hilti technology manual

8/12/2019 HILTI Technology Manual

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Fastening Technology Manual


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Only the information and data given in the latest issue ofthe brochure are valid. The ascertained data only applieswhen Hilti products are used. All rights are reserved.Also, no extracts may be published or contents copiedwithout our expressed permission.


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Certified quality system

The quality of the products, software and services offered by Hilti and which is assured byour Group-wide quality management, covers our manufacturing, technical documentation,services and advice. In view of the fact that our quality system meets the stringentrequirement of ISO 9001 and the European directive EN 29001, the Swiss Association forQuality Assurance, SQS Berne, has issued the corresponding SQS certificate to Hilti. Inthis way, this independent association for quality inspection and surveillance has verifiedthe comprehensive quality system of the Hilti drilling and electric tools, DX, anchor and

construction chemicals divisions.

As a result of bilateral agreements between various national certification authorities, theSQS certificate is recognized by most European states.


Hilti Quality Systemcertified according to

Quality is a commitment


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Table of Contents

Pages

Anchor Technology ................................................................................... 1-28

Base material ............................................................................................................... 1 Why does an anchor hold in base material? ................................................................ 4

Setting anchors ............................................................................................................ 6

Loadbearing behaviour ................................................................................................ 8

Long-term behaviour .................................................................................................. 24

Corrosion................................................................................................ 29-63

How does Hilti solve the corrosion problem ............................................................... 30

Application examples / recommendations .............................................................. 30 Critical special applications .................................................................................... 39

Theory for consideration ............................................................................................ 41

Fundamental aspects of corrosion ......................................................................... 41

Types of corrosion and corrosion phenomena ....................................................... 45

Fastener protection against corrosion .................................................................... 51

Fire Prevention ....................................................................................... 65-72

Introduction ................................................................................................................ 65 Fire prevention regulations ......................................................................................... 67

Hilti fire prevention products ....................................................................................... 68

Product information .................................................................................................... 68

Testing/Inspection of products .................................................................................... 71

Anchor Fastening Design ..................................................................... 73-108

Safety concept ........................................................................................................... 73

Anchor fastening design ............................................................................................. 77 Influence of concrete compressive strength and direction of loading ........................ 79

Influence of depth of embedment .............................................................................. 82

Influence of edge ....................................................................................................... 85

Influence of distance between anchors ...................................................................... 88

Check for anchor breakage ........................................................................................ 91

Examples of applications ........................................................................................... 94

Computer programme for anchor fastening design .................................................. 103

Procedure for anchor fastening design .................................................................... 104

Summary of required formulae ................................................................................ 105

List of abbreviations ................................................................................................. 108


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Pages

Anchor Selection Table....................................................................... 109-112

Criteria Relevant to Safety .........................................................................113

Product Information ............................................................................ 114-198

HUC ......................................................................................................................... 114

HSL-TZ..................................................................................................................... 118

HSL-GR.................................................................................................................... 122

HSC ......................................................................................................................... 125

HST-R ....................................................................................................................... 130 HSA .......................................................................................................................... 134

HSA-K ...................................................................................................................... 139

HKD-S ...................................................................................................................... 145

HPS-1....................................................................................................................... 148

HRD-U...................................................................................................................... 150

HUD-1 ...................................................................................................................... 152

HRA ......................................................................................................................... 154

HY150-HAS(R)......................................................................................................... 156 HY150-HIS-(R)N ...................................................................................................... 160

New Design Method ...............................................................................................164

HDA.......................................................................................................................... 172

HVA-HAS(R) ............................................................................................................ 179

HVA-HIS-(R)N .......................................................................................................... 186

HVA-Rebar ............................................................................................................... 192

HY150 Rebar ........................................................................................................... 198

Appendix 1 list of test reports .......................................................... 199-200

Appendix 2 Application reference in Hong Kong ............................. 201-202

Hilti engineering / Testing services / Download pages ...............................203


Table of Contents


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1. Base material

The wide variety of building materials used today providedifferent anchoring conditions for anchors. There is hardly abase material in or to which a fastening cannot be made with aHilti product. However, the properties of the base material playa decisive role when selecting a suitable fastener/anchor anddetermining the load it can hold. Base materials have beendescribed comprehensively in brochure A1 (Base materials forfastenings).

The main building materials suitable for anchor fastenings have

been described in the following.

1.1 Concrete

Concrete is synthetic stone, consisting of a mixture of cement,aggregates and water, possibly also additives, which isproduced when the cement paste hardens and cures.Concrete has a relatively high compressive strength, but only alow tensile strength. Steel reinforcing bars are cast in concreteto take up tensile forces. This is then referred to as reinforcedconcrete.

b,D . .... . calculated compressive stress b,Z ...... calculated tensile stressf ct ...... concrete tensile strength

Different anchoring conditions

A mixture of cement, aggregatesand water

Cracking from bending

Stress and strainin sections withconditions I and II

Section a-a Section b-b


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Anchor Technology

If the tensile strength of concrete is exceeded, cracks form,which, as a rule, cannot be seen. Experience has shown that thecrack width does not exceed the figure regarded as admissible,w =~ 0.3 mm, if the concrete is under a constant load. If it issubjected predominately to forces of constraint, individualcracks can be wider if no additional reinforcement to restrict thewidth of cracks is provided in a concrete component. If aconcrete component is subjected to a bending load, the crackshave a wedge shape across the component cross-section andthey end close to the neutral axis. It is recommended thatanchor systems which have a follow-up expansion feature andare of the force-controlled type e.g. HSL-TZ, HST or undercut

anchor systems e.g. HUC and HSC, be used in the tension zoneof concrete components. Other types of anchors can be used ifthey are set at such a depth that their anchoring section ispositioned in the compression zone.

Anchors are set in both low-strength and high-strengthconcrete. Generally, the range of cylinder compressivestrength, fcc200, is between 20 and 50 N/mm

2. Expansionanchors should not be set in concrete which has not cured formore than seven days. If anchors are loaded immediately afterthey have been set, the loading capacity may only be taken tobe the actual strength of the concrete at that time. If an anchoris only set and then loaded later, the loading capacity can betaken to be the strength determined at the time of applying theload.

Cutting through reinforcement when drilling anchor holes mustbe avoided. If this is not possible, the design engineerresponsible must be consulted first.

1.2 Masonry

Masonry is a heterogeneous base material. The hole beingdrilled for an anchor can run into mortar joints and cavities.Owing to the relatively low strength of masonry, the loadstaken up locally cannot be particularly high. A tremendousvariety of types and shapes of masonry bricks are on themarket e.g. clay bricks, sand-lime bricks or concrete bricks, allof different shapes and either solid or with cavities. Hilti offers arange of different fastening solutions for this variety of masonrybase material e.g. HPS, HRD, HUD, HIT etc.

If there are doubts when selecting a fastener/anchor, your localHilti salesman will be pleased to give assistance.

If cracks in the tension zone exist,

suitable anchor systems arerequired.

Observe curing of concrete whenusing expansion anchors.

Avoid cutting reinforcement.

Different types and shapes

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When making a fastening, care must be taken to ensure that a

layer of insulation or plaster is not used as the base material.The specified depth of embedment (anchoring depth) must bein the actual base material.

1.3 Other base materials

Gas concrete: this is manufactured from fine-grained sand asthe aggregate, lime and/or cement as the binding agent, waterand aluminium as the gas-forming agent. The density isbetween 0.4 and 0.8 kg/dm3 and the compressive strength 2-6N/mm2. Hilti offers the HGN and HRD-G anchors for this basematerial.

Lightweight concrete: this is concrete which has a low density 1800 kg/dm3 and a porosity which reduces the strength ofthe concrete and thus the loading capacity of an anchor. Hiltioffers the HRD, HUD, HIT, etc anchor systems for this basematerial.

Plasticboard/gypsum panels: these are mostly non-supporting

building components, such as wall and ceiling panels, to whichless important fastenings are made. The Hilti anchors suitablefor this material are the HLD and HHD.

