Fixings continue to be developed for the ever changing construction industry with many major brands including Fischer Fixings, Rawlplug, ITW Construction Products, Hilti & JCP delivering new products ever year.
Choose the right fixing
The choice of fixing used is usually determined by the building material you are fixing into. Substrates as they are called include concrete, masonry, brick, block (either solid or hollow section) gypsum or fibre boards. There are many other types but we will concentrate on the common types found on most construction sites or house builds. If you know your substrate, but do not know the fixing you require, use the fixing substrate guide to help you make the right selection.
Fixings can be split into a number of categories, each with their own individual benefits. On this page you'll find a small amount of information to give you an overview, followed by an link to a more comprehensive guide to each product.
Different types of fixings
Mechanical anchors are quicker to install than chemical resin fixings as there is no waiting time for the fixing to cure. Mechanical anchors expand locally, nearly all are required to be fixed a safe distance from any edge and a minimum distance apart. Any reduction in the recommended minimum distance apart would result in reduced perfomance and should be avoided if possible. Mechanical anchors are often torque controlled anchors and should be installed using a torque wrench. Mechanical anchors are available in steel, stainless steel & galvanised as required. Click for the full range of high performance mechanical anchors.
Shot fired nails or direct fixings utilize modern nail technologies to fire hardened pins into steel, concrete & brick. Direct fixings fall into two main categories - Gas fired fuel injection system and Powder Actuated Tools or PAT. Gas fired Spit Pulsa Nailing tools fasten cable management, wiring accessories and drywall track up to 6 times faster than conventional methods. PAT cartridge systems are low velocity, indirect acting tools. This means that once the explosive cartridge is detonated, the energy produced drives a captive piston forward, which in turn drives the fastener into the substrate. Both systems will not operate until actuation pressure is applied, making the tools safe to handle and reduce accidental miss fires.
The substrate and its quality is decisive for selection of the fixing: the building material and anchor base. A differentiation is made between concrete, masonry and panel materials.
Generally, these aspects should be considered during installation:
The edge distance and axial spacing as well as the component thickness and width must be thought about to make sure the fixing will hold the required load. Otherwise the construction material may split and cracks appear. If information on edge distances and axial spacing is needed please call the Fix8 sales team who will be happy to help.
The hole depth must, with a few exceptions be larger than the anchoring depth because full load capacity is only ensured if the screw has enough room to project beyond the tip of the plastic fixing. You will find that most products will show this information but if anymore information is needed please contact Fix8 sales.
Hole cleaning after drilling, by blowing out or suction is fundamental. A hole that is not cleaned out reduces load capacity. The drilling dust has a negative impact on the fixing in the hole causing the fixing to not anchor correctly.
The Installation Types
There are three different methods of installation
Flush in this instance the fixing is usually flush with the construction surface. The installation sequence generally involves the following steps:
- Pre-mark the drill holes
- Drill, clean holes, set fixing flush with the surface.
Push-through installation is especially recommended if the fixture is already in its final position or where fixings are secured with a large number of fixings.
- The holes in the item to be mounted can be used as a template, since their hole diameters are at least as large as the drill diameter in the construction material.
- The fixing is inserted into the hole through the item to be mounted and then expanded.
Stand-off installation is used to fasten items to be mounted at a specific distance from the anchor base. To do this, usually metal anchors with metric internal threads are used to hold screws or threaded rods with lock nuts.
Useable length and anchoring depth: in addition to the type of installation the useful length and anchoring depth of the respective fixing should be considered during installation.
The useable length (clamping thickness) of the fixing and screw should correspond to the thickness of the item to be mounted. On anchors that have internal threads, this can vary by selection of the screw length. However, in push through installation and with bolt anchors, the maximum useful length is specified by the fixing. There are a large variety of useful lengths available. If the anchor base is covered with plaster or insulating material, screws or fixings must be chosen with a length that corresponds at least to the plaster thickness, plus the thickness of the item to mounted.
The anchorage depth corresponds, for plastic and steel fixings to the distance between the upper edge of the load bearing component to the lower edge of the expansion part and for chemical anchors to the lower end of the threaded rod.
There are different bearing mechanisms that transfer the forces that act on the fixing into the base material.
With friction connection the expansion part of the fixing is pressed against the hole wall: the outer tensile loads are held by friction.
With form locking the fixing geometry matches the shape of the substrate and or of the drill hole.
With adhesive bonding the resin adheres the fixing with the anchor base.
With excess stress, incorrect installation or a substrate with inadequate load bearing capacity, the following failures can occur:
Fracture of the anchor base caused by
- tensile load "N" or shear load "V" being to high
- inadequate strength of the anchor base
- setting depth being too low
Component splits due to
- component dimensions being too small
- the edge and axial spacings being too small
- expansion pressure being too high
Fixing pulls out due to
- failure of the frictional or bonded connection due to high load or incorrect installation.
Steel failure due to
- fixing and/or steel strength too low for the applied load.
Cracks In Concrete Components
Cracks can occur anywhere in concrete at any time. Factors involved in this are loads like dead loads, traffic or wind loads, shrinkage and creeping of the concrete or external influences like earthquakes or ground movements that result in tensions, deformations and thus cause crack formation.
