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Problem 4

Grouting post-tensioned ducts

A major concern with existing bridges andsome post-tensioned floors has been qualityof grout and protection of tendons. Aproperly grouted duct will protect tendonsfrom corrosion by providing an alkaline

environment around and between thetendons. However, such factors with thegrout as poor flow, segregation, airentrapment combined with proceduralproblems such as maintaining pumping

pressure, protection of end plates and correctchoice of ducting have led to many challegesfacing the industry.

The selection of a suitable duct grouthas been simplified by the introductionof the CARES Appendix PT10 that nowrecommends solely the use of a pre-bagged premixed grout for use in post-tensioning ducts. This in a waysupercedes BS EN 447: Specification forcommon grout, which allowed the useof site-mixed grouts. weber.cem ductgrout was developed several years ago

in conjunction with the work done inthe preparation of Concrete SocietyTechnical Report TR47: Durable Post-tensioned Concrete Bridges. It has beenused successfully on many major civilengineering and building projectswithout any problems.

Choosing the right grout

Voids in ducts

Chemical reaction within the ducts and tendons

Corrosion of the end anchorages

The best way to ensure complete fillingof the ducts and hence protection ofthe tendons is to use a top qualitygrout. The favoured option is to use acombined colloidal mixer and pumpwith a constant pressure monitoringsystem. This ensures a steady flow ofgrout with a measured quantity ofwater without segregation and withoutair entrainment. BS EN 446 providesdirections on the correct groutingprocedures to be adopted by thespecialist grouting contractor.

Incorrect selection of ducting materialcan lead to chemical attack as well asswelling of the grout. Recent testscarried out on galvanised steel ductshave shown that gas evolution as aresult of a chemical reaction betweenzinc or aluminium and the alkalinegrout can result in swelling of theducting and possible disruption of theconcrete surrounding the ducts. Plasticducting has been in favour in manycountries worldwide.

End plates can be susceptible tocorrosion as they are exposed to theelements. They can be coated with aresin coating such as weber.tec EPbonding aid to resist formation ofanodic sites on the steel.

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For detailed instructions, please refer to the relevantproduct data sheet. For further information, please contactour Technical Helpline on 01525 722110.

Solution 4

Correct grout selection and application

Problem: Grouting post-tensioned ducts

Recent moves by Authorities to improvequality of both post tensioned bridge andfloor construction have established anumber of specifications that define

grouting procedures, test methods andspecify the grout material to be used. Thesehave now been published and the grout canbe certified under the CARES scheme.

Because post-tensioned grouting is aspecialised and critical operation it shouldonly be undertaken by approved andtrained operatives and contractors.

weber.cem duct grout is a quality-controlledpremixed cementitious grout developed forthe grouting of tendons, bars and anchors ininternal prestressing ducts. weber.cem ductgrout complies with the special requirements

of clause 8 of Concrete Society TechnicalReport No 47 Durable Bonded Post-tensionedConcrete Bridges. weber.cem duct grout is apre-bagged special non-shrink grout whichdoes not segregate or bleed.

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Applications• Floor slabs

• Bridge beams

• Cement silos

• LNG tanks

StandardsTR 47

BS EN 445

BS EN 446

BS EN 447

ETAG

CARES UK Appendix PT10

Mixingweber.cem duct grout needs only tobe mixed with clean, potable waterto give high flow consistencyrequired. Add up to 7·75 litres waterper 25 kg bag. Do not over-water.Mixing should be carried out in ahigh-shear, specialist grout mixer.Water is added to the mixer, followed by the dry powderand mixed until a homogeneousgrout consistency is achieved. Keepthe mixed grout in slow, continuousagitation until it is pumped into theduct. The temperature of freshlymixed grout shall be between 5°Cminimum and 30°C maximum.

Product required