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FAQ's

Frequently Asked Questions

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Frequently Asked Questions

Resin Bonded Anchors

1.Why do some manufacturers recommend tightening resin anchors with a torque wrench when they are not tightened to set them?

It is just as important to use a torque wrench to tighten an anchor set in resin as it is when tightening a torque controlled expansion anchor. Using a torque wrench set to the manufacturer's recommended torque for the base material concerned, on any anchor, will ensure that the bolt material is not overstressed and that the required clamping force is generated through the fixture to guarantee that the fixture will not move. On a resin anchor it will also protect the resin bond from being overstressed. Just think - once the nut on the stud of a resin anchor is finger tight against the fixture, turning the nut by just one more complete turn will tend to pull the anchor rod from the structure by one thread pitch. This could be enough to break the bond. For use in cracked concrete a new type of resin anchor is becoming available - the "Torque controlled resin anchor" or "Resin expansion anchor" which must be set with a torque wrench to make it function correctly - so tightening with a torque wrench on ALL anchors is the best policy.

 

2.And why do they recommend reducing the torque in brickwork or blockwork?

In weak materials the bond may be overstressed by the load induced by the torque appropriate for the loads achievable in concrete so if the installation torque is quoted only for concrete this value should be reduced in proportion to either the reduction in design load or the reduction in base material strength.

 

3.What are the different merits of capsule systems compared with injection systems?

Injection systems are more versatile, as they can be used in both concrete and masonry whereas capsule systems are difficult to use in masonry. Capsule systems are also fussier to use in that you need special anchor rods and setting equipment and must use capsules which match the hole diameter. As injection systems can be used to deliver a variable amount of resin into the hole they cater much better with the problems of masonry and can be used for varying embedment depths but usually when designing anchors for concrete the same embedment depths are used for both systems and many manufacturers sell the special anchor rods designed for capsule systems for use with injection systems as they have useful features such as embedment depth marks.

When it comes to safety critical applications into concrete then capsule anchors (of the "Spin-in" type rather than the "Hammer-in" type) have some useful advantages over injection anchors.

  1. Capsule systems are less prone to poor hole cleaning (you should always clean the hole thoroughly with both types). This is because the spin-in type tends to draw any dust sticking to the hole sides into the mix whereas the injection type simply lays the resin against the side of the hole and any residual dust will prevent the bond from forming. During drilling some dust will stick to the sides of the hole and blowing alone will not remove it. ALL holes for ALL resin anchor types should be thoroughly cleaned by blowing, brushing and blowing. Omitting the brushing operation in capsule systems can result in approximately 30% loss of ultimate strength. Omitting the brushing operation in an injection system can lead to 70 - 80% loss.
  2. Capsules contain exactly the right amount of resin. Judging the correct amount with injection systems is slightly hit and miss, for the first few holes at least, and will usually err on the side of excess to ensure there is at least enough. Capsules also involve less mess.
  3. Capsule systems are generally stronger. Injection formulations are getting stronger all the time but this is still generally true. OK, so the strongest of all resins is currently pure Epoxy, only available in injection format, but this is a specialist resin for special applications.
  4. Capsule systems cure faster. Comparisons of curing times of capsule systems Vs injection systems from the same manufacturer will generally show capsules to cure between 2 and 3 times faster. Systems vary so always check the particular manufacture's times.
  5. Capsule systems involve no environmental pollution. With injection systems wastage occurs in three ways - when some resin is pumped to waste to ensure thorough mixing, when too much is pumped into the hole and when the nozzle is thrown away. Together with the cartridge itself this represents environmental pollution.

For applications which are less safety critical Injection Systems may be preferred because:

  1. There is no need to buy special anchor rods or setting tools. There is no need to buy specific capsules to match the rod diameter.
  2. They are generally cheaper.

Injection anchors are better for masonry because they can cater for the inevitable voids due to poorly filled mortar joints or porous materials. They are available with special accessories such as mesh sleeves to control the resin in hollow and perforated materials. Capsule anchors of the "Hammer-in" type are intended for rebar applications (starter bars) and should therefore be used only in concrete and for that purpose. There are however special injection systems for installing starter bars which lend themselves to this special application.

