Unless otherwise stated in the Drawings, the concrete mix shall be designed mix. However, prescribed mix may be used provided: -
(a) The work is of minor nature or involving a small quantity of concrete, (b) Prior approval of the S.O. is given, and
(c) The strength of the concrete is still the responsibility of the Contractor.
When Portland Pulverized-Fuel Ash Cement or Portland Slag Cement is specified to be used, the concrete mix shall be of designed mix only.
3.1 Prescribed Mix
Prescribed mix shall be as detailed in Table 5.The mix prescribed in the table does not require the use of admixture.
For small volume concreting work, volume batching is permitted provided prior approval of the S.O. is obtained. The proportion shall be as specified in Table 5A.
3.2 Designed Mix
Designed mix shall comply with the recommendations of MS1195. The minimum cement content and maximum free water: cement ratio to be used shall be as shown in Table 6 and this shall supersede Table 14 of BS 5328.
The Contractor shall comply with the following requirements:
a) Notify the S.O. whether the designed mix is to be produced as site mix or ready mix. b) If the Contractor chooses to use ready mix concrete, he shall notify the S.O. the name of the
supplier, location of the ready mix plant, journey time taken to transport the concrete to the Site and production capacity of the plant.
c) Submit a mix design report covering all concrete mixes to the S.O. for approval. The designed mix shall comply with the requirements specified in Table 6.
d) The Contractor shall ensure that the S.O. be permitted to visit or station his representative at the plant at any stage of the concrete production.
e) The sampling, making, curing and testing of the cubes shall be carried out by the Contractor in accordance with MS 30 or as directed by and in the presence of the S.O. or his representative. Test cubes shall be stored by the S.O. at a secured place. The Contractor shall provide all facilities in connection therewith.
When Portland Pulverized-Fuel Ash Cement complying with MS 1227 or Portland Slag Cement complying with MS 1389 is used, depending on the percentage combination used as allowed by MS 1226 or MS 1387 respectively, it may be necessary to increase the total cement content to achieve concrete of equal strength development when compared with concrete using Portland cement complying with MS 522. Tests on trial mixes shall be carried out in accordance with 3.2.3 and the results shall be approved by the S.O.
In all cases, it is the responsibility of the Contractor to ensure that the designed mix shall be durable, workable, comply with the strength grade and other requirements as specified in the Drawings.
3.2.1 Target mean strength
The concrete mix shall be designed to have at least the required minimum cement content, as specified in Table 6, and to have a target mean strength greater than the required grade of concrete by at least the current margin.
The current margin for each particular type of concrete shall be determined by the Contractor and shall be taken as the lesser of:
a) 1.64 times the standard deviation of cube tests on at least 100 separate batches of concrete of nominally similar proportion of similar materials and produced over a period not exceeding 12 months by the same plant under similar supervision, but not less than 2.5 N/mm2 for concrete, of grade 15 or 3.75 N/mm2 for concrete of grade 20 or above.
b) 1.64 times the standard deviation of cube tests on at least 40 separate batches of concrete of nominally similar proportions of similar materials and produced over a period exceeding 5 days but not exceeding 6 months by the same plant under similar supervision, but not less than 5 N/mm2 for concrete of grade 15 or 7.5 N/mm2 for concrete of grade 20 or above.
Where there are insufficient data to satisfy (a) and (b) above, the margin for the initial mix design shall be taken as 7.5 N/mm2 for concrete of grade 15 and 12 N/mm2 for concrete of grade 20 or above. This margin shall be used as the current margin only until sufficient data are available to satisfy (a) or (b) above. However, subjected to the approval of the S.O., when the specified characteristic strength approaches the designed target mean strength of concrete made with a particular aggregate, a smaller margin not less than 5 N/mm2 for concrete of grade 15 or 7.5 N/mm2 for concrete of grade 20 or above may be used for the initial mix design.
3.2.2 Suitability of Proposed Mix Proportions
The Contractor shall submit for the approval of the S.O., prior to the supply of any designed mix, the following information:
a) The nature and source of each material,
b) Appropriate existing data as evidence of acceptable previous performance for target mean strength, current margin, workability and water: cement ratio; OR full details of tests on trial mixes carried out in accordance with 3.2.3, and
c) The proposed quantities by weight of each material per cubic meter of fully compacted concrete.
3.2.3 Trial Mixes
Unless otherwise stated and approved by the S.O in writing that the Contractor has complied with the submission specified in 3.2.2, trial mix shall be carried out. The Contractor shall give early notice, to enable the S.O. to be present at the making of trial mixes and preliminary testing of the cubes. The Contractor shall prepare trial mixes, using samples of approved material typical of those he proposes to use in the Work, for all grades to the approval of the S.O. prior to commencement of concreting.
