Concrete mix and grade of concrete:- Research on the construction of Reinforcement Concrete(RCC), concrete mix, concrete properties, Reinforced concrete, class of concrete and complex task related to concrete, there is a series of steps and rules followed for proper High strength RCC structure construction and for there Durability.
Class of Concrete mix and Grade of concrete
A. General: All structural concrete conform to the appropriate class and grade for strength and exposure conditions of the appropriate ACI, IS or BSS standards.
B. Unreinforced Concrete for Foundation and Floor Slabs and other structural Parts (M10, M15, and M20): Furnish forms and place foundation concrete as specified. Concrete is placed in sections according to the construction drawings or as specified by the Employer’s Representative.
Related:- How Concrete is Made ( New Research)
Contraction joints are not the subject of separate payment items. Construction joints in the foundation slab are only allowed if directed by the Employer’s Representative. On connection to walls, water stops shown on the Drawings or as specified by the Employer’s Representative is cast into the fresh concrete. Visible concrete surfaces are finished in fair-faced concrete. This item includes the construction of all recesses, block outs and voids.
C. Concrete Mixes: Concrete is consist of cement, graded aggregate, and water thoroughly mixed and completed to provide the strengths as ACI, IS Code detailed below:
Nom max size of Agg Proportion Min. Compressive strength
of aggregate at 28 days
A 38mm 1:2:4 150kg/cm2
AA 19mm 1:2:4 150kg/cm2
B 38mm 1:3:6 100kg/cm2
C 38mm 1:4:8
The mix proportions shown in the table above are given as a guide to the mixes ordinarily needed to achieve the specified strengths. The actual mix designed after tests depends upon the closeness of control which the contractor is prepared to exercise in production and upon the quality of the materials that obtained.
The contract rates included in the Bill of Quantities for all types of concrete are to allow for any additional cement which are required to achieve the specific strengths and to produce a dense impermeable and stable concrete.
Preliminary trial mix design of concrete will be carried out by the Contractor in the presence of the Employer’s Representative to determine the design mixes which can satisfy the specification with the available materials. These mixes are designed with due regard for the workability necessary to allow the constructor to place and compact the concrete with the equipment be proposes to use in any particular situation.
D. Water Cement Ratio:
The water cement ratio for the various types of concrete is determined by the preliminary tests and in no case, the water ratios used be allowed to exceed those determined by the preliminary tests.
The contractor should have knowledge of the moisture content of all sand and coarse aggregate as they reach the mixer and shall make such adjustments to the mix, as are necessitated by changes in the moisture content of all aggregates.
- A. Unless otherwise specified and/or approved by the Employer’s Representative, concrete should compose of Portland cement and aggregate and water as specified, well mixed and brought to the proper consistency.
- B. Grout is composed of Portland cement, sand and water all proportioned and mixed to the consistency approved by the Employer’s Representative. A nonshrink compound is required by the Employer’s Representative where this is deemed necessary, in which case an extra payment shall be agreed upon between the Employer’s Representative and the Contractor
- C. For nonstructural concrete and blinding, the proportion of Portland cement to sand and aggregate should not be less than 1 to 8 by volume. The minimum weight of cement in each cubic meter of concrete placed is 225 kg.
- D. For structural concrete, the “Design Mix“ is used. The “Design Mix” is established from the test mixes to attain the stipulated characteristic strength. The minimum quantity of cement in each cubic meter of concrete placed is 325 kg.
- E. The proportion in which various ingredients are to be used is as determined and approved from time to time during the progress of the works and tests made of the aggregates and the resulting concrete. The mix proportion and the appropriate water/cement ratio are determined on the basis of producing concrete having suitable impermeability, durability, workability and required strength without the use of an excessive amount of water or cement. The new water/cement ratio of the concrete (inclusive of the water within or absorbed by the aggregate) are not exceed 0.54 by weight for structural concrete, and not exceed 0.66 by weight for other classes. Tests for the concrete are made by the Employer’s Representative and the mix proportions is changed whenever necessary for the purposes of securing the required economy, workability, density, impermeability, durability or strength, and the Contractor is not be entitled to additional compensation because of changes.
- F Additives: Plasticizers, waterproofing compounds, and the other additives are not used unless approved in writing by the Employer’s Representative.
- G. The Contractor at his expense provide the Employer’s Representative with such equipment and activities as may be necessary for procuring and testing representative samples of the concrete or cement to the satisfaction of the Employer’s Representative.
