Validación por caso de estudio

K.L. Radhika

Assistant Professor, Department of Civil Engineering, UCE (A), O.U, Hyderabad A.P.

ABSTRACT

A Self compacting concrete (SCC) is the one that can be placed in the form and can go through obstructions by its own weight and with out need of vibration. The concept of SCC resulted from research into in-situ concrete piling and the filling of other inaccessible areas where compaction is essential but difficult. The required workability for casting concrete depends on type of construction, selected placements and consolidation methods, complex shape of the formwork and structural design details, that affect the degree of congestion of reinforcement. Originally developed in Japan, self compacting concrete technology was made possible by the much earlier development of super plasticizers for concrete. A highly flowable concrete is not necessarily SCC, because self compacting concrete should not only flow under its own weight but should also fill the entire form and achieve uniform consolidation without segregation. In the present work firstly, M40 grade of SCC was developed using different percentages of fine aggregate to total aggregate ratio i.e. 0.50, 0.55, 0.60 using Nansu method of mix design. Secondly the influence of fine aggregate to total aggregate ratio for above developed SCC, on mechanical properties i.e. Compressive strength, Split tensile strength and Flexural strength, for 7 and 28 days curing period, maintaining water powder ratio constant are investigated. It was observed that as the FA/TA ratio is increased from 0.5 to 0.60 the flowable property and mechanical properties i.e compressive strength and split tensile strength has increased.

INTRODUCTION

Self-compacting concrete was developed in Japan in the early 1990s. Okamura [1] pointed out that SCC can flow into any corner and pass through the spacing of reinforcement without vibrating. The major advantages of SCC over conventional concrete are: high mobility,high resistance to segregation and no need of vibration.Okamura and Ozawa [1] also pointed out that in SCC mixes not only the coarse aggregate content might be limited but also the pozzolanas and superplasticizer might be used to prevent segregation and increase flowability. Self-consolidating concrete (SCC) is an advanced type of highly flowable, non-segregating concrete that is able to flow under its own mass without vibration and through congested reinforcement. The advantages of SCC include the elimination of vibration, reduced labor costs, improved consolidation, improved formed surface finish, improved jobsite safety, and increased construction speed. Compared to conventional, vibrated concrete mixtures, SCC typically exhibits higher sand-to-aggregates ratios (S/A), higher paste volume and in some cases smaller maximum aggregate size. In addition, SCC utilizes relatively high dosages of high-range water-reducing admixture (HRWRA) and, in some cases, a viscosity modifying admixture (VMA). The extent to which SCC mixture proportions must be changed relative to comparable vibrated concrete mixtures is determined to a large extent by the aggregate characteristics. By selecting the proportion of aggregates

for SCC, the paste volume can be minimized. In cases where favourable aggregates are unavailable, it is often still possible to adjust the mixture proportions to accommodate challenging aggregates and achieve the required self-consolidating properties.

NEED FOR THE PRESENT STUDY

The main property that defines SCC is high workability in attaining consolidation and specified hardened properties.

Before it satisfies the hardened properties it should also satisfy the fresh properties in terms of filling ability, passing ability and resistance to segregation. The self compatibility is largely affected by the characteristics of the materials and mix proportions. The strength of SCC is provided by the aggregates binded by the paste at hardened state, while the workability of SCC is provided by the binding paste at fresh state. Therefore the contents of coarse and fine aggregates, binders, mixing water and superplasticizer will be the main factors influencing the properties of SCC. In Nansu method(3) of mix design the volume ratio of fine aggregate to total aggregates ranges from 50% to 57% to achieve the fresh properties of SCC.

And as per EFNARC specifications the coarse aggregate content normally ranges from 28% to 35% by volume of mix to achieve the fresh properties of SCC. The need for the present work is to find the optimum content of FA/TA ratio that satisfies the fresh properties and also the hardened properties of SCC maintaining water powder ratio constant.

Influence of Fine Aggregate to Total Aggregate Ratio on Mechanical Properties of Self Compacting Concrete BASIC PROPERTIES OF SELF-COMPACTING

CONCRETE

Fresh SCC must possess the key properties including filling ability, passing ability and resistance to segregation at required levels. The filling ability is the ability of the SCC to flow into all spaces within the formwork under its own weight. Without vibrating the concrete, SCC has to fill any space within the formwork and it has to flow in horizontal and vertical directions without keeping air entrapped inside the concrete or at the surface. Passing ability is the ability of the SCC to flow through tight openings such as spaces between steel reinforcing bars, under its own weight. Passing ability is required to guarantee a homogenous distribution of the components of SCC in the vicinity of obstacles. The resistance to segregation is the resistance of the components of SCC to migration or separation and remains uniform throughout the process of transport and placing. To satisfy these conditions EFNARC(2,9) has formulated certain test procedures as described below.