Sampling and testing procedures shall be in accordance with MS 26. Three separate batches of concrete shall be made. The workability of each of the three trial batches determined by means of the slump test or compacting factor test or vebe consistometer test or as specified in accordance with MS 26, shall be appropriate to the proposed uses and methods of placing and compaction of the mix and shall be approved by the S.O. Six cubes shall be made from each batch. Three from each set of six shall be tested at an age of 7 days and three at an age of 28 days. The average strength of the nine cubes tested at 28 days shall exceed the specified characteristic strength by the current margin minus 3.5 N/mm2. The approved trial mixes shall then be designated as the ‘designed mix’ and its corresponding workability as the ‘designed workability’.
During production, the S.O. may require additional trial mixes to be made before a substantial change is made in the materials or in the proportion of the material to be used. Trial mixes need not be carried out when adjustments are made to the mix proportions in accordance with 3.2.4.
3.2.4 Control Strength of Designed Mixes a) Adjustment to Mix Proportions
Adjustment to mix proportions shall be made to the approval of the S.O. in order to minimise the variability of strength and to maintain the target mean strength. The specified limits of minimum cement content and maximum free water: cement ratio shall be maintained. Changes in cement have to be declared.
An increase or reduction of cement content by greater than 20 kg/ m³, or of fine aggregate by greater than 5%, or any changes in the type or dosage of admixture, or any changes in the type of cement, or consistency is considered a change in mix design.
b) Change in Current Margin
A change in current margin used for judging compliance with specified characteristic strength becomes appropriate when results of a sufficiently large number of tests show that the previously established margin is significantly too large or too small.
Recalculation of the margin shall be carried out in accordance with 3.2.1. Although a recalculated margin is almost certain to differ numerically
from the previous value, the adoption of the recalculated value will not generally be justified if the two values differ by less than 18% when based on tests on 40 separate batches or less than 11% when based on tests on 100 separate batches or less than 5% when based on tests on 500 separate batches.
On the adoption of a recalculated margin, it shall become the current margin for the judgment of compliance with the specified characteristic strength of concrete produced subsequent to the change.
3.3 Requirements for Concrete 3.3.1 Workability
The workability of the fresh concrete shall be judged by its suitability for the condition of handling and placing so that after compaction, it surrounds all reinforcement, tendons and ducts and completely fills the formwork.
Workability of the concrete shall be within one of the following limits: i) Slump
± 25mm or ± one third of the ‘designed workability’, whichever is greater. ii) Compacting Factor
± 0.03 where the ‘designed workability’ is 0.9 or more.
± 0.04 where the ‘designed workability’ is between 0.8 and 0.9. ± 0.05 where the ‘designed workability’ is 0.8 or less.
iii) Vebe
± 3 seconds or ± one fifth of the ‘designed workability’ whichever is lesser.
3.3.2 Concrete Grade
The grade of concrete to be used in the work shall be as stated in the Drawings and /or in the Bill of Quantities.
(Concrete shall be designated as GRADE X/Y where ‘X’ is the numerical value of the characteristic strength at 28 days in N/mm2, and ‘Y’ is the nominal maximum size of aggregate in mm. For prescribed mix, a suffix ‘P’ shall be added after ‘X’). 3.3.3 Cement Content
Cement content in this specification shall refer to the total quantities of cement as approved in 2.1, or the total quantities of cementitious materials comprising Portland cement and p.f.a or g.g.b.s.
3.3.3.1 Minimum Cement Content
The minimum cement content shall be in accordance with Table 6, unless otherwise shown in the Drawings.
3.3.3.2 Maximum Cement Content
The maximum cement content shall not exceed 550 kg/ m3 unless otherwise shown in the Drawings or as approved by the S.O.
3.3.4 Total Chloride Content
other source shall not in any circumstances exceed the limits in Table 7 expressed as a percentage relationship between chloride ions and weight of cement in the mix.
When necessary, tests shall be carried out in accordance with BS 1881: Part 124 for each grade of concrete, to demonstrate that these limits are not exceeded. 3.3.5 Maximum Sulphate Content
The total estimated sulphate content of any mix, including that present in the cement shall not exceed 4% by weight of cement in the mix. Where necessary, tests shall be carried out in accordance with BS 1881: Part 124 for each grade of concrete to demonstrate that this limit is not exceeded.