pretressed anchors

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Anchor Head Stressing head + bearing plate (anchor plate) Tendon is anchored tendon force is transmitted to the structure   Head should be designed to permit the tendon to be stressed and anchored at any force up to 80% of the characteristic tendon strength and should permit force adjustment up or down during the initial stressing phase. should be restressable (load adjustments 10 possible) should be detensionable should permit an angular deviation of 5o from the axial position with no effect on ultimate capacity. In such cases the stress in the shortest strand should limit the acceptable working load.If the design requires uniform stresses within the tendon, monostrand stressing is essential. Centralizers and spacers should be provided. Construction   1.Method of drilling (with or without flushing) 2.Tendon installation 3.Grouting system   Time period of the above operations (1,2&3) may influence the capacity of the anchorage. Drilling Any drilling procedure that can supply a stable hole free of obstructions may be used. minimum disturbance or disturbance most beneficial to capacity Care should be taken not to use high pressures with any flushing in order to minimize the risk of hydrofracture particularly in buit-up areas.(Open return within BH is desirable.) Drill hole entry point: 75 mm accuracy. O.C. soils and several hours waiting  check swelling tolerance o drill up tolerance is 2.5o except in case of closely spaced design + staggered design. o 10o to facilitate grouting Overall drill hole deviations 1/30 (1.9o) should be anticipated. After drilling full length and thoroughly flushed out to remove any loose material  probe into the hole. Even cased drillingprobe whether e.g. saturated silts and fine sands moved inside. 1 m overdrill is the trick in such cases. Drilling –tendon installation- grouting in the same day otherwise delays  ground deterioration in especially O.C. fissured clays and marls. a)changes in soil type b)drilling rates c)water levels d)flushing losses or gains stoppages must be recorded.

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Tendon Installation (Homing)  Tendons should be stored indoors in clean dry conditions.  If they are outdoors, should be stacked off the ground and be completely covered by a waterproof branda/tarpaulin (air circulation and avoid condensation)  Tendons should not be dragged through surface soil and handled with care (carried by someway)  Minimum grout cover in BH’s : centralizers min 10 mm between centralizers min 5 mm 1-3 m spacing  Spacers for multiunit tendons : min 5 mm spacing  Sleeve or nose cone at the bottom of tendon to reduce the risk of BH damage during homing.  Check that there is; -no damage to tendons, components -no corrosion If the assembly is more than 200 kg, use mechanical systems otherwise damage may occur. (Funneled entry pipe may be sometimes used to ease the homing operation.) Another check : Take out the tendon and inspect what has happened! Centralizers, spacers, smear of clay, damage, distortion, etc. Grouting Grouting performs one or more of the following functions: i)                    To form the fixed anchor in order that the applied load may be transferred from the tendon to the surrounding soil. ii)                   To augment the protection of the tendon against corrosion iii)                 To strengthen the soil immediately adjacent to the fixed anchor in order to enchance anchorage capacity   In permeable soils the loss of grout over the fixed anchor length should be checked observing the controlled grout flow coupled with a back pressure. The efficiency of fixed anchor grouting can be finally checked by monitoring the response of the soil to further injection when back pressure should be quickly restored.   In general if the grout volume exceeds 3 times the BH volume for injection pressures less than total overburden pressure, then general void filling is indicated which is beyond routine anchor construction.

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Preparation of grout : *Weigh dry mass of cement, water (lt) *Mechanical mixing at least 2 minutes (homogeneous mix) *Thereafter the grout should be kept in continuous movement e.g. slow agitation in a storage tank. As soon as practicable after mixing, the grout should be pumped to its final position. Do not use after its initial setting time. High speed colloidal mixers (1000 rpm) and paddle mixers (150 rpm) are used. High speed mixers are preferable. Pumps should be -of the positive displacement type. -capable of exerting discharge pressures of at least 1000 kPa. Rotary screw (constant pressure) or reciprocating ram and piston (fluctuating) pumps are acceptable in practice.   Before grouting all air in the pump and line should be expelled and suction circuit of the pump should be airtight. During grouting, the level of grout in the supply tank should not be drawn down below the crown of the exit pipe, as otherwise air will be injected.

