Self compacting Concrete

Prof.K.Vijai, Associate Professor and Dr. R.Kumutha, Professor and Head Department of Civil Engineering, Sethu Institute of Technology, Kariapatti, (TN)

The use of Self–compacting Concrete (SCC) is spreading world wide because of its very attractive properties in the fresh state as well as after hardening. Several attempts have been made in the recent years to study about the strength and behavior of SCC. However, only few studies have been conducted on the strength and behaviour of structural elements made using Fibre Reinforced Self–compacting Concrete (FRSCC). Therefore, an attempt has been made in the present investigation to study the effect of recron polyester fibres on the strength and behavior of FRSCC structural elements subjected to flexure. A mix proportion of SCC was arrived by using trial and error method. Totally, nine trial mixes were investigated and w/c ratio, cement content, water content, was maintained constant for all the mixes. The variable in this study is percentage of volume fraction (0.25, 0.5 and 0.75) of recron fibres of 12mm length. Finally, six beams were cast, out of which two were FRSCC beams, two were conventional concrete beams with fibres and two control beams without fibres one made with conventional concrete (CC) and the other one made with SCC. The effect of addition of recron fibres in the compressive strength and the flexural behavior of conventional and SCC beams has been studied.

Introduction

SCC is now an emerging technique in the field of concrete technology. SCC is an innovative idea to tackle the problem of concreting through dense reinforcement. SCC is unique, because of its properties, like fill ability, flowability, pumpability, and make production of concrete more industrialized. The use of cementitious fines like fly ash makes the concrete economical. It becomes necessary to develop a compaction free production system thereby reducing the overall cost of the project, improve the quality of the work, and providing safety in the work environment. SCC possesses high flowability, resistance to segregation, passing ability which enables the concrete to fill in through the dense reinforcement. In addition to the properties of fresh concrete, SCC should also possess the properties of hardened concrete, in order to ensure the hardened concrete properties. The use of SCC will lead to a more industrialized production, reduce the technical costs of in situ cast concrete constructions, improve quality, durability, pump ability and reliability of concrete structures and eliminate some of the potential for human error. It will replace manual compaction of fresh concrete with a modern semi-automatic placing technology and in that way improve health and safety in and around the construction site. SCC must have a great filling ability, a high segregation resistance during and after placing the concrete and a great filling ability through dense reinforcement and around other obstacles such as recesses and embedded items.

The use of fiber-reinforced concrete or cement composites to enhance the performance of structural elements has been the subject of many research projects during the past few decades. Numerous types of fiber-reinforced concrete or cement composites reinforced with steel, polymeric, glass, and carbon fibers have been evaluated for structural applications. As one might suspect, not all fiber-reinforced concrete or cement composites behave in a similar manner, and thus proper material selection is critical to achieve the desired structural performance.

Steel fibre reinforced concrete (SFRC) is a composite material in which short discrete steel fibres are randomly distributed throughout the concrete mass. Extensive research work on SFRC has established that the addition of steel fibres to plain cement concrete (PCC) improves its strength, durability, toughness, ductility, post-cracking load resistance, etc [1-2].Owing to the favourable characteristics of SFRC, its use has steadily increased during the last two decades all over the world and its current fields of application includes airport and highway pavements, earthquake-resistant and explosion resistant structures, mine and tunnel linings, bridge deck over lays, hydraulic structures, rock-slope stabilisation, etc[3].

Behavior of Steel fibre Reinforced Self–compacting Concrete Flexural Elements has been studied by Ganesan N, Indira P.V and Santhosh Kumar P.T and it was found that the addition of steel fibres improved the crack load and ultimate strength of the concrete [4]. Steel fibres are added to improve tensile strength and fracture properties of concrete. Such an addition results in imparting ductility to an otherwise brittle material. The addition increases the strain capacity and imparts improvement in ductility also known as pseudo-ductility. The ductility factor increased up to a volume fraction of steel fibres of 0.5% for all the aspect ratios. A literature review indicated that while several researchers studied the effect of addition of steel fibres on the strength characteristics of conventional concrete and self compacting concrete, no study has been conducted so far to investigate the effect of addition polyester fibres on the flexural strength of conventional and self compacting concrete. Therefore, an attempt has been made to study the effect of addition of recron 3S polyester fibres on the flexural strength of normal and self compacting concrete. Test results obtained in this investigation are presented and discussed.