- H. The frequency of the tests is determined by the Employer’s Representative on the basis of placement rate and the structure, but not more often than necessary to make sure that the concrete being placed conforms to the Specification and the design requirements.
Materials for Concrete
Concrete Reinforcement Steel:
The Contractor-furnished steel reinforcement bars conform to IS Code or equivalent.
- Water: The Water in the concrete, mortar, and grout is free from objectionable quantities of silt, organic matter, alkalis sulphates, and other impurities. The water to be used in all concrete, mortar, and grout is subject to the approval of the Employer’s Representative.
- General: All aggregate materials used for the Contract and for all related purpose, and is required by the Employer’s Representative, are consist of materials herein specified and the requirements set forth herein shall apply except where such stipulations and requirements are specifically modified by the Employer’s Representative for any particular item of work.
- Handling and Stockpiling:
- The Contractor can excavate as required, load, transport, unload, handle and stockpile sand and aggregates as required to perform the construction of the Works and structures herein. All methods employed by the Contractor for unloading, loading, handling and stockpiling of sand and aggregates and the location and arrangement of all stockpile areas are approved from the Employer’s Representative.
- The Contractor clear and evenly grade for drainage all sites destined for stockpiling and is handle stockpiling operations of sand and aggregates so that the segregation and breakage will be kept to a minimum and that stockpiled materials will not be contaminated with soil or other foreign material due to surface floods of groundwater.
- The Contractor required at his own expense to reprocess any sand or aggregates which may become segregated or contaminated due to improper stockpiling and lack of adequate protection as directed by the Employer’s Representative.
- The Contractor conducts all stockpiling operations in such a manner as to deposit all materials directly in final position in the stockpiles and in layers of not more than 1.5-meter height. Sand and aggregates not be moved from place to place in the stockpiles except as superficial leveling may be necessary to provide suitable roadways for trucks in placing successive layers, and the Contractor should provide effective means to prevent breakage of aggregates caused by trucks operating over stockpiles. Dumping over the ends or sides of the stockpiles are not be permitted
- Sand: All natural sand required for the construction of the works is furnished by the Contractor and may be obtained from river deposits or other approved natural sources. If natural sand is obtained from sources not owned or controlled by the Employer, the Contractor makes all necessary arrangements with the source owner and pay all rentals or other costs connected therewith.
- Samples: The Contractor submits to the Employer’s Representative, for preliminary tests and approval of the material, a representative sample of about 15 kg of natural sand proposed for use at least 14 days before the use of such materials is required. Approval by the Employer’s Representative of sources of natural sand should not be construed as constituting approval of all materials taken from the sources, and the Contractor is responsible for the specified quality of all such materials used in the work.
- The deposit of natural sand is cleared by the Contractor of all vegetation and other objectionable matters, and all unsuitable soil, sand, and gravel should be removed. The deposit should developed and operated so as not to detract from the usefulness of the deposits. The materials are screened and washed as necessary to produce natural sand meeting the requirements set forth herein
- Cleanliness and Quality: Sand or fine aggregates should clean and free from clay lumps, soft and flaky particles, alkali, organic matter, loam, mica and injurious amounts of deleterious substances. The sum of percentages of all deleterious substances must not exceed 5% by weight. Sand shall be sharp, cubical, hard and durable. Loss of fine aggregate subject to five cycles of sodium sulphate soundness test should not exceed 5 percent.
- Fine Aggregates: Fine aggregate should be natural sand or sand derived by crushing gravel or stone and should be free from coagulated lumps; sand derived from stone unsuitable for coarse aggregate are not used as fine aggregate. The following grading are taken for general guidance.
Percentage Weight Passing B.S. Sieves
|B.S. 410 Test Sieve
|Grading Zone 2
- All natural and blended sand should be subject to testing by the Employer’s Representative to determine whether the sand produced conforms to the requirements of the specification. The Contractor is furnished, without charge, such assistance as the Employer’s Representative may require in obtaining representative samples for testing purposes and in inspecting plant facilities and operations of the Contractor. The Contractor at his expense furnishes the Employer’s Representative with all the equipment needed in order to make the above tests and analysis.
- Coarse Aggregate: Coarse Aggregate is crushed or uncrushed gravel or crushed stone. The pieces should be angular or rounded in shape and could have granular or crystalline or smooth (but not glossy) non-powdery surfaces. Friable, flaky, and laminated pieces of mica are only present in such quantities as not to affect adversely the strength and durability of the concrete. The nominal sizes of the coarse aggregate shall be 38 mm to 5 mm in structural Concrete and well graded to the satisfaction of the Employer’s Representative.
