Precast concrete’s place in today’s sustainable society
By Kirk Stelsel
To promote its new buy-back program, Best Buy is running a commercial that pokes fun at the ever-changing landscape of consumer electronics. Frustrations boil over as people find out the TV, laptop, tablet or phone they just purchased will soon be outdated – an experience we can all relate to.
Changes in construction-related industries may not happen quite as fast as consumer electronics, but important shifts have taken place that demand the attention of anyone looking to stay ahead of the curve. Perhaps the most important in recent times is the growing momentum of the green building movement.
Just 20 years ago, using the word “green” in a conversation would have most likely been in reference to landscaping, money or the actual color. Today, sustainability is a major consideration for new construction. The formation of the U.S. Green Building Council in 1993 and the prevalence of its LEED Rating System are evidence enough of shifts in building requirements.
For those in the building and specifying communities, this means taking a hard look at sustainability during the design and material selection process. This has set off a chain reaction that has resulted in building materials being scrutinized in a whole new manner. In addition to cost, strength and aesthetics, things like life cycle assessment, environmental impact and ecological benefits are becoming more commonplace requirements.
For the precast concrete industry, this new focus is welcomed by manufacturers who recognize that precast concrete has a lot to offer in sustainable designs – and it’s only getting better. Through new mix designs, additives and curing techniques, precast is becoming stronger, more versatile and more environmentally friendly than ever.
A Natural Choice
In its most basic form, concrete is nothing more than portland cement, water, sand and crushed rock. The fact that concrete is made from widely available natural resources attributes to its prevalent use and why its history can be traced back as far back as the ancient Greeks and Egyptians. The Romans used volcanic ash to create a hydraulic binder known as pozzolana cement to build structures like the Coliseum and Pantheon, among the most recognizable structures on earth today nearly 2,000 years after they were built.
Today, the concrete industry uses portland cement, developed in 1824 by Joseph Aspdin, as its primary binder. Despite using natural resources, the process of making portland cement requires heating clay and limestone (lime and silica constitute almost 90% of cement’s mass) to high temperatures, giving off CO2 as a byproduct. The cement industry has made strides in reducing emissions, but competition has also emerged to create alternative low-carbon or carbon-negative cements. If successful, these new products will further reduce the carbon footprint of the cement industry and, in turn, the concrete industry.
In order to achieve economies of scale and acceptance by the precast industry, these new technologies, which are still at a developmental stage, are undergoing long-term testing to prove their practicality for mass production. While these processes can take many years, several startups have built or plan to build pilot plants.
Another option for precast producers is the use of supplementary cementitious materials (SCMs) such as fly ash or blast furnace slag. These materials create a pozzolanic reaction, similar to the Roman use of volcanic ash. SCMs are industrial byproducts – fly ash comes from the combustion of coal used in electric power plants and slag from the production of iron – but the concrete industry has created a way to recycle these byproducts in an environmentally beneficial manner. The use of SCMs as a partial replacement for portland cement can increase precast concrete’s sustainability, strength and durability.
A strong case for precast concrete
One of the most intrinsic benefits of precast concrete is the durability it offers. Precast products are made in a quality-controlled environment, and the finished product has a tested strength when it arrives on site. Once in place, precast products can achieve a service life in excess of 100 years, depending on the product type and use.
While precast is already one of the most durable building materials available, recent innovations have made it possible to achieve even better results. The development of mix designs with higher strength, including ultra high-performance concrete (UHPC) has created a new sector of products that provide much greater compressive and flexural/tensile strengths in remarkably thin cross sections for design flexibility. These concretes can reach compressive strengths up to 33,000 psi and flexural/tensile strengths up to 7,000 psi, and its very low permeability offers great advantages for durability. Use of this technology for precast concrete is ideal for infrastructure work such as bridges that are subjected to consistent heavy loading and exposure to de-icing salts. In addition, its use in architectural elements provides a long service life, an elegant look through a reduction in material, and complex shapes that are customizable in color and texture.
Another approach to higher-strength concrete, pioneered by the National Research Council in Ottawa, is the use of porous shale soaked in water as a replacement for sand. This method facilitates the use of a low-water mix design. Reducing water increases strength, but it can also lead to rapid drying that causes shrinkage and cracks. The use shale keeps the concrete hydrated from the inside as it cures, despite the low-water mix.
Researchers around the globe have also been developing self-healing concrete. Much like a starfish, self-healing concrete has regenerative properties that allow it to recover from minor damage. Research has branched off into two segments, one using a synthetic solution contained in microcapsules, the other a biological approach that uses bacteria trapped in the concrete. By setting off a reaction that produces filler to close micro cracks, both technologies enable self-healing concrete to regain strength and keep water from reaching steel reinforcement.
At the Massachusetts Institute of Technology’s Concrete Sustainability Hub, recent studies have also uncovered a way to increase the service life of concrete. Researchers at MIT who are studying concrete at the nano level have determined that creep, one of the primary factors in the deterioration of concrete under load, can be slowed. During creep, calcium-silicate-hydrates shift into voids previously occupied by water. Adding silica fume particles to fill these gaps results in a higher density concrete with a significantly longer lifespan. In addition to lasting longer, this concrete is also stronger, creating the possibility of using less material to make structures. The product is still many years from the market, but it holds great potential.
Used in conjunction with the quality-focused production of precast plants, each of these advancements can give precast an even longer lifespan. Reducing the frequency of replacement further contributes to the sustainability of precast concrete.
Precast with an appetite
The sun’s ultraviolet rays typically get a bad rap for the damage they can cause. Extended exposure to UV rays can fade and break down materials and cause skin cancer, but UV rays can also do a lot of good. For the precast industry, the addition of titanium dioxide (a photocatalytic agent) to the mix design creates a self-cleaning, smog-eating surface when exposed to UV rays.
While smog-eating precast may seem a bit far-fetched, cement producer Essroc Italcementi has already put the technology into production. Its product was used for a basketball practice facility at Louisiana State University, the I-35 West gateway structure in Minnesota (see Precast Solutions, Fall 2009), and various other projects in the United States, Canada and overseas.
The UV rays that hit the surface of the concrete trigger a chemical reaction that breaks down organic pollutants, such as nitrogen oxide, into water, oxygen and harmless salts. Nitrogen oxide, a common component of smog, causes respiratory diseases in humans and damage to plants and trees.
A promising future
Concrete is the most-used man-made material on earth, and for good reason. Its low cost, high durability and range of applications make it indispensable in many forms of construction, and precast concrete only enhances those benefits. Whether it’s residential, commercial or infrastructure work, precast concrete can be called upon to fill the need.
As the industry continues to evolve through the efforts of independent researchers, cement and concrete companies and academics, precast concrete will become an even more sustainable building material. The outcome of this work will be a more eco-friendly product with greater strength and durability to better shape and protect the world around us.
Kirk Stelsel is NPCA’s communication manager.