Prescribed Mix - Compliance Requirement

Production & Supply Concrete To Site

The Parameters used in the trial mix must be maintained during actual production & supply concrete to the site. Adjustment are allowed within Cl. 3.2.4

It is the responsibility of the Contractor to ensure the supplier produce and deliver to the site, concrete in compliance with the requirements of the specifications and providing information as required by SPK

However, for the smooth flowing of concrete works at the site, the following process of approval and verification shall be clarified and agreed upon between S.O and Contractor. (* Delete where applicable)

1. Batching Plant : The Contractor shall ensure his supplier permits access for the S.O to enter the premises periodically to inspect the production of concrete.

2. Delivery Note : The time of batching for all concrete delivered to site shall be noted. Maximum allowable duration is Two Hours (2 jam). Concrete must immediately be rejected after the expiry of this duration. ( Nota : Duration of 2 hours means the time between batching and completion of placing vibrated concrete)

3. Batching Record : For every load of concrete delivered to Site and shall be given to the Contractor and jointly verified for compliances by the S.O. *during delivery / at the end of the day's concreting / within 2 days

4. Workability : Verification of every load of concrete is by slump test. * truck driver / Contractor / S.O shall carry out the test and verified for placement or reject by * Contractor / S.O

5. Test Cubes : Prepared by * truck driver / Contractor / S.O. Record and markings by *truck driver / Contractor/S.O Must be cured and kept in curing bed constructed at/near S.O's site office. Testing at *supplier's lab / IKRAM / Independent lab. Testing witnessed, verified and recorded by *S.O, Contractor & Supplier / Contractor & Supplier / Supplier only. Test must be carried out at the 7th and 28th day.

6. Additional Cubes : For earlier removal of forms other than specified in Table 10 or for pre-stressed concrete before transfer. However, Cl 4.9 & 8.4 must be complied. Two cubes required from each sample (nota: The curing of the cube shall not followthe normal condition but to simulate the actual field condition and kept separately from the curing bed)

7. Rate of Sampling : Must comply with Cl 4.3. Three (3) cubes taken from each sample. One(1) tested for 7 day strength and Two(2) for 28 day strength. First day of concreting, twelve (12) cubes from four (4) samples must be taken regardless of volume for each grade of concrete.

8. Tempreture : Cl 5.6 shall be observed. For Cl. 5.5-c *plastic sheet / tarpaulin shall be used for cast in-situ slabs, prestressed slab, slab toppings, cast insitu beams and post tensioned beams.

9. Curing & Protection : Must start by final set time of concrete ie 600min. Method of curing is by *wet gunny sack / impermeable sheeting / continuous wetting of surface / membrane forming curing compound. Curing period shall not less than 5 days.

Trial Mix

Refer to Previous Table

Trial Mix must be witnessed and verified by the S.O and conforming with MS 26 and Cl. 3.2.3-Section D. Informations must be recorded using the relevant Borang SPK.

• If a current marging of 12 N/mm2 ia adopted. The Target Mean Strength = 52 N/mm2. The average 28-day strength from the nine (9) cubes obtained from the three (3) batches of concrete shall exceed (52-3.5) N/mm2 i.e 48.5N/mm2

• The S.O shall give the approval in writing

Trial Mix (Contoh Perkiraan)

1. Nilai purata konkrit, fm = 46.8 N/mm²

2. Jidar semasa(Current Margin) = 7.5 N/mm²

* Pensampelan dan prosedur ujian hendaklah berdasarkan kepada MS 26 atau BS1881

* Purata kekuatan konkrit berdasarkan kepada keputusan ujian mampatan bagi 9 kiub pada hari ke 28 > atau = gred + jidar semasa - 3.5 N/mm² (cl 3.2.3)

3. fm (campuran semasa) = 40 + 7.5 - 3.5 = 44.0 N/mm² < 46.8 N/mm²

4. Ujian ini berjaya dan kerja-kerja konkrit boleh diteruskan dengan syarat nisbah campuran dan bahan (materials) yang digunakan semasa bekalan ke tapak adalah sama dengan yang digunakan semasa "Trial mix" ini.

Concrete Mix (Current Margin)

Current Margin

The current margin of the proposed designed-mix must be supported by previous results of similar mix proportions and analysed for compliance with Cl. 3.2.1- Section D

1. Margin = 1.64 x s or not less than 3.75 N/mm2
   
   From the results of 100 separate batches produced within 12 months from the same plant for similar proportion of similar materials

2. Margin = 1.64 x s or not less than 7.5N/mm2
   
   From the results of 40 separate batches produced within 12 months from the same plant for similar proportion of similar materials.

Otherwise, the S.O shall decide that the current margin adopted is 12 N/mm2 and the adopted target mean strength shall be derived from this value until sufficient data are available to justify for lower current margin, but not less than 7.5 N/mm2.

Characteristic Strength of Concrete

It is the strength of material below which not more than 5% of the results are expected to fall. That is, at least 95% of the samples will have a strength more than the characteristic value.

Target Strength of Concrete

In order to ensure that the specified minimum strength at 28 days is attained in 95% of all test samples, the concrete mix must be proportioned to give a mean strength at 28 days exceeding the specified strength by 1.64 times the standard deviation.

Standard Deviation (SD)

Initially, the value of the standard deviation may be taken based on past data or experience or judgement and subsequently from the accumulated results of the test samples. The standard deviation must be recalculated from the test results obtained after any change in the source or quality of materials and the mix shall be adjusted as necessary to comply with the requirements.

The Standard Specifications for Building Works-1989 vs Standard Specification for Building Works-2005

Both specifications are entirely different when specifying concrete and its compliance requirements, causing confusions amongst designers and supervisors.

The Standard Specification for Building Works-2005 is in compliance with:

1. MS 523, (which is a total adoption of BS 5328), complies with the design requirements of BS 8110 whereas the Standard Specification for Building Works -1989 adopts MS 7.5 which was based on the design requirements of CP 114 and has long been withdrawn by SIRIM.

2. BS 6089, which specifies the acceptance criteria for testing of hardened concrete, especially when dealing with non-compliance whereas the Standard Specification for Building Works -1989 leaves the non-compliance issue to the discretion of the S.O or his Representatives.

3. Emphasize on the use of designed mix and allows provision for prescribed mix whereas Standard Specification for Building Works -1989 does not allow the use of designed mix unless prior approval is given by the S.O.

4. Two types of concrete mix allowed which are Designed Mix and Precribed mix

Concrete

What does 28 -day strength mean?

Answer:
Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.

JKR Concrete Specification

TAHUN 2010

Selamat datang tahun 2010. Semoga tahun ini akan memberi kekuatan bagi blog ini meneruskan agenda yang terlewat pada tahun 2009.

Pada tahun 2010 ini, blog ini akan berusaha memperbanyakkan artikel mengenai keperluan bagi seorang Jurutera Awam/Struktur.

Komen saudara adalah dialu-alukan.

Sekian, Wassalam.