In addition to the previously named building materials, a largevariety of others e.g. natural stone etc, can be encountered inpractice. Furthermore, special building components are alsomade from the previously mentioned materials which, becauseof the manufacturing method and configuration, then result inbase materials whose peculiarities must be given carefulattention e.g. hollow ceiling floor components etc.

Descriptions and explanations of each of these would gobeyond the bounds of this manual. Generally though,fastenings can be made to these materials. In some cases, testreports exist for these special materials. It is alsorecommended that a discussion be held in each case by thedesign engineer, company carrying out the work and Hiltitechnical staff.

In some cases, testing on the jobsite should be arranged to

provide proof of the suitability and the loading capacity of theselected anchor fastener.

A plaster coating is not a base

material for fastenings.

Gas concrete

Lightweight concrete

Plasterboard/gypsum panels

Variety of base materials

Jobsite tests


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Anchor Technology

2. Why does an anchor hold in base material?

There are three basic working principles which make an anchorhold in a building material:

Friction

Keying

Bonding

Combination of working principles

Many anchors obtain their holding power from a combinationof the above-mentioned working principles.

For example, an expansion force is exerted by an anchoragainst its hole wall as a result of the displacement of a conerelative to a sleeve. This permits the longitudinal force to betransmitted to the anchor by friction. At the same time, thisexpansion force causes permanent local deformation of thebase material, above all in the case of metal anchors. A keyingaction results which enables the longitudinal force in theanchor to be transmitted additionally to the base material.

Friction

Keying

Bonding

Combination of workingprinciples

The tensile load, N, istransferred to the basematerial by friction, R. Theexpansion force, Fexp , isnecessary for this to takeplace. It is produced, for

example, by driving in anexpansion plug (HKD).

The tensile load, N, is inequilibrium with thesupporting forces, R,acting on the basematerial, such as with theHUC anchor.

An adhesive bond isproduced between theanchor rod and the holewall by a synthetic resinadhesive, such as with theHVA anchor.

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In the case of expansion anchors, a distinction is madebetween force-controlled and movement-controlled types. The

expansion force of force-controlled expansion anchors isdependent on the tensile force in the anchor (HSL heavy-dutyanchor). This tensile force is produced, and thus controlled,when the tightening torque is applied to expand the anchor.

In the case of movement-controlled types, expansion takesplace over a distance which is fixed by the geometry of theanchor in the expanded state. Thus an expansion force isproduced (HKD anchor) which is governed by the modulus ofelasticity of the base material.

The synthetic resin of an adhesive anchor infiltrates into thepores of the base material and, after it has hardened andcured, achieves a local keying action in addition to the bond.

Force-controlled and movement-controlled expansion anchors

Adhesive/resin anchor


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Anchor Technology

3. Setting of anchors

The user of an anchor system should receive instruction on how to setanchors in the case of anchor fastenings where safety is at stake. (Themanufacturers instructions or those of the approval authority must beobserved.)

Generally, anchors are set, or anchor fastenings are made, in thefollowing way.

Mostly, the anchor hole is drilled using a rotary hammer or cam-actiondrill and carbide-tipped drill bits. Hilti supplies matched programmes

of rotary hammers and drill bits as well as diamond core bits anddrilling rigs which are suitable for the anchors in each case. Steeldetectors can be used to indicate the position of reinforcing barsbefore anchor holes are drilled. If the user comes up against areinforcing bar with a carbide-tipped drill bit (wrongly positioned hole),he will notice this because of the slower drilling progress, greatervibration and, possibly, slipping of the safety clutch of the rotaryhammer drill. Drilling should then cease to avoid any damage to thebuilding component and to protect the drill bit. If the engineerresponsible for the structure permits the reinforcing bar to be cutthrough, as an exception, diamond bits will cut through the

reinforcement cleanly.

The depth of hole required for each anchor can be found in the settingdetails given in information about the anchors. In particular, this depthmust be kept to in the case of anchors set flush with the work surface,such as the HKD anchor, HSC safety anchor and the HVA adhesiveanchor. The hole may be drilled deeper without second thoughts andwithout influencing the loadbearing behaviour in the case of anchorswhich are correctly positioned automatically by the bolt head or nutwhen they are inserted into the hole.

Perfect functioning of an anchor will be ensured if its hole is carefullycleaned to remove dust and fragments, for instance by using a jet ofair from a suitable source.

An anchor can be set either before the part to be fastened is put intoplace by presetting the anchor or after the part is in place by so-calledthrough-fastening. In the latter case, the hole in the base material isdrilled through the predrilled and correctly positioned part to befastened, the anchor is inserted through this part into the basematerial and then expanded.

If chemical anchors are used, allowance must be made for the curingtime before a tightening torque or a working load can be applied.

Producing the anchor hole

Hole depth

Cleaning of hole

Setting the anchor

Curing time

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The operations for making most anchor fastenings are finished

when the nut or bolt is tightened. The applied tightening torque

is converted to a prestressing force (preload) in the anchorwhich pulls the part being fastened tightly against the base

material (clamping force).

Wrongly positioned holes are those which cannot be drilled to

the required depth bacasue the drill bit comes into contact with

a reinforcing bar or the hole is produced in the wrong place.

As a recommendation, new holes for adhesive anchors,

undercut anchors and force-controlled metal expansion

anchors should be drilled at a distance 3 x d (drill bit nominaldiameter) away from the wrong holes, provided that these havebeen filled with repair mortar.

If the wrong hole has not been filled with repair mortar, the new

hole should be drilled 2 x the depth of the wrong hole awayfrom it.

If the reinforcement has been damaged or destroyed, design

evidence must be provided to the effect that the reduction in

the loadbearing capacity of the building component can be

accepted.

Tightening the anchor nut or bolt

Wrongly positioned holes


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Anchor Technology

4. Loadbearing behaviour

4.1 Type of loading

The type (direction) of loading is defined by the angle, , whichis formed by the longitudinal axis of the anchor and the

direction of the applied load.

When under a shear or combined load, the anchor is also

stressed in bending because the point of load application is

outside the point where the anchor is clamped in the base

material.

Compressive load on base material in section Z-Z

Bending moment M, relative to the Y-Y axis

No bending moment, M, must be allowed with the standard

fastenable thicknesses (thickness of the part to be fastened)

given in the information about the anchors because the

recommended shear or combined load was determined during

tests using just this thickness of the part to be fastened.

Direction of loading

Point of load application

Allowance for bending momentnot necessary with standardfastenable thicknesses

Accordingly, the load is pure

tension when = 0, pureshear when = 90 and a

combined load (inclinedtension) at 0 < < 90.

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When selecting the size of anchor for a stand-off fastening

subjected to a shear load, V, or a combined load, F, the

bending moment, M, must be allowed for.

(The bending arm is the distance from the point of load

application to the surface of the supporting base material plus

the diameter of the anchor bolt/rod)

4.2 Mode of loading

The working load acting on an anchor can be a sustained static

load or a load which varies with time. In design work, a

distinction is made between a predominantly dead load and

Stand-off fastening

Sustained static load

Pulsating load

Alternating load

Load a not predominantly dead load

i.e. a dynamic load. If a load

only varies in the tensile range

or only in the compressive

range, it is referred to as a

pulsating load .

If a load varies in both thetensile and compressive

ranges, it is referred to as an

alternating load .

A shock load is characterised

by a rapid loading rate and a

short time in which the load

acts of only a few

milliseconds.

Dynamic loads can be caused

by, for example:

- machine foundations

- crane rails

- bridges

- pipelines

- railway tracks.

Static load

Pulsating load

Alternating

Time

Load

Load

Time

Time


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Anchor Technology

4.3 Load-displacement behaviour

If an anchor fastening has been correctly designed and properly

made, it displays an essentially elastic behaviour under the

working load. When the load is applied to the anchor fastening andremvoed from it, the displacement is reversible. If the working load

is exceeded, the anchor fastening must not fail with a brittle

behaviour.