Experimental Programme

Materials used

Ordinary Portland cement having a specific gravity of 3.09 was used in the casting of the specimens. Crushed Granite aggregates with a maximum nominal size of 12mm and a specific gravity 0f 2.66 were used as coarse aggregates. Locally available natural river sand with a specific gravity of 2.48 was used as the fine aggregate. Fly ash obtained from the nearest thermal power station was used as a filler material in SCC. Conplast SP430 was used as an admixture to improve the workability of concrete.Recron-3S polyester fibres of 12mm length having a tensile strength of 400 N/mm2to 600 N/mm2were used in fibre reinforced SCC.

Test Methods for SCC

Properties of Fresh Concrete

In order to flow and fill through the dense reinforcement the SCC must possess certain properties like flow ability, fill ability, resistance for segregation. The major fresh concrete properties of SCC are Flow ability, Stability and Fillability. Table 1 lists the test methods to study the properties of SCC in fresh state and their acceptance criteria as per the guidelines provided by EFNARC [5]. The L-box test gives an indication of the resistance of the mixture to flow round observations in the L-box mould as shown in Figure 1. This test also detects the tendency of the coarse aggregate particles to stay back or settle down, when the mixture flows through closely spaced reinforcements. The slump flow test judges the ability of concrete to deform under its own weight. Viscosity of the mortar phase is obtained by a V-funnel apparatus as shown in Figure 2.

L-Box Test

Sequential procedure for arriving mix proportion of SCC

Initially, a normal mix with 100 mm slump is targeted without the use of superplasticizer simply by adjusting the water-cement ratio. In order to arrive at the normal mix of conventional concrete, the ACI method of mix design was used. The concrete mix was designed for a compressive strength of 30 MPa. A vertical slump of 160mm to 180mm is then aimed by adding superplasticizer to the normal mix. If any segregation, especially bleeding takes place at this stage, as judged visually, a part of coarse aggregate was replaced with fine aggregate. The percentage of replacement was chosen to be small, say 5 percent, by weight. To proceed towards achieving SCC, coarse aggregate is then replaced with a filler material namely fly ash, by weight starting from a value of 5 percent, 10 percent, 15 percent etc, until a slump flow of 500mm – 800mm was achieved by slump flow test. Mixes which were arrived, based on the sequential procedure are given in the Table 2.

Mix proportions for various trials

The mix ratios conforming to the slump flow specifications were tested for the V funnel and L Box tests. For the mix to get qualified as SCC it should satisfy the specifications of V funnel and L Box also. Then the fresh properties of concrete were also tested by the addition of recron fibres in the arrived SCC mix. Test results of fresh concrete are given in Table 3. Control SCC mix was obtained by replacing 25% of coarse aggregates by fly ash and by adding superplastcizers of 1% by weight of cementitious materials. When 0.25% and 0.5% of recron fibres were added to the control SCC mix, it satisfies the fresh concrete properties suggested by EFNARC guidelines and it was taken for the investigation of flexural behavior of beam. When 0.75% of recron fibres were added to the control SCC mix, fresh concrete properties given by EFNARC guidelines were not fulfilled and hence it was not taken for further investigation.

