PRESTRESSED CONCRETE

TECHNICAL SPECIFICATIONS

7.1 GENERAL

The following specifications on Prestressed Concrete shall be treated as additional clauses to those required under Section 4 "Structural Concrete" including all clauses and subclauses under it.

7.2 CONCRETE MIX

Only specially designed concrete mixes complying with the requirements specified in the Drawings shall be used for prestressed concrete work. Submit, for the approval of the SO Rep, details of the concrete mix proposals at least 6 weeks in advance of concreting work so as to allow strength tests from trial mixes to be made. Trial mixes shall be repeated until a satisfactory mix as regard to the cube strength and workability has been achieved. Once the SO Rep has approved the mix, it shall be used for that work and no departure shall be made from it either in properties of materials or in their relative proportions unless authorised by the SO Rep who may require further trial mixes to be made.

All sampling and testing of constituent materials, fresh and hardened concrete, shall be carried out in accordance with the provisions of the appropriate Singapore or British Standard. If the strength of cube test results representing the prestressed concrete work fails to achieve the required strength, the Contractor shall be subject to charges and shall carry out further tests and/or rectifications to ascertain the in-situ strength of concrete as specified under Clause 4.16, 4.17 and 4.18 including all subclauses under them.

Notwithstanding that all tests have been complied with, member which cracks during or after prestressing operations to an extent which in the opinion of the SO Rep renders it unfit for its purpose in the work shall be removed from the work and replaced.

7.3 PRESTRESSING STEEL

The prestressing steel, wire or strand used in prestressing tendons shall comply with the requirements of BS 2691, BS 3617, BS 4486, BS 4757 and BS 5896. High tensile steel wires or high tensile steel strands shall be supplied in coils of a sufficiently large diameter so that it shall retain its physical properties and shall be reasonably straight when unwound from the coils.

A copy of the Manufacturer's test certificates covering each coil of prestressing steels to be used shall be provided to the SO Rep. If test certificates relating to the material is not available, the Contractor shall arrange for testing of samples. The SO Rep may require the Contractor to carry out further test on the steel of each batch. All such testings shall be carried out by an approved testing authority.

All wires or strands shall be cut from certified coils. Keep proper records so as to enable identification of the wires with the coil from which they are cut. The manufacture and assembly of tendons shall be done in accordance with the best quality of workmanship appropriate to the prestressing system being used.

Tendons shall be made to the required length plus an allowance, where applicable, at each end for stressing operation. Splicing of strands or wires forming a tendon shall not be permitted. Flame cutting of wire or strand within 80mm of where tendon will be gripped by the anchorage or jacks shall not be permitted. Care shall be taken to ensure that the flame does not apply on the anchorage or on the tendon, within 25mm of the anchorage.

A durable metal label on which shall be stamped the length of the tendon and the coil number of the wire or strand used, shall be tied to each tendon. Care shall be taken that tendons of any type are not damaged, kinked or bent. Tendons shall be kept free from loose or thick rust, oil, grease, tar, paint, mud, or any other deleterious substances but a thin film of rust will be permitted.

7.4 ANCHORAGE DEVICES

Anchorage devices shall be subjected to the approval of the SO Rep. Each anchorage device shall be capable of transmitting a force not less than the ultimate tensile strength of the tendon without overstressing the concrete. Spiral or other reinforcement specified by the manufacturer as being required for this purpose shall be considered part of the anchorage devices and shall be of the size and dimensions recommended by the Manufacturer for the prestressing system being used. Certificates of origin shall be provided for all anchorage devices.

No damaged or defective anchorage devices shall be used. All steel parts shall be protected from corrosion and all threaded parts and fittings shall be protected by wrappings or plugs until used. The anchorage devices shall be kept free from mortar, loose rust, grease, tar, paint, oil, mud or any other coatings.

7.5 SHEATHING

All Sheathing shall be subject to the approval of the SO Rep. Sheathing shall be of the type suitable for the prestressing system to be used and shall be strong enough for withstanding the placing and compaction of the concrete without suffering damages or deformation. Internal and external diameters of sheathing shall be within the limits specified in the design or approved by the SO Rep. The sheathing and all splices shall be mortar tight. Steel sheathing shall be galvanised or lead coated. The Manufacturer's value for the friction coefficients of the proposed sheathing shall be stated by the Contractor. Enlarged portions of the sheathing at couplings or anchorages shall be of sufficient length to provide for the extension of tendons.

Grout holes shall be provided at both ends of the sheathing and shall be at least 10mm diameter. Each of the grouting holes shall be equipped with a plug valve or similar device capable of withstanding a pressure of 1 N per mm² without loss of water, grout or air. Additional vents, with plug valves, shall be provided at suitable points in the sheathing, including high and low points, for the purpose of bleeding air and water to ensure the complete filling of the duct with grout. The number and location of these additional vents shall be as specified in the design.

Sheathing for prevention tendons shall consist of plastic tubing or other approved material of a quality, diameter and thickness approved by the SO Rep. Sheathing shall continue through end forms for at least 25mm.

