STRUCTURAL PRECAST PRE-STRESSED CONCRETE
1.01. Quality assurances
1. Comply with the provisions of the following codes, specifications and standards, except as otherwise shown or specified.
a. ACI 318 “Building Code Requirements for Reinforced Concrete”.
b. Precast Concrete Institute, PCI Design Hand Book MNL 120
c. PCI Quality Control for precast and prestressed concrete MNL 116
d. PCI Manual for Quality Control of Arch. Precast Concrete MNL-117
e. British Standard, BS 8110 when applicable.
A. Manufacturer’s Data
1. Submit copies of manufacturer’s specifications for all manufactured materials and products. Include manufacturer’s certifications and laboratory test reports as requested.
2. Include erection procedure for precast units, sequence of erection and required handling equipment.
B. Shop Drawings
1. Submit shop drawings, complete information for the fabrication and installation of precast concrete units. Indicate member dimensions and cross section, location, size and type of reinforcement, including lifting devices necessary for handling and erection.
2. Provide layout, dimensions and identification of each precast unit corresponding to the sequence and procedure of installation. Indicate welded connections, detail inserts, connections and joints including accessories and construction at opening in precast units.
3. Provide location and details of anchorage devices that are to be embedded in this and other construction.
4. Provide manufacturer’s complete design calculations prepared by a professional engineer.
1. Submit one full size sample of each required precast concrete unit to the project site at start of installation work. Acceptable samples to be incorporated in the construction.
1.03. Delivery, storage and handling
A. Deliver precast concrete units to the project site in such quantities and at such times as will assure the continuity of the installation. Store units at the project site to limit physical damage and so that markings are visible. Lift and support units at the designated points only.
1.04. Job Conditions
A. Examine all parts of the supporting structure and the conditions under which the precast concrete work is to be erected. Do not proceed with installation until unsatisfactory conditions have been corrected.
B. Design modifications may be made only as necessary to meet filed conditions and to ensure proper fitting of the work and only as acceptable to Client/Main Contractor.
PART 2 – PRODUCTS
A. Provide forms and where required, form facing materials of metal, plastic, wood or other acceptable materials that in non-reactive with concrete and will produce the required finish surfaces.
B. Accurately construct forms, mortar-tight and of sufficient strength to withstand all pressures due to concrete placing operations, temperature changes and when prestressed, pretensioning and detensioing operations. Maintain form work to provide completed precast concrete units of the shapes, lines and dimensions indicated, within the specified fabrication tolerances.
2.02. Reinforcing Materials
A. Reinforcing Bars: ASTM A615 and as follows:
1. Provide Grade 60 Bars, where otherwise shown.
B. Spiral Reinforcement: ASTM A82, plain cold drawn, size and configuration as indicated.
C. Steel Wire: ASTM A82, plain, cold-drawn, steel.
D. Welded Wire Fabric: ASTM A497, welded steel wire fabric.
E. Prestressing Strand Standard
BS 5896/3 Standard Strand – 1700 – 9.3 – Relax 2. or ASTM A416
F. Support for Reinforcement: Provide supports for reinforcement including bolsters, chairs, spacers and other devices for spacing, supporting and fastening, reinforcing, complying with PC and CRS recommendations.
1. For exposed to view concrete surfaces, where legs of supports are in contact with forms, provide supports with legs which are plastic coated.
2.03. Concrete Materials
A. Cement: Ordinary Portland cement confirming to standard ASTM C150 (latest edition).
B. Cement may be accepted on the basis of a manufacturer’s mill certificate that the cement furnished meets the physical and chemical requirements of ASTM Designation C 150.
C. Aggregates: ASTM C33 and as herein specified. Provide aggregates from a single source. Do not use aggregates, which have not been tested in approved mixes.
1. Fine Aggregates: Clean, sharp, natural sand, free from loam, clay, lumps or other deleterious substances.
2. Coarse Aggregate: Clean, uncoated processed aggregate containing no clay, mud loam or foreign matter, as follows:
a) Crushed stone, processed from natural rock or stone.