An advantage is the type of behaviour where a large amount of

displacement takes place in the range of the ultimate load. Thus, in

the case of force-controlled expansion anchors, the visible re-

expansion, or follow-up expansion, under a tensile load indicates

that the working load of the force-controlled expansion anchor has

been exceeded. This is why these anchors are referred to as safety

anchors.

The following fundamental load-displacement behaviour exists

independent of the type of loading (direction of the load) or the

type of anchor used:

Load-displacement behaviour

The gradient, B, depends on the stiffness (rigidity) of the anchor.

Hence, an anchor which has a short rod/bolt or a shallow

anchoring depth, for example, will have a steeper characteristic

curve than an anchor which has a long rod/bolt and a large depth

of embedment.

The prestressing force, P, which is set up when the anchor is set or

the part to be fastened is put into place, has a major influence on

the load-displacement behaviour of an anchor when it is

subsequently loaded. After an anchor has been prestressed once to

Po ,it has an elastic behaviour up to this level on being reloaded. Theload, NE represents a kind of limit to elasticity. It corresponds to the

prestressing force Po applied when the anchor is set. Proportions of

plastic displacement, such as expansion movement of force-

controlled expansion anchors or settlement of key-action anchors

Elastic behaviour under working

load

Beyond the working load

Prestressing force influencesload-displacement behaviour

Load

Ultimate load

Overloading

Working loadDisplacement

Permanentdisplacement

Reversibledisplacement

NE

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are taken up in advance by the prestressing process.

It can thus be said that a load which is smaller than the existing

prestressing force only results in slight additional loading of theanchor and produces no displacement worth mentioning.

This statement has been explained and depicted in detail in the

following. To this end, the entire system, consisting of anchor,

base material and part fastened, must be looked at more

closely.

The prestressing force, P, the clamping force resulting from it,

Fcl , which presses the part fastened against the base material,

and the external working load applied to the part fastened, NA,all act in the complete system.

The following figs. 1-6 show the load-deformation behaviour

when the anchor is prestressed and the external working load

is subsequently applied.

Consideration of entire system

Fig. 1:

This shows the system of base material

and anchor represented by different elastic

springs (unloaded state) base material

(concrete) anchor (steel)

Fig. 2:

Load-deformation characteristic for steel

and concrete

Steel: elastic spring - large deformation

Concrete: Stiff spring - small deformation

anchor (steel)

Deformation

base material(concrete)

Load

concrete

stee

l


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Anchor Technology

Fig. 3:

The required tightening torque (prestressing

force) is applied. As a result:

The anchor steel elongates, ls The concrete is compresses, lc The bolt/rod is prestressed, P = Fs A clamping force, Fcl , builds up between

base material and part fastened.

Fig. 4:

The prestressing force causes the bolt/rod

to elongate, ls. At the same time, aclamping force, Fcl , is produced.

Bolt/rod elongationthrough external load

Fig. 5:An external load, NA, is applied to the part

fastened:

The prestressing force, Fs,increases and the bolt/rod

elongates further, ls *. The clamping force, Fcl , decreases,

so does compression of the base

material.

Fig. 6:This is the load-displacement behaviour

after applying the external load, NA.

This load causes further elongation of the

bolt/rod and a reduction in the clamping

force, Fc. It is then zero (Fcl = 0), when the

additional bolt/rod elongation is equal to

the concrete compression, lc, caused bythe pretensioning force, P.

ltot. = ls + ls*

Fs = P + Fs = Fcl + NA.

clampingforce

Bolt/rod elongationthrough prestressing

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Anchor Technology

4.4 Failure modes

4.4.1 Effects of static loading

The failure patterns of anchor fastenings subjected to a

continually increased load can be depicted as follows:

The weakest point in an anchor fastening determines the

cause of failure. Modes of failure, 1, break out, 2, anchor pull-

out, 3, failure of anchor parts, occur mostly when single

anchors at a suitable distance from an edge or the next anchor

are subjected to a pure tensile load. These causes of failure

govern the max. loading capacity of anchors. On the other

hand, a small edge distance causes the modes of failure 4,

edge break, and 5, splitting of building components. The

ultimate loads are then smaller than those of the previously

mentioned modes of failure. The tensile strength of the base

material for the fastening is exceeded in the cases of break out,edge break and splitting.

Basically, the same modes of failure take place under a

combined load. The mode of failure 1, break out, becomes

more seldom as the angle between the direction of the applied

load and the anchor axis increases.

Generally, a shear load causes a conchoidal area of spall on

one side of the anchor hole and, subsequently, the anchor

parts suffer a bending tension failure or shear break. If the

distance from an edge is small and the shear load is towards

the free edge of a building component. However, the edge

break away.

Failure patterns

Causes of failure

Combined load

Shear load

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Anchor Technology

The width of a crack in a concrete component has a major

influence on the tensile loading capacity of all fasteners, not

only anchors, but also inserts, such as cast-in headed studs. A

crack width of about 0.3mm is assumed when designing

anchor fastenings. The reduction factor which can be used for

the ultimate tensile loads of anchor fastenings made in cracked

concrete compared to uncracked concrete can be assumed to

be 0.6 - 0.65 for the HSL-TZ, for example, or 0.65 - 0.70 for the

HSC and HUC anchors, for instance. Larger reduction factors

for ultimate tensile loads must be anticipated (used in

calculations) in the case of all those anchors which were set in

the past without any consideration of the above-mentioned

influence of cracks. In this respect, the safety factor to allow forconcrete failure when the concrete is cracked is not the same

as the figure given in product information i.e. all previous

figures in the old anchor manual. This is an unacceptable

situation which is being eliminated by carrying out specific

tests with anchors set in cracked concrete and adding suitable

information to the product descriptions.

Cracks in concrete have no influence on the ultimate shear

loads worth mentioning.

Since international testing conditions for anchors are being

based on the above-mentioned crack widths, no theoretical

relationship between ultimate tensile loads and different crack

widths has been given.

The statements made above apply primarily to static loading

conditions. If the loading is dynamic, the clamping force and

prestressing force in an anchor bolt/rod play a major role, as

described in section 4.3. If a crack propagates in a reinforeced

concrete component after an anchor has been set, it must be

assumed that the prestressing force in the anchor will decreaseand, as a result, the clamping force of the part fastened will be

reduced (lost). The properties of this fastening for dynamic

loading will have deteriorated. To ensure that an anchor

fastening remains suitable for dynamic loading even after

cracks appear in the concrete, care must be taken that the

clamping force and prestressing force in the anchor are

maintained. Suitable measures to this effect can be sets of

springs or similar devices. In this respect, reference shuld be

made to section 5 because also the Relaxation of prestressingforceplays a role.

Reduction factor for crackedconcrete

Prestressing force in anchorbolts/rods

Loss of prestressing force due tocracks

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4.6 Influence of type of loading

Static loading tests have provided the basis for determining

the influence of the type of load i.e. the direction of the applied

load. These tests were carried out using different anchor

systems and different angles of load application.

Different types of anchors have different ultimate tensile and

shear loads and modes of failure owing to their different

designs. There can also be different causes of failure

depending on the direction of loading.

Ultimate loads under various types of loading, taking a safety anchor (HSC-A

M8*40) set away from component edges as an example.

As a rule, a combined load acts at an angle of 90 0. Therelationship between the recommended load and the angle of

load application has been presented in product information

sheets in the form of a so-called interaction diagram.

Influence of direction of appliedload

Modes of failure depending on

type of loading

O

30,0

27,5

25,0

22,5

20,0

17,5

0 10 20 30 40 6050 70 80 90 Angle []

Ultimate loads [kN]

Bending tension failure underinclined tensile loading

Shear failure under lateralloading


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4.7 Influence of concrete strength

If the concrete base material has a low strength, an anchorfastening subjected to pure tensile loading nearly always fails

because a cone of concrete breaks out.

As the strength of the concrete increases so does the ultimate

load of the anchor fastening unless the strength of the anchor

material is exceeded. If the anchor does not break, the ultimate

load increases proportionally with the tensile strength of the

concrete. Usually, however, the strength of concrete is given asthe compressive strength. It is therefore obvious that the

ultimate load, Nu , of an anchor fastening is given in relation to

the concrete compressive strength.