 

4. How do I work out how many fixings I can get from one injection cartridge?

In summary what you do is divide the volume of the cartridge by the filled volume of each hole and take an allowance of 20% for wastage (first trigger pull to waste to ensure even mixing, resin left in the nozzle, excess needed to ensure the space is at least filled). This will give you the maximum theoretically possible from each cartridge. This may reduce if you use several nozzles per cartridge and if you are generous in filling the holes. The table below has data to help you do this, including maximum holes filled from a typical 380ml cartridge with some typical hole diameters and depths. For a different cartridge size change the number pro rata.

Formula

 

 5. There are so many different types of resin. When do I use which type?

There are two key points.

  1. Follow what the manufacturer says about which base materials the resin is suitable for and
  2. Follow their installation instructions regarding installation temperature ranges and curing times - they can be very different.

Basically most resin formulations are suitable for most general purpose usage. The original "(Unsaturated) polyester" resin is still the most commonly supplied and has proven satisfactory for most applications but it does lose strength in permanently damp concrete*. For those situations any of the modern formulations such as vinyl ester (epoxy acrylate) and hybrid mortars (synthetic resin plus cementitious mortar) will be satisfactory. Some of these also have slightly higher service temperature limits than polyester but these temperatures are reached only in special applications. True epoxy resins (not to be confused with epoxy acrylate) are now available in injection systems. These are among the strongest resins available and may be used in larger hole diameters to improve anchorage strength without increasing hole depth but they take much longer to cure.

* All resin formulations may have a problem in damp brickwork or stonework due to the inherent weakness of the substrate itself.

 

6. I notice some resin anchors come in foil packs rather than glass capsules. Does the foil interfere with the resin bond?

No. In fact resins have been supplied in "sausage skin" format for decades for special applications with a proven record of reliability. The foil is dispersed within the resin as the anchor rod is spun into it and has no effect on bond strength.

 

 

 

 

 

 

 

 

Fixings to Blockwork

1. How can I make sure the blockwork fixings I want to use are right for the blockwork in the structure I am faced with?

Knowing the sort of blockwork in the construction helps (both the type and strength). Then you can look at our Guidance Note on Fixings to Brickwork and blockwork or speak to one of the members to get a recommendation. In an existing structure it is very difficult to tell the nature of the blocks let alone their strength so initial tests may well be worth carrying out to check the suitability of any proposed fixing and determine the allowable load.

See Guidance Note:  Procedure for site testing Construction Fixings.

 

2. I know the blockwork is aircrete. What is the most suitable fixing?

As with most types of block, be they aircrete (aerated concrete) or some sort of light weight aggregate, a resin fixing will probably be strongest but may be time consuming and relatively expensive. There are special nylon anchors made for blockwork some specifically for aerated concrete, these are designed to grip a large area of the block.

 

3. Are there any special measures for drilling holes in blockwork to ensure fixings work at their best?

Try to be as gentle with the blockwork as possible. Using a hammer drill will almost certainly open up the hole dramatically over size and weaken the fixing. With the lightest materials such as aerated blocks use an HSS drill bit on rotary only. This will drill a hole close to the intended diameter. If that is not available use a conventional masonry drill on rotary only. Some manufacturers recommend the use of a punch of the right diameter to create the hole as this compresses the material around the hole reinforcing the structure.

 

CE Marking

1. Must I use CE marked products for anchoring?

CE marking is not mandatory in the UK - yet - so you do not have to use a CE marked product but it is a way of demonstrating compliance with the Building Regulations and will safeguard your liability if correctly specified and installed. However if you are specifying anchors for use in other markets within the EU where CE marking is mandatory then it is illegal for suppliers to sell products without CE marking for applications covered by the scope of an ETAG once the transitional period has finished - as it has for most anchor types. See Guidance Note -  European Technical Approvals for Construction Fixings

 

Throughbolts

1. How can I remove throughbolts from concrete after use?

Only with difficulty. 
If you know before installing them that they may not be required at a later date then you should drill the hole about 20mm deeper than the total anchor length. After use they can then be knocked into the hole which can be grouted up. If you must remove existing anchors then they can be jacked from the concrete using a hydraulic cylinder as long as the surface immediately surrounding the anchor is reinforced with a thick plate and the reaction forces from the jack are directed through that. This will prevent a large cone of concrete from being pulled out of the surface. If anchors set externally are cut off when no longer needed they may corrode if they are not stainless steel causing damage and staining.