The following grading are taken for general guidance percentage by weight passing 85 sieves:
|Normal Size of graded aggregate
|38 mm to 5 mm
|19 mm to 5 mm
The Contractor provide such means and equipment as are required accurately to determine and control the amount of each separate ingredient entering the concrete mix. Such means and the equipment and its operation at all times be subject to approval of the Employer’s Representative. All dry materials entering the mixer are measured by weight or calibrated volume boxes.
A. The concrete ingredients are mixed in a batch mixer or portable mixer for not less than 1 ½ minutes after all ingredients (except for the full amount of water) are in the mixer. The mixing time is increased where the batch mixer exceeds a capacity of 1.5 m3.
The Employer’s Representative reserves the right to increase the mixing time when the charging and mixing operations fail to produce a concrete batch throughout which the ingredients are evenly distributed and consistency uniform. The concrete is uniform in composition and consistency from batch to batch except when changes in composition, consistency are required.
Water is added prior to, during and following the mixer charger operations. Excessive overmixing requiring the addition of water to preserve the required concrete consistency is not permitted.
B. Hand mixing of concrete is not permitted, except that the Employer’s Representative may approve such operations for small size concrete works upon request of the Contractor.
C. Truck mixers are permitted only when the mixers and their operation are such that the concrete throughout the mixed batch and from batch to batch is uniform with respect to consistency and grading. Any concrete retained in truck mixers so long as to require additional water to permit satisfactory placing are discarded at the expense of the Contractor.
D. Any mixer that at any time produces unsatisfactory results is repaired. Mixers in centralized batch and mixing plants are arranged so that mixing action in the mixers can be observed from a location convenient to the mixing plant operator’s station. Mixers are not loaded in excess of their rated capacity unless specifically authorized.
Formwork (Shuttering and Centering)
A. Design and Material: Forms conform to the various shapes, lines, grades, and dimensions of the concrete structures as shown on the Drawings or as determined by the Employer’s Representative. The material used and the design of the forms subject to approval by the Employer’s Representative before the construction of the forms is started; provided however that such approval is not relieved the Contractor from his responsibility for the adequacy of the forms or from his obligation to remedy any defects which may develop or become apparent with use of the forms.
The Employer’s Representative at any time condemn any sections of forms found deficient in any respect, and the Contractor should promptly remove the condemned forms from the work and replace them at his expense. Drawings showing the general design and dimensions for forms for structures are not be submitted by the Contractor to the Employer’s Representative for approval unless the Employer’s Representative orders such submission.
B. Form Constructions: Forms to confine the concrete and shape it to the required lines are used wherever necessary. Forms are of metal, of metal lined timber, plywood lining, tempered pressed wood lining, or of smoothly planed boards, in good condition as required to produce the surface finish specified herein
- The surface of concrete exposed to view on completion of the works are fair faced. The forms for such surfaces are made of metal and are true in every respect to required shape and size and are of sufficient strength and rigidity to maintain their position and shape under the loads and operations incident to placing and vibrating the concrete. All forms on waterway surfaces are planed and cleaned to eliminate marks in so far as it is practicable.
- Suitable and effective means are provided in the construction of forms for holding adjacent edges and ends of panels and sections tightly together and inaccurate alignment so as to prevent the formation of ridgeline offsets or similar surface defects in the finished concrete. Forms, when erected, should be grout tight. Only in specific cases, where it requires smoothing of the corners and curves, wooden forms are allowed upon the approval by the Employer’s Representative.
- Surface Treatment: Adequate and suitable means for removing the forms without injury to the surface of the finished concrete is provided. Before concrete is placed, the surface of the forms is oiled with commercial form oil that will effectively prevent sticking of the concrete to the forms and is not stain the concrete. Bond breaking materials or processes are used only after approval by the Employer’s Representative. Care is taken in applying oil to avoid contract with reinforcing steel with resulting loss of bond
C. Chamfering: Chamfer strips are placed in the corners so as to produce beveled edges on permanently exposed concrete surfaces. Interior angles on such surfaces and edges at formed joints are not require bevelling unless the requirement for bevelling is so indicated on the Drawings.