Influence of concrete compressive strength on ultimate load of anchor

The relationship between the concrete compressive strength

and the characteristic ultimate load of the anchor fastening i.e.

5% fractile value from tests, is given in the brochure B2,

Anchor fastening design.

Concrete breakage

Anchor break

Ultimate load, Nu

Concr

etebr

eaka

ge

Anchor breakage

Compressive strength, fcApplication range

Crater-like breakage of concrete with anchor under tensile load

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A shear load acting towards the edge of a building component

has a particularly unfavourable influence on the loadbearing

capacity of an anchor fastening. If anchor fastenings are made

near an edge, where c < ccr , reinforcements should be

provided in the component edge at the level of the anchoring

depth. This should be at least 0.25 times the anchor load (for

all types of loading). The admissible stress in the steel, s,adm ,should be used in the design calculation.

4.10 Multiple-anchor fastenings

If the load to be carried is distributed among several anchors

by a fastened part, such as a bracket, reference is made to a

multiple-anchor fastening.

The distance between anchors, scr , is the distance at whichthe concrete that breaks out with a single anchor without

influencing the neighbouring anchors. If the anchoring spacing

is equal to or less than scr , the ultimate load of the single

anchor will be reduced because the cones of concrete which

break out with the anchors overlap each other.

Shear load in direction ofbuilding component edge

Multiple-anchor fastenings

Small anchor spacings reduceultimate loads of single anchors.

Multiple-anchor fastening


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Break-out cones of concrete influencing each other

The anchor spacing, scr , at which the concrete can break away

with an anchor without influence from another anchor, isdiameter, d. This is approx. 3.5 times the anchoring depth, hef, for metal expansion anchors and undercut anchors, but 1.5

times the anchoring depth for adhesive/resin anchors.

Influence of distance between anchors on ultimate load of anchor

If the anchor spacing which is less than the specified min.distance, smin, is used, the concrete can already be destroyed

when the anchor is set, either because of cracks which run

from one anchor to another when a force-controlled anchor is

expanded or by local destruction of the concrete when the

tightening torque is applied to force-controlled anchors. In

view of this, there should be no reduction in the min. anchor

spacing which, depending on the type of anchor, is between

0.5 and 2 times the anchoring depth. If, however, small anchor

spacings are unavoidable, a solution can be found in individual

cases by setting neighbouring anchors at different depths.

Before doing so, however, advice should be obtainedfrom Hilti.

Anchor spacing for undisturbedbreak-out of concrete with

anchor

Reduction of min. anchorspacing

Ultimate load, Nu

Smin ScrZone of influence

Distance betweenanchors, s

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If several multiple-anchor fastenings are made next to each

other on a building component and no shear reinforcement has

been provided to take up the additional load resulting from

them, the distance between each multiple-anchor fastening, ac=~ 2 scr , must be observed.

4.11 Influence of reinforcement

The tests to determine the application conditions were carried

out using unreinforced concrete components. The results are

thus on the safe side because, in practice, concrete structures

are reinforced. It is known from research work thatreinforcement has no significant influence on the ultimate loads

of anchor fastenings provided that this reinforcement is not

specifically positioned stirrup-type reinforcement. The

reinforcement can have a positive effect in that, for example,

the width of cracks is kept small or the sudden (brittle) breaking

of component edges is avoided. Similarly, stirrups, coils or

close-mesh surface reinforcement positioned away from edges

can have a favourable effect on the load-displacement

behaviour of an anchor fastening. No generally valid figures for

the influence of reinforcement can be given, however, because

of the many varying factors, such as type, amount, and

positioning of the reinforcement as well as the position of the

anchor relative to the reinforcement.

Multiple-anchor fastenings nextto each other need shear

reinforcement.


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Immediately after a load is applied to an anchor, a relatively

large amount of creep takes place. In this time, the stresses inrelatively highly loaded points of the concrete decrease, the

stress distribution changes and the creep stabilises. The

influence of the magnitude of the load is shown clearly by the

results of long-term static loading tests after 100 days of

loading in each case at different load levels.

Influence of magnitude of load on creep of M8 metal anchor loaded for 100

days

If the sustained load exceeds a certain level, the creep

progresses until the fastening fails. The loads recommended

by Hilti, Nrec , have been set in a range according to the current

level of knowledge, however, in which creep has a negligible

effect on an anchor fastening.

The influence of the creep of concrete on the prestressing

force in the anchor bolt/rod has been described in the following

section 5.2.

5.2 Dynamic effects

This subject has been discussed in detail in the brochure A2,

Factors influencing fasteners.

The fatigue of a material can already occur at relatively low

loads if it is exposed to sustained dynamic loading. According

to experience, the concrete is never affected, but always the

steel. The number of load cycles which produces failure isprimarily dependent on the stress amplitude, a , i.e. the max.to min. stress.

Creep of concrete

Material fatigue

Creep after

100 days (mm)

3

1.5

2.5

2

1

0.5

00

0.5 1 1.5 2 2.5 3

Load, relativeto max. workingload, Nrec


Anchor Technology

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Tests have been shown that the measured loss of prestressing

force in anchors/fasteners of different anchoring principles

such as cast-in headed studs, undercut anchors and

expansion anchors, is comparable with respect to time.

In practice, this means that the remaining prestressing force

after a considerable time is only 30 - 40% of the initial figure.

One way of making up the lost pretensioning force to a certain

extent is to retighten the anchor. Anchors which have been set

in not fully cured concrete should be retightened when the

concrete has cured because (fresh) concrete displays a greater

amount of creep.

The statements made above only apply to uncracked concrete.

If the concrete is cracked, it must be anticipated that the

prestressing force in the anchor bolt/rod will decrease even

more because of a widening crack.

The prestressing force in an anchor set in cracked concrete

can only be maintained by taking special measures, such as by

using tensioning components (springs). Care must be taken in

this case that the anchors always remain accessible.

5.3 Corrosion

Protection against corrosion

Anchors corrode (rust) mostly on the part protruding from the

base material. All anchor parts made of metal, which is not

stainless steel, are protected against corrosion e.g.

galvanizing.

If no regulations enforced by authorities have to be observed,

the following recommendations can be made for long-lasting

fastenings:

Small reduction in prestressingforce due to retightening ofanchor

Pretensioning force in crackedconcrete

Application conditions Protection against corrosion

galvanizing to 5 - 10 microns

hot-dip galvanizing to 45 microns

stainless steel

(austenitic CrNi steel)

inside rooms without particular exposure to dampness

with sufficient concrete coverage

inside applications in damp rooms with occasionalcondensation and in coastal area

outside applications with only slight atmospheric pollution

inside applications with only slight atmospheric pollution

outside applications with very corrosive atmospheric pollution


Anchor Technology

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Avoidable damage by corrosion

Hilti product quality

Many years of experiencethrough ongoing studies

Applied research

Introduction

Roughly a fifth of the worlds annual steel production is needed

for the replacement of steel components damaged bycorrosion. Most of this damge could be avoided in the light ofthe current level of technology. This means that greatimportance must be attached to selecting suitable protectionagainst corrosion, among other things the use of a specialmaterial, and this is the more economical apporoach in thelong term. Higher initial costs are soon compensated in mostcases by longer life, reduced surveillance and less repair work.

Where fastening systems are concerned, safety aspects are

most important in addition to the economic implications. thesafety requirements, which have become more stringent inrecent years, and the latest research findings are taken intoaccount when Hilti develops new products.

Series of tests and experiments are carried out to uphold ahigh level of product quality and to coninually improveproducts.

The usual short-term tests, such as the salt spray test,alternating climate test (condensation test) or Kesternich test

are most suitable for quality control purposes, but they do notallow conclusions to be drawn directly about behaviour inpractice. In view of this, Hilti products are additionallysubjected to stiff free weathering tests. Today, experiencefrom up to 13 years of weathering in three different climaticzones is available. The weathering locations are in Schaan(rural atmosphere), Rouen (industrial atmosphere) and LeHavre (marine atmosphere). These studies have helped toprovide insight into the way corrosion attacks fastenings.This is an outset requirement for developing optimizedprotection against corrosion. In addition to these studies,existing fastenings are regularly examined which can beviewed as a kind of long-term quality assurance.

