We recently told you about chemical anchors, after which we received requests from you for more details. Today we will talk about how to choose a chemical anchor, how to use it, and accessories. After all, it is no secret that in many respects the reliability of fasteners depends on the correct selection for certain supporting structures and compliance with the rules of use.
What is a chemical (liquid) anchor
In construction, chemical anchors came from the mining industry. It was there that the need to develop a high-quality and simple roof bolting system to support the roof in stable rocks was especially acute. Through anchors with spacers are too complicated and not reliable enough, but the idea of gluing a metal rod inside the hole took root very firmly.
Naturally, in construction, chemical anchors are used for a slightly different purpose. Basically, the need for their use is dictated by the structure of building structures. Materials such as hollow ceramic blocks, shell rock and aerated concrete do not have sufficient density and hardness to ensure reliable fixation of finishing substructures, furniture and equipment. But if the pores of the material are filled with a liquid composition, which hardens over time, at the time of accepting the load, the pin has an almost monolithic coupling with the building material.
Modern chemical anchors are not just a pin and a tube of glue, manufacturers develop all fastening technology from scratch and provide the market with comprehensive solutions. So, for more technologically advanced work, special brushes and scrapers for cleaning holes, glue dispensers, two-component mixers and even drilling equipment can be used. The adhesive composition itself is selected individually for specific conditions of use, including the material of the walls. Naturally, all this directly affects the cost: the difference in the price of anchors of different systems and brands can be up to 10 times.
Types and scope
For use in civil engineering, 3 types of chemical anchors are most widely used. Each of them includes 15–20 names of various brands, sometimes quite serious differences can be observed in the device and application technique.
The first group – anchors for gluing reinforcement and studs into concrete. Adhesives are used differently: for reinforcement, the inclusion of corrosion inhibitors and deoxidizers is practiced, the consistency of the glue is usually thick. The threaded rod, due to the large number of small ribs, requires a more fluid polymer complex with a high degree of hardening. Chemical anchors for concrete are distinguished by the highest manufacturability: the arsenal of special tools may contain chemical agents for processing boreholes and reinforcement, as well as mechanical devices for pushing in the stud.
The second group is ampoule anchors. They are usually used where high drilling accuracy and cleanliness of the borehole can be guaranteed. The ampoule is better than the injectable composition in the sense that there is no need to control the degree of filling. A slight error in the free volume of the hole and the capsule is compensated by the ability of the adhesive to expand during curing. As a rule, the compositions in capsules are two-component, the hardener is mixed with the resin when screwing in a special design threaded rod. The distribution of the hardener is very even, while the use of mixers is mandatory when injecting the adhesive. Ampoule anchors cannot be used in masonry materials such as PKB, that is, with large cells of vertical orientation: the composition will simply flow down.
The third group is injection anchors. Their popularity is due to their versatility: there is no need to compare the number of capsules and hairpins, to monitor their safety. Manual injection of the mixture is optimal for fastening anchors in tapered holes that widen towards the bottom. There are also disadvantages: in addition to the impossibility to control the completeness of filling, the glue is subjected to drainage under the influence of gravity. Reduce consumption and achieve more efficient distribution in all directions using mesh bushings.
The main question is how to guarantee full compliance of the anchor characteristics with the existing conditions and tasks? The easiest way is to study the annotation for a single product, which indicates the acceptable materials and types of structures, hole sizes, point locations and method of fastening, temperature and humidity ranges, and most importantly, tables of permissible loads for different materials. True, such a detailed approach is necessary mainly only in construction design. To hang a storage water heater on a cinder block wall, a universal anchor will be enough. We also draw the attention of our readers to the limited service life, which is especially important for atmospheric hardening adhesives..
Drilling and preparation rules
There are three ways of making holes for chemical anchors, two of them are suitable for fastening critical structures and assemblies. The main difference is that the hole for critical fasteners is made with high precision, thus guaranteeing the minimum consumption of glue and its careful distribution over the hole. In general, the borehole diameter is 1.5–2 times larger than the stud thickness, however, some manufacturers have other recommendations on this matter..
