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High Strength Concrete (HSC)

posted Jun 16, 2018, 8:13 AM by jeffery jim


1. Recently, i am a little bit overwhelm with the strength of the concrete we achieved in this project. Before I go into details or technical in concrete design mix, let me explain the fundamental about concrete. Before a concrete is utilize in a project, the contractor will propose a design mix which is the basis ratio for batching ingredients in order to achieve target strength.

2. Targeted strength of a concrete is divided into 2 types. Characteristic strength is the standard strength for the concrete as a basis for design purposes. Target mean strength is the limit that is specified in order to have 95% of the concrete (statistical) population to be as high or higher than the characteristic strength. For this to be achieved, design mix shall include and additional 1.64s factor of the characteristic strength or 12kN/m² (for G30 and above) as the cut-off limit to ensure that only 5% of the concrete population is below the characteristic strength. Safe to say, 95% of the concrete are more than the desired characteristic strength.

3. That is the fundamental of strength grades or criteria for concrete. Secondly, the required free cement/water ratio should be as low as possible, JKR prescribed anything less than 0.50 free cement/water ratio in order to restore robustness of the concrete against elements and delamination/degradation agents. Such prescription is in accordance to the development of BS8110 from 1985 until 1997 which recommend the lowering of this ratio in order to reduce voids and subsequently leads to denser and more robust concrete instead of using higher grade concrete due to its proprietary which only gives higher free cement/water ratio.

4. The aggregate is a normal sandstone and/or mudstone range with a rather low and marginal Aggregate Crushing Value (ACV) which is close to 20% crushed. No granite rock was use. All aggregates passed elongation and flakiness test, so there is no dispute on the dimension apart from retained at the desired sieve.

4a. The cement used for these grades trial mixes is limited to Ordinary Portland Cement (OPC) which is available for the masses within Sabah. No, there is no trace of grout in the sample. I know how grout's composition and the failure mode under uni-axial compression. No additional additive included except for generic retardant and superplasticizer, and the free cement/water ratio set as 0.45 to 0.47.

5. Now, about the concrete which achieved higher than the target mean strength. In my case, the desired characteristic strength is 30kN/m² and the target mean strength is supposedly above 42kN/m². However, what achieved at site is consistent grade 50kN/m². So what is the plausible or possible reason to such phenomenon? It is a great achievement but it comes with a cost. How do you explain such high strength in non-laboratory or controlled condition? This is an outcome from a batching plant mix and consistently affect all grades (from G20 to G35). In other word, almost 98% confident level for concrete population for G25 to achieve its characteristic strength and 95% of G35.

6. Now, with all of these parameters set as almost normal, how did the 7-day result close to the characteristic strength and the 28-day strength gone by more than 5kN/m² higher than the targeted mean strength?

7. The answer lies to the gradation of the aggregates. Somehow, i believe that i have found one of the best grading zone for aggregate in order to achieve high strength concrete. The only parameters that remain in play will be aggregates of these mixes.

8. The followings are clues from H.T. Nam Le, L.H. Poh, S. Wang, M.-H. Zhang, Critical parameters for the compressive strength of high-strength concrete, Cement and Concrete Composites (2017).

- Belloc [7] reported that the type of coarse aggregate may affect the compressive strength of HSC though no direct relationship between the compressive strength of coarse aggregate and that of HSC is established in their study. In addition, there is evidence that aggregates with smoother surfaces are more likely to induce interfacial cracks, resulting in a decrease in the compressive strength of HSC [5, 8]. The texture of the aggregate surface also has a direct influence on the bond strength of the ITZ [9, 10].

- Coarse aggregate size also has an important influence on the strength of HSC. As mentioned in the ACI report on HSC [8], smaller coarse aggregate sizes generally lead to higher concrete strengths, due to the lower stress concentrations around the particles. A similar phenomenon is also observed by Hassan, et al. [11] in their numerical study on a single inclusion block, where the strength of the composite reduces with an increase in the inclusion size. In some ultra-high strength concretes, coarse aggregates are completely removed [12, 13].

- Following these observations and studies, the current recommendations to further improve the compressive strength of HSC include: reducing the mismatch in elastic moduli of phases, selection of stronger coarse aggregates, reducing the maximum aggregate size, or elimination of coarse aggregates [8, 14].

- A significant amount of work has also been done on improving the mechanical properties of high-strength mortar (HSM) matrix in order to increase the compressive strength of HSC. To the authors’ knowledge, no research has been conducted on the influence of reduced mismatch in elastic moduli between phases, e.g. by increasing the elastic modulus of the matrix with a given compressive strength, on the compressive strength of HSC. It should be noted that a change in the elastic modulus of the mortar matrix results in a corresponding change in concrete, which can affect the strain capacity of concrete [15].

9. If you want to break the code, you have to read more journals. There is no two ways about it. The mix is probably accidental, the knowledge behind is not.

Video: G25 concrete achieved compressive strength of 42.44kN/m² in 28 days. I am a happy engineer today.

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