By Angus Stocking
With its amazing compressive strength, concrete has a history in large-scale construction going back to ancient times – the Romans even used forms in a manner that resembles modern precast manufacturing. Today, researchers continue to study Roman concrete to determine how structures built thousands of years ago have performed so well while withstanding the test of time.
But the Romans never learned to reinforce concrete to compensate for its relatively low tensile strength. Today, there are four major ways to reinforce concrete: rebar, welded wire fabric, prestressing/post-tensioning and fiber. These technologies allow the impressive and versatile uses of concrete we associate with modernity.
The use of steel rods is mankind’s oldest technology for reinforcing concrete, going back to the 15th century. But the first use in construction wasn’t until 1853, when François Coignet used the material in a four-story home in Paris.
Embedding steel bars into concrete increases the material’s tensile strength, allowing cured concrete to be used in applications like precast beams and deck plates. This works well not only because of steel’s inherent tensile strength, but because steel and modern concrete have very similar coefficients of expansion. As temperature changes cause the two materials to expand and contract, they remain at the same size relative to each other and undue stress and cracking is avoided.
For effective steel reinforcement in any application, the size or area, strength and precise placement of bars must all be carefully considered. Fortunately, this is a well-studied area, and precast manufacturers have access to comprehensive specifications and well-developed codes and tools when designing new elements (1).
Two additional conditions must be in place for effective reinforcement: good bonding and corrosion protection.
Bonding means reinforcing steel must adhere to concrete so it does not shift or slide independently. This was a difficult problem until modern times. A relatively simple innovation, deformed steel bars (usually ribbed) increase friction between steel and concrete. These were not commonly used until the early 1900s. Bonding and splicing techniques are well understood today, and designers and detailers have good tools for predicting the performance of various bonding schemes.
Corrosion remains a challenge for steel-reinforced concrete structures. The problem is straightforward – rust occupies a volume 2 1/2 times greater than the steel it oxidizes. When this expansion happens inside concrete, it is extremely destructive.
Concrete provides some corrosion protection on its own, but water intrusion combined with minimal concrete coverage may eventually lead to corrosion. Various techniques are used to prevent corrosion, including coatings, deeper placement of reinforcing members and less permeable concrete. In some applications, notably seaside structures and bridge decks – which have to deal with salty air, de-icing salts and flexure cracking – epoxy-coated, galvanized or stainless steel rebar is used.
In recent years, use of fiber-reinforced polymer rebar for precast structures has increased, especially in aggressive environments like those mentioned above. Because FRP rebar is manufactured from composite materials, it is corrosion-resistant, resulting in increased service life and enhanced durability.
FRP is lightweight but typically exhibits higher tensile strength than traditional steel. This makes it useful in a variety of applications, including bridge decks, caissons, seawalls and more. Still, the increased costs associated with the material have limited even more widespread adoption.
Because rebar is the oldest known concrete reinforcing technology, the design knowledge and skill needed to use it is readily available and the infrastructure of production and distribution is widespread. Consequently, rebar is relatively inexpensive when compared to other reinforcement techniques. And from a technical perspective, simple steel bars are often the most effective structural reinforcement available.
But there are downsides. Rebar is a heavy material compared to modern alternatives, which makes construction and fabrication potentially more labor intensive. The weight itself can be a limiting factor in large precast structures as well. And there are limits to rebar’s structural supporting capacities, which is why the ambitions of modern builders and architects sometimes dictate alternative reinforcement technologies.
Welded wire reinforcement arose as a direct response to the perceived shortcomings of rebar. The material, which looks a bit like steel fencing, “is produced from a series of longitudinal and transverse high-strength steel wires, resistance welded at all intersections (2).” The resulting steel lattice is stronger, by weight, than simple bars for the same reasons lightweight wood trusses are stronger than heavy beams. Welded wire’s design strength is typically compared to grade 60 reinforcing bars. However, actual steel tensile strength is certified to greater standards and can be used to reduce original design steel areas (and hence weight) when permitted by specification.
The gridded nature of the material provides welded wire excellent bonding characteristics with concrete. In addition to the welded wire nodes, there are many surfaces – in multiple orientations – for concrete to grab onto. For additional bond, welded wire can be produced with deformed wire rather than traditional smooth wire.
Because it’s already fabricated into large sheets to meet the required steel design, welded wire is easier for precast production staff to place and secure than rebar, where each bar must be individually placed and tied. This can lead to reduced labor and time savings and may also limit pinch and strain injuries.
“Precasters have been early adopters of welded wire innovations,” said Todd Hawkinson, a consultant to the Wire Reinforcement Institute. “Really, welded wire is more of a shop technique than a construction site technique. Forming the cages and other shapes needed requires the kinds of tools and space found in precast plants.”
Welded wire is more suitable for thin-walled precast structures like utility vaults and concrete pipe because it relies on thin wires at close spacing rather than larger diameter rebar at greater spacing (3). Additional advantages include better crack control and improved weldability because welded wire is typically made from low carbon, cold-drawn steel.
Compared to rebar, the chief disadvantage of welded wire for precast manufacturers is added expense, including greater initial investments in equipment. In most reinforcement applications, welded wire fabric must be formed into cylinders, cages, boxy stirrups and other specialized shapes to suit particular needs. The hydraulic benders and cutters needed to work with sheets of material can be expensive.
Another disadvantage of welded wire arises from its lightness. The material has to be precisely positioned, but is more easily displaced during concrete pours, making it difficult to place and secure.
