Evaluation of strength at early ages of self-compacting concrete with high volume fly ash

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Abstract

Self-compacting concrete (SCC) demands large amount of powder content and fines for its cohesiveness and ability to flow with out bleeding and segregation. In the present investigation, part of this powder is replaced with high volume fly ash based on a rational mix design method developed by the authors. Because of high fly ash content, it is essential to study the development of strength at early ages of curing which may prove to be a significant factor for the removal of formwork. Rate of gain in strength at different periods of curing such as 12 h, 18 h, 1 day, 3 days, 7 days, 21 days and 28 days are studied for various grades of different SCC mixes and suitable relations have been established for the gain in strength at the early ages in comparison to the conventional concrete of same grades. Relations have also been formulated for compressive strength and split tensile strength for different grades of SCC mixes.

Introduction

Self-compacting concrete (SCC) represents a milestone in concrete research. SCC is a highly flowable, non-segregating concrete that can spread in to place, fill the formwork and encapsulate the reinforcement without any mechanical vibration for consolidation. SCC was originally developed at the University of Tokyo, Japan during the year 1986 by Prof. Okamura and his team to improve the quality of construction and also to overcome the problems of defective workmanship. A prototype of SCC for structural applications was first completed in 1988 and was named “High Performance Concrete”, and later proposed as “Self Compacting High Performance Concrete”. A committee was formed to study the properties of SCC, including a fundamental investigation on workability of concrete, which was carried out at the University of Tokyo, Japan [1].

SCC represents one of the most outstanding advancement in concrete technology during the last decade. Due to its specific properties, which are achieved by the excellent coordination of deformability and segregation resistance, SCC may contribute to a significant improvement in the quality of concrete structures and open up new fields for the application of concrete. The use of SCC offers many benefits to the construction practice: the elimination of the compaction work results in reduced cost of placement, a shortening of the construction time and therefore in an improved productivity. The application of SCC also leads to a reduction of noise during casting, better working conditions and the possibility of expanding the placing time in inner city areas. Other advantages of SCC are the improved homogeneity of the concrete and the excellent surface finish without blowholes or other surface defects, due to the optimised combination of the individual components of the concrete mix [2]. The designation “self-compacting” is based on the fresh concrete properties of this material and therefore the degree of compactability, deformability and viscosity of different mix compositions were investigated very frequently. Manu and Subramanian [3] reported the developments in the area of self-compacting concrete and the existing level of research along with a number of issues. Mix proportioning technique is one of such issues to be addressed.

To produce a homogeneous and cohesive mix, self-compacting concrete demands a large amount of powder content compared to conventional vibrated concrete. Jagadish et al. [4] reported that SCC often contains powder in the order of 450–600 kg/m3 of concrete. Due to its rheological requirements, filler additions (both reactive and inert) are commonly used in SCC to improve and maintain the workability, as well as to regulate the cement content and to reduce the heat of hydration. Part of this powder content can be effectively replaced by mineral admixtures like fly ash, ground granulated blast furnace slag, silica fume, etc. [5]. Independent of the fact that SCC consists basically of the same components as normal vibrated concrete, there exist clear differences regarding the concrete composition in order to achieve the desired “self-compacting properties”. On the one hand, SCC has to reach a high segregation resistance and on the other hand a high deformability. Therefore, the content of ultra fine materials in SCC is essentially higher. The use of fly ash in this regard provides benefits such as reduction in the water requirements with increased workability and increased strength at later ages of curing, which cannot be achieved through the use of additional Portland cement.

Binu et al. [6] presented a rational method of mix design based on the material characteristics for various grades of SCC by incorporating high volume fly ash as mineral admixture. Fly ash based SCC has proven records of long term strength and durability as it imparts a continuous hardening system to the concrete. Jagadish and Ranganath [7] studied the effect of fly ash on the long term strength in high strength self-compacting concrete. Because of high fly ash content, it is essential to study the development of strength at early ages of curing which may prove to be a significant factor for the removal of formwork.

Section snippets

Materials used

The following materials were used in the experimental investigation. The properties of constituent materials are listed in Table 1, Table 2:

  • Cement: Ordinary Portland cement (53 Grade) with specific gravity of 3.14 confirms to IS 12269:1987 (ASTM C 150-85A).

  • Fine aggregate: Locally available river sand of specific gravity 2.64, fineness modulus of 2.17, bulk density of 1320 kg/m3 which confirms to Zone II as per IS: 2386 (Part I).

  • Coarse aggregate: Crushed granite coarse aggregate of 12 mm down size

Mix design

The conventional design of concrete mix is based on the assumption that particles of different sizes fill up larger voids effecting densest packing with the cement paste providing the necessary hardened cement paste to form concrete of design strength under a particular degree of control. This conventional concrete is basically a three constituent material matrix, cement being in a finely powdered form to facilitate fast chemical reaction with water and hence creating the necessary dense

Fresh and hardened concrete properties

Different ingredients were batched by weight as per the mix proportions given in Table 3 and mixed well in a pan mixer of capacity 60 kg and the workability tests such as slump flow test, V-funnel test, L-box test and GTM screen stability test as per specifications were carried out to test the flowability, filling ability, passing ability and segregation resistance as per specifications. The workability test results (Table 4) are found to be with in the prescribed limits as per specifications

Results and discussion

Graphs are plotted for the gain in compressive strength at early ages of curing of various grades of SCC. Fig. 1 shows the rate of gain in strength for AS30–AS70 from 12 h to 28 days of curing. Similar curve is plotted for BS30–BS70 in Fig. 2. AS30–AS70 series shows comparatively higher strength than BS30–BS70 series because of higher cement content and flowability. The exclusion of quarry dust in AS30–AS70 series enables higher flow and self-compacting characteristics with an increase in

Relationship between compressive strength and split tensile strength of SCC

Based on the test results on the compressive strength and split tensile strength of various grades of different SCC mixes at different ages of curing graphs are plotted with tensile vs. compressive strength and suitable linear relations are developed between tensile strength and compressive strength for various grades of SCC ranges from 30 to 70 MPa. Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 show the relation between tensile strength and compressive strength of various grades of SCC. Relation

Conclusions

  • 1.

    SCC mixes are prepared for different grades ranges from 30 to 70 MPa with all required rheological characteristics such as flow ability, filling ability, passing ability and segregation resistance.

  • 2.

    Relations have been established for the gain in compressive strength at early ages of curing (12 h to 28 days) for different grades of SCC mixes and the relations are compared with the IS Code formula for conventional concrete as per IS: SP 23-1982.

  • 3.

    The proposed equation for the compressive strength is

Acknowledgements

The authors sincerely thank the Principal and Management of B.S. Abdur Rahman, Crescent Engineering College, Chennai 600 048, for their constant encouragement and support to carry out this experimental investigation.

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