 

Anchors and Fire

1. Some expansion anchors have plastic parts in the sleeves. Are these OK to specify for an application where a fire rating is necessary?

Yes they are.

These plastic parts are usually only there to help with the installation - they provide an anti-rotation feature to ensure the anchor expands properly while the bolt is being turned and a crush feature to make sure that any small gaps are pulled down and that the clamping force is directed through the fixture and not through the sleeve of the anchor. They are redundant after installation and of no consequence in the case of a fire. [These features are provided by means other than plastic parts on some anchor types.]
See Guidance note:  Fixings and Fire.

 

2. What happens to resin anchors in a fire - do they burn or melt?

They do neither.
In fact the performance of resin anchors in fire tests is very little different to that of all steel anchors. The bolt head or nut at the surface is usually the weak point where failure occurs in a fire test.

See Guidance Note:  Fixings and Fire.

 

 

Corrosion

1. I have to specify fixings for a ceiling above a swimming pool. Will A4 stainless steel handle the high chloride concentrations present?

Sadly no.

The presence of chlorides at elevated temperatures promotes what is known as "Stress corrosion cracking" which even affects grade A4 stainless steel. Special stainless alloys have been developed to cope with this and other highly aggressive situations. Some manufacturers make a limited range of anchors available from this material.

See Guidance Note:  Fixings and Corrosion.

Fixings into Brickwork

1. Can I fix anchors into the mortar joints?

Most anchor manufacturers would prefer that you didn't. The danger is that the mortar may be weak or even non-existent in a perpendicular joint. So if you fix into the meat of the brick you will get the maximum support around the fixing. If there is a good reason why you want to fix into the joint - on a listed building for instance, then follow the following guidelines:

  1. Check with the manufacturer of the fixing you want to use!
  2. Try to use a fixing with a diameter larger than the width of the joint
    - this way you will bear on the bricks and not rely solely on the mortar joint
    - but not so large that it may crack the brickwork!
  3. Fix into the junction where the perpendicular joint meets the horizontal one (if you have the choice) otherwise on a horizontal joint.

Obviously in many applications you cannot see the joints anyway as they are hidden behind plaster. In these cases you should ensure that multiple fixings are being used so that the failure of one fixing cannot lead to the failure of the whole fixture or system.

In critical applications site tests may be necessary to determine the strength of fixings set in joints.

See Guidance Note -  Fixings for Brickwork and Blockwork

 

2. RESIN ANCHORS IN SOLID BRICKWORK My company regularly needs to attach fabricated structures to brick walls, that are usually of solid (9") construction. We have decided to standardise our approach by using injection resin anchors so as not to crack the bricks that we are fixing into. What is the best embedment depth to install this system to? If I drill through into the back leaf should I use a mesh sleeve to avoid losing resin into the void between the leaves?

Good question. The best approach is to decide whether to fix into the front leaf or the back (remote) leaf and set the drill depth in order to optimise your choice. The strongest fixing will be achieved by drilling as deep as you safely can into the back leaf (in a typical 9" solid brick wall this means 180mm - any deeper and you may break out at the back). Pump your injection resin to the base of the hole (this assumes you have cleaned the hole using a BRUSH as well as a blow out pump and that you have pumped at least the first trigger pull of resin to waste to be sure the two components are mixed) and aim to fill the hole to about half to two thirds full, withdrawing the nozzle from the drill hole as you go. Insert the anchor rod with a pushing and twisting motion and remove any excess resin from around the surface of the hole. If no excess comes out it may be because some of the resin has become lost in the void between the two leaves. This should not matter. The anchorage will gain strength from the back leaf pulling against the front leaf. For lower loads aim to fix only into the front leaf. Drill as deep as you can without spalling the back from the brick into the void between the leaves where there may be no mortar. This will be to a depth of 70 - 80mm depending on the power of drilling machine used (the more powerful it is the shallower you should drill). So "How do I know how strong these fixings will be?" Site testing I'm afraid is the only way. (See Guidance Note: Procedure for site testing construction fixings.) Whatever you do don't try to install resin anchors into solid brickwork with an embedment depth of 90 - 120mm You will lose most of the resin into the void and the fixing may be very weak. Don't forget to leave the anchor undisturbed for the full curing time before tightening or loading. "What about mesh sleeves?". With this method in "Solid" brickwork you shouldn't need them even if they are frogged bricks. You will need sleeves if you are fixing into perforated bricks but then the construction is most likely to be a cavity which is discussed in the question below. (See also Guidance Notes - Resin bonded anchors and Fixings for brickwork and blockwork.)