Still, there are many reasons welded wire is so widely used in precast manufacturing – it’s strong, versatile and easy to work with. Comprehensive specifications and good design tools are available, including many from WRI.
Imagine a row of 20 concrete blocks aligned end to end. To join them, you run a piece of rebar through the voids, fill it with concrete and leave it to cure. If lifted from the end, the blocks would function as a single mass, but would sag under their collective weight. Now imagine the same scenario, but replace the bar with high-strength steel tendons placed between abutments on each end and stretched to 70-to-80% of ultimate strength. Now place the concrete in the void and allow it to cure before releasing the tension on the cables. In this scenario, the concrete’s compressive strength is used to aid tensile strength.
That’s the idea behind post-tensioned concrete reinforcement – concrete’s inherent compressive strength is used to increase tensile strength. Post-tensioning tendons (multi-ply cables) are set in slabs or other members and allowed to overlap forms. The tendons are sleeved and/or greased so that they don’t bond with poured concrete. After the concrete has sufficiently cured, the tendons are stretched tight with hydraulic jacks and wedged in place to maintain their tight grip. In some applications, cementitious grouts are then used to fill voids around the tendon, bonding it to the new concrete. Depending on the precast product, post-tensioning can occur at the plant prior to shipment or on the job site.
Prestressing is subtly different. Tendons are still used, but solid anchors apply tension (stress) prior to pours. After curing, the tendons bond with the new concrete and can be cut away from anchors. On most construction sites, prestressing is not practical because sufficiently strong and stable anchor points are not available. It’s more common in precast manufacturing plants, where anchor points can be built in place. Both prestressing and post-tensioning are sometimes referred to as “active” reinforcement due to the stretched, elastic nature of the reinforcing steel.
The advantage is greater initial strength leading to reduced deflections with thinner concrete sections. Concrete beams reinforced with post-tensioning or prestressing can be designed to span longer distances than beams reinforced with other methods because they can generally be thinner and lighter. Additionally, the more slender beams provide extra clearance space and may be more aesthetically satisfying for context-sensitive designs. Slabs reinforced with active methods have been known to exhibit fewer and smaller cracks. In ambitious projects, active reinforcement can be used to make structural elements with complex curves and other difficult geometry.
Disadvantages arise from the relative complexity of active reinforcement production and installation practices. Reinforcing elements remain under high stress for the duration of the project, meaning ordinary causes of failure – like corrosion – can have dramatic effects.
Even with these difficulties, prestressed and post-tensioned reinforcement remains vital in modern construction and precast manufacturing. Simply put, it can make the seemingly impossible, possible.
Properly speaking, fiber reinforcement is not a new technology. The Romans sometimes used horse hair to make concrete less likely to crack. That’s the same basic idea underlying fiber reinforcement – fibrous material is used to increase concrete’s tensile strength.
Historically, fiber has been used in the precast concrete industry to enhance durability, but not as a true replacement for traditional reinforcement. But in recent years, researchers have focused heavily on developing design methods to allow for the use of fiber as primary structural reinforcement. The establishment of ASTM C1765, “Standard Specification for Steel Fiber Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe,” in 2013, and the forthcoming ASTM C1818, “Specification for Rigid Synthetic Fiber Reinforced Concrete Culvert, Storm Drain and Sewer Pipe” has helped lay the groundwork for the use of various fiber types as a reinforcement option in the future.
Although fibers encompass a wide variety of materials – including nylon, cellulose and many more – the most common types currently used in the precast industry are steel, polypropylene and fiberglass.
Generally made from carbon or stainless steel, steel fibers work to prevent cracking in concrete products. Manufacturers have developed varying geometries for steel fibers, which anchor into the concrete in different ways depending on their shape. While the most common application for steel fibers is floor slab construction, use has expanded in recent years to include other precast products such as tanks (4).
Polypropylene fibers are part of a wider category of synthetic fibers which provide many of the same advantages to precast products as steel alternatives. Polypropylene is made from strands of fine monofilament. Typically, polypropylene fibers possess the same physical characteristics as steel and are also used to prevent cracks and enhance durability. Applications for polypropylene have widened to include septic tanks, burial vaults and additional precast structures.
Glass fibers differ from the aforementioned types in that they are most commonly used for architectural applications. When glass fibers are added to a concrete mix, decorative elements and cladding systems can be manufactured as thin as 1/2 inch with minimal weight, reducing loading and providing excellent thermal properties. The material also boasts some of the same characteristics as other fiber reinforcement types, increasing tensile strength and making the concrete resistant to cracking.
Despite the advantages fiber can offer precast products, some issues still need to be addressed before it can become an even more effective reinforcement technology. Sizes and shapes of fibers vary, making it difficult to determine which type is the optimal choice for a specific project, especially in the absence of a more complete set of standards. Additionally, more work needs to be done to ensure fibers are evenly distributed throughout precast structures in a manner that can be replicated and relied upon from mix to mix.
Compared to rebar or welded wire, fiber reinforcement is not a mature industry. It’s more like the still-evolving software sector, which means new equipment and workflows can be made suddenly obsolete. Even so, fiber reinforcement’s versatility guarantees it a place in modern precasting, and research and innovation continues to open up exciting possibilities.
For a technology that is the foundation of modern civilization, concrete reinforcement has a surprisingly short history. Most of the important developments occurred in the 20th century or are happening now. For those who make a living forming concrete, this means significant change is still happening and the future offers exciting possibilities.
Angus Stocking is a licensed land surveyor who has been writing about infrastructure since 2002.