Percentage by weight passing BS Sieves BS 410
Test Sieve Grading Zone 1 Grading Zone 2 Grading Zone 3 Grading Zone 4
in mm
3/8 9.52 100 100 100 100
3/16 4.76 90 - 100 90 - 100 90 - 100 (95) - 100
No. mm
7 2.36 60 - (95) (75) - 100 (85) - 100 (95) - 100 14 1.18 30 - (70) (55) - 90 (75) - 100 (90) - 100
Microns
25 600 15 - 34 35 - 59 60 - 79 80 - 100
52 300 5 - (20) (8) - (30) (12) - (40) (15) - 50
100 150 0 - (10) 0 - (10) 0 - (10) 0 - 15
Store the fine and coarse aggregates separately and in such a manner that segregation of the various size particles shall not occur, form the stockpiles on a platform of weak concrete, timber or similar approved hard standing and keep aggregates clean and free from foreign substances.
2.37 SAMPLES OF AGGREGATES
Methods of sampling and testing of aggregates shall comply with SS 73 and BS 812. If required submit samples of the aggregates to the SO Rep at least two weeks before the Contractor begins to deliver materials to the Site. Materials shall not be delivered until samples have been approved. Samples of fine aggregates, each of about 0.015 cu m shall be submitted in air tight containers and samples of coarse aggregate each of about 0.030 cu m shall be submitted in suitable containers. Label all samples plainly with descriptions of the material and source of supply. Furnish additional samples as directed.
Test sieves shall comply with SS 74 and BS 410.
2.38 CEMENT MORTAR
Cement mortar for general purposes shall consist of one volume of Portland Cement to three volumes of fine aggregates unless otherwise specified or as directed by the SO Rep. Testing of cement mortar shall comply with BS 4551.
Thoroughly mix the materials in a dry state either by hand on a close jointed wooden platform or in a mechanical mixer of an approved type and sufficient water shall then be added and the whole mixed to a mortar of even and proper consistency. The proper consistency is that when moisture is excluded under a slight pressure, the mortar binds together into a solid lump. Use mortar immediately after it has been prepared and on no account shall it be used after it has commenced to set.
2.39 CONCRETE
Unless otherwise stated, concrete mix refers to "designed mix". In designed mixes, the strength shall be the main criterion specified, with compliance to minimum and maximum cement content, and maximum water/cement ratio. The workability of the mix shall be such that the measured slump shall not deviate from the agreed value by more than + 25mm or one third of the value, whichever is greater, or Clause 16.6 of BS 5328. Mix design shall be based on a target mean strength not less than the specified characteristic strength plus 7 N per mm2. The Contractor shall design the mix and shall be responsible to ensure that the mix supplied will give the required properties including strength. Unless otherwise specified for protection against aggressive soil conditions, designed concrete mixes shall comply with the specifications in the Table below : Design of the mix shall ensure no segregation, no excessive bleeding and ability to achieve specified surface finish. Segregation and bleeding shall be determined by visual inspection.
"Nominal mix" may be used in place of "designed mix" provided :
the works are of minor nature or involving a small quantity of concrete.
the approval of the SO Rep is given.
the strength of the concrete is still the responsibility of the Contractor.
The Contractor may adopt the prescribed nominal mix and preparation of ingredients as shown in the table below :
(b) Semi-dry sand (30% voids) of bulk density 1600 kg per m3.
(c) Coarse aggregate (45% voids) of bulk density 1400 kg per m3.
The whole of the concrete shall be dense, compact and thoroughly homogeneous. It shall be placed in foundations, walls, floors, slabs, moulds or shuttering after mixing as soon as practicable and no concrete shall be placed without being thoroughly tamped and compacted. Complete all tamping well before the initial setting of the cement commences.
The SO Rep reserves the right to instruct the Contractor to modify, alter and improve the concrete mix from time to time during the progress of the Works.
2.39 CONCRETE (CONT’D)
Keep the fine and coarse aggregates separately until actually mixed in the hopper of the concrete mixer, which shall be of the batch or other approved type. Mix each batch for a sufficient period to produce a mixture of even and proper consistency.