Test result of fresh Concrete

Flexural Behavior of Conventional and Self– compacting concrete

Self compacting Concrete
To study the flexural behavior, totally six reinforced concrete beams of size 1200mmx100mmx150mm were cast, out of which two were FRSCC beams, two were conventional concrete beams with fibres and two control beams without fibres one made with conventional concrete and the other one made with SCC. Two numbers of 10mm diameter Fe 415 grade steel bars were provided as longitudinal reinforcement and two legged 6mm diameter stirrups were provided at 150mm spacing in the mid span. The spacing of the stirrups was kept as 80mm in the shear zone. The beams were simply supported and subjected to two point loading as shown in Figure 3. A hydraulic jack of 15T capacity was used to apply the load. The dial gauges were fixed at the centre of the beam and under the load points to record the deflection of the beam during test. The deflection of the beams at mid span and under the load points were measured at every 0.2T intervals of loading. At every loading stage, cracks appearing on the surfaces were marked and crack widths were measured using a crack width measuring microscope The beam was loaded up to failure. The test results for conventional concrete and SCC beams are given in Tables 4 and 5. From the test results, load-deflection characteristics were studied and given in Figures 4 and 5. Figures 6 and 7 show the specimens after failure.

Test result for conventional Concrete
Self compacting Concrete

Self compacting Concrete

Discussion of test results

From the load-deflection curves, it has been noted that when recron polyester fibres are added, energy absorption capacity increases for both conventional and self compacting concrete. The compressive strength of conventional concrete and Self– compacting Concrete was found to be improved by the addition of Recron 3S fibres. Self–compacting Concrete shows higher compressive strength when compared to conventional concrete and this trend was applicable in fibre reinforced concrete also. In case of conventional concrete the compressive strength enhanced by about 9.8% and 4.4% for 0.25% and 0.5% of volume fraction of fibres respectively. In self compacting concrete compressive strength of concrete increased by about 12% and 6.9% for 0.25% and 0.5% of volume fraction of fibres respectively.

The first crack load in conventional concrete beam is 9 kN. When 0.25% and 0.5% fibres are added in conventional concrete beam the first crack load is enhanced to 10 kN and 12 kN respectively. Similarly in SCC beams the first crack load is 8 kN and it is increased to 12 kN and 14 kN due to addition of 0.25% and 0.5% fibres respectively. Because of addition of 0.25% and 0.5% of fibres the load carrying capacity of conventional concrete beam is enhanced by 4.4% and 8.4% whereas the load carrying capacity of SCC beams is increased by 2.8% and 5.3% respectively. The stiffness was also found to be increasing gradually both in Conventional as well as in SCC Beams.

Concrete Beam

Conclusions

Based on the results of this experimental investigation, the following conclusions are drawn:

The compressive strength of SCC was found to be improved by the addition of recron fibres to the mix.Also the SCC shows higher compressive strength when compared to Conventional concrete. The first crack load was also found to be increased in SCC than the CC beams. The ultimate load carrying capacity of the beam has also increased due to the addition of recron fibres in the mix. The stiffness has also increased gradually both in conventional as well as in SCC due to the addition of fibres. The SCC shows good properties than the conventional concrete properties. The SCC does not require compaction or it can be compacted with a very little effort. SCC is very useful in concreting through the congested reinforcement. The use of SCC increases the pump ability of the concrete; this reduces the labour cost and increases the safety at the work site. It improves the economy of the concreting work. Therefore, In general practice also the use of SCC proves to be advantageous.

References

  • B Singh, P Kumar and S K Kaushik. .High Performance Composites for the New Millennium. Journal of Structural Engineering, Vol 28, no 1, April-June 2001, pp 17-26.
  • N Ganesan and K P Shivananda. Strength and Ductility of Latex Modified Steel Fibre Reinforced Concrete Flexural Members. Journal of Structural Engineering, Vol 27, no 2, July 2000, pp 111-116.
  • V Ramakrishnan. .Materials and Properties of Fibre Reinforced Concrete.Proceedings of the International Symposium on Fibre Reinforced Concrete, Vol 1,December 16-19, 1987, Madras, pp 2.3-2.23.
  • Ganesan N, Indira P.V And Santhosh Kumar P.T Ultimate Strength Of Steel Fibre Reinforced Self Compacting Concreete Flexural Elements, The Indian Concrete Journal, December 2006.
  • Poulson, B EFNARC (2002) “Specification And Guidelines For Self-Compacting Concrete”
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