The protection against corrosion of special fastenings ischecked during specific field tests.

Conditions in road tunnels have been investigated by the HiltiCorporation in the Mt. Blanc Tunnel in cooperation with theSwiss Federal Institute of Technology, Zurich. A range ofmaterials highly resistant to corrosion were subjected to

these conditions in addition to the classicalstainless steels1.4305 (A1), 1.4391 (A2) and 1.4401 (A4).

Further field tests are also being run in road tunnels inSwitzerland and on power plant chimney stacks in Germany.

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Corrosion


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Corrosion

A) How has Hilti solved the corrosion

problem in practice?1. Application examples/recommendations

Generally, the following recommendations can be given forselection of the right protection against corrosion for

fastenings.

In the following, a detailed guide for selection of the right

protection against corrosion for fasteners has been given forspecific applications on the basis of commented examples.

The selected applications have been arranged according tothe following structure:

Building construction General construction/finishing Cladding/Roofing

Building services (house and building installations) Plumbing, heating, air-conditioning and ventilation Industrial installations Electrical installations

Non-building construction/civil engineeringRoad construction/bridge buildingTunnel constructionWaterway construction/dock and harbour installations

Special construction Industry/chemical industry Power plants Chimney stacks/waste incineration plants

Waste water treatment plants Parking buildings Indoor swimming pools Stadiums

Surrounding conditions Protection

Inside rooms without particular influence of moisture Galvanized5 10 microns

If covering of concrete is sufficient

Fastenings in damp inside rooms with occasional Hot-dip galvanizedexposure to condensation and in coastal vicinity 45 microns

Fissaggi allaperto in atmosfera poco aggressiva

Inside fastenings exposed to heavy condensation Stainless steel(Austenitic Cr Ni

steel)

Outside fastenings in corrosive atmosphere

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Cladding/Roofing

Profile metal sheetingCurtain wall claddingFastening of insulating materialFraming of cladding

Rural, small-townatmospherewihout industrial emissions,

low SO2 content

High-alpine atmosphereslight air pollution, lowtemperatures

Town atmospherehigh SO2 e NOx pollution,chlorides from road salt canaccumulate on not directlyweaterhed parts

Industrial atmospherevery high SO2 pollution,under circumstancesadditional corrosivesubstances

Coastal atmospherehigh chloride content, undercircumstances combined

with industrial emissions

Not directly weathered fasteners e.g. behindcurtain wall cladding, or exposed to the risk ofcorrosion if chlorides (road salt) can accumulate. Ifaccompanied by a high SO2 concentration, evenstainless steel can suffer corrosion.

Inside fastening

Outside fastening

Insulation material

Inside fastening

Outside fastening

Insulation material

Inside fastening

Outside fastening

Insulation material

Inside fastening

Outside fastening

Insulation material

Application ConditionsBuildingconstruction

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Plumbing, heating,air-conditioning andventilationPipescable trays

Air ducts

Electrical installationsCable traysLightingAerials

Industrialinstallations

Crane installationsBarriers/fencesConveying equipmentMachine fastening

Dry inside rooms, heated, no condensation

Damp inside rooms, poorly ventilated rooms,

cellars/basements, shafts/conduits, occasionalcondensation from highly humidity andtemperature fluctuations

Frequent or long-lasting condensation e.g. green-houses;not closed, half-open inside rooms, open sheds

Dry inside rooms, heated, no condensation

Damp inside rooms, poorly ventilated rooms,cellars/basements, shafts/conduits, occasionalcondensation from highly humidity andtemperature fluctuations

Frequent or long-lasting condensation e.g. green-houses;not closed, half-open inside rooms, open sheds

Buildingservices

Application Conditions

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Buildingconstruction/Bridgebuilding

Traffic signsCrash barriersConnecting structures

Tunnel construction

Sheeting in tunnelsReinforcing mesh for sprayedconcreteTraffic signsSupply linesAir ductsLighting

Directly weathered

Chlorides washed off directly weathered partsby rain

Frequent heavy exposure to road salt If drying is poor Heavy exposure to chlorides If safety requirements are stringent

Civil eng.Application Conditions

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Waterways & harbourinstallations

Fastenings on quay walls,harbour and dock installations

Off shore

Drilling rigs

High humidity, dampness etc.

Chlorides

Frequently in combination withindustrial atmosphere

On rigs

Under water: Pay attention to effects ofcathodic protection system of rig!

Fastening of pipes etc. in concrete tanks(alternating oil - sea water contents)

Fastenings in diret contact with conveyedmedium (gas, oil, salt water)

Application ConditionsCivil eng.

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Industry/Chemicalindustry

Power plants

Dry inside rooms

Corrosive inside rooms (fastenings inlaboratories, steelworks, plating plants etc.)

Very corrosive vapours

Outside fasteningsVery high SO2 pollutionAdditional corrosive substances

Chemical industry

Extremely stringent safety requirements andlong-life stainless steel

For fastenings where high risk is involved

Application ConditionSpecialstructures

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Chimney stacks ofpower plants andwaste incinerationplants

LaddersPlatformsStepsLightning conductors

Waste watertreatment plants

In lower area

In exit areaConcentration of acids at exit, often highchloride content in fossil power plants

Applications in the atmosphere:high humidity, digester/sewage gas

Underwater applications:- Community waste water- Industrial waste water

The following applies to electrical insulation:

Note:

Contact between the fastener/anchor andconcrete reinforcement must be preventedotherwise contact corrosion can rsult becausewater purification tanks have common

grounds/earths

Application ConditionsSpecialstructures

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Parking buildings

Indoor swimmingpools

Stadiums

Heavy contamination with chlorides (road salt)brought in by vehicles, many wet-and-dry cycles

In rural atmosphere

In town atmosphere

- Accessible fastenings which can bechecked, such as for seating

- Fastenings where safety is at stake andthey cannot be checked e.g. in roofs

Application ConditionsSpecialstructures

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2. Critical special applications

Indoor swimming pools, leisure centres:

Various research programmes are being run to evaluate thesuitability of steels for fastenings in indoor swimming pools. Itcan be said from the start that the Ni alloys of the Hastelloygroup and titanium are suitable, based on the extensiveexperience of the chemical industry.

At the time of printing this section of the manual, nothingdefinite can be said about which of the steels having a highmolybdenum content, such as the materials 1.4539 or

1.4529, can withstand expsure to corrosive surroundngs. Inthe case of an actual fastening, it is recommended that thepossibility of supplying special solutions should be checkedwith the Hilti engineer responsible.

Road tunnels:

Typical of conditions in road tunnels are heavy depositshaving a high chloride content, high humidity and, frequently,condensation. The film of moisture on metal surfaces is veryoften acid. Depending on the length of a tunnel, the traffic

frequency, the number of tunnel conduits etc., steelscontaining approx. 4.5% Mo will be sufficient, but steelscontaining 6% Mo (material 1.4529) should be used forfasteners when safety is at stake. These steels are regardedas being sufficiently resistant to stress corrosion cracking.

The conditions existing in road tunnels are being studied inthe Mt. Blanc Tunnel by the Hilti Corporation in cooperationwith the Swiss Federal Institute for Technology. A range ofmaterials highly resistant to corrosion have been included inthis study in addition to the classicalstainless steels 1.4305(A1), 1.4301 (A2) AND 1.4401 (A4). After only 11 months ofexposure, heavy pitting and crevice corrosion could beobserved on the CrNi steels (A1 and A2) as well as on theCrNiMo steels containing 2 - 3% Mo (A4). The steelscontaining approx. 4.5% Mo withstood the conditionsconsiderably better, but they too were not free fromcorrosion.

