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for Building Works -1989 adopts MS 7.5 which was based on the design requirements of CP 114 and has long been withdrawn by SIRIM.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.