Permeable concrete for sustainable construction
Permeable concrete, also known as pervious concrete, is a special type of concrete which is high in porosity, thus making the infiltration of water into the ground easier. While regular concrete contains an admixture of coarse aggregate, sand and water, permeable concrete contains little or no sand in order to make it porous.
According to a research report titled "Groundwater management in Bangladesh: An analysis of problems and opportunities" by USAID, published in February 2015, the increasing usage of groundwater is leading to the depletion of groundwater level at an alarming rate. Moreover, the lack of a proper drainage system in the country is leading to clogged drains, thus, preventing water from passing through and restoring the levels of groundwater.
"Currently, the groundwater level is 60 metres. On average, the level of groundwater is going down by three metres per year. It is estimated that by 2050, the groundwater level may reach 120 metres, making its extraction extremely difficult. Moreover, the sinking risk of the city is also increasing as water is unable to pass through the drainage system," informs Dr Md Tarek Uddin, PEng., Professor, Department of Civil and Environmental Engineering, Islamic University of Technology (IUT). He adds, "To cater to the 18 million people living in Dhaka city, approximately 2,300 million litres of water are required per day, 75 percent of which comes from groundwater. Therefore, enhancing the use of permeable concrete is one of the most suitable ways to overcome the challenges of depleting levels of groundwater."
The significance of permeable concrete lies in the fact that it allows water from precipitation and other sources to pass through porous surfaces, thus helping to resolve the problem of clogged drainage system and restoring groundwater.
A building is usually built comprising 60-70 percent of an area. The remaining 30 percent is supposed to be free and therefore permeable concrete can be used in that area for better drainage of water, adds Dr Tarek.
Though using permeable concrete is a step towards ensuring sustainable infrastructure, its usage should be limited to certain types of construction. Pavements and walkways, parking lots, open spaces around buildings and streets which don't have to carry heavy loads are usually ideal for using permeable concrete. Highways and streets, where the load of vehicles and traffic congestion are high, should be avoided when using this porous concrete as their strength is lower than the usual concrete used in most roads.
Dr Ishtiaque Ahmed, Professor, Department of Civil Engineering, BUET, explains, "Since sand is not included in the admixture for creating permeable concrete, the resultant pores after constructing roads are not capable of carrying heavy load; cracks may occur due to such heavy loads." While the strength of regular concrete should be a minimum of 2000 psi (which may increase according to the load they should carry and also due to the variation of the admixture), the strength of permeable concrete typically ranges from 1200-1700 psi, he adds.
Permeable concrete should also be avoided in reinforced concrete where rods and steel bars are used as water infiltration will lead to the corrosion of these elements.
While talking about the cost using permeable concrete, Abu Sadeque, former director, Housing and Building Research Institute (HBRI), shares that the cost of permeable concrete is approximately 20-25 percent higher than the regular concrete. However, the long-term benefits of using permeable concrete will outweigh the high costs in terms of ensuring better ecological balance, he opines.
To lessen the cost of using permeable concrete, Dr Ishtiaque Ahmed suggests that stone grains can be used to prepare the concrete mixture.
Citing the example of widespread usage of permeable concrete in countries like Canada and China, Dr Ishtiaque urges the concerned authorities to learn from their examples to enhance use of permeable concrete in Bangladesh. Our engineers also need to be made aware of the benefits of using permeable concrete, he adds.