Only the CrNi steels containing 6% Mo, the nickel alloys andtitanium proved resistant, even to stress corrosion cracking(see brochure B.4 for assessments)

Fasteners of this material have been supplied by Hilti forrepairs in the Mt. Blanc Tunnel.


Corrosion

39


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Corrosion

Chemical industry:

Ni alloys from the Hastelloy group are suitable for specialapplications in the chemical industry e.g. in acid depots.

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B) Theory for consideration

1. Fundamentals of corrosion

1.1 Main corrosion terms and definitions

The term corrosion has many different meanings for thelayman. They include rust and the damage which corrosioncauses. To standardize corrosion terminology, the main termshave been simplify (DIN 50900, ISO 8044). These main termshave been explained in the following, taking a fastening asthe basis. Generally, a fastening consists of the part to be

secured e.g. a bracket, the fastener itself, e.g. an anchor, andthe base material in which the fastener holds e.g. concrete. Itis sufficient to review the properties of these components foran evaluation of the loadbearing behaviour.

When evaluating the resistance to corrosion, thesurroundings must also be taken into account, such as theatmosphere, rain, dust etc. in our example. The constituentsof the surroundings influencing the fastening make up what iscalled the corrosive medium. They can be, for example, a filmof moisture including salts from dust deposits, condensation

or also the alkaline system, also termed corrosion system,covers, by definition, all involved metal parts and allconstituents of the corrosive medium whose propertiesinfluence corrosion.

The combination of the properties of metal parts and thecorrosive medium in the corrosion system decides whetheror not corrosion can take place.

When deciding on the material or the protection againstcorrosion for metal parts - those of the medium are mostly

given in adance in the construction industry - allowance mustbe made for the system requirements regarding functioning,safety, life and also appearance. In many cases, it is notnecessary for corrosion to be stopped completely, but onlyfor it to be reduced to an acceptable amount.

Corrosion is defined as the reaction of a metal with itssurroundings which causes a measurable change to themetal and can result in impairment of the functioning of ametal component or an entire system.

What constitutes a fastening

Evaluation of resistance tocorrosion

Corrosive medium

Corrosive system

Selection of materials to suitrequirements

Corrosion


Corrosion

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Corrosion

The properties of the surrounding medium play a major rolewhere the risk of corrosion in a system is concerned.Generally, the constituents of the medium can be readilydetermined and defined in most cases if it is present involume, for example, water, chemicals etc. In the case offilm-like electrolytes, such as those which occur when dewforms in the atmosphere, this can only be done with a greatdeal of outlay. Consequently, a way round this is searched forusing indirect methods and on the basis of practicalexperience by putting atmospheres into categories:

Rural atmosphere Town atmosphere Industrial atmosphere and A marine climate

These categories are only an aid though, because localconditions can differ very widely, depending on:

Design features e.g. single of double-skin cladding, curtainwall cladding, the height of a structure

Environmental conditions e.g. the formation of deposits,the frequency of the action of rain etc.

The location of the structure e.g. protected from the wind.

Recently, therefore, the classification of corrosiveness of theatmosphere is carried out primarily on the basis of theduration of moisture exposure as well as the SO2 andchloride pollution. The range of atmospheres which can beevaluated in this way begins with air-conditioned insiderooms and extends to tropical outside climates where there isserious air pollution.

1.4 How does corrosion take place?

A distinction is made between three types of corrosivereaction:

Chemical reaction e.g. oxidation Physical metal reaction e.g. embrittlement of steel caused

hydrogen diffusing onto it Electrochemical reaction which takes place with an

exchange of an electrical charge. This requires anelectron-conducting medium - an electrolyte such aswater - where a film of moisture can be sufficient.

Medium determines risk ofcorrosion

Categories of atmosphere

Corrosive reaction

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Corrosion

Sometimes, locally different conditions in the surroundingsare responsible for an irregular rate of removal. For example,

that part of an anchor which is in the hole is exposed todifferent conditions that the part outside in the atmosphere.Often, the critical zone is the point of transition because themoisture exposure here is longest.

Fig. 3: Patch (selective) corrosion

Pitting:

This phenomena shows as small pits on pinholes caused bylocal corrosion resulting from corrosion cells.

This type of corrosion is observed primarily on materials

which form a protective layer against corrosion (passive layerof coating) e.g. stainless steels, aluminium alloys, anodizedaluminium and nickel alloys. In certain corrosive conditions,especially if chlorides are present, this layer is destroyedlocally and, depending on the grain structure and purity of thesteel, the results are different forms of pitting.

Fig. 4: Pitting

Differences in surroundingconditions

Protective layer penetrated

locally

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This type of corrosion can also take place when materialshave been metal plated and the plating is higher in the

electromotive force series than the substrate e.g. chrome ornickel or steel.

If building components have a large area, their loadbearingcapacity is often not impaired by a certain amount of pitting.In the case of wire-like or rod-like components, however,pitting can very well result in hazardous reductions in cross-section.

Crevice corrosion:

According to DIN50900, the definition of this type is locallyaccelerated corrosion in crevices resulting from corrosioncells caused by different concentrations of the corrosionmedium (electrolyte). Differences in the access of air canalso be included here (see Fig. 5). Apart from crevicesresulting from the design of a structure, deposits e.g. dust,can also form crevices. In the case of stainless steels,crevices formed by non-conductive materials e.g. plasticwashers, are particularly critical. In fact, this is all the morecritical the narrower the crevice is. For the CrNi and CrNiMosteels generally used in construction, the critical crevice

width is less than 1 micron.

Fig. 5: Crevice corrosion

Risk dependent on cross-section

Corrosion in crevices

Critical crevice width

Sufficient oxygen(cathode)

Material base

Oxygen starved(anode)

Fastened steelcomponent


Corrosion

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Corrosion

Intercristalline corrosion:

Selective corrosion advances around the grain boundariesand can result in the disintegration of the grain structure intoindividual grains. If a building component is stressed, inter-cristalline corrosion can also appear in the form of cracks. inthe case of CrNiMo steels, this type of corrosion is mostlycaused by poor heat treatment and, often, by carelesswelding. The susceptibility of these steels depends on thecarbon content and the carbide forming constituents (Ti, Nb).If parts are to be welded, steels which are deep carburized orstabilized should be given preference.

Contact corrosion:

This corrosion is accelerated by the combination of twometals which have different electrochemical behaviour. Themetal higher in the electromotive force series is protected,whereas the one of lower potential suffers an acceleratedattack of corrosion (see also chapter 3.5.3).

Fig. 6: Contact corrosion in water treatment plant

Microbiological corrosion:

Microbes can cause and acelerate corrosion owing to theoften corrosive produts of metabolism. A well knownexample is the damage to waste water conduits caused bybacteria which produce sulphuric acid. Only in recent years

has it been realized that this type of corrosion causes muchmore damage than previously imagined.

Disintegration of grain structuredue to wrong heat treatment

Steel reinforcement

Products of metabolism causecorrosion

Steel reinforcement

Galvanizing

Waste water

A2 stainless steel

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Corrosion

Vibrational corrosion (Corrosion fatigue)

As a result of the interaction of corrosion and alternatingmechanical stressing, transcristalline cracks can appear. Inthis case, the cracking, unlike stress corrosion cracking, isindependent of critical marginal conditions. Any combinationof metal and medium can be involved. In constrast toexposure to an alternating load in a dry atmosphere (fatigue),the most widely used materials have no fatigue strength inelectrolytes i.e. there is no min. stress below which no fatiguefailure occurs after any number of load cycles.

Corrosion and alternatingmechanical stressing.

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3. Fastener protection against corrosion

Basically, the means provided for protection againstcorrosion must inhibit the sequence of corrosion, corrosionphenomena and corrosion damage in a given system. Witheconomic aspects in mind, this should take place in such away that, as specified in the requirements profile, no damageoccurs or, if so, it only occurs after the planned life of asystem. Fundamentally, two different approaches can betaken:

Active protection against corrosion. Passive protection against corrosion.

Stable, resistant materials are used for active protectionagainst corrosion, whereas access of the corrosive medium(electrolyte) to the building components is prevented or madedifficult in the case of passive protection. For example,passive protection can take the form of coatings, sealing orother designed measures.