D. Water: Complying with ACI301.
1. Air-entraining Admixture: ASTM C260
2. Water-reducing Admixture: ASTM C494m, Type A.
3. Retarding Admixture: ASTM C494, Type B.
F. Calcium Chloride: Do not use calcium chloride.
2.04. Grout Materials
A. Cement grout: Portland Cement, ASTM C150, Type II, Aggregate ASTM C144 with special gradation requirement.
B. Concrete grouting
2.05. Proportioning and Design of Precast Concrete and Site Concrete
A. Prepare mix designs for each type of concrete required. Mix designs to be prepared by qualified precast manufacturing plant personnel.
B. Proportion mixes by either laboratory trial batch or field experience methods, acceptable to Client using materials to be employed on the project for each type of concrete required, complying with ACI 211.1.
C. Concrete shall comply with the applicable provisions of Section 03300 except for the following specific requirements.
1. Produce standard weight concrete consisting of the specified Portland cement, aggregates, admixtures and water to produce the following properties.
a) Compressive cylinder strength: 25 Mpa minimum at 28 days.
b) Aggregate size: 10mm or 20mm.
c) Minimum cement content: 380 Kg. per cubic meter of concrete.
d) Slump at point of discharge: Zero slump for Hollow core and 75mm slump for normal precast.
2. Cure compression test cylinder or cubes using the same method as will be used for the precast concrete work.
D. Proposed mix for each type of concrete prior to start of precast unit production to be as follows:
Hollow Core Slabs: Design Mix
Site Concrete: Design Mix
E. Mortar bedding (Optional only for Subcontractors discretion)
Where deemed required on beam bearing, can consist of prior layer or post packing of gap under slabs with dry mortar mix.
3 : 1 sand cement with 0.3 water ratio.
PART 3 – EXECUTION
A. General: Fabricate precast concrete unit complying with the manufacturing and procedures, quality control recommendations, specified for the types of units required.
B. Jobsite mixing: Mix materials for concrete in an acceptable drum type batch machine mixer. For mixers of one cubic meter or smaller capacity, continue mixing at least 1½ minutes, but not more than 5 minutes after all ingredients are in the mixer, before any part of the batch is released. For mixers of capacity larger than one cubic meter, increase the minimum 1½ minutes of mixing time 15 seconds for each additional cubic meter, or fraction thereof.
1. Provide a batch ticket for each batch discharged and used in the work, indicating the project identification name and number, date, mix type, mix time, quantity and amount of water introduced.
C. Ready-mix Concrete; Comply with the requirements of ASTM C94 in Section 03300 and as here specified.
1. Delete the references for allowing additional water to be added to the batch for material with insufficient slump. Addition of water to the batch will not be permitted.
2. During hot weather, or under conditions contributing to rapid setting of concrete, a shorter mixing time than specified in ASTM C 94 may be required.
a) When the air temperature is between 29 degrees centigrade and 35 degrees centigrade, reduce the mixing and delivery time from 90 minutes to 75 minutes and when the air temperature is above 35 degrees centigrade, reduce the mixing and delivery time to 60 minutes.
D. Holes for opening larger than 100mm diameter or 100mm square in accordance with final shop drawings. Other smaller holes will be field cut by trades requiring them after approval of site management.
E. Clean reinforcement of loose rust and mill scale, earth oil and other materials, which reduce or destroy bond with concrete.
F. Accurate position, support and secure reinforcement against displacement by formwork, construction or concrete placement operations. Locate and support reinforcing by metal chairs, runners, bolsters, spacers and hangers as required.
G. Place reinforcement to obtain at least the minimum coverage for concrete protection. Arrange, space and securely tie bears and bar supports to hold reinforcement in position during concrete placement operations.
3.02. Concrete Finishes
A. Surface used for developing composite action with insitu concrete shall, when the concrete has set but not hardened, have the surface brushed with a stiff brush, just sufficient to remove the outer mortar skin and expose the larger aggregate without disturbing it.
B. Formed surfaces not exposed: Normal plant run finish produce in forms that impart a smooth finish to the concrete. Small surface holes caused by air bubbles normal form joint marks and minor chips spalls will be tolerated but not major or unsightly imperfections, honeycomb or structural defects will be permitted.
3.03. Installation, General
A. Refer to other precast concrete and prestressed concrete sections for installation requirements for those particular precast units.