Irresponsible fasteners are used in the materials of load-bearing walls, the strength grade of which is at the M100 mark or below. The strength characteristics of the material itself do not allow for critical fasteners, therefore, the maximum that a chemical anchor is capable of in such conditions is to withstand the static load of the hinged system. Holes are drilled with a conventional hammer drill and a drill of the required size. Since most materials of this kind have extremely high porosity, dust is removed from the borehole by blowing air under pressure – the dust is simply clogged into the deepest pores, reducing the depth of spreading of the glue. An ordinary rubber bulb will suffice for purging, although many craftsmen successfully use compressed carbon dioxide cylinders or special hand pumps..
Responsible fasteners are used to connect parts of the supporting structure: fastening frame walls to a concrete base, or when installing cantilevers of a hinged system with a significant concentrated, sometimes dynamic load. Drilling of bore-holes is carried out mainly by the bumpless method using various devices:
- Straight jig – eliminates the runout of the drill and ensures its perpendicular position relative to the surface.
- Oscillating jig: after passing the pilot hole with a special drill, the hole is expanded to a cone. Thus, the pull-off force is no longer perceived by the adhesive seam itself, a significant part of the load is transferred to the wall material.
- Hollow drill bits – make it easier to clean up the hole and get debris out of it.
The most important criterion affecting the quality of anchorage with a chemical anchor is the cleanliness of the borehole. The presence of dust is categorically unacceptable; due to poor wettability, it prevents the adhesive from contacting the monolithic layer. So, if the adhesive composition is designed to fill small pores in materials with a closed cell system, blowing can only aggravate the situation. First you need to clean the hole with a metal brush, then blow out the rest of fine dust with gas under pressure. This eliminates the possibility that dust will clog into the pores, which should be filled with glue..
The holes can also be flushed, which is commonly practiced when preparing particularly deep holes in closed cell materials. For washing, aqueous solutions of surfactants are used, providing intensive formation of foam, which is removed from the holes by compressed air. Please note that for some types of anchors, the hole is considered suitable for fastening only for a limited time after cleaning..
As already mentioned, it is most simple to use ampoule anchors: when the stud is screwed in, a special protrusion at its end quickly mixes the resin with the hardener until completely homogeneous. But with manual introduction, there are a number of subtleties:
- For materials with large pores (PCB, cinder block), the use of a mesh sleeve is absolutely mandatory. It is inserted into the hole before the glue is injected, after which the injected glue is evenly distributed in all directions..
- When injecting two-component adhesive, it is imperative to use a mixer. It is a thin tube, divided by a partition into two channels, through which the base and hardener are supplied in a strictly defined proportion. For each composition, use a different type of mixer.
- For high-quality filling of the borehole or sleeve, use special dispensers. They squeeze out the adhesive with considerable force, due to which the air is completely forced out of the hole. Filling with glue must be carried out in the full volume of the hole.
The insertion of the stud with the injection method of glue injection is done manually; for a length of over 500 mm, it is recommended to use special conductors with mechanical feed of the stud under great effort. For ampoule anchors, the pin is clamped into the drill chuck and slowly inserted into the hole at medium speed.
After inserting the stud, the glue hardens, at this moment the core must remain motionless. The average solidification time at a temperature of 15–20? С is 30–40 minutes, at low negative temperatures the process can last 8–10 hours. In general, the lower temperature threshold for the use of chemical anchors is -5 ° C; in colder conditions, curing may not occur at all. At the same time, there are also special anchors that can solidify at temperatures and -20? C.
Special attention should be paid to the fact that not all chemical anchors provide tightening fastening. In this case, due to the absence of prestressing of the pin, permanent deformation may appear. Thus, when fixing the hinged element, it should be located close to the plane of the building structure, so that there is no stretching “neck”, the surface of which is not bound by strong glue. This does not cause a critical decrease in the strength of the entire structure, however, in the presence of spacers, it is better to put them in immediately and tighten the stud with an effort of half the elastic deformation limit.