Concrete of a desired strength for any specific application can be obtained by using any of the accepted methods of design. However, the addition of mineral admixtures like fly ash in concrete requires a systematic design procedure for obtaining the required strength. Dr. P. Dinakar, Assistant Professor, School of Infrastructure, Indian Institute of Technology, Bhubaneswar demonstrates that it is possible to design a fly ash concrete for the required strength through “efficiency concept” and the same can be extended for the design of self compacting concrete (SCC)”.
The recent introduction of self-compacting concrete (SCC) also referred to as “Self-Consolidating Concrete” has been one of the most important developments in building industry. The advantage of SCC is that it settles into the heavily reinforced even deep and narrow sections by its own weight and can consolidate itself without necessitating internal or external vibration. At the same time it can also maintain its stability without leading to segregation and bleeding. However, SCC demands a large amount of powder content compared to conventional vibrated concrete to produce a homogeneous and cohesive mix.
The common practice to obtain self-compactibility in SCC is to limit the coarse aggregate content and the maximum size and to use lower water–powder ratios together with new generation super plasticizer. During the transportation and placement of SCC, the increased flowability may cause segregation and bleeding which can be overcome by providing the necessary viscosity, which is usually supplied by either increasing the fine aggregate content, by limiting the maximum aggregate size, by increasing the powder content or by utilizing viscosity modifying admixtures.
Thus, the disadvantage of SCC is its cost associated with the use of chemical admixtures and use of high volumes of portland cement. One alternative to reduce the cost of SCC is the use of mineral additives such as limestone powder, natural pozzolans, fly ash and slag which are finely divided materials added to concrete as separate ingredients either before or during mixing.
As these mineral additives replace part of the portland cement, the cost of SCC will be reduced especially if the mineral additive is an industrial by-product or waste. It is well established that the mineral additives, such as fly ash and slag, may increase the workability, durability and long-term properties of concrete. Therefore, use of these types of mineral additives in SCC will make it possible, not only to decrease the cost of SCC but also to increase its long-term performance.
Proposed method for proportioning fly ash self compacting concrete
The inexpensive, abundantly available and low efficiency fly ash is a ready to use powder (unlike the GGBS or limestone powder that needs grinding) that can be directly added as an additional cementitious constituent. The proposed method attempts to extend this concept and realize the development of high volume, high strength and high performance self compacting concrete. The advantage of this method is that at higher replacement percentages of fly ash the self compacting concretes are more economical than the use of superplasticizers in SCC. However, this requires specific adjustments to all the other ingredients like sand, coarse aggregate, superplasticizers and water to arrive at an optimal mix proportion.
In the mix proportioning of conventional concretes, the water content is fixed based on the maximum size of the aggregate and/or aggregate grading. In the case of SCC, the quantity of total fines (powder) is of importance. In view of this, fix the total cementitious materials (TCM) content preferable around 550kg/m3. To understand the behaviour of SCCs one can also choose this in the range of 500-600kg/m3 (EFNARC, 2005).
Calculation of Efficiency of Fly Ash and Fly Ash Content
As per the methodology proposed (Figure 1), the fly ash content can be varied between 15-75% for the design of normal fly ash concretes. The maximum percentage replacement possible for normal fly ash concretes was decided according to the strength requirements (Figure 2). The same concept has been extended for the design of SCC with fly ash. In this procedure the 28 day efficiency curve shown in Figure 1 is used for calculating the efficiency of fly ash for any replacements varying between 15-75%. And, the percentage replacement of fly ash is chosen as per the strength requirement using Figure 2.
Recent experimental results have shown that it is possible to replace even higher percentages if one was to modulate the aggregate gradings and the filler proportions to minimize the water content needed. The water cement ratio determined for the conventional concrete from Figure 3 is used to calculate the water content of SCC.
The total aggregate content can be assessed according to the absolute volume method. The fine aggregate content in the total aggregate is generally recommended to be in the range of 48-55% (EFNARC, 2005). Alternatively, one can follow the continuous grading curves, if required (Fig 4 & 5). In the proposed methodology a combined aggregate grading as recommended by the DIN 1045 standards is utilized.
The chemical admixtures have the most profound impact on the behaviour of fresh SCC. Dosage of admixtures is adjusted in such a way to obtain initial slump flow values greater than 550 mm, which is necessary for the production of a highly flowable SCC as per EFNARC (2005) guidelines.
Once all the calculated proportions fall within the recommendations, the self compacting concrete can be developed. Three different concretes of strengths 30, 60 and 90 MPa have been designed with the mix design methodology as explained for fly ash replacements varying between 30 to 70%. The mix details are presented as below.
The fresh concrete mix exhibited slump flow values between 670-730 mm and showed no signs of segregation. The slump flow values obtained has a direct bearing on the volume of fly ash used. At high volume replacements of fly ash SCCs exhibited highest slump flows. Similar trend was observed in V-funnel flow times also.
The compressive strengths were evaluated for different periods of time for SCCs. From the results it can be seen that the concretes of above 90 MPa strengths at 28 days can be produced with the available high grade (C53) cements alone at lower water cement ratios. The designed target strengths were easily obtained for all the concretes. In case of 30 MPa concrete strengths even more than the design strengths were obtained.
The results of the 30 MPa concrete shows that even at 70% replacement the strength gain rate is almost similar to that of normal concretes. The results of 60 MPa SCC with 50% replacement of fly ash showed lower strengths compared to the normal concretes, but still achieved the 28 days designed strength. The results of high strength normal and self compacting fly ash concretes of 90MPa show that the strength gain rate is comparatively low after 28 days for 90 MPa concrete. It can also be observed that 90 MPa SCC with 30% replacement showed lower strength at early ages but almost an equal strength of 90 MPa at 28 days.