Performance of Flowable Concrete in Underground Structures


 Flowable concrete gives special combinations of uniformity, performance and consistency requirements that cannot be possible by traditional normal slump concrete. It is superior concrete than the traditional concrete with high workability, no segregation, no bleeding, and it is also suitable for use in the structures with long-distance pumping i.e., more than 120-meter length, for example, in NATM tunnels, cross-passages, and the concrete structure of the station box. Flowable concrete can flow itself without vibration or a little bit of vibration like surface tamping at outer surface of the formwork for achieving a smooth surface finish after de-shuttering. Percentages of concrete ingredients that are used in concrete to ensure a balance between deformability and stability. Type of selected materials and the fixed ratio of concrete materials can be affected on the fresh and harden properties of the concrete. The article presents experimental techniques for mix designing of flowable concrete for grade M40 and application of the same concrete mix at the cast in situ Base slab, Roof slab Rcc walls for underground metro stations, NATM tunnel & cross passages in Mumbai Metro Project, package -UGC-07. The test results for conformity requirements of M40 grade flowable concrete such as flowability, viscosity, passing ability of fresh concrete, durability test and harden concrete tests determined, and results are included here.


Introduction
Rheological properties of owable concrete or self-compacting concrete are advanced than traditional normal slump concrete and hardened properties of concrete is same as traditional normal slump concrete [1]. Flowable concrete is suitable for a fast rate of concrete placement, with faster construction times and ease of ow around dense reinforcement without vibration or little bit surface tamping to get better surface nish. It ensures good homogeneity, pumping ability, best surface nish, consistent concrete strength, and durability of the concrete structure. "The workability of concrete describes the owability, mobility, and stability of fresh concrete.
Speci c measurement limits of Slump, Slump ow, Flow table cannot be xed and its totally depends on placing condition of site, however concrete should be homogenies, cohesive, no settlement and no bleeding in mix" [2]. The hardened concrete properties like compressive strength, exural strength, split tensile, etc. Are speci cally stricken by composition of concrete ingredients. In Mumbai region river sand is not available and therefore construction industries are totally depending on manufactured sand. In M-sand it is di cult to maintain required fractions of sieve sizes for getting appropriate zone of sand i.e., zone II as per IS383:2016 [3]. According to site requirements only owable concrete was suitable due to long distance of pipe pumping and congested reinforcement in the structure. Hence owable concrete was developed by conducting several numbers of concrete mix trials and concluded that trials were achieving standards requirements for owability by slump ow, and sieve segregation resistance tests as per speci cation mentioned in European standard [4]. Chemical admixtures i.e., superplasticizers required for optimized the cementitious content and minimize the W/C ratio for production of owable concrete [5]. Appropriate selection of supplementary cementitious materials such like GGBS, Ultra ne GGBS-Alcco ne, Crystalline growth admixture etc. are also required for improving durability, decreasing permeability, surface nish ability and improving the overall hardened properties of concrete.

GGBS
Ground granulated blast furnace slag (GGBS) of M/s JSW obtained from Pen, Raigad, Maharashtra, India. The physical and chemical properties of GGBS are given in the Table 1 and Table 2, respectively.

Ultra ne GGBS -Alcco ne
Ultra ne GGBS commercially available as Alcco ne-1203 is a low calcium silicate-based mineral additive which is generally used as a replacement of silica fume in high-performance concrete [6]. Alcco ne 1203 is a slag based SCM having ultra-neness with optimized particle size distribution [7].

Aggregates
In Mumbai region, Maharashtra, India, river sand is not available and therefore construction industries are totally depending on manufactured sand.  Table 5 shows the physical properties of the coarse and ne aggregates.

Concrete Mix Compositions
The concrete mix is designed as per absolute volume method according to the Indian standard -Concrete mix proportioning Guidelines [13] to meet environmental exposure condition of Mumbai city, Maharashtra i.e., considered sever condition. The properties of the concrete ingredients differ from one state to another state of the same country, and it is totally depending on actual environmental exposer condition of site and quality of available concrete ingredients for making suitable concrete mix [14].
Concrete mix compositions are presented in Table 6 & Table No-7.

Test Methods [1]
Flowable Concrete is able to ow under its self-weight without segregation, bleeding without vibration and these behavior of concrete can be ensured by Flowability tests, and Sieve segregation resistance tests of concrete to determine the ow ability, passing ability and segregation resistance of Flowable according to BS: EN 12350 Part 8, 9, 10, 11, and 12[15]  Results are found more satisfactory in owability requirements and passing ability tests Trial mix no TR 4 has achieved all the requirements of lling ability but could not achieve the requirements of passing ability. In trial No-TR 5 achieved all the requirements of lling ability and passing ability but values are found closer to the requirements. Trial mix no TR5 has been nalized based on above-mentioned tests results values with considering TR5 can be achieved all the parameters of owing and passing ability at site if apply some vibration during casting the structure.
Details of nal NATM Lining owable concrete M40 Grade mix design trial mix no-TR5 is as below. Mix proportion of selected mix design TR5 implemented at site for Tunnel NATM concrete and NATM cross passages and total 11426 cum concrete quantity is executed from 25th Feb 2020to 30th june'2021.
The owing ability and passing ability of concrete mix are checked according to conformity criteria for the properties of Flowable concrete [4] at site and observed results are as below. All the parameters of checks are not checked at the site due to the fact shifting of all checking out equipment couldn't nearly possible at some point of concreting at the site, but slump ow, T 500 mm checks, and the temperature had been executed for 1904 transit mixers and their common values are cited in above table.
A few tests could not be conducted at the site laboratory due to the non-availability of su cient testing apparatus at in house laboratory.
Consequently, samples of specimens were sent to a third-party laboratory for undertaking assessments areas underneath.   Concrete cubes have been casted for testing of compressive strength for ongoing working structures and results are found satisfactory. Samples taken for cube compression strength of working structures, average compressive strength, standard deviation, and coe cient of variation are represented in the graph as beneath. In trial No-TR 9 achieved all the requirements of lling ability and passing ability values are found nearer to the requirements. Trial mix no TR 9 has been nalized based on above-mentioned tests results values with considering TR9 can be achieved all the parameters of owing and passing ability at site with apply some vibration during casting the structure to get the satisfactory surface nish.
Details of nal RCC works Flowable Concrete M40 Grade mix design trial mix no-TR 9 is as below.   A few tests could not be conducted at the site laboratory due to the non-availability of su cient testing apparatus at in house laboratory. Consequently, samples of specimens were sent to a third-party laboratory for undertaking assessments areas underneath.  The authors are thankful to Director Projects of Mumbai Metro Rail Corporation and PMT Head of Shanghai Tunnel Engendering Co., Ltd for their motivational inputs and supports.
Compliance with ethical standards Con ict of interest -On behalf of all authors, the corresponding author states that there is no con ict of interest.  Sieve Segregation resistance Test Figure 6 Graphical representation for Compressive Strength of site casted cubes for 7 days, 28 days and 56 days  Actual site photographs during concrete placing at site showing two stages of concrete pouring and no's of bends to reach at placement point.