 

3. RESIN ANCHORS IN CAVITY BRICKWORK My company has a contract to fix bracketry for aerials, masts and flag poles to brick structures using injection resin. We are concerned to use the right technique for cavity walls. Do we fix only in the outer leaf or try to bridge across using mesh sleeves?

There are several problems here - so the short answer is "Involve your fixing manufacturer". The first thing to do is to check that the cavity construction can take the imposed load, they don't always have the integrity needed for this, it depends on the nature of both leaves and of the wall ties used to tie them together. A simple pull test locally against the structure will not necessarily tell you that the structure is strong enough overall. We suggest a structural engineer looks at this for each different structure and decides whether or not the required load can be taken by the outer leaf only or must be shared by both. The direction of loading must be considered in this exercise. A tensile load will obviously be directed into whichever leaf the fixing is anchored, but a shear load will be transferred into the front leaf irrespective of where the anchorage is made.

Fixing to the outer leaf.

If loads can be taken by the outer leaf then the specific method depends on whether the bricks are solid or perforated. If bricks are solid (including frogged) be careful to drill to no more than 75mm or you may break out the back of the brick - use an appropriately powered drilling machine. Use a threaded stud up to M12 with no sleeve and follow the manufacturer's instructions. If bricks are perforated you will need to use a mesh sleeve, these are typically 85mm long and made of steel or plastic, and require a larger drill hole, say 15 or 16mm diameter (depending on make) drilled to about 90mm. You will probably break out the back of the brick, this does not matter as the sleeve controls the resin to bond with the brick where it can and interlock with the perforations where it can't.

Fixing across the cavity.

You should not try to fix into the remote leaf alone. If you do then when you try to tighten the fixing you will pull the two leaves together and crack the structure. Bridging across the cavity and bonding into both leaves with a long mesh sleeve is possible but problematic. Again the nature of the construction, especially the inner leaf, which may not be visible, must be known. Materials used in the inner leaf to provide good thermal insulation may be very poor as a material to fix into. Load testing prior to fixing may tell you the potential strength into this leaf but is itself an awkward job. For remedial wall ties techniques on these lines do exist. The fixing is usually made into the remote leaf first which, after the resin has cured, may be tested because the loads are low and unlikely to collapse the cavity. The bond with the outer leaf is then completed. This may not be possible with higher loads using long mesh sleeves. If both inner and outer leaf are bonded at the same time it is impossible to test the strength of the bond in the inner leaf. You also have to be careful not to provide a path for water/condensation (angled drilling may solve this) or cold bridging. The stiffness of an anchorage made across both leaves may also be a potential problem - check with an engineer.

 

4. POSITIONING ANCHORS IN BRICKWORK I've looked at your Guidance Note on brickwork and blockwork. This has useful guidelines on positioning, does this apply to resin anchors as well as expansion anchors or may they be relaxed for resins?

Yes, it does apply to resins and while resins may be preferable in brickwork because they do not impose expansion stresses it is difficult to relax the guidelines. In fact if we were writing that Guidance Note today we might even go further:

  • Anchors should only be set in structures strong enough to take the required loads i.e. for significant loads this means load bearing walls of solid (9") brickwork. Highly loaded anchors should not be set in non-structural walls e.g. parapets or under windows, without the permission of an engineer responsible for the structure. Anchors may only be set in the single leaf of a cavity construction for low loads.
  • No anchor should be located in a brick at the edge of a wall. This means anchors should be set at least 280mm from the edge of a wall or reveal. (Allow more if plastered or rendered.)
  • No anchor should be placed closer than 1000mm to the top of an unrestrained wall
  • No more than one anchor to be located in a single brick.
  • Anchors used to support significant loads should not be located in adjacent bricks unless they are set at an embedment depth sufficient to locate the main bond in the remote leaf.
  • No resin anchors to be located in mortar joints.

