an almost impermeable barrier. At the same time, it gives the fresh concrete a thick, pourable consistency that can be tremied (placed underwater) into deep, narrow trenches. Engineers typically keep bentonite content below about 17% by mass, since higher amounts start to erode what little compressive strength the mix has. The water-to-cement ratio runs far higher than in normal concrete, often between 1.25 and 3.65, which further reduces strength while boosting workability.
Why Engineers Use It in Dams
The primary job of plastic concrete is forming cutoff walls, which are vertical barriers built into the foundation beneath a dam. Water naturally tries to flow underneath and around a dam through porous soil layers. If that seepage isn’t controlled, it can carry fine soil particles with it, a process called piping, which gradually undermines the structure. A cutoff wall blocks that path.
At the Coquitlam Dam in British Columbia, for example, engineers built a plastic concrete cutoff wall 0.8 meters wide, 150 meters long, and roughly 20 meters deep beneath the dam’s core. The wall’s purpose was to reduce water pressure at the downstream toe of the dam and prevent silt from being carried into underlying sand and gravel layers. Plastic concrete was chosen specifically because the dam also needed to survive earthquakes. A rigid conventional concrete wall would crack during seismic shaking, defeating its purpose. The plastic concrete wall could deform along with the surrounding soil without losing its seal.
This combination of flexibility and impermeability makes plastic concrete the standard choice for seepage control in earthen dams, levees, and sometimes around contaminated sites where groundwater flow needs to be redirected.
Mix Design and Placement
A plastic concrete mix starts with cement, water, bentonite, and fine or coarse aggregates. The proportions look unusual compared to structural concrete. Because the water-to-cement ratio is so high, the fresh mix behaves more like a thick slurry than the stiff paste you’d see in a foundation pour. Workability is measured by viscosity rather than slump. Engineers generally consider the mix acceptable when its viscosity stays under 100 pascal-seconds and ideal when it drops below 50.
Placement typically happens using the tremie method: a pipe is lowered to the bottom of a slurry-filled trench, and the plastic concrete is pumped down through it, displacing the slurry from the bottom up. This prevents the fresh concrete from mixing with groundwater or trench fluid on the way down. The trenches themselves are often excavated using clamshell or hydromill equipment, with bentonite slurry holding the trench walls open during digging.
Long-Term Performance
Because plastic concrete sits underground, permanently submerged in or near groundwater, its durability depends on resisting chemical attack from dissolved minerals and maintaining its seal over decades. The same bentonite that makes the mix flexible also helps here. Swollen bentonite particles fill microscopic pores and keep the permeability extremely low, limiting the amount of potentially aggressive groundwater that can penetrate the material.
Strength testing on plastic concrete has been carried out at 28 days, 56 days, 91 days, and even four years after placement. The material does gain some strength over time, but it remains in the low-strength category by design. The real performance metric isn’t how much load it can carry. It’s whether it continues to block water without cracking as the dam and its foundation settle, shift during earthquakes, or respond to changing reservoir levels.
Recycled Plastic in Concrete: A Different Concept
The phrase “plastic concrete” sometimes comes up in discussions about sustainability, where recycled plastic waste replaces some of the stone or sand in a conventional concrete mix. This is a fundamentally different material with different goals. Here, “plastic” refers to the ingredient, not the flexibility of the finished product.
Researchers have tested replacing sand with shredded PET (the plastic in water bottles) and replacing coarse aggregate with shredded polyethylene (used in packaging). The results are mixed. PET replacement actually improves workability: replacing 15% of the sand with PET increased slump by 30%. Polyethylene went the other direction, reducing workability by up to 19% at the same replacement level.
The tradeoff is strength. Replacing sand with PET at levels from 2.5% to 15% reduced compressive strength by 7% to about 31% compared to normal concrete. Polyethylene coarse aggregate replacements showed similar losses, up to roughly 32%. Tensile strength dropped by as much as 22%, and flexural strength fell by up to 60%. These are significant reductions, which means recycled plastic concrete works best in non-structural applications like sidewalks, barriers, or lightweight panels where full structural strength isn’t required.
Some studies have found that adding short plastic fibers (rather than using plastic as aggregate) can actually improve durability. Small doses of PET or polyethylene fibers reduced water permeability and drying shrinkage cracking, and improved resistance to acid attack, sulfate exposure, and chloride penetration. This fiber-reinforcement approach treats plastic as an additive rather than a replacement, keeping the base concrete largely intact while gaining some crack resistance.
How to Tell Which Meaning Applies
If you’re reading about dam construction, foundation seepage, or cutoff walls, “plastic concrete” refers to the bentonite-cement material designed for flexibility and impermeability. If the context is sustainability, waste reduction, or green building materials, it almost certainly means concrete containing recycled plastic as an ingredient. The two share a name but solve entirely different engineering problems.