Post-installed reinforcing bars in concrete structural applications are typically executed with injection adhesive systems – solutions which depend on a combination of design and jobsite requirements. As a design engineer, it is crucial that you also consider the installation method, as this can influence both design values and installation effectiveness.

In this article, we compare epoxy and hybrid systems and provide you with some insights into two setting parameters: base material temperature during installation and drilling method. These factors should be taken into consideration when selecting the right adhesive for your case. Discover how to ensure your next rebar design with injection systems is on the right track to being jobsite-proof, and the benefits you can derive over other possible solutions (see: Concrete connections, when and why injection systems make the difference).

Jobsite condition 1: base material temperature during installation

The temperature of the concrete during the rebar setting affects two product installation parameters: the working time and the curing time (note that exposure to fire is not considered here).

The working time begins immediately after injection and is the period during which the rebar must be inserted and can be adjusted. When the working time is over, the fixing should not be touched until the full curing time is observed. If setting goes beyond the defined working time, the rebar will not be properly installed, or the load performance compromised. This parameter depends on the product features and the base material temperature: the higher the temperature, the shorter the working time, and vice versa. Epoxy mortars such as the HIT-RE 500 V4 offer a longer working time than hybrid systems and are therefore usually more appropriate for jobsites located in warm regions or for large diameter bars in combination with deep holes in civil projects, requiring a longer time to complete the setting.

The only hybrid mortar to present an adequate working time in the presence of hot temperatures and deep embedment (significantly longer than other hybrid mortars and not much shorter than epoxy mortars) is the HIT-HY 200, making it suitable for a wide range of concrete works.

Civil jobsites may require the use of mortars with a longer working time to ensure appropriate installation.

The curing time is the time required to develop the full capacity. This parameter depends on the base material temperature in a similar way to the working time.

Hybrid systems typically feature an accelerated curing time and may be the best option to increase installation speed – and thus jobsite productivity – in applications with small to medium bar diameters, short holes and low temperatures. For this reason, hybrid mortars are also known as “fast-cure” systems.

As mentioned above, since the HIT-HY 200-R offers a longer working time (in Hybrid mortars range), productivity gains are also possible when using this system for deeper embedment and/or larger sizes in warm environments.

Jobsite condition 2: drilling method

Post-installed reinforcing bars typically require deep embedment to satisfy anchorage length requirements. The deep holes for installing reinforcing bars are usually drilled using one of the following four methods (with varying effects on product performance):

1. Hammer drilling method
2. Hammer drilling method with hollow drill bit in combination with vacuum cleaners (automatic cleaning system)
3. Diamond wet coring method
4. Diamond wet coring method combined with hole roughening

The first method involves using rotary-impact drills equipped with cruciform carbide bits. The drill bit produces a non-uniform hole surface. Most of the mortars available on the market derive their holding strength from the micro-keying of the adhesive to the rough concrete and are therefore suitable for this drilling condition.

Hammer drilling

As a second alternative, the rotary-impact drilling machine can be equipped with hollow drill bits (automatic cleaning system) in combination with certified vacuum cleaners. This increases application reliability and speed by eliminating the most load-affecting and time-consuming step in the installation process: cleaning the hole before injection of the adhesive. As in the previous case, both epoxy and hybrid systems can be used if they qualify for this condition, such as the HIT-HY 200-R and HIT-RE 500-V4.

Hammer drilling method with hollow drill bit in combination with vacuum cleaners

The third case is diamond wet coring, which is used whenever the previous solutions are not practical. For large sizes and deep embedment, for example, core drills can produce very long, straight and big holes with the use of extensions. In contrast to hammer drills, which fracture the concrete with impact energy, core drill bits utilize diamond fragments to abrade the concrete. This method typically produces a very smooth hole, with the result that some adhesive systems are not suitable for use with core-drilled holes. Approved epoxy mortars for diamond core installation such as the HIT-RE 500 V4 should always be preferred in such cases. Nevertheless, even when the product is qualified for diamond coring, due to the smooth drilled hole surface, the bond strength for certain rebar diameters is reduced compared to hammer-drilled holes. Roughening the hole enables the use of the hybrid system HIT-HY 200-R when this drilling technique is used, as explained below.

Diamond wet coring method

Finally, diamond wet coring combined with hole roughening involves the use of a tool to roughen the sides of the smooth, diamond-cored hole. This procedure increases the mechanical interlock between mortar and concrete to achieve the same load performance as hammer-drilled holes. This advantage is offered by the epoxy mortar HIT-RE 500 V4 and the hybrid mortar HIT-HY 200-R. This new set of approvals is valid for post-installed rebar connections under many conditions (e.g. static, seismic) – consult the European Technical Assessment (ETA) approval for the rebar sizes covered.

Diamond wet coring method combined with hole roughening

Rule of thumb for the selection of adhesives based on the drilling method

Alongside the above factors, other conditions (such as short-term and long-term temperature classes, range of sizes, qualified embedment depth and cleaning methods) must also be considered.

In practice, you should always consult the ETA rebar approval of the adhesive, check the compatibility of the products with the jobsite circumstances, combine these with the design requirements (e.g. static, seismic) and make the best choice to ensure full compliance. The PROFIS Engineering software is an ideal tool to help you accomplish your design tasks, it is customised to AS 3600 rebar design method (supported by AEFAC technical notes). By entering the jobsite and design constraints as input data, you will be supported throughout the calculation process and be able to access the above-mentioned documents in just a few clicks. You can also find more information in our rebar design guidelines and our on-demand webinar on using PROFIS Rebar.

 Are you handling a case you want to discuss with us? Let us know in the comments.