The protective measures taken by Hilti with fasteners havebeen shown in table 1.

Table 1

Purpose of protection againstcorrosion

What protection againstcorrosion is used by Hilti?

Protection against corrosion Measures with fasteners

Plastics Polyamide, polypropylene,

POM, epoxyacrylate polyethylene

Organic coatings Epoxy resin

Galvanized steel Mechanical zinc plating & chromated

Galvanized & chromated

Sendzimir galvanized

Hot-dip galvanized

Corrosion-resistant materials Stainless steelsSpecial alloys

Additional measures Sealing caps


Corrosion

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Corrosion

3.1 Plastics

Plastic anchors for light-duty fastenings are generally madeof polyamide. This material has very good chemicalresistance.

The products for fastening insulating material aremanufactured in polypropylene or polyethylene. Thesematerials have good chemical resistance, but they are notstable in UV light in the long term.

Special requirements exist for the rail anchor and these aremet optimally by the plastic POM, such as very good

electrical insulation properties, high strength and goodchemical resistance.

Epoxy acrylate and a modified epoxy acrylate are used forchemical fastenings i.e. the adhesive anchor and the Hiltiinjection technique. The resin, hardener and filler have beenfine tuned to each other so that shrinkage, creep and waterabsorption are very small. The resistance to alkalis, salinesolutions and acids is very good.

3.2 Organic coatings

Organic coatings are only used to a limited extent by Hilti toprotect fasteners against corrosion. Only certain nails fortemporary fastenings have this coating.

Organic coatings constitute, virtually without exception,passive protection against corrosion i.e. they prevent or delaythe corrosive medium from reaching the surface of the metal.If protection is to be 100%, the coating must therefore beabsolutely impervious i.e. pore free and dense, while havingan optimal bond. In field practice, these conditions are

difficult to achieve for several reasons.

Their surfaces are often involved in the working principlebased on frictional properties under high mechanical loadingwhich govern proper functioning. In view of this, organiccoatings of only limited thickness can be used.

If the coating is thin i.e. less than 20 microns, it is virtuallyimpossible for it to be without pores. These coatings onlyprovide temporary protection against corrosion, therefore,because rusting below the coating will begin in a relatively

short time if defects and a damp atmosphere exist at thesame time. If active pigments are used e.g. Zn or Al,adequate resistance to corrosion is achieved.

Plastic anchors

Insulation fasteners

Rail anchor

Chemical fastenings

Temporary fastenings

Full protection difficult

Only limited coating thickness

Temporary protection against

corrosion

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3.3 Zinc-plate steel

Most fasteners are zinc plated. Zinc on steel is an idealcombination as protection against corrosion. Plating withzinc can be carried out easily and economically on a largescale by various processes. The protection it providesagainst corrosion can be adapted to suit different practicalrequirements by selecting a suitable plating thickness. Sincecorrosion in a certain atmophere progresses linearly withrespect to time, the protection against corrosion is directlyproportional to the plating thickness (see Fig. 8).

Fig. 8: Mean life of zinc plating (as per 11)

Most fasteners galvanized

Plating thickness governs

duration of protection

50

40

30

20

10

080 1006040200

Mean

life(years)

Rural

and

cle

ancoastala

tmosphe

re

Towna

tmosphere

Industrial

atmosph

ere

Seawater

Plating thickness (microns)


Corrosion

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Corrosion

3.3.1 Corrosion of zinc-plated steel

Generally, zinc and zinc-plated steel corrode uniformly overthe surface. The products of corrosion of pure zinc platinghave a white to grey colour (this is sometimes called whiterust), whereas those of iron-zinc alloy platings are red brown.Red rust is the name given to the products of corrosion ofsteel which appear at defects or after the zinc plating hasweathered away. The products of corrosion of zinc, chieflybasic zinc carbonate, form a protective layer which clearlyshows further corrosion. This protective layer weathers awayslowly in the atmosphere. Its rate of removal is linear withrespect to time. The rate of removal of zinc suffering

atmospheric corrosion is roughly 10 times smaller than that ofsteel.

In an atmosphere heavily laden with SO2 e.g.near industry, theprotective layer cannot form completely. Owing to reaction withSO2 and oxygen in the air, zinc sulphate forms which is readilysoluble in water so that it is then washed away by rain. Thisresults in the rate of zinc corrosion beng considerably higher inan industrial atmosphere than in a rural or town atmosphere.The limits to the use of zinc plating, especially of hot-dipgalvanizing, are reached here because of the restricted

thickness on threaded parts. Hot-dip galvanizing has also notproven satisfactory as protection against corrosion whereheavy condensation and poor ventilation exist e.g. for manykinds of double-skin claddng or in damp thermal insulation.

Zinc platings provide cathodic protection because they havea lower electrochemical potential then steel i.e. they protectthe underlying steel even if the plating has been slightlydamaged. The protection offered, however, decreases rapidlywith increasing extent of the damage (see Fig. 9a and 9b)

Products of corrosion

Rate of removal

Hindered protective layer nearindustry

Limits to use

Cathodic protection

Zinc platingprocesses

Cathodic protection Steel corrodes outside of zincZinc corrodes instead of steel

Fig. 9a Fig. 9b

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3.3.2 Zinc plating processes

The zinc plating processes used by Hilti are as follows:

Mechanical zinc plating:

During the mechanical plating processes, zinc powder inwater-filled drums is hammeredonto steel parts by galssbeads. An electric current is not applied. The max. achievableplating thickness is approx. 20 microns. Mechanical zincplating causes no hydrogen embrittlement, not even at thesteel hardnesses usual for studs and nails. As a result, it isnot necessary for fasteners to be subsequently baked.

Galvanizing:

When galvanizing, pure zinc from a zinc salt solution isdeposited on the steel when a direct current is applied. Theplating bonds very well, but the thickness is limited toapprox. 25 microns. Galvanizing is used primarily forthreaded parts for which mechanical zinc plating cannot beused.

Both galvanically and mechanically plated fasteners have amin. plating thickness of 5 microns and are blue chromated.This gives them adequate long-term protection againstcorrosion if they are used in dry inside rooms. If exposed tomoisture, however, the protection is limited (Fig. 10).

Sendzimir galvanizing:

During the sendzimir process, steel strip first has its surfacecleaned by a special annealing process. It is then drawncontinuously through a bath of molten zinc. The platingthickness, generally 20 microns on both sides, is achieved by

wipingthe strip with a jet of air or steam.

Plating thickness approx. 20microns



Process Product

Mechanical zinc plating Studs and nails

Galvanizing Threaded studs, anchors,special-nails

Sendzimir galvanizing Anchors, installationcomponents

Hot-dip galvanizing Anchors, accessory products


Corrosion

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Corrosion

Hot-dip galvanizing:

When hot-dip galvanizing, the individual parts are dipped in abath of molten zinc. The plating thickness is govened by thesubmersion time and the constituents of the steel. Smallparts are galvanized in drums and then centrifuged to removesurplus zinc. The plating consists of an iron-zinc alloy layerand a layer of pure zinc. On threaded parts, the platingthickness is between 45 and 60 microns.

Greater resistance to corrosion in very damp and corrosiveconditions is provided by sendzimr galvanizing (anchors and

parts made of strip or sheet metal) and hot-dip galvanizingbecause of the thick layers and the better resistance tocorrosion of the iron-zinc alloy layer.

Fig. 10: Rate of corrosion as a function of air humidity

3.4 Corrosion-resistant materials

Apart from brass, which is used for light to medium-dutyfastenings in damp rooms, stainless steels are used mostlyfor corrosion-resistant fasteners and connectingcomponents.

The most widely used types (97%) are the austenitic CrNi andCrNiMo steels. Decisive for their use, apart from their ideal

combination of resistance to corrosion, mechanicalproperties and the economics, are regulations, codes etc.from authorities. Special materials must be used for specialfastenings where the requirements for resistance to corrosionare most stringent.

