Reinforced thermoplastics enable affordable storm-resistant structures

(Published on July-August 2008 – JEC Magazine #43)

HABIB JOSEPH DAGHER, P.E. DIRECTOR, ADVANCED STRUCTURES & COMPOSITES LABORATORY THE UNIVERSITY OFMAINE

ED PILPEL, PRESIDENT, POLYSTRAND, INC.

Composites and wood may seem like unlikely allies but the University of Maine’s Advanced Structures & Composites Laboratory and Polystrand, Inc. have joined forces to develop new and improved material systems for construction. A recent breakthrough resulting from this alliance is a technology platform for high-strength beams and panels that can be cut and nailed like traditional wood components. One enabler for this breakthrough is the use of reinforced thermoplastics that easily accept the nails and screws used in traditional construction. Beams and panels made with laminated wood and reinforced thermoplastics can be used in much the same way that carpenters and framing specialists have traditionally assembled homes and commercial structures in the U.S.

When composites were developed for construction applications in the past, they were typically made with thermosetting polymers, either alone or in combination with other materials including wood. While the assemblies were very strong, it was nearly impossible to drive a nail or screw into them. That led to the development of new assembly techniques that added cost and were unfamiliar to the craftsmen building the structures. Components made from thermosetting polymers also tended to be available in specific sizes that were difficult to cut, leading to limited flexibility on the job site. We are now using reinforced thermoplastics and find they can be fabricated with the same tools that contractors use with ordinary wood. Thermoplastics have other advantages in construction such as the absence of volatile organic compounds (VOCs). This can be an issue for thermoset composites in hot climates where offgassing in close living quarters can cause discomfort for occupants. Thermoplastics are also easy to manufacture and recycle, which are important benefits in the move to sustainable construction.

Andrew sparks analysis

The University of Maine was first asked to help improve the storm resistance of homes and commercial buildings after Hurricane Andrew in 1992, the second-most-destructive hurricane in U.S. history. Andrew caused $26.5 billion in damage (worth about $40 billion today) with most of that damage occurring in south Florida. The lab was asked to study the structural failures in south Florida to identify weaknesses and recommend solutions for the construction industry.

It is well-known that wood can be brittle and becomes more so as it ages and dries. It was discovered that building components were much more ductile or malleable when wood was combined with polymers. This discovery led to advanced engineered wood systems. The researchers analysing the effects of Hurricane Andrew in South Florida expected to find that wind pressures had simply overcome wood’s ability to resist.

Actually, they learnt that joints were also a problem. This is a critical area for construction that could be improved by using reinforced polymers. The ASC Laboratory initially developed material systems using fibreglass and thermosetting polymers. While they made the structures much stronger, they were also expensive and unfamiliar to the people building homes and commercial structures. This led to hybrid wood products and new techniques for assembling joints.

Centre for composites

Research on engineered wood continued and soon prompted the Laboratory to set up the Advanced Engineered Wood Composites (AEWC) Centre. This laboratory helped formalize the research and development process and provided resources. A breakthrough of sorts came when the AEWC Centre was asked to help develop a modular ballistic protection system (MBPS) to protect military troops from blast and fragmentation threats in mobile tent camp environments. The Centre worked under contract and in partnership with the Natick Soldier Research, Development & Engineering Centre.

The challenge was to balance threat protection capabilities, ease of deployment and cost. The system needed to be highly mobile while remaining quick and easy to assemble. It also needed to withstand a specific blast force and fragmentation threat, in addition to having a reasonable cost structure. As the system was intended for use in hot environments, the offgassing of materials containing styrene would have been a problem. That led the AEWC Centre to look into reinforced thermoplastics, such as the tapes from Polystrand, Inc.

Nowadays, the cost of such materials is reasonable and the manufacturing process is capable of large-scale, volume production. There is no inherent roadblock to adding any capacity whatsoever for the system. In collaboration with its partners, the Centre was able to develop wood and reinforcedthermoplastic composite panels that can be mounted inside a traditional military tent frame. Requiring no tools, the MBPS can be used by as few as four soldiers to up-armour a 6 x 10 m tent in less than an hour.

A final generation of the MBPS was shown at Composites & Polycon 2007, the trade show and conference hosted by the American Composites Manufacturers Association. The MBPS won both the Best of Show and People’s Choice Awards for Composites Excellence (ACE).

The MBPS was developed with military use in mind but the material concept can potentially be used in any situation where blast protection or energy absorption of wind forces is needed for temporary structures, i.e. in homeland security, police, prison and private security applications.

Testing shows great strength

The MBPS tests demonstrated the high strength and resilience of the reinforced thermoplastic and wood composite system. Not only did the structures remain intact after blast testing, but video made it possible to carefully examine the panel performance during blast overpressure. Panels with reinforced thermoplastics were able to bend and recover while their wood counterparts came apart after a relatively small amount of pressure.

Tests performed at the AEWC Centre showed that reinforced thermoplastic and wood composites absorbed energy two to six times better than unreinforced wood structures.

In the bending mode, the composite panels were found to absorb six to seven times more energy than comparable unreinforced wood products.

In August 2007, the Centre tested two demonstration buildings and found they could withstand overpressures higher than the strongest recorded hurricanes. Explosives were used to create the overpressures and it was found that they created pressure that was more than 100 times the force of a 160-kilometre-perhour wind. In brief, reinforced thermoplastic and wood composite panels are much better at energy management than traditional building materials, and this holds true whether the overpressure is produced by a manmade blast or Mother Nature.