Latest Advances in the Development of GFRP Rebar: End Heads Made of Polymeric Concrete

Reinforcement with Schöck ComBAR

Latest Advances in the Development of GFRP Rebar: ComBAR second-generation headed end bars with a core diameter of 16 millimeters. Source: Schöck

Irvine Creek Bridge: ComBAR bars were installed in the PL-3 barrier walls. Source: Schöck

Straight ComBAR bars were used in the slabs and headed ComBAR bars were used to connect the approach slabs with the bridge deck of the 3rd Concession Road Underpass. Source: Schöck

Quantum Nano Center in the university city of Waterloo: Schöck ComBAR reinforcing bars were installed in the edge of the foundation slab. Source: Aecon Buildings

A common problem with all fibrous bar-shaped construction materials, including wood, is the fact that bent elements are much weaker than straight elements. Through the need for a glass fiber reinforced polymer (GFRP) reinforcement element that can transfer shear forces and allows for a reduction of the development length of straight bars, the German construction elements supplier Schöck developed the first bar end head for GFRP bars worldwide. Since their first installation, these heads have been continuously improved. Now a second-generation Schöck ComBAR bar end head is available with significantly improved mechanical properties and even better durability.

In Canada, the installation of glass fiber reinforced polymer (GFRP) rebars is becoming increasingly common in situations where conventional steel reinforcement reaches its limits due to its tendency to corrode in chemically aggressive environments. Until recently, the installation of GFRP bars was often hampered by the fact that bent bars have to be produced in the factory as they cannot be bent at the site. Furthermore, bent GFRP bars are much weaker than straight bars due to the redirection and associated rearrangement of the fibers in the bend. As a result, stirrups for shear reinforcement and end hooks to reduce the development length of straight bars are not economically feasible in GFRP. A newly developed end head made of polymeric concrete now allows the installation of headed GFPR bars as shear reinforcement, and makes it possible to significantly reduce the development lengths of straight GFRP bars.

In 2007, the GFRP rebar system Schöck ComBAR was introduced to the Canadian construction market, when the bars were installed in the barrier walls of the Irvine Creek Bridge near Fergus, Ontario. This project is also the very first time that GFRP bars with headed ends were installed in Canada. Whereas this first generation of ComBAR heads was developed to allow the usage of straight headed ComBAR bars as shear reinforcement in diaphragm walls, the straight bars on the inside face of the barrier walls at the Irvine Creek Bridge were installed with an end head at the bottom to reduce their development length in the deck slab, avoiding the use of hooks.

Technical Details

As part of the continuous development process of the ComBAR rebar system, Schöck has developed a second-generation headed bar end which is currently being introduced to the world market. A newly developed material, more durable than the material used in the first generation of heads, allows faster and more flexible production. More importantly, it will be possible to specify higher design loads for the end anchorage of straight bars, as the geometry of the heads has been further improved.

The second-generation head is made of a thermosetting polymeric concrete with a compressive strength far greater than that of normal grade concrete. It is cast onto the end of the straight ComBAR bar and hardened at elevated temperatures. The concrete mix contains an alkali-resistant vinyl ester resin, the same material used in the straight ComBAR bars, and a mixture of fine aggregates.

The maximum outer diameter of the new ComBAR end heads is 2.5 times the diameter of the bar. The outer head diameter for the 16-millimeter core diameter bar, for example, is 40 millimeters. The head of the 16-millimeter bar is approximately 100 millimeters long. It begins with a wide disk which transfers a large portion of the load from the bar into the concrete. Beyond this disk, the head tapers in five steps to the outer diameter of the blank bar. This geometry guarantees optimal anchorage forces and minimal transverse splitting action in the vicinity of the head.

Short-term tensile strength tests on 16-millimeter ComBAR bars with headed ends yield a mean value of the tensile strength or embedment strength between 100 and 110 kilonewton (kN). The guaranteed or characteristic value of the tensile strength is well above 80 kN, the tensile stress is above 400 MPa.