Plating thickness approx. 45 - 60microns

Stainless steels for corrosion-resistant fasteners

Special materials for morestringent requirements

4

3

2

1

10 20 30 40 50 60 70 80 90 100

Rel.rateofcorrosion

Rel. humidity (%)

Clean air

Polluted air 0,01 % SO2

Critical moisture content

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Only seldom do building materials contain corrosivesubstances. In the course of time though, concrete, brick,

insulating material etc. can pick up damaging substancesfrom the surroundings e.g. chlorides. Concrete can only beexpected to pick up large amounts of chlorides if there isdirect exposure e.g. via cracks, or through frequent wet anddry cycles (capillary effect). If fastenings are in a corrosiveatmosphere, it can be assumed that concrete base material isless critical than the surrounding medium. Acid, corrosiveprecipitation from the atmosphere and condensation areneutralized by the alkalizing effect of concrete.

In construction, the cases known to date of stainless steel

being damaged by stress corrosion cracking were caused bypoor workmanship when manufacturing the stainless steelfasteners. An exception in this respect is the damage whichhas occured in indoor swimming pools.

Conditions similar to those producing stress corrosioncracking in indoor swimming pools, must, according to thecurrent level of knowledge, be expected in any event withspecial applications, such as chimney stack construction forpower plants, in the chemical industry and, possibly, in roadtunnels. Recent studies have shown materials in the A4 group

to be resistant when used for cladding installation in generalbuilding construction [9].

Further processing on jobsites, such as heating, forming andwelding stainless steel fasteners cannot be permitted. Onlythis will ensure that the resistance to corrosion and themechanical properties specified by the manufacturer aremaintained. Subsequent treatment, such as oiling or coating,must not be carried out either because this can change thefunctioning, loadbearing behaviour and resistance tocorrosion of fasteners.

Furthermore, the use of tools made of unalloyed steel, suchas pliers, brushes etc. must be avoided. Rusting particles onthe stainless steel could initiate corrosion.

Building materials are seldomcorrosive

Damage from poor workmanship

Incorrect processing ofstainless-steel fasteners


Corrosion

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Corrosion

3.4.3 Materials for special fastenings(In indoor swimming pools and tunnels)

Hilti has carried out in-depth studies of fastenings whereparticularly corrosive conditions exist, such as in road tunnels,indoor swimming pools etc.. Fasteners, especially anchors,which are made of materials suitably resistant to corrosion, canbe supplied for these special applications on request.

Aluminium and copper as well as their alloys are only suitable forfasteners in exceptional cases owing to their low strength. Apartfrom this, their resistance to corrosion hardly exceeds that ofstainless steels. On the other hand, several highly alloyed

CrNiMo steels and special alloys are available.

These steels and Ni alloys have been listed in table 4, roughly inthe order of decreasing resistance to corrosion. The order is theresult of a laboratory test, the FeCl3 test.

Table 4: Materials highly resistant to corrosion[percentage by weight]

A term - activator total - is closely linked to this test (see Fig. 11).

This is understood to be the total of the alloying constituentscontributing to resistance to corrosion multiplied by a certainfactor.

The following formula has long been well known:

AT = Cr + 3,3 Mo

Special materials on request

Activator total

Material/ Cr Ni Mo N Fe Others Standard/

Des. Regulation

2.4062 20.022.5 Rest 12.514.5 2.06.0 W: 2.53.5 VdTV WB 479

1.4529 19.021.0 24.026.0 6.07.0 0.100.25 Rest Cu: 0.51.5 SEW 400

Avesta

254 SMO Approx. 20 Approx. 18 Approx. 6.1 Approx. 20 Rest Cu:Approx.0.7 Avesta

1.4462 21.023.0 4.56.5 2.53.5 0.080.20 Rest SEW 400

1.4539 19.021.0 24.026.0 4.05.0 0.040.15 Rest Cu: 1.02.0 SEW 400

1.4439 16.518.5 12.514.5 4.05.0 0.120.22 Rest DIN 17440

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Fig. 11: Critical pitting temperature (CPT)as function of activator total inFeCl3 test [12]

Recently, nitrogen has also been incorporated in the activatortotal, but no agreement exists on the multiplication factor tobe used. Values of 0, 13 and 30 are found. In view of this, the

activator total must not be used as the sole criterion whenevaluating the resistance to corrosion. It can only serve as arough guide.

The selection of one of these materials for use for criticalapplications in the construction industry must be basd ondifferentiated aspects.

3.5 Other measures for protection against corrosion

3.5.1 Sealing caps

As an additional protective measure, sealing caps areavailable, particularly for DX fasteners.

The purpose is to keep a corrosive medium away from thefastener e.g. rain or dew. Care must be taken to ensure thatsealing is really tight over the entire time a cap is used. Thesealing cap must be seated properly. If it does not sealcompletely, the opposite effect than that desired can, undercircumstances, result because, if the air humidity is high,

infiltrated moisture condenses when the temperature drops.This then dries only very slowly.

Activator total as arough guide

Sealing cap

Complete sealing necessary

+

0

++

++

+

+

1.4539

+

+

90

80

70

60

50

40

30

20

1025 3020 35 40 45 50 55

Activator total (% Cr+3.3 % Mo)

CPT (C)

1.4529 (6 % Mo)

1.4571 (A4)

(Duplex)

Inconel 625

+


Corrosion

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3.5.2 Designs to counteract corrosion

When building components and systems are designed,aspects of protection against corrosion should be allowed foron principle whenever possible. Consideration must alwaysbe given to the entire system, not just an anchor.

Special attention must then be given to crevices and gapsresulting from the design. Crevices can lead to a greater riskof corrosion. Whenever possible, therefore, they must beavoided.

To maintain the passivating layer of stainless steels, an

oxidant, generally oxygen in the air, must have access to thesurface of the steel. Oxygen diffusion is hindered in a tightcrevice filled with a corrosive medium so that the passivelayer than breaks down locally ad permits a strong attack ofcorrosion. Crevices betwen materials which are notelectrically conductive i.e. plastics, deposits etc., are morecritical than those between metals.

The tighter the crevice, all the more critical it is. The criticalrange of crevice width is between several 100ths to 10ths ofa micrometer. This is why deposits of dust, for example, are

more critical than the gap between an anchor and the holewall.Often, crevices occurring with fastenings are specific to thesystem. Consequently, materials in the A2 group if there is noexposure to chlorides.

3.5.3 Avoidance of contact corrosion

If two or more metals are used in combination with eachother so that current can flow from one to the other, attention

must be paid to their electrochemical compatibility. In acertain medium e.g. a humid atmosphere, every metal has aparticular electrochemical potential. If the potentials ofconnected metals differ, a current flows in a similar way tothat in a battery. The metal of lower potential corrodes morestrongly, preferentially at the point of contact. The metal ofhigher potential is protected. This action is called contactcorrosion.

Design of building components

Crevices increase the risk ofcorrosion

The tighter the crevice, the morecritical it is

Pay attention to electrochemicalcompatibility

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Most decisive for the rate of corrosion is the ratio of areas ofthe two metals. One of the worst cases is a small area of the

metal of lower potential and a large area of the metal ofhigher potential. A review of important metal combinationsfor use in construction, especially for fastenings, is given intable 4. It is assumed here that the fastener has aconsiderably smaller area than the part fastened. The tableshows where it is anticipated that the attack of corrosion willbecome worse when the fastener is in contact with theindicated material. Some of this data was determined on ourtest rigs in different climates.

If a combination of metals cannot be avoided, contact

corrosion can be eliminated by using electrical insulation e.g.plastic washers or sleeves or suitable coverings.

If fastenings are made under water e.g. in waste watertreatment plants or in the sea, care must be taken to ensurethat there is electrical insulation betweenthe fastener and theconcrete reinforcing bars. Extensive damage repeatedlyoccurs in such cases.

Heavy corrosion of fasteners Moderate corrosion of fasteners

O Slight or no corrosion of fasteners

Table 5: Extent of contact corrosion withvarious combinations of materials

Rat

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