Used Plastic Takes Place of Stone in Concrete Blocks

Employees at Conigliaro Industries, Inc. pour Plas-crete into a mold. The mix is 50 percent recycled plastic material. The mold makes a 2' x 2' x 4' Plas-crete block (see the picture on the right) that can be used in place of similar concrete blocks.

- Photo courtesy of Conigliaro Industries

Keeping post-consumer plastics out of landfills has kept one Massachusetts company busy developing new products containing recycled plastics.

Conigliaro Industries, Inc., in Framingham, Massachusetts, is a materials recovery facility that recycles all plastics it brings into its plant. It separates the Number 1 PET, (Polyethylene Terephthalate), and the Number 2 HDPE (High Density Polyethylene), plastics for uses such as asphalt mix and export.

The other plastics, Numbers 3-7- PVC (Polyvinyl Chloride), LDPE (Low Density Polyethylene), PP (Polypropylene), PS (Polystyrene) and other (Polycarbonate, Acrylic, Acrylonitrile-Butadiene-Styrene and Mixed Plastics), are sorted from the stream. Conigliaro is looking for new uses for these plastics.

An item Conigliaro has developed and that is gaining in popularity is Plas-crete building blocks. These are precast concrete blocks. Conigliaro has replaced the pea stone in the concrete mix with ground recycled plastics.

Mike Bowen, Plas-crete System sales associate, explained that the company wanted to find a way to keep the Numbers 3-7 plastics out of landfills. Conigliaro Industries received a state grant to investigate replacing the heavy virgin stone in concrete with plastic. He said what was developed was a concrete block that was strong and sturdy, yet lighter and easier to handle than regular concrete blocks.

When Conigliaro makes a 2' x 2' x 4' Plas-crete block, they use 250 pounds of scrap plastic. These blocks weigh 1,850 pounds. A regular concrete block the same size weighs about 2,700 pounds.

Mr. Bowen said, "These blocks are lighter in weight and can be handled easily. You don't need special equipment to move them. All you need is a Bobcat loader and another person to seat the blocks. A regular concrete block that size requires larger cranes or loaders to be moved and placed. You are usually talking about a four-man crew, meaning more in labor and equipment costs."

Transporting these blocks is less expensive as well. "You may only get eight or nine cement blocks on a 48' flatbed. We can get 28 Plas-crete blocks on the same size flatbed trailer," Mr. Bowen said.

The precast concrete market Conigliaro is making blocks for include retaining walls, wall blocks, traffic diversion barriers, building foundations and parking stops. The company is looking at other areas where precast concrete is used.

In the Plas-crete mix, which is a blend of virgin sand, ground plastic, water and Portland cement, the plastic does not completely adhere to the cement as stone does.

"You get a honeycomb effect. This gives it an ability to expand and contract with the weather," Mr. Bowen said. "This should prevent the cracks and splits that regular concrete gets."

The state of Massachusetts has replaced some aging wooden road salt sheds with walls made of the Plas-crete blocks.

A road salt shed in the state of Massachusetts utilizing Plas-crete blocks.
-Photo courtesy of Conigliaro Industries

Mr. Bowen said, "This is a better option for the state. The salt piles naturally spread out and push against the walls. The loading equipment used to pick up the salt from the shed usually ends up pushing against the walls, too. These walls will hold up better than wood walls."

To prepare the plastic for use in Plas-crete, the clean scrap plastic is shredded using a ShredPax AZ-80 shredder. Material is run over a rotary magnet to remove any remaining metals. It is then sent to a Herbold granulator.

Each ingredient material for the Plas-crete is placed in a separate feed unit on the plant. The plant is equipped with controls so that each material can be measured; producing batches according to specified customer needs. The materials are blended together by an auger, which leads to a mold, where the uncured Plas-crete is vibrated into place. The Plas-crete is allowed to cure for 12-24 hours prior to removal from the mold and it is ready to be sold.

Mr. Bowen said, "If a customer needs a heavier block or a block with a different PSI-rating, we can make the blocks to the customer specifications."

Conigliaro is looking at different uses for precast concrete. "The uses for this product are only limited by the imagination and the ability to buy molds. Molds are very expensive. This product could also be used in cement mixer trucks, like regular cement." Conigliaro is studying the possibility of getting molds for freeway noise reduction barriers and burial vaults.

"Concrete is most adequate for noise reduction. A concrete barrier requires concrete posts with slats that slide into the posts. The 12-foot high barriers require 8,000 blocks per mile. We could probably make the blocks for about half the price of regular concrete," Mr. Bowen said.

The United States military is testing the Plas-crete and a couple of concrete companies are doing tests on the product. "The concrete companies want to make sure the concrete doesn't melt," said Mr. Bowen. "That's not going to be a problem. The blocks have tested up to a temperature of 2,000 degrees Fahrenheit. The way the product works, most of the plastic gets drawn toward the center of the blocks."

Conigliaro Industries is mostly concerned with keeping the plastic out of landfills.

Mr. Bowen said, "We want the plastic out of the waste stream. Concrete companies don't use plastic, because they are not set up to shred the material. It takes a long time to develop a shredding system like ours. Right now we have a pile of 200 tons of shredded plastic. We would be willing to supply this plastic for free to concrete companies at this point. We want it to be used."