ANCHOR EVOLUTION

From a piece of steel to an engineering marvel

The Trendsetter

Traditional post-installed anchoring systems comprised of basic grout systems and simple deformation-controlled systems. Applications for post-installed anchoring systems were very limited as there was lack of understanding amongst the engineers on this concept. Due to lack of proper design guideline, the design specifications recommended only the anchor diameter based on material strength of the anchor. Hilti entered the Indian market with limited range of post-installed connections in 1997. With few anchor experts on board, Hilti started to educate the construction fraternity about post-installed connection technology. Since then the market has seen an increased demand for post-installed connections due to boom in construction industry and increase in complexity of applications. Basic post-installed anchoring systems have also developed into well engineered products like hybrid adhesive anchoring systems, expansion anchoring system, direct fastening systems etc. to cater to these complex applications and meet productivity requirements. Post-installed connections have now become an integral part of any type of construction project and are used for a wide range of applications starting from as basic as door-window frame fixing to as complex as strengthening applications.

Apart from productivity, sustainability and safety have become key topics in the construction market, which has paved way for the next generation of post-installed connection systems.  Performance, reliability, safety, ease of installation and green technology are some key parameters that are now considered before selection of post-installed connection systems.

SAFEset

For example, inadequate cleaning of holes during installation can significantly reduce performance of traditional adhesive anchoring systems. Whereas, the new SAFEset adhesive anchoring system by Hilti allows installation in unclean holes without significant load drop and at the same time reduces installation time. SAFEset adhesive anchoring systems achieve this by combining best attributes of adhesive and mechanical anchoring systems. In SAFEset adhesive anchoring systems, the traditional threaded rod is replaced by rod with cone shaped helix which as acts wedge of expansion anchors. When the rod is torqued, a series of wedges exert expansion forces on the hardened hybrid adhesive. Engineers prefer SAFEset systems over traditional adhesive anchoring systems as it scores high on performance, reliability, safety and productivity. Though it is a new technology, it is being adopted by the engineering community very rapidly.

HUS3

Hilti's HUS3 mechanical anchoring system provides a more reliable solution for tension dominated applications. It works on a combination of undercut and confinement principle in concrete just like a cast-in undercut and rebars undercut. Unlike expansion anchors, HUS3 anchors do not require a minimum torque to ensure optimum performance, thus making HUS3 a safer and reliable solution. Cutting edge of HUS3 offers fast installation and a secured setting. Due to less installation time, 50% increase in productivity can be achieved when compared to traditional expansion anchors. HUS3 comes in hexagon and countersunk head versions to suit every type of application offering higher loads with aesthetics in both interior and exterior applications. As HUS3 does not introduce significant expansion forces in the base material, they can be spaced closer together and also close to edge. This feature enables a design engineer to optimize base plate design to save material cost and at the same time achieve the desired level of reliability.

Post-installed Anchoring systems

Apart from anchoring products, the assessment and design method of post-installed anchoring systems have also evolved over the years. Just a material strength test is no longer sufficient to determine suitability of use of an anchor for a given application. As per the latest international standards, the anchoring systems are thoroughly assessed to evaluate performance in both ideal and non-ideal service conditions. Depending on the assessment programme selected, attributes like performance under sustained loading, in crack, in water-filled holes, in improperly cleaned holes etc. are evaluated. Based on this assessment, anchors are assigned different safety and design factors which are used for design. The assessment reports also clearly define the load and service condition for which the anchor can be used. A limit state design approach is adopted for design of anchors – capacity of the connection is checked for different modes of failure. Concrete cone breakout, concrete edge breakout, concrete pryout, splitting, pullout etc are some failure modes that are checked in addition to the material strength. Traditional design specification containing recommendation for anchor diameter is no longer sufficient. The design specification as per the latest guidelines typically includes information like anchor type, diameter, embedment, location etc. At present, there are no performance evaluation and design standards for post-installed anchors in India. In absence of national standards, engineers refer to International standards like ETAG 001 – "Guideline for European Technical Approval of Metal Anchors for Use in Concrete", ACI 318 – "Building Code Requirement for Structural Concrete and Commentary" etc, for design guidelines. Hilti has an engineering team and design software to support the consultants in anchor design using these standards. However, there is an urgent need to develop anchor design and assessment standards to cater specifically to Indian construction environment.

Installation methodology

Due to more detailed designs compared to tradition specifications, the installation methodology has also evolved. For anchor connections to perform as per design, they have to be constructed exactly as per the engineering drawings. To be able to do so, measuring or scanning tools are employed.  The actual reinforcement layout is mapped using scanning tools like Hilti PS 1000 or PS 250.The scan data from PS 1000 and PS 250 can be transferred to a PCfor further analysis to determine rebar diameter, concrete cover etc.In case of overlap of reinforcement with anchor location, the anchor location has to be shifted after seeking approval from the design engineer. Hilti also allows transfer of scan results to its design software "Profis anchor" to aid the design engineers to select suitable location for anchor installation.

Application and Installation

Despite well engineered products and stringent international design guidelines, construction fraternity still faces challenge in terms of quality of application. Hilti products do offer features like embedment mark, identification mark, etc to aid in inspection. However, there is a need to outline good inspection programmes to enforce quality. Post-installed connections are not given their share of attention during inspections. In some sites, a certain percentage of connections are proof loaded to check the performance of anchors. However, there are no mandatory programmes in place to train installers, to periodically review quality of installation tools and to inspect anchor installation.

Author: Kamalika Pal, Assistant Manager – Codes and Approvals; Hilti India Private Limited. E mail kamalika.pal@hilti.com