Long-term durability tests in highly alkaline saturated concrete at 60°C show that the 16-millimeter headed ends are able to sustain a load of 65 kN for more than 5,000 hours. The five percent quantile of the long-term tensile or embedment strength is in the order of 54 kN for the 16- millimeter heads. Tests under these conditions are internationally accepted as accelerated short-time tests to simulate 100 years of design service life under normal Canadian environmental conditions. In other words, the above value corresponds to the characteristic value of the embedment strength for a design service life of 100 years.

In extensive bond creep tests it was shown that the creep behavior of the ComBAR headed ends is far superior to that of straight ComBAR bars. At the same load, the total creep of a bar with a headed end is about half  that of a straight bar without head.

ComBAR heads are currently available for 12, 16 and 32-millimeter core diameter ComBAR bars. As the ComBAR heads are cast onto the straight bars, their length is only restricted by the space available in the production facility at Schöck. For the sake of easy handling, standard lengths should be limited to 3.5 meters.

Historical Development

Development of the ComBAR heads began in 2003, when ComBAR was bid for the reinforcement of the diaphragm walls at the Amsterdam, The Netherlands, North-South Line subway project. The great advantage of FRP reinforcing bars over conventional steel bars in tunneling is the fact that they can be penetrated directly by the tunnel boring machine (TBM). This leads to substantial savings in the overall construction cost and time. While straight ComBAR bars were already available at that time, a reinforcement element was needed to transfer the shear forces in those sections of the diaphragm walls reinforced with GFRP. As bent FRP bars are comparatively weak, Schöck began to develop a bar end head to produce double-headed bars similar to those used in steel reinforcement to transfer punching shear loads in slabs and shallow beams – another of Schöck`s fields of expertise.

Since then, heads for smaller diameter bars have been developed and the material composition of the heads improved. The latest step – the development of the second-generation heads – began in 2008 and was completed for the 12 and 16-millimeter diameter bars in 2009.

Applications

Bent bars of fibrous materials such as wood are generally much weaker than straight bars of the same material. As a result, an alternative to bent GFRP bars was needed to serve as shear reinforcement and to allow for the reduction of the embedment length of straight bars. To that end, the German construction components and GFRP reinforcement supplier Schöck has developed bar end heads for its glass fiber rebar system Schöck ComBAR. These are the first and until now only available GFRP end heads worldwide. They are made of a material similar to that used in the straight ComBAR bars. Shear forces in slabs, beams and walls can now be transferred using double headed ComBAR bars. The embedment length of straight bars can be significantly reduced by providing end heads on one or both ends of the bar. The strength as well as the durability and the creep behavior of ComBAR bars with headed ends are far superior to those of bent bars.

ComBAR headed bars have been installed in several projects in Canada. One of the most notable is the Irvine Creek Bridge near Fergus, Ontario, executed in 2007. ComBAR bars with end heads were installed in the PL-3 barrier walls with a restricted space for bar development in top slab. Another project in 2008 was the approach slabs of the 3rd Concession Road Underpass, where straight ComBAR bars were used in the slabs and headed ComBAR bars were used instead of stainless steel hooks to connect the approach slabs with the bridge deck. ComBAR bars were also installed in the 800mm thick foundations slab in the Quantum Nano Center in Waterloo, Ontario in 2008. Here the length at the slab edges was reduced. The vertical edge of the slabs was reinforced by using double-headed ComBAR bolts. Last but not least, the Walker Road Overpass near Windsor, Ontario, should be mentioned, where ComBAR headed bars were installed in the PL-3 barrier walls in 2008.

Outlook

Long-term durability testing of the second-generation ComBAR end heads for bars with a core diameter of 16 millimeters will be completed by the end of 2009. Final guaranteed characteristic values for the design as end anchors will be specified in early 2010. Test results for bar end heads for ComBAR bars with a diameter of 12, 25 and 32 millimeters will be available in 2010.
For more information, please visit www.schoeck-canada.com or www.dako-pr.com.