Tests on Self-Compacting Concrete

The slump flow equipment (Fig 1) is currently widely used in concrete practice, and the method is very simple and straightforward. Thus the slump flow combined with T50 was selected as the first priority test method for the filling ability of SCC. The V-funnel (Fig 2) or Orimet tests are recommended as second priority alternatives to the T50 measurement. The passing ability of fresh SCC can be tested by U-box (Fig 3) or J-ring (Fig 4).

Fig. 1: Slump Flow

Fig. 2: V-Funnel

Fig. 3: U-Box

Fig. 4: J-Ring

The fresh properties of SCC as per EFNARC Specifications are shown in the table-1.

Table 1: Fresh Properties of Self-Compacting Concrete as PER Efnarc Specifications

S.

No Method Unit Efnarc

Limit 1. Slump Flow by Abrams

Cone mm 650-800

2. T50 cm Slump Flow sec 2-5

3. V-Funnel sec upto 12

4. Time Increase, V-Funnel at

T5 Minutes sec 0-3

5. J-Ring mm 0-10

6. U-Box mm 0-30

EXPERIMENTAL PROGRAM

The experimental program can be identified in two stages, firstly to develop SCC mixes for M40 grade for various percentages of FA/TA ratio i.e. 0.50%,0.55%,0.60%

which satisfies the fresh properties as per EFNARC specifications. Secondly to study the influence of various percentages of FA/TA ratio i.e. 0.50%,0.55%,0.60% for above developed SCC, on mechanical properties such as Compressive strength, Split tensile strength and Flexural strength, for 7 and 28 days curing period. The tests were conducted according to IS: 516-1959(7) to attain the mechanical properties of concrete.

Materials Used Cement

Ordinary Portland cement of 53 grade ultratech (5)

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development confirming to IS : 12269 and tested as per IS:4031-1988

was used. The normal consistency, fineness and specific gravity of the cement used are 33%, 9% and 3.07 respectively.

Aggregates

Fine Aggregate used was locally available river sand confirming to Zone-II as per IS : 383-1970(6) was used.

Locally available machine crushed granite chips of 12mm and maximum size of 20mm was used as coarse aggregates.The physical properties of fine aggregate and coarse aggregates are given in the table – 2

Table 2: Physical Properties of Fine and Coarse Aggregates S. limited, Hyderabad, which was originally obtained from Vijayawada Thermal Power Station.The silica content was estimated to about 96%. Fly ash used in this study was basically to improve workability and cohesiveness of concrete. The physical properties were tested as per IS 3813-1999 and presentated in the table -3

Table 3: Physical Properties of Fly Ash S.

No Properties Values

1 Specific Gravity 1.95

2 Specific Surface Area 338m²/kg 3 Particles Retained On 45 Micron

Is Sieve

24%

4 Lime Reactivity-Avg Comp.

Strength 8.4 N/mm²

5 Comp. Strength at 28 Days 84 N/mm² Water

The water(13) which is used for mixing concrete was clean and free from harmful impurities such as oil, alkali, acid etc.

Superplasticizer

High range water reducing admixture called superplasticizer are used for improving the flow or workability for decreased water cement ratio without sacrificing in the compressive strength. These admixtures when dispersed in cement significantly decrease the viscosity of the paste by forming a thin film around the

cement particles. In the present study GLENIUM B233 conforming to ASTM C494, IS 9103;1999 is used.

Viscosity Modifying Agent (VMA)

The use of VMA as admixture gives more possibility of controlling segregation when the amount of powder is limited. This admixture helps to provide very good homogeneity and reduces the tendency to segregation. In the present work Stream2 VMA is used.

Mix Design

Nansu method of mix design for SCC was used to arrive at initial trial mixes for M40 grade concrete for various percentages of FA/TA ratio i.e. 0.50%,0.55%,0.60%, maintaining water powder ratio constant,and these mixes were modified accordingly as per EFNARC specifications to achieve the optimum mix proportions satisfying fresh and hardened properties and also economy.The principal consideration of the proposed method is to fill the paste of binders into voids of the aggregate framework piled loosely.

Casting and Curing of Specimens

The program consisting of casting and testing six standard cubes of size 150mmx150mmx150mm for compression test, six standard cylinders of size 150mm diameter and 300mm height for conducting split tensile strength and six standard prisms of size 100mm x 100mm x 500mm for conducting flexural strength. Total of 18 pecimens were cast at the same time under identical temperature conditions and all the specimens were demoulded after 24 hours of air drying and kept in water for 28 days for water curing. All these tests were conducted according to IS:

516-1959 after the curing period of 7 and 28 days.