B. Bearing Pads: Install bearing pads where indicated, to be used only precast units being erected. Set pads on level, uniform bearing surfaces and maintain in correct position until precast units are placed.
C. Power Actuated Fasteners: Do not use power-actuated fasteners for surface attachment of accessory items in any precast, prestressed unit unless approved in writing by the precast manufacturer.
D. Grouting shear keys; After precast concrete units have been placed and secured, grout open spaces at connection and joints as follows:
1. Cement grout consisting of 1 part Portland cement, 3 parts sand and only enough water to properly mix and for hydration.
2. Provide forms or other acceptable method to retain the grout n space until sufficiently hard to support itself. Pack spaces with stiff grout material, tamping until voids are completely filled. Place grout to finish smooth, plump and level with adjacent concrete surfaces. Keep grouted joints damp for not less than 24 hours after it has taken its initial sets. Promptly remove any grout material for exposed surfaces before it hardens.
3.04. Quality Control Testing
A. The Contractor shall employ a separate testing laboratory to evaluate precast manufacturer’s quality control and testing method.
B. The precast manufacturer shall allow the Contractor’s testing facility access to materials storage areas, concrete production equipment and concrete placement and curing facilities. Cooperate with Contractor’s testing laboratory and provide samples of materials and concrete mixes as may be requested for additional testing and evaluation.
C. Dimensional tolerances:
1. Units having any dimension smaller or grater than required and outside the specified tolerance limits, will be considered deficient strength and subject to additional testing as herein specified.
2. Precast units having any dimension greater than required will be rejected if the appearance or function of the structure is adversely affected, or if the larger dimensions interfere with other construction. Repair or remove and replace rejected units as required to meet the construction conditions.
D. Strength of Units
1. The strength of precast concrete units will be considered potentially deficient if the manufacturing processes fail to comply with any of the requirements which may affect the strength of the precast units, including the following conditions:
a) Failure to meet compressive strength tests requirements.
b) Reinforcement and pre-tensioning and de-tensioning of tendons of pre-stressed concrete, not conforming to specified fabrication requirements.
c) Concrete curing and protection of precast units against extremes in temperature not as specified.
d) Precast units damaged beyond repair during handling and installation.
E. Testing precast concrete units for strength
1. When there is evidence that the strength of precast concrete units does not meet specification requirements, the concrete testing service shall take cores drilled from hardened concrete for compressive strength determination, complying with ASTM C42 and as follows or where core test is not suitable, testing by a full load test.
a) Take at least 3 representative cores from precast units of suspect strength from locations directed by Client/Main Contractor.
b) Test cores in a saturated surface dry condition per ACI 318 if the concrete will be wet during the use of the completed structure.
c) Test cores in an air dry condition per ACI 318 if the concrete will be dry at all times during use of the completed structure.
d) Strength of concrete for each series of cores may be considered satisfactory if their average compressive strength is at least 90% of an if no single core tests less than 80% of the 28 day design compressive strength.
e) Test results will be made in writing on the same day that tests are made, with copies to client, contractor and precast manufacturer. Include in test reports the project identification name and number, date, name of precast concrete manufacturer, name of concrete testing service, identification letter, number and type of member or members represented by core tests, nominal maximum size aggregate, design compressive strength, compression breaking strength and type of break (corrected for length, diameter ratio), direction of applied load to core with respect to horizontal plane of the concrete as placed and the moisture condition of the core at time of testing.
2. Patching: Where core test results are satisfactory and precast units are acceptable for use in the work, fill core holes solid with low slump concrete or patching mortar and finish to match adjacent concrete surfaces.
F. Defective work
1. Precast concrete units which do not conform to the specified requirements, including strength, tolerances and finishes, shall be replaced with precast concrete units that meet the requirements of this Section. The contractor shall be responsible for the cost of corrections to any other work affected by or resulting from corrections to the precast concrete work.
3.05. Preparation of Supports
Contractor to prepare bearing on support beams to ensure all are uniformly level, fill and aligned to within the allowable tolerance. Where beam is not uniform or structurally inadequate, contractor to make good to suit proper placement and support for slabs.
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