D. Centering: Forms are properly secured in position so as to prevent floating or another movement during the placing of concrete. They are supported during placement of concrete on concrete piers, metal pedestals or by other approved means. Form supports should be carried to a firm foundation so that no settlement of the forms will be possible during construction.
Fabrication and Cleaning Reinforcing Steel
A. Reinforcement bars before being placed is free from loose mill scale rust, oil, grease and any coatings that will destroy or reduce the bond. When after placing there is a delay in placing the concrete, the steel bars are re-inspected and cleaned when required by the Employer’s Representative.
B. The steel bars should accurately be bent and formed to the dimensions indicated on the Drawings included herein or on the approved reinforcing steel detail drawings.
C. The steel bars are not straightened or be bent in a manner that will injure the material. Bars with kinks or bends not shown on the Drawings are not used. All bars shall be bent cold. Heating of the bars for bending is permitted only if the entire operation is approved by the Employer’s Representative.
Placing of Reinforcement
A. General: Reinforcement bars are accurately positioned as shown on the Drawings and secured against displacement by using iron wire ties or suitable clips at intersections, and is supported as required by concrete or metal supports. Supports for horizontal reinforcement are used so that there will be no sagging of the bars or mesh. Where portions of the supports are exposed on concrete surfaces designed to receive a smooth finish, the supports are made of non-corrodible metal.
B. Cover: The minimum concrete cover to reinforcement is 30 mm or as specified by the Employer’s Representative, but in no case will the minimum concrete cover be less than 1.5 bar diameters. Bars are bent and fixed with suitable spacers to ensure that the minimum cover is maintained. The ends of tying wires are bent inwards or cut off so as to have 30 mm minimum concrete cover.
C. The minimum distance between parallel bars is 25 mm, but in no case be less than 1.5 times the maximum size of the coarse concrete aggregate, unless particular instruction is given by the Employer’s Representative.
When it becomes necessary to splice reinforcement bars at points other than shown on the Drawings, for floors, walls, and columns the minimum overlaps of splices are thirty times the steel diameter of the larger bar and is subject to the approval of the Employer’s Representative.
Preparation for Placing of Concrete
A. No concrete is placed until all formwork, reinforcement, installation of parts to be embedded, bracing of forms and preparation of surfaces necessary before placing have been approved by the Employer’s Representative. No concrete is placed in water without the written permission of the Employer’s Representative, and the method of depositing the concrete should subject to his approval. Concrete is not placed in running water until the concrete has sufficiently hardened. All surfaces of forms and embedded materials that have become encrusted with dried mortar or grout from concrete previously placed are cleaned of all such mortar or grout before the surrounding or adjacent concrete is placed.
B. Cleaning of Surface: Immediately before placing concrete, all surfaces upon or against which the concrete is to be placed is free from standing water, mud, debris or loose material. The surfaces of absorptive materials against or upon which concrete is to be placed are moistened thoroughly so that moisture will not be drawn from the freshly placed concrete.
C. Blinding Course: Where shown on the Drawings or where required by the Employer’s Representative, the Contractor lay a blinding course of non-structural concrete. The surface of the blinding course is finished to the dimensions shown on the Drawings and is dense and smooth. No concrete is placed on the blinding course until at least 24 hours after laying. No structural concrete is placed against the ground without either formwork or blinding.
D. Construction Joint Surfaces: Concrete surfaces upon or against which concrete is to be placed, and to which new concrete is to adhere, and which have become so rigid that the new concrete cannot be incorporated integrally, are defined herein as “construction joints”. The surfaces of construction joints are clean and moistened before being covered with fresh concrete or mortar. Cleaning consist of the removal of all loose or defective concrete coatings or foreign material. The surface of construction joints should cleaned by wet sandblasting, scraping, chipping or other approved methods and then washed thoroughly (e.g. with high-pressure air-water jets) to the satisfaction of the Employer’s Representative immediately prior to placement of fresh concrete. All pools of water are removed from the surface of construction joints before placing of the following concrete layer.
E. Placing Equipment: Concrete is compacted, except when the Employer’s Representative decides otherwise, with the help of concrete vibrators of approved type and capacity. Transporting concrete inside the forms with the help of vibrators is not allowed. Before placing of any concrete the Contractor should present to the Employer’s Representative for approval the list of all equipment available for the particular pour of concrete, including number, capacity and the state of repair of the said equipment. The methods and equipment used for transporting and placing of concrete are such that concrete having the required composition and consistency will be delivered to and placed into the work, without objectionable segregation or loss of slump.
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