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An injection pressure of 20 kPa/m depth of ground is common in practice.Where high pressures that could hydrofracture the ground are permitted careful monitoring of grout pressure and quantity over the fixed anchor length is recommended.   If on completion of grouting, the fluid grout remains adjacent to the anchored structure then the shaft grout should be flushed back 1 to 2 m to avoid a strut effect during stressing.   Quality control to grout prior to injection : -initial fluidity by flow cone or flow trough -density by mud balance -bleed by 1000 ml graduated cylinder (75 mm diameter) Record 1.Air temperature 4.quantity of grout injected 2.age of constituents 5.tests conducted etc. 3.grouting pressure   Anchor Head After final grouting and testing, cutting of the tendon should be done by disc-cutter (without head). Projected tendons should be protected against accidental damage.   Stressing Stressing is required to fulfill two functions, namely; i)   To tension the tendon and to anchor it at its secure load ii)  To ascertain and record the behaviour of the anchor so that it can be compared with the behaviour of control anchors, subjected to on-site suitability tests. Stressing operation means: 1. Fitting of the jack assembly on to the anchor head. 2. The loading or unloading of the anchor including cyclic loading where specified. 3. Complete removal of the jack assembly from the anchor head.

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Experienced crew is essential. Calibration is essential.Apart from pressure gauges on the jacks load cells are recommended like in case of pile testing.Jacks must be calibrated at least every year. Accuracy < 0.5 % Loading-unloading friction hysterisis should be determined in the tests. Load cells should be calibrated after every 200 stressings or after 60 days in use whichever is more frequent. If complementary pressure gauges used simultaneously indicate no significant variation  calibration interval is up to 1 year.   Pressure gauges : calibrate after every 100 stressings or after every 30 days (whichever is more frequent) On every contract specify method of tensioning to be used and the sequence of stressing (and level) No tendon should be stressed beyond 80% of the characteristic strength. Grout should reach 30 MPa strength. In sensitive soils (clay,marl etc.) number of days before stressing may be longer. After stressing this load will be the readings for future readings, then perform check-lift load measurement.   Provide safety during stressing.

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Corrosion Protection There are cases of corrosion (localized)failures (35 in the literature)   *All permanent anchors *All temporary anchors exposed to aggressive conditions should be protected. Degree of protection depends on: 1) Consequence of failure 2) Aggressivity of the environment 3) Cost of protection 4) ... Overall protection is required. Table. Proposed classes of protection

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-Purpose of outer barrier is to protect inner barrier against the possibility of damage during handling and placement. -Protective systems should aim to exclude a moist gaseous athmosphere around the metal by totally enclosing it within an impervious covering or sheath. -Cement grout injected in-situ to bond the tendon to the soil does not constitute a part of a protective system. (differential strains, cracks etc.)   Non-hardening fluid materials such as greases also have limitations such as corrosion protection media; Because; i) Fluids are susceptible to drying out (usually accompanied by shrinkage and change in chemical properties) ii) Fluids are liable to leakage iii) Fluids having virtually no shear strength are easily displayed and removed from the tendon or metal pieces. iv) Their long-term chemical stability not known with confidence.   These aspects require that non-hardening materials are themselves protected or contained by a moisture proof robust form of sheathing which must itself be resistant to corrosion.   Nevertheless, non-hardening fluids such as grease fullfill an essential role in corrosion protection systems, in that 1..They act as a filler to exclude atmosphere from the surface of a steel tendon, create the correct electrochemical environment and reduce friction in the free length. Also used on anchor head. 2..Use of thicker metal sections for the tendon is not a solution because corrosion does not uniformly operate.