Immediately after mixing, convey the concrete to the place of final deposit as rapidly as possible by methods which will prevent the segregation, loss or contamination of any of the ingredients. In all cases place the concrete and compact within 20 minutes of the addition of water to the mix. Any concrete left unplaced after this time or showing any indication of drying out or taking its initial set shall be immediately rejected and removed from the Works.
Transport the concrete in clean metal buckets, barrows, dumpers or trucks and obtain the written approval of the SO Rep before any method involving the use of concrete pumps, placers, pipelines, chutes or spouts may be used.
For transporting concrete by means of chutes, segregation should be minimised by providing a downpipe at the end of the chute. Chutes shall be of metal or metal-lined, preferably round-bottomed, and large enough to guard against overflow. Slopes shall not be flatter than 1 to 3 or steeper than 1 to 2 for ordinary mixes. The SO Rep may allow the use of steeper slopes for stiff mixes.
Hand mixing shall be allowed only for small quantities of concrete and subject to the mixing being satisfactorily performed and approved by the SO Rep and to the addition of 10% extra cement.
Adjust the proportions of water and aggregates according to the moisture content of the aggregate.
Add only sufficient water to the cement and aggregates during mixing to produce a concrete having sufficient workability to enable it to be well consolidated, to be worked into the corners of the shuttering and around the reinforcement and to have the specified strength. When a suitable amount of water has been determined, maintain the resulting consistency throughout the Works and conduct tests from time to time to ensure consistency. The method of determining the consistency of concrete shall either be by slump test or by the compactor tests as defined in SS 78 and BS 1881. Supply all labour and materials necessary to carry out the tests as directed. All concrete which has too high a water content shall be rejected straightaway.
No concrete shall be re-mixed after it has once become partially set or hardened but shall be rejected and removed from the Works.
2.40 DEPOSITING AND COMPACTING CONCRETE
Do not tip the concrete from a height of more than 2 m but lowered in chutes or skips. It shall be placed in regular courses, in layers of not more than 250mm thick, punned down, beaten and neatly levelled to a smooth surface. It shall not be covered up until passed. When the Works is interrupted, the surface of the unfinished concrete shall be thoroughly cleaned, watered, coated with cement grout and covered with a layer of cement mortar, at least 10mm thick, immediately before further concrete is added and whenever practicable, wooden battens shall be inserted in the unfinished concrete and withdrawn to form a continuous key for further concrete, both in horizontal and vertical joints.
At all junctions between new and existing concrete work, clean, roughen, coat and spread with at least 10mm thick of cement mortar to the surface of the existing concrete; immediately before the new concrete is deposited. No concrete shall be placed unless the SO Rep has inspected the receiving surfaces and is satisfied with the conditions for concreting.
Concrete shall be normally compacted by means of approved immersion vibrators of minimum 5000 cycles per minute or otherwise well rammed between and around the steel reinforcement by hand rammers to ensure compactness and to avoid air-holes, or voids and to produce smooth surfaces, free from honey combs. Remove any water accumulating on the surface of newly placed concrete by approved means and
2.41 PROTECTION AND CURING OF CONCRETE
Allow no load of any kind, however light, on concrete which has not properly matured. Do not allow walking on or laying of planks on concrete until it has been properly matured and ready to carry load.
Allow a period of 24 hours between the laying of the concrete foundation and the laying of pipes or kerbstones before backfilling.
After being placed in position, all fresh concrete shall be protected from rain, sun and drying wind, and shall not be subjected to any vibrational disturbance other than that arising from compacting. The exposed surfaces of concrete, other than concrete foundation and haunching for kerbstones and pipes, shall be cured with an appropriate curing compound as approved by the SO Rep. The curing compound shall be applied by mechanical sprayer and comply strictly with the manufacturers' instruction. If the curing compound is exposed to rain or water within 4 hours from the time the application is completed, the Contractor shall re-apply the area affected with appropriate curing compound again.
The curing compound shall not be used in concrete containing Portland Blast Furnace Cement or Sulphate - Resisting Cement. In such instance, other appropriate curing method shall be adopted.
