Civil engineering researchers, sustainable construction advocates and innovators in the zero-waste movement are on the hunt for the most effective and efficient ways to use and reuse concrete.
The world is literally covered in concrete and with an annual production of 3 tons per person globally it is the second most consumed material on earth, after water. Its strength and durability are coveted by construction crews from New Zealand to Zimbabwe.
But, it is also a significant source of greenhouse gas emissions.
“It (concrete) accounts for around 5 percent of global anthropogenic emissions of carbon dioxide and affects a wide range of sustainability issues, including climate change, emissions to air and water, natural resource depletion and worker health and safety,” according to Philippe Fonta, the managing director of the Cement Sustainability Initiative (CSI) & Tire Industry Project (TIP), at the World Business Council for Sustainable Development (WBCSD).
That’s why researchers and innovators are exploring “greening concrete,” or reducing the amount of waste concrete on construction sites and encouraging money-saving methods to recycle concrete collected at demolition sites.
The effort to reduce the amount of old concrete that clogs landfills is being embraced by many countries throughout the world. In the Netherlands, for example, concrete waste is banned from landfills and instead must be recycled, except for some residual process waste, according to CSI’s report on Concrete Recycling. The report also said that 38 states in the U.S. use recycled concrete aggregate for road sub-base and 11 recycle it into new concrete.
“Recycling or recovering concrete has two main advantages: (1) it reduces the use of new virgin aggregate and the associated environmental costs of exploitation and transportation, and (2) it reduces unnecessary landfill of valuable materials that can be recovered and redeployed,” Fonta said.
Recycling concrete isn’t the solution in all cases. Its effectiveness is determined at each site.
“Regarding recycling of concrete from construction and demolition waste, there are technically many possibilities. One has to be sure to balance the impacts compared to virgin aggregates. Recycling requires energy to transport, process, store and then often more transport. Concrete can be processed back into very clean aggregates with the right equipment and demolition practices. This can then be used like any other aggregates. The best solution will always be the one with least transport and handling,” Fonta said.
The National Ready Mixed Concrete Association’s Vice President of Sustainability Tien Peng explained the factors to consider when using Recycled Concrete Aggregate (RCA).
“The use of RCA in concrete may or may not be the best solution, depending on availability of virgin materials, quality of existing concrete, cost to produce quality RCA, cost to landfill old pavement, cost to build with RCA concrete,” Peng said.
A leader in effective concrete recycling is the Colorado-based Recycled Materials Company Inc. RMCI specializes in construction services for the removal of concrete, processing of concrete rubble into specification aggregates, subgrade preparation and sub-base installation for interstate highway and airport projects, according to its website. RMCI gained attention with the demolition and recycling of 6.5 million tons of aggregate at Denver’s former Stapleton International Airport.
“The project, known as The World’s Largest Recycle Project, covered 1,400 acres of runways, taxiways and aprons and the total aggregate recycled there could build a two-lane highway from Denver, Colo., to Chicago,” according to RMCI.
The Construction Materials Recycling Association estimated in 2012 that about 140 million tons of concrete are recycled annually in the United States. Road base is the most common end use.
Another way to reduce concrete waste is to mix only the amount needed for a particular job.
Peng said that on average 5 percent of ready mixed concrete produced in the United States is returned to the concrete plant.
“Return concrete can be made into blocks. It can also be stored, crushed and reused as aggregate, so long as there is storage space,” Fonta said. He said the latest equipment on the market can now recycle the water along with the solid aggregate in uncured concrete.
Currently a Canadian-based manufacturer is selling a sustainable and reusable landscape block mold that converts waste concrete into a useable product. Steve Thorpe, the founder of Formablok, said he had been toying with the idea for about 30 years. His first prototype was made out of steel in 2011.
“My main thinking behind Formabloks has been onsite waste concrete recovery. We all order more concrete than we need. If we are pumping we add an extra yard to the pour as well. All this leads to waste on the ground of construction sites,” Thorpe said. “The primary idea was to build a universal ‘full floating’ block that could be placed with minimal tools and skills."
After some trial and error, the company came up with a mold made of plastic and aluminum that is lightweight, rust-proof and durable.
Now Thorpe sells four designs (half or full block with either a dowel or flat top) that form interlocking blocks. Once the concrete hardens, the forms are stripped and reused. Footing, cutting, core filling and mortaring is not necessary when using the Formablok. Thorpe’s design recently received a U.S. patent.
“Production of segmental blocks makes sense because they can be easily handled and stored onsite until the final landscaping portion of the project,” he said. “There are very few sites built today, residential or commercial, that don’t use segmental blocks somewhere within the building envelope.”
On his website, Thorpe includes step-by-step tutorials for using the blocks.
“The way our forms come apart allows users to add custom liners and monograms. We have some forms that have about 500 pours on them and are still as good as the day they were made.” he said.
Formablok molds have been sent as far as Seychelles and South America.
“By making a segmental concrete block, you produce a product that almost everyone can use for many applications,” Thorpe said. “People seem to be building mostly anything you can imagine when it comes to landscaping, but I just sent some to Jamaica and the guy is intending to build a cottage with them.”
The CSI is working on methodologies to evaluate the environmental and social impact of concrete, including manufacturing, the use phase and the end-of-life stage/recycling phase.
“This will allow us to compare different materials and constructive solutions over the full life cycle, and will enable us to understand the offsets, for example, a larger footprint in the manufacturing stage that is more than compensated by savings in the use phase,” Fonta said
In the future, these efforts will encompass an in-depth examination of the entire production cycle of concrete, starting with improved biodiversity management in pits and quarries to new uses for old concrete.