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Trial Mix (What most engineers do not know)

posted Jun 10, 2019, 7:53 AM by jeffery jim

If you have no problem with JKR's Concrete Specification for Road Works, you actually have problem in understanding the literature on concrete development. The latest JKR Standard Specification for Building Works 2014 is relatively acceptable.

The problem arises when we talk about trial mix is the batching count and processes. Here are some comments I made in relation to issue mentioned-above.

We refer to the mentioned Clause 9.3.2.3 – Trial Mixes of the Project Specification Paragraph 3 where;
“Sampling and testing procedures shall be in accordance with MS26. Three separate batches of concrete shall be made. The Workability of each of the three trial batches determined by mean of the slump test of compacting factor test or Vebe consist meter test or as specified in accordance with MS26, shall be appropriate to the purposed uses and methods of placing and compaction of the mix and shall be approved by the S.O. Six cube shall be made from each batches. 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 nine cubes tested at 28 days shall exceed the specified characteristic strength by current margin minus 3.5N/mm². The approved trial mixes shall then be designated as the “designed mix”and its corresponding workability as the “designed workability.”

The above mentioned clause of the Project Specification is an adaptation from Clause 3.4.3 of BS5328 - Part 3: 1990: Specification for the Procedures to be used in producing and transporting concrete which highlights the method of conducting trial mixes in a small and controlled batches. The following is the citation from the mentioned clause;
“When trial mix is required, the Number of LABORATORY or SITE MIXED batches shall be specified or agreed between the purchaser and producer using the materials which are proposed for the work. The workability of each trial batches shall be the same as the proposed supply within the tolerance given in 3.5 (Max free water/cement ratio) of BS5328 Part 4. THREE cubes shall be made for each batch for test at 28days. The average compressive strength of the three cubes tested at 28 days shall exceed the specified characteristic strength by at least 10N/mm².”

Similar have been recorded in MS26 - Part 1: 1991: Method of Testing Concrete part 1: Method of Sampling Fresh Concrete (Revision 1)which is obsolete where Clause 1.2.1.2 highlights the definition of a “batch” as the quantity of concrete mixed in one cycle of operations of a batch mixer or the quantity of concrete conveyed ready-mixed in a vehicle, or the quantity discharged during 1 minute from a continuous mixer. This construed and if not compliment or satisfied the above-mentioned Clause 3.4.3 which refer to trial mix conducted in a control or small quantity. Subsequent Clause 1.3 – Mixing and Sampling Fresh concrete in the Laboratory from MS26 Part 1: 1991 have the whole context of trial mix in a smaller scale in a laboratory where batching of fresh concrete is either executed by the mean of pan mixer (Clause 1.3.4.4.1) or drum mixer (clause 1.3.4.4.2). This laboratory testing is also mentioned in BS1881 – Part 125: 1986: Methods for Mixing and Sampling Fresh Concrete in the Laboratory. For the recent and in place MS26 – Section 1: 2009, the content is similar to BS EN 12350 Part 1: 2009, samplings are taken as either spot sample or composite sample where samples obtain can be either from batch or mass of concrete. The new addition of definition of a “batch” is considered as “conveyed READY-MIXED in a truck mixer when the load requires more than one cycle of a batch mixer or more than a minute of operation of a continuous mixer.” This shows that level of acceptance when referring to ready-mixed concrete from batching plant when conducting trial mixes.

Similar changes are reflected from the obsolete Standard Specification of Building Work (SSBW) 2005 and the latest revision, 2014. The SSBW 2005 did include verbatim Clause 9.3.2.3 of the Project Specification in its Section D: Concrete Works – Sub-clause 3.2.3. However, in SSBW 2014, designed concrete should comply with requirements specified in its Table D6 and D6A. As for the concrete strength from the Initial Test, shall only comply and/or satisfy Table D8 or D9 with adequate margin or margin by twice the expected standard deviation, of at least a margin of 6N/mm² or 12N/mm² and only three specimens from each three batches (overall 9 cubes) shall be tested. The citation of three batches also reflects on the test condition of 27±2°C which is in a laboratory (controlled) by either a pan mixer or a drum mixer.

Through passages from references mentioned above, early Trial Mixes were mean to be conducted in laboratory instead of using batching plant until the release of BS EN 12350 Part 1: 2009 and MS 26 Section 1: 2009. We don’t condone any ready-mixed supplier to conduct Trial Mix using hand-mix or the use of drum mixer since the result may not be reflective of the performance and/or capability of the batching plant. Extending to that, the use of such method in conducting the Trial Mix may defeat the purpose stipulated in Section 9: Concrete of the Project Specification, Sub-clause 9.3.2 and 9.3.2.1 where the target mean strength is based on the performance of each ready-mixed plant for 100 separate batches not exceeding 12 months and/or 40 separate batches not exceeding 6 months.


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