2.42 CONCRETING AGAINST SURFACE OF EXCAVATION
Place concrete only against surfaces of excavation when the receiving surfaces are clean, firm and free from any accumulation of water. Should the receiving surfaces deteriorate as a result of the Contractor's delay in placing concrete, the Contractor shall, as directed, cut the loose material and top-up with compacted firm material or lean concrete.
Unless otherwise specified, concrete sealing to foundations shall be 75mm Grade 15 concrete.
Wherever in the opinion of the SO Rep that ground conditions permit, the timbering shall be removed before concreting and the concrete placed in the walls for the full width from the shuttering to the face of the excavation in one operation. The Contractor's attention is drawn to the desirability of keeping the working space or overbreak to a minimum in all cases.
Where concreting in one width is called for by the SO Rep, payment for any extra concrete involved will be priced at the Rates and the appropriate item for rough shuttering will be omitted.
2.43 CONCRETING THROUGH WATER
Concreting shall not be deposited under water without the prior consent of the SO Rep. Submit details of the proposed method of placing concrete through water for the approval of the SO Rep. Design a concrete mix for this purpose with a high workability of approximately 150mm slump and an increased cement content of 25%.
2.44 FINISH OF CONCRETE
Upon removal of the shuttering and with approval of the SO Rep, make good honeycombed surfaces immediately by a method approved by the SO Rep and superficial water and air holes shall be filled in.
Unless instructed to the contrary, rub down the face of exposed concrete placed against shuttering with a carborundum stone immediately upon removal of the shuttering to remove fins or other irregularities. Float the face of concrete for which shuttering is not provided (other than slabs) to give a finish equal to that of the rubbed down face where shuttering is provided. No cement wash, mortar or paint may be applied to any concrete surface without the permission of the SO Rep. Generally concrete finishes as struck from the shuttering are required.
Lay all floors, slabs and inverts to the required depths, thickness, inclinations and curvatures shown on the detail Drawings. The falls shall be even so that no pockets occur and the concrete well rammed, consolidated and formed to a true and even slope by means of wooden screeds and straight edges, and immediately thereafter be floated over with a wooden float, any hollows or rough places unavoidably left after screeding being filled up with fine concrete. Subsequently and just before setting, finish off the concrete to a smooth and even surface with metal floats, unless otherwise directed by the SO Rep.
Do not lay concrete floors and inverts in two courses, except where specified or by special permission of the SO Rep in writing. Where screeds are required or approved, key and roughen the structural concrete to ensure proper bond.
First class plane surfaces free from ripples, hollows, air holes or exposed aggregate is required.
2.45 CONCRETE ADDITIVE
Suitable additives may be used in concrete only with the approval of the SO Rep. Apply the concrete additives as approved in accordance to the manufacturer's recommendation and instructions.
When directed by the SO Rep, carry out verification on the suitability of concrete additives by trial mixes and laboratory tests.
2.46 PREMIX AND READY-MIX CONCRETE
If the Contractor wishes to use ready-mix concrete, obtain prior approval from the SO Rep who will, inter alia, require to be satisfied that the proposed supplier will adhere to the requirement specified in the Specifications and that the quality and control of the mix are satisfactory and comply with SS EN 206 and BS 5328. The SO Rep shall have access during all working hours of the ready-mix plant. The ready-mix concrete shall comply in all respects with the specified requirements for concrete.
The concrete shall comply in all respects with the specified requirements for concrete and the Contractor shall arrange and pay for the making of all the requisite tests.
Transport premix concrete only in continuous mixing lorries of an approved design and type. Transport the concrete dry and the water shall be added to the mix only after arrival at the Site and immediately before placing.
Transport ready-mixed concrete only in continuous mixing lorries complying with the manufacturer's operating standard. Appropriate retarder when used shall be subject to the approval of the SO Rep. No additional water shall be added to the ready-mixed concrete.
2.47 TESTING OF CONCRETE CUBES
The methods of sampling and testing concrete shall comply with BS 78, BS 1881 and BS 5328. Provide a minimum of 4 numbers of standard metal moulds. No concreting Works shall be allowed to commence unless the required number of moulds is present at the Site. No extension of the Time for Completion shall be allowed for stoppage of work due to the Contractor's failure to comply with this requirement.