A great example of this is a bridge. Bending occurs due to the pressure forces applied to the bridge deck. A compressive zone will occur at the top of the bridge deck, while tensile forces and strains will occur in the lower part of the bridge deck. However, concrete is unable to bear noticable tensile forces but steel reinforcement bars can. But whilst steel bars are capable of bearing these tensile stresses, they elongate as they do so. This doesnt affect the steel but will cause incalculable cracks which are hardly visable to the naked eye to form in the concrete. This is called the cracked tensile Zone.
Fixings suitable for cracks
With anchorings in concrete, it is almost always assumed that cracks are present in the anchoring area that influence the bearing capacity of the fixings. However it is very complicated, ifimpossible to prove whether the concrete is cracked or non cracked. For safety reasons, the use of fixings that are suitable for cracks is basically recommended to designers and tradesmen. Fixings with the so-called CC approval from the DIBt and with approval according to ETAG 001 for concrete have proven their suitability in cracks and can thus be used without restriction in the tensile and compressive zones of the concrete. The following special fixings are suitable for use with cracks:
Fixings acting with machanical interlock like ZYKON anchors that are used in undercut holes. These anchors have a conical part that optimally engage even with a continuing crack or under shock load.
Expanding fixings automatically adapt to the hole expanded by crack formation in that their cone is pulled deeper into the expansion part and the expansion diameter thereby increases. These fixings are alos suitable for absorbing shock loads. The first approved plastic fixing for cracked concrete is the Fischer long-shift fixing SXS that represents a special case.
Development of Corrosion
Corrosion is a chemical reaction in which metal deteriorates. The less noble the metal (electrochemical potential) the more intense the material damage. In this process it is either converted into flaking rust of worn away in places. Different appearance patterns and the most frequent types of corrosion in fixings and anchors are explained below:
Surface Corrosion in this case, the metal corrodes relatively uniformly over the entire surface or over a part of the surface. An example of this is the invisible rusting due to condensation of an anchor in the transition area from the anchor plate to the hole. The result is that the connection that apears completely intact from the outside fails abruptly.
Contact Corrosion If metals with different nobility are in contact with each other in a conductive medium, the less noble metal always corrodes. As a consequence, stainless steel is usually not endangered. What is decisive is the surface ratios of the two types of metal: the greater the surface area of the most noble metal in comparison to the less noble, the greater the corrosion becomes. For example, if large stainless steel sheets are screwed with galvanised screws, the screws will be highly attacked within a small amount of time. In contrast using stainless steel screws for galvanised sheets is not critical.
Stress Corrosion Cracking if internal or external tensile stresses occur, there can be strain and corrosion of the metal. In this process a crack develops and grows due to mechanical stresses which increase, thus preparing a path for progressive corrosion. For example it occurs with A4 steel in an atmosphere containing chlorine (indoor swimming pools etc.). Generally stress corrosion is not visible with fixings and usually leads to sudden failure of the anchoring.
There are different methods for protecting fixings from corrosion. The most important methods include:
Zinc Plated is the most frequently used corrosion protection for metal fixings. It consists of a metallic coating with a layer thickness between 5um and 10um. The Zinc plating is carried out either with a blue passivating that gives the anchor a silvery appearance or yellow chromated. Since the zinc plating is worn off over time, it only offers adequate corrosion protection in dry interior rooms.
Fixings of stainless steel A4 (material no. 1.4401 or 1.4571) are suitable for fixings in damp rooms, in open air, in industrial atmospheres or near the sea ( but not directly in sea water). These steels are alloys with a chrome content of at least 12% that forms a passive layer on the steel surface that protects against corrosion.
Fixings of special alloys (e.g steel, material no 1.4529) are used in especially aggressive environments containing chlorine, in road tunnels or with direct sea water contact. In this case, the chrome content of normal stainless steel drops below 12%. The protective passive layer dissapears and the anchor becomes susceptible to corrosion. On the other hand, the special alloys are very corrosion resistant in these highly aggressive media, due to their relatively high percentage of molybdenum. With an alloy percentage of 50%, they clearly surpass the usual unalloyed, low alloyed or high alloyed steels with a maximum of 30% alloy percentages. This means the steel 1.4529 alloyed with chrome, molybdenum and nickel has an alloy percentage of 58%. The rest consists of iron and carbon. Because of this high percentage of expensive alloy addictives, the manufacturing of these steel types are therefore rather costly.
Approvals, Markings And Their Importance
For the approval of resin anchors suitable for use in contact potable water
For the approval of foams and sealants which have been treated in accordance BS476: Part 20
This is issued by a European approval authority (eg. DIBt) on the basis if the guidelines for european technical approvals (ETAG) ETA: European Technical Approvals/Options 1 -12CE: European conformity mark confirms the compliance of the building product (eg fixing) with guidelines for the European Technical Approvals. Products with the CE mark can freely traded in European economic Market.
General Building Authority Approval
German approval, issued by the DIBt, Berlin for anchorings in concrete to be dimensioned according to Method A (CC method) Proof of compliance of the building product with the general building authority approval. Confirmed by a material testing facility.
General Building Authority Approval
German approval, issued by the DIBt, Berlin. Proof of compliance of the building product with the general building authority approval. This is confirmed by a material testing facility.