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Protective Systems There is a variety of protective coatings or coverings. The principles of protection are the same for all parts of the anchorage (details are different) : tendon bond length, free length and anchor head   Free Length Inject solidifying fluids to enclose tendon or by pre-applied coatings or combination of both. Protective system should permit reasonably unhibited extension of the tendon during stressing, and thereafter, if the anchor is restressable. Greased and sheathed tendons are a popular solution in such circumstances. No metallic coatings are recommended. Bond Length Requires the same degree of protection and transmits high tendon stresses to the ground. Strength and Deformability Characteristics No creep and no cracking is desired in bond length. Epoxy and polyester resins may be used in encapsulations. cementitious grouts are cheaper. Stress\strain behaviour of resins and plastic duets (compatibility) must be considered. for effective load transfer ducts are corrugated   Restressibility should be possible.

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TESTING There are three categories of anchor testing: 1. proving tests 2. on-site suitability tests (identical conditions similar to working loads) 3. on-site acceptance tests 1. Proving Tests Several variables (fixed end length and others) Thisi is a rigorous test program : Procedures\Soil conditions\Materials\Level of safeties all studied in detail (e.g. grouting different ways)   2. On-site Suitability Tests These tests are performed under identical conditions similar to working anchors. They are loaded in the same way and at the same level.They are performed (In advance of main contract or on selected working anchors. Period of monitoring should be sufficient to ensure that prestress or creep fluctuations stabilize.)   3. On-site Acceptance Tests Every anchor should be tested; Check transfer of load to fixed zone Check capacity of anchor Apply greater load than design load in shorter time Compare with on-site suitability tests which are performed in longer time (long term behaviour. Proof Load : Temporary anchors 1.25 Permanent anchors 1.50 Short duration : To save time and money

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1. Proof load-Time Data *This cycle may include deformations due to wedge ‘pull-in’; bearing plate settlement, initial fixed anchor displacement. **Take 5 min readings.

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If proof load does not reduce more than 5% in 15 min (after allowing for any movement of the anchored structure) Anchor is OK. If not, two further proof load cycles; if fail again 5% criterion new load   Alternatively: Proof load can be maintained by jacking and the anchor head monitored after 15 min. in which case the criterion is 5% Xe (elastic displacement of the tendon=Displacement monitored at proof load – displacement at datum load i.e. 10%Tw)    If the tests fail diagnosis a.Grout-tendon b.Grout-soil c.? 2. Apparent Free Tendon Length     At : steel cross-section Es : elastic modulus of steel Xe : elastic displacement of the tendon (disp. Monitored at proof load-disp. at datum load i.e. 10%Tw) During destressing stage of 2nd cycle. T : Tproof-10%Tw    Apparent free tendon length should not be less than 90% of actual free tendon length in design Apparent free tendon length should not be more than actual (intended) free length+50%of tendon bond length Apparent free tendon length should not be more than 110% of the actual free tendon length   III is for a.short encapsulated bond lengths b.fully decoupled tendons with an end plate or nut   If outside the limits  Diagnose (Es may be 10% less in strands) If behaving elastically may be considered OK. (i.e. when the lengths are near the criteria.)

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3. Short Term Service Behaviour Residual load criteria

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Using properly calibrated load cells and logging equipment residual load may be monitored at 5, 15, 50 mins. If rate of load loss reduces to 1% or less per time interval after allowing for temperature, structural movements, relaxation of the tendon  Anchor is OK.   If the rate is more than 1%  further readings up to 10 days If does not satisfy the criterion a. abandone and replace b. reduce in capacity c. subject to remedial stressing programme   Alternative to load monitoring  Displacement-Time data at the residual load at the specific observation periods in the table. Rate of displacement should reduce to 1% e or less per time interval. (resort to the table)         If prestress gains (more than 10% Tw each time) are recorded; A) Insufficient anchor capacity or overall slope failure B) Capacity  destress and provide additional support Initial residual load x apparent free tendon length e = area of tendon x elastic modulus of tendon