Strength reduction and strength-scaling relations of M40 concrete with untreated bamboo chips as partial coarse-aggregate replacement

Concrete is the most widely used building material on Earth, a mixture of cement, water, sand, and crushed stone that hardens into a rock-like solid. The crushed stone, known as coarse aggregate, makes up the largest portion of the volume and provides the bulk of the material's strength. Because this stone is quarried from the ground, it is a finite resource that requires significant energy to extract and transport. In many parts of the world, finding affordable alternatives to this stone is a major engineering challenge. One such alternative is bamboo, a fast-growing grass that is abundant and cheap in countries like Bangladesh. While bamboo has long been used as a structural support or as a fiber added to concrete to prevent cracking, using it as a direct replacement for the heavy stone chunks has remained a mystery. The central question is whether the concrete can still hold weight if a portion of its stone skeleton is swapped out for plant material, and if so, how much strength is lost in the exchange.

A researcher at the Chittagong University of Engineering & Technology set out to answer this by testing a specific type of concrete designed to reach a target strength of 40 megapascals, a standard used for many structural buildings. They took untreated, seasoned bamboo chips—pieces of the plant wall cut into sizes similar to the stone they were replacing—and mixed them into the concrete in place of the crushed stone. They did not treat the bamboo with chemicals or special coatings; they simply used the material as it was found in nature. The researcher created five different batches of concrete. One batch contained no bamboo at all, serving as a control, while the others replaced 4, 8, 12, and 16 percent of the stone volume with bamboo chips. They then poured these mixtures into molds to create cylinders and beams, cured them in water for 28 days, and subjected them to rigorous testing to see how much force they could withstand before breaking.

The results showed that the bamboo had a predictable and significant impact on the concrete's ability to carry weight. As the amount of bamboo increased, the compressive strength—the force required to crush the material—dropped steadily. The control mix without any bamboo reached a strength of 44.2 megapascals. When 4 percent of the stone was replaced, the strength fell to 35.8 megapascals. At 8 percent replacement, it dropped further to 29.3 megapascals. By the time the replacement reached 16 percent, the strength had plummeted to 15.4 megapascals, a loss of 65 percent compared to the original mix. The researcher found that for every single percent of bamboo added, the compressive strength decreased by nearly 1.8 megapascals. This decline was so consistent that it formed a straight line when plotted on a graph, suggesting a direct and reliable relationship between the amount of bamboo and the loss of strength.

However, the story was different for the other types of strength that matter in construction. Concrete is often subjected to bending forces, such as when a floor slab sags under weight, or to splitting forces, where the material is pulled apart from the sides. The researcher tested these properties by bending beams and splitting cylinders. They discovered that while the bamboo still reduced these strengths, the drop was not as severe as it was for crushing strength. At the highest replacement level of 16 percent, the ability to resist bending fell by 46 percent, and the ability to resist splitting fell by 41 percent. This means that while the material became much weaker under direct pressure, it retained a larger portion of its ability to handle bending and pulling forces. The workability of the mix, measured by how easily it flowed when poured, changed very little, dropping by only 3.5 millimeters even at the highest bamboo content, indicating that the bamboo did not make the concrete difficult to handle during construction.

A key part of the study was checking whether the standard rules engineers use to estimate concrete strength still applied when bamboo was present. Building codes typically use a specific mathematical relationship to guess the tensile or bending strength based on the crushing strength. The researcher found that this relationship held true even with the bamboo chips. The way the bending and splitting strengths changed relative to the crushing strength followed the same pattern as ordinary concrete. This is a crucial finding because it means engineers do not need to invent new formulas to design with this material; they can simply treat bamboo-concrete as a weaker version of regular concrete and use the existing safety margins. Furthermore, the actual bending strength of the bamboo mixes was slightly higher than what the standard code predicts, suggesting the material is safe for use within those established limits.

The study also compared these results with previous research that used bamboo chips that were either completely dry or soaked in water. The performance of the untreated, seasoned bamboo used in this study fell right in the middle of those two extremes. This suggests that the moisture content of the bamboo is the most important factor controlling how much strength is lost. Dry bamboo tends to absorb water from the concrete mix, swell, and then shrink as it dries, creating tiny gaps and cracks that weaken the bond. The bamboo used here was seasoned, meaning it had already lost some moisture, which likely prevented the extreme swelling seen in dry chips but did not offer the full stability of water-soaked chips.

The practical takeaway from this research is that replacing a small portion of the stone with bamboo is a viable option for certain types of construction. When 4 to 8 percent of the stone was replaced, the concrete retained between 66 and 81 percent of its original strength, resulting in a final strength between 29 and 36 megapascals. This is strong enough for ordinary structural elements like beams and columns in standard buildings. However, once the replacement level went above 8 percent, the strength dropped below the threshold for most structural uses, limiting the material to non-structural applications like partition walls or lean concrete for foundations. The researcher concluded that while bamboo offers a sustainable way to reduce the demand for quarried stone, the amount used must be carefully controlled, and the moisture condition of the chips is the primary factor that builders can manage to ensure the concrete remains safe and strong.