On each concreting day, for every 20 cubic metres or part thereof of each grade of concrete, at least one test shall be taken. At least 4 cubes shall be cast for each test. Two cubes shall be tested at 7 days and the remaining two cubes shall be tested at 28 days for compliance of the specified concrete strength.
Testing for Grade 15 Concrete is not required unless directed by SO Rep.
Compliance of the specified concrete strength shall be judged by tests made on cubes at an age of 7 days and 28 days respectively. Two test cubes shall be made for each test at the age required. The cube strength shall be calculated from the maximum load sustained by the cube at failure. The appropriate strength requirement shall be considered to be satisfied if :
(i) the average of the two cubes is not less than the specified minimum crushing strength; and (ii) the difference of the two cube strength is less than 15% of the average crushing strength.
The test cubes shall be cured in accordance with SS 78, BS 1881 and BS 5328. Concrete shall be placed in standard steel mould in layers approximately 50mm deep and each layer compacted by using a steel bar 300mm long weighing 1.8 kg and having a ramming face of 25mm square. The number of strokes per layer shall not be less than 35 for 100mm cubes or as per SS requirements. The cubes shall be tested at the HDB’s Engineering Materials Laboratory or other approved accredited testing laboratory with equivalent or better testing facilities, auto-capturing and computerized monitoring system with analysis capabilities. The Contractor shall submit the information pertaining to the proposed laboratory to SO Rep for approval. All contract information and test results shall be uploaded to EPTC e-cube website.
The Contractor shall provide lockable humid boxes of adequate size to store all the test cubes for the
2.47 TESTING OF CONCRETE CUBES (CONT'D)
The Contractor shall provide all necessary assistance to hand over the cubes to the HDB’s appointed contractor for the collection of cubes.
The Contractor shall provide facilities to enable the identification of test cubes by radio frequency or similar means. The Contractor shall provide at least one web-based radio frequency identification (RFID) reader capable of reading the RFID tags on the concrete cube from a minimum distance of at least 40 mm. The reader must be able to read all 125kHz RFID tags, including tags starting with hex codes of '2800' and
‘2868’. The Contractor shall include the appropriate interfacing software to a web-based controller.
The Contractor shall logon to a website of the Employer to register the concrete cube data and tag IDs using the RFID reader. The registration of cube data and tag IDs shall be done within the same casting day. The appropriate RFID tags shall be placed at the corner of the cubes while the cubes are still fresh.
The top surface of the RFID tag shall be level with the concrete cube surface. The Contractor shall protect the RFID tags on the concrete cube while demoulding. For damage or loss of RFID tag, the Contractor shall be charged the replacement cost based on the Employer's purchase price
The Employer shall deduct the charges payable by the Contractor from any monies due or becoming due to the Contractor or recover the same from the Contractor as a debt due by the Contractor to the Employer.
2.48 FAILURE OF TEST ON CONCRETE TEST CUBES
For each batch of concrete, make two test cubes for each test at 7 days and 28 days respectively. The crushing strength to be achieved shall be as specified in Clause 2.39 "Concrete". Notwithstanding that the test on the 7 days cubes pass the strength as specified in Clause 2.39 "Concrete", the test on the 28 days cubes shall still be required to be carried out.
The SO Rep shall exercise his rights under Clause 1.6 "Nuisance and Irregularities" to impose charges against the Contractor for each test that fails to attain the required crushing strength at 28 days as specified in Clause 2.47 "Testing Of Concrete Cubes". The SO Rep may also require that part of the Works represented by the cubes that has failed, be taken down or cut and replaced to comply with the Specifications or that additional measures shall be provided to make the Works secure.
2.49 CONCRETE WALLS
Concrete walls generally shall be constructed in alternate bays not exceeding 7m in length. The height of any lift of concrete shall not exceed 2.5m unless otherwise permitted by the SO Rep. Walls shall be truly
Concrete walls generally shall be constructed in alternate bays not exceeding 7m in length. The height of any lift of concrete shall not exceed 2.5m unless otherwise permitted by the SO Rep. Walls shall be truly