7.6 PLACEMENT OF TENDONS (a) Post-tensioned Tendons

Unless otherwise stated in the Drawings or approved by the SO Rep, tendons shall be sheathed and placed in position prior to concreting. Where permanent tendons cannot be placed, temporary tendons or other methods approved by the SO Rep shall be used to stiffen the sheathing.

Tendons shall be handled with care and shall be pulled through the sheathing in such a manner as to avoid damage or contamination to either the tendon or the sheathing.

Sheathing shall be securely fixed at points sufficiently close together to maintain a smooth tendon profile throughout the length of the tendon within a tolerance of 3mm of the position shown. If in the opinion of the SO Rep, the sheathing is liable to be damaged or deformed during concreting work, the Contractor shall take additional precautions to maintain the position and shape of the sheathing.

Any temporary opening in the sheathing shall be plugged and all joints between sheathing and any other part of the prestressing system shall be effectively sealed to prevent the entry of mortar, dust, water or other deleterious matter. Sheathing shall be neatly fitted at joints without projection or reduction of diameter, and the joints shall be mortar tight.

Immediately after the whole of the concrete has been placed, all tendons therein shall be pulled back and forth for about 300mm to ensure that they are perfectly free inside the sheaths. Ducts stiffened by temporary tendons shall be checked by passing through a tendon identical with the permanent tendon.

7.6 PLACEMENT OF TENDONS (CONT'D) (a) Post-tensioned Tendons (Cont'd)

Unless otherwise specified, the minimum concrete cover to the outside surface of any sheathing shall be 50mm for beam soffits and 40mm elsewhere. This minimum cover shall be increased by 15mm for members in contact with earth or water or over salt water and by 25mm for members in contact with salt water.

Each anchorage device shall be set square to the line of the corresponding prestressing tendon and shall be positioned securely to prevent movement during concreting. The anchorage devices shall be cleaned to the satisfaction of the SO Rep. Any mortar or concrete which adheres to the bearing or wedging surfaces shall be removed immediately. Provision shall be made for covering anchorages with reinforced concrete after completion of stressing. The minimum cover shall be 50mm over the anchorages and 40mm over the reinforcement.

(b) Pretensioned Tendons

Tendons shall be located as shown on the Drawings and suitable devices shall be provided to ensure that the correct positioning of the tendons is maintained during casting.

When tendons are being placed, particular care shall be taken so that the tendons shall not come into contact with the oiled surface of the forms or be otherwise soiled. Any oil, etc which might collect on the tendons shall be removed by cleaning the affected tendons with a petrol-soaked rag or by other approved methods.

The anchorages shall be such as to prevent any slip during the casting or curing operations.

Unless as shown otherwise in the Drawings, the concrete cover to the surface of any tendon shall not be less than twice the tendon diameter or 25mm whichever is the greater. Such minimum cover shall be increased by 15mm for members situated in earth or water or over salt water.

7.7 CONCRETING

Concrete in one precast unit shall be placed in one operation continuously without interruption. The Contractor shall provide such protective cover as and when required to avoid stoppage due to sudden rain. No unit shall be removed from the mould or erected until sufficiently matured to ensure that no damage shall be done to the unit. For post-tensioned construction, where necessary, temporary openings shall be provided in the formwork to enable placing and adequate compaction of concrete, especially around and underneath sheathing and anchorages.

Care shall be taken to avoid damaging the sheathing. Vibrators shall not come into contact with the sheathing and if the sheathing is damaged during concreting, the whole or a portion of the concrete cast may be rejected by the SO Rep. Sheathing shall be cleaned out within half an hour of completion of each concreting operation by blowing oil-free compressed air through the length of the sheathing. The concrete shall be moist cured until the prestress is applied.

7.8 CONCRETE STRENGTH AT TRANSFER

No post-tensioning of tendons or the release of pretensioned tendons shall take place until the concrete has attained the minimum strength required as stated in the Drawings or as specified. These requirements shall be ascertained from tests on concrete cubes made and cured under the same conditions as the concrete member. Sufficient cubes shall be made so that if the stipulated strength is not achieved, further cubes will be available for such additional testing as may be required.

7.9 TENSIONING EQUIPMENT

All tensioning equipment shall be approved by the SO Rep prior to use. Hydraulic pumps shall be power driven and the power unit shall be so adjusted that the rate of extension of the tendon is within the limits recommended or as directed by the SO Rep. Dynamometers, and each set of equipment comprising pump, jack and pressure gauge shall be calibrated as directed by the SO Rep and the true force determined from the calibration. Dynamometers, pressure gauges and scales shall permit force and elongation to be determined within an accuracy of 2%.

Pressure gauges shall only be used as a substitute for dynamometers and shall be concentric scale gauges complying with the requirements of BS 1780 which requires gauges to be accurate to within 1% of their full capacity. They shall be selected so as to be used between the 50% and 90% of their full capacity and shall not be less than 200mm in diameter. Where pressure gauges are used, a suitable device shall be fitted so as to protect the gauge against sudden release of pressure. Provision shall also be made by such means as a "tee" connection, for the attachment of a second gauge which shall be attached and used as a check whenever so requested by the SO Rep.