RESULTS AND DISCUSSIONS Mix Proportions

Trial mixes were carried out based on traditional Nansu method of mix design for M40 grade self compacting concrete for various percentages of FA/TA ratio i.e.

0.50%,0.55%,0.60%, maintaining water powder ratio constant and then these mixes were modified accordingly as per EFNARC specifications to achieve optimum mix proportions satisfying fresh properties. The details of the fresh properties and mix proportions and are shown in Table 4 and 5.

Tests on Hardened Concrete

Compression strength test, Flexural strength test and split tensile strength test has been conducted confirming to IS 516-1959(7), for M40 grade self compacting concrete for various percentages of FA/TA ratio i.e.

0.50%,0.55%,0.60%, maintaining water powder ratio constant, in the Universal Testing Machine of capacity 200MT. The results of various mechanical properties for the curing period of 7 and 28 days are shown in the

table-Influence of Fine Aggregate to Total Aggregate Ratio on Mechanical Properties of Self Compacting Concrete

6. It has been observed that as the ratio of FA/TA is increased from 0.50 to 0.60 compressive strength and split tensile strength also increased for 7 and 28 days curing period,but different trend was observed in the case of flexural strength where the strength increased as the ratio increased from 0.50 to 0.55 and on further increase in the ratio from 0.55 to 0.60 the flexural strength decreased for both 7 and 28 days curing period. The above results show that as the FA/TA ratio increased from 0.50 to 0.60 the compressive strength also increased by 13.41% and 7.40% and split tensile strength increased by 13.02% and 15.0% for 7 and 28 days curing period.The flexural strength increased by 7% and 8.57% when the ratio increased from 0.50 to 0.55, on further increased to 0.60%

the strength decreased by 2.33% and 3.17% for 7 and 28 days curing period.

Table 6: Hardened Properties of M40 Grade SCC with Various FA/TA Ratios

 As the ratio of FA/TA increased, its filling ability and resistance to segregation also gradually increased.

 As the ratio of FA/TA is increased from 0.50 to 0.60 compressive strength and split tensile strength also increased for 7 and 28 days curing period,but different trend was observed in the case of flexural strength where the strength increased as the ratio increased from 0.50 to 0.55 and on further increase in the ratio from 0.55 to 0.60 the flexural strength decreased.

 The maximum compressive strength and split tensile strength was observed for the FA/CA ratio of 0.60%.

 The flexural strength of concrete increases with the “Self-Compacting Concrete” Journal of Advance Concrete Technology Vol.1, No.1,5-15, April 2003.

[2] EFNARC, “Specifications and Guidelines for Self-Compacting Concrete”, EFNARC,UK, February 2002, pp.

1-32. concrete” Journal of marine science and technology,vol-38,No.4,Dec-Feb,pp.25-33 Indian Standards, New Delhi.

[7] IS: 516-1959 “Methods of Tests for Strength of Concrete”, Bureau of Indian Standards, New Delhi.

[8] M S Shetty “Concrete Technology” text book.

Table 4: Fresh Properties of M40 Grade SCC With Various FA/TA Ratios

FA/ TA Ratio

Filling Ability Passing Ability Segregation

Resistance

Table 5: Final Mix Proportions of M40 Grade SCC with Various FA/TA Ratios FA/TA

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development

[9] “Specifications and guidelines for self-compacting concrete.” Published by EFNARC in February 2002(4) [10] J.C. gibbs and W. zhu strength and hardened concrete self

Compacting Concrete Uneasily of Paisley, Scotland, United Kingdom, 1^st International RILEM Symposium on Self Compacting Concrete, Stockholm Sweden September 13-14-, 1999.

[11] Snobi. M and P.F bartos Hardened SCC and the reinforcement ACM Advanced Concrete and Masonry Centre. University of Paisley, Scotland, Sweden 1^st

International RILEM Symposium on Self Compaction, Concrete, Stockholm, Sweden September 13-14, 1999 [12] Ozawa K., Kunishima, M., Maikawa, K. and Ozawa, k,

“Development of High performance Concrete Based on the Durability Design of Concrete ‘Structures”Proceedings of the second East-Asia and Pacific Conference on Structural Engineering and Construction (EASEC-2), Vol.

1 pp. 445-45, Jan’1989

[13] IS: 516-1959 “Methods of Tests for Strength of Concrete”, Bureau of Indian Standards, New Delhi.

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development (ACEID-2014), Vasavi College of Engineering, Hyderabad, A.P. 6 - 7 February, 2014. pp.81-88.