 

 

 

Fixings to Plasterboard

1. What is the most suitable fixing into plasterboard with a small cavity behind it?

This sounds like plasterboard set on dabs of plaster leaving a gap of only say 10mm. In this case conventional cavity fixings needing more free space may not work. Some universal fixings can work in this situation They work by tieing a knot in themselves behind the board. It may be necessary to drill into the structure behind the board initially with a masonry drill to give the back of the fixing and the screw somewhere to go. Alternatively if the substrate behind is brickwork a conventional screw and plug may be the answer with the plug set fully back into the main structure and the screw long enough to reach into it and expand it fully - take care not to pull the plasterboard back to the substrate. A frame fixing or hammer-in fixing may be a good solution as these will not tend to pull the plasterboard back so much.

Quoted load values

1. Values published by different manufactures for fixings of the same type and size seem to vary a lot. Why is this and how can I get a meaningful comparison?

There are two aspects to this. First make sure that you are comparing values on a like for like basis. Not only do different manufacturers use slightly different methods to test the performance of fixings, they sometimes use different methods to determine the allowable loads. The best comparison will be that given by values quoted in European Technical Approvals as these have been tested and assessed on the same basis.

In the absence of ETAs on one or both products being compared start by checking that the same test methods have been used. Comparing Mean Ultimate (average failure) loads is a useful starting point as these indicate the maximum load achievable but some manufacturers do not quote them preferring instead to quote the "Characteristic" load. This is the mean ultimate load less an allowance for the consistency of failure of the product and will always be somewhat lower than the mean ultimate so don't be tempted to compare "Ultimates" with "Characteristics". The safe working load (recommended load) will be determined by applying a safety approach, usually to the characteristic load. This approach used to be a "Global safety factor", typically three, but the more enlightened manufacturer will use a "Partial safety factor" approach as per the ETAG. In this approach different partial safety factors are applied to take into account different aspects of the fixing's performance in tests and aspects of the base material etc. It means safe working loads are more finely tuned to the way the fixing actually works giving the specifier more confidence in the allowable loads. The way Partial Safety Factors work compared to Global Safety Factors does mean that the quoted allowable loads will be different so find out on which basis they are quoted, see Guidance Note European Technical Approvals for construction fixings.

The other aspect - remember there are two - is that fixings which may look similar may in fact be very different. Many fixings have been copied from one very good idea. Sometimes the copy will be well developed and even better than the original and sometimes it will simply look like the original but not actually function properly at all. Anchors supplied by CFA members will all be thoroughly developed and tested and so can be relied upon. The best assurance is an independent assessment of the product's performance and the best of those is an ETA.

See Guidance Note -  European Technical Approvals for construction fixings.

 

2. STEEL GRADES AND PERFORMANCE. How does the grade of steel used to manufacture an anchor relate to the level of performance that the anchor can achieve?

The grade of steel will have been carefully chosen by the manufacturer to match the strength of the overall anchorage, so anchors capable of high loads will almost certainly be made from high tensile steel such as 8.8 or stronger. The choice of steel grade only becomes an issue when the user supplies the bolt or threaded rod which forms part of the finished anchorage. So with socket anchors for instance always use the strength grade quoted by the manufacturer. With most internally threaded socket anchors there is no point in using a stronger grade of bolt or rod as the shell or threaded component of the anchor will govern the quoted strength. With resin injection anchors it is particularly important to use rods supplied by the manufacturer if at all possible. If not - because a special length is needed for instance - be sure to get threaded rods of at least the same grade. Rods for capsule anchors should always be supplied by the manufacturer as they will usually have special features, such as chamfers and hexagon driving ends along with a depth mark to ensure correct installation.

Testing Fixings

1. I have been asked to arrange site tests of a specified fixing. Should these be to failure or some sort of proof load?

It depends on the purpose of the test.

If the objective is to check the suitability of a particular fixing
in a particular base material and the manufacturer cannot tell you or you need to determine the allowable loads in a material of unknown strength such as old brick or stonework then you need to test to failure. Carry out at least three tests to check suitability, five to find allowable loads. These tests may damage the structure so must be carried out away from areas where working anchors are to be fixed. If damage must be avoided then load fixings carefully aiming for a large safety margin above the applied load i.e. 4 times. This may still damage the structure but is less likely than going all out for failure loads.