7.10 TENSIONING PRECAUTIONS

All tensioning operations shall be carried out in the presence of the SO Rep in accordance with the best practice applicable to the particular prestressed method proposed. The tensioning operation shall be performed only by personnel trained and experienced in this type of work.

Care shall be taken during tensioning to ensure the safety of all personnel engaged on the work and of other persons in the vicinity. Jacks shall be secured in such a manner that they will be restrained should they loose their grip on the tendons. No person shall be permitted to stand behind the jacks or close to the line of the tendons while tensioning is in progress. The operation of jacks, the measurements of elongation and associated operations shall be carried out in such a manner and from such positions that the safety of all concerned is ensured. During actual stressing operations warning signs shall be prominently displayed. Where, in the opinion of the SO Rep, there is danger to the general public or to property, special precautions in the form of substantial barrier shall be required.

7.11 TENSIONING FORCE

Unless otherwise specified, the tensioning forces required shall be that shown in the Drawings. The tensioning force applied to any tendon shall be determined by direct measurement of the force and checked by measurement of the elongation of the tendon. The secant modulus determined from test samples or shown on test certificates shall be used when interpreting the measurement of elongation.

Allowance shall be made for any anticipated draw-in at the anchorage devices on release of the jack, for elastic shortening of the member and for anticipated losses due to stressing of subsequent tendons. The final forces in each tendon, as measured by the dynamometer, shall be within an accuracy of plus or minus 3% of the values stipulated.

If, on completion of tensioning each tendon to the required force, the check measurement of elongation differs from its required value by more than 5%, the SO Rep may direct that some or all of the following steps be taken :

(a) Recalibration of equipment;

(b) Testing of tendon material to check secant modulus;

(d) Lubrication of tendons to reduce friction losses. Only water soluble oils shall be used in ducted systems and these shall be washed out before grouting;

(e) Where only 1 jack is used previously, the tendon shall be tensioned from both ends using 2 jacks;

(f) Other methods as directed by the SO Rep.

7.12 PRETENSIONING

The abutments and casting beds for pretensioning of tendons shall be strong enough to withstand the required total prestressing force with adequate safety. The abutments shall be so constructed as to provide protection at all times against injury or damage caused by slipping occurring in anchorages during or after tensioning. Provisions shall be made for free and adequate movement of the member at points of support in the direction of the expected shortening due to prestress without impairing the rigidity of the forms during casting operations. Where necessary, provision shall be made in the design of the casting bed for the use of external vibrators.

In the case where tendons are to be deflected, submit details of the proposed layout for each line of tendons together with values for the portion of the force to be provided by the initial tensioning and for the portion which will be provided by the elongation resulting from subsequent deflection of the tendons.

Tensioning shall not commence until the SO Rep has approved the values submitted.

Special care shall be taken to apply tensioning force smoothly at an even rate and to determine its value with considerable accuracy. Tendons shall be marked for measurement of elongation and shall also be marked at both the jacking and dead ends of the stressing bed so that any slip may be observed. Should a slip of any of the tendons in a group of tendons tensioned together occur, the tensioning of the whole group shall be released, tendons reset, and the whole group tensioned again. Alternatively, the tensioning of the group of tendons can be completed without the slipped tendon(s) which can be subsequently tensioned.

The prestressing force shall be transferred from the jack to the abutment of the stressing bed immediately the required force (or elongation) has been reached and the pressure in the jack shall be relaxed before any other operation commences. Tendons shall be deflected, where shown in the Drawings and the required force maintained, by use of approved fixing devices strong enough to hold the tendons firmly in their proper positions especially during concreting and curing, until the concrete has attained the required strength.

In order that the required force is maintained after the tensioned steel has been anchored, an allowance shall be made in assessing the value of the applied force for loss of stress due to yield in the anchorages and for slip when the steel is wedged after tensioning. In long line beds, due consideration shall be given to the friction caused by the varying shape and number of diaphgrams.

Prior to concreting, a check shall be made of the accuracy of alignment of forms and of the distance centre to centre bearings, overall length, width, positioning of end blocks, tendons, sheathing and anchorages and their connections, reinforcement, bearings, dowels and any other fittings shown in the Drawings. Concreting of a member shall not commence until the approval of the SO Rep has been obtained.

Prior to transfer of the prestressing force from the abutments of the casting bed to the members, all tendons shall be tested for tightness and any loose tendon found shall be reported to the SO Rep who will decide whether the members are acceptable. All tendons shall be marked at each end of every member so as to check the draw-in and any slip of the tendons. The procedure of release shall be continuous and shall be performed in the shortest possible time without interruption. The prestress shall be transferred to the members in such a manner that the tendons are released gradually and, preferably, simultaneously.

In document Standard Specifications for Building Works and Other Installations - 2012 (Page 191-199)