If the objective is to check the quality of installation
of actual fixings used on the job then proof loads should be carried out. Limit test loads to between 1.25 and 1.5 times the applied load (or the manufacturer's recommended load if the applied load is not known at the time of tests) this will avoid damaging the anchorage in the base material but will show up any badly set fixings. A sample of at least 2.5% of the total fixings on the job should be tested and at least three. For safety critical applications it may be appropriate to start at a higher sample rate which can be reduced as confidence is established.

See Guidance Note:  Procedure for Site Testing Construction Fixings.

Plastic Anchors

1. PLASTIC AND NYLON? What is the difference in properties between nylon and plastic and how does this affect the performance of a fixing in different applications?

"Plastic" is a general term that includes "Nylon". If a fixing is referred to by the manufacturer as "Plastic" it is probably made of Polypropylene and not Nylon. If it is nylon it will probably say so or be referred to as a "Polyamide" or "PA6" . Polyamide material has several benefits over polypropylene which is why it costs more to manufacture and hence to buy. Nylon has a higher load capacity than polypropylene and is more durable in several ways, it exhibits less creep, has a wider service temperature range ( -40 to +80ÂșC) better UV resistance and, by virtue of being self extinguishing, is capable of better fire ratings than other plastic materials. There is a huge wealth of data on the long term performance of the particular polyamide (PA6) that has been used for fixings for several decades, this is why it is the most commonly used.

 

2. PLASTIC PLUGS - SCREW SIZES. What diamater/length should the screw used with a nylon or plastic plug be in relation to the diameter/length of the plug itself?

Both the length and diameter of the screw - along with the TYPE e.g. wood screw, self tapping etc. - should be specified by the manufacturer in their literature and on the packaging. Some manufacturers relate the plug designation to the screw size (the original fibre wall plugs introduced the first fixing system where the plug, the screw and the drill all had the same number, i.e. a No.10 screw went into a No. 10 plug in a hole drilled with a No. 10 drill bit. Drill bits are no longer identified in this way.) Others use the designation of the required drill diameter, i.e. a 10mm plug requires a 10mm drill hole but it will take a range of screw sizes, (the larger the stronger). It is therefore advisable to check the manufacturer's guidance prior to installation. To determine the correct length of the screw use the following simple equation:
Screw length = length of the plug + thickness of the part being fixed + the diameter of the screw.
It is always important to ensure that the screw penetrates the end of the plug in order for the plug to achieve its maximum level of expansion. This assumes the plug is set flush with the surface of the substrate, if it is set deeper then increase the screw length by the extra depth. Certainly if the screw size is not correct then the overall fixing will either be weak or you may have difficulty inserting the screw.

 

Anchor Selection

1. EDGE AND SPACING DISTANCES. What is the general principle behind the determination of edge / axial spacings for a specific anchor type? What is the most suitable anchor type to use when the axial / edge spacings are small? Is the performance of an anchor reduced if the axial / edge spacings are smaller than recommended by the manufacturer?

The key factor in determining the edge and axial spacing criteria for an anchor is the fact that any anchor type needs a certain amount of the base material around it a) to get a grip of the structure and b) to transmit the loads into the it. If a really good fixing is pulled from the concrete during a tensile test it will pull a cone of concrete out with it. For well designed expansion and undercut anchors this cone will develop from the base of the anchor. So, to ensure full performance, this cone has to be undisturbed. Edge distances and centre spacings designed to maintain full recommended loads are referred to as "Critical edge" and "Critical spacing" distances. Most anchor types may be used at closer distances referred to as "Minimum edge" and "Minimum spacing" distances. These are absolute minimum dimensions - you may not go closer. They may only be used with design techniques which also reduce the allowed loads. These performance criteria will have been determined by a comprehensive test programme. One type of expansion anchor, the "Drop-in" anchor, which is set using a hammer and special punch, exerts such significant shock loads into the concrete that edge distances may not be reduced below the "Critical edge distance". The anchor types best suited to small edge and spacing distances are resin anchors, and, to a lesser extent, undercut anchors. Resin anchors exert no setting stresses into the base material so may be set much closer than many other types but they still rely on a volume of base material to transfer the applied load so, as with all anchors, the manufacturer's data must be followed.