1.0 INTRODUCTION TO SCC
Self-Compacting Concrete (SCC), a relatively new category of high performance concrete, is proportioned in such that the concrete freely passes around and through reinforcement, completely fills the formwork and consolidates under its own weight without segregation. The high flowability of SCC makes it possible to fill the formwork without vibration
[Khayat, 1999; Khayat et al., 2004].
Developed in Japan in the late 1980’s [Ozawa, et al., 1989], SCC has been a topic of research and development in many locations, especially in Japan and Europe [Ouchi, et al., 2003]. SCC has been successfully used in numerous applications where normal concrete is difficult to place and consolidate due to reinforcement congestion and difficult access. Precast, prestressed bridge elements, such as AASHTO Type III girders, have congested reinforcement and tight dimensional geometry, and therefore can benefit from the use of SCC.
Three basic characteristics are required to obtain SCC: high deformability, restrained flowability and a high resistance to segregation [Khayat, et al., 2004]. High deformability is related to the capacity of the concrete to deform and spread freely in order to fill all the space in the formwork. It is usually a function of the form, size and quantity of the aggregate, and the friction between the solid particles, which can be reduced by adding a high range water-reducing admixture (HRWR) to the mixture. Restrained flowability represents how easily the concrete can flow around obstacles, such as reinforcement, and is related to the member geometry and the shape of the formwork.
Segregation is usually related to the cohesiveness of the fresh concrete, which can be enhanced by adding a viscosity-modifying admixture (VMA) along with a HRWR, by reducing the free water content, by increasing the volume of paste, or by some Combination of these factors.
Two general types of SCC can be obtained:
(1) Concrete with a small reduction in the coarse aggregate, containing a VMA.
(2) Concrete with a significant reduction in the coarse aggregate content without any VMA.
SCC has been claimed to offer many advantages for the precast, prestressed industry including elimination of noise and problems related to concrete vibration, lower labor cost per member, and faster casting, thereby increasing productivity. Due to the low water-cement ratio, SCC should have improved to durability and strength.
Generally, SCC contains a higher cementitious materials and lower water-cement ratio than conventional concrete, and so can provide relatively high strength. The paste usually includes fly ash, slag, silica fume, or other supplementary cementitious materials, or an inert filler such as limestone powder. The paste content of SCC is also relatively high, with a reduction in the size and quantity of coarse aggregate. These factors are typically associated with increased creep and shrinkage, and may be related to a reduction in elastic modulus.
WHAT IS SELF-COMPACTING CONCRETE (SCC)?
It is a concrete that can be compacted by its own weight and fills every corners in the formwork and the placing can be done without vibrating compaction. In the plastic state it is very homogenous, cohesive and very flowable.
1.1 WHY IT IS NEEDED?
Concrete is a versatile material extensively used in construction applications throughout the world. Properly placed and cured concrete exhibits excellent compressive-force-resisting characteristics and engineers rely on it to perform in a myriad of situations. However, if proper consolidation is not provided, its strength and durability could be questionable. To help alleviate these concerns, Japanese researchers in the late 1980’s developed a concrete mixture that deformed under its own weight, thus filling around and encapsulating reinforcing steel without any mechanical consolidation.
- Self-Compacting Concrete offers new possibilities and prospects in the context of durability and strength of concrete.
- As a result of the mix design, some properties of the hardened concrete can be different for SCC in comparison to normal vibrated concrete.
- Mix design criterions are mostly focused on the type and mixture proportions of the constituents.
- Adjustment of the water/cement ratio and super plasticizer dosage is one of the main key properties in proportioning of SCC mixtures.
1.2 AIM AND OBJECTIVES
The aim of this study of self-compacting concrete using Plantain leaf ash as partial replacement of ordinary portland cement is to obtain self-compacting concrete satisfying EFNARC guidelines and make comparison of self-compacting concrete to normally compacted concrete in terms of workability and compressive strength.
The above aim will be accomplished by fulfilling the following research objectives:
- Determining the effect of Plantain leaf ash as partial replacement of cement on the properties of SCC in
- FRESH STATE (Filling ability and Passing ability)
- HARDENED STATE (Compressive strength)
- Obtaining specific experimental data to understand fresh and hardened properties of self-compacting concrete.
- Developing SCC using Plantain leaf ash as partial replacement of cement in varying dosages satisfying European standards and to study their behaviour.
- Determining whether the properties observed in (1) are structurally sufficient for its application according to relevant standard as a construction material.
- Assessing the implication of its usage as a construction material in the built environment
1.3 SCOPE OF STUDY
For this study, concrete with varied percentages of Plantain leaf were used in producing self-compacting concrete in terms of filling ability and passing ability and were compared with normally compacted concrete.. The key parameter in the study is:
- i. The workability characteristics using slump flow test, V-funnel test, L-box test and compressive strength characteristics at 14, 21 and 28 days using 45 cubes of 150mm X 150mm X 150mm were determined.
1.4 SIGNIFICANCE OF STUDY
- To produce concrete of high and significant strength and durability to be used for all construction structures.
- To effectively utilize and solve the problem of the storage and disposal of plantain leaf ash.
- To minimize maintenance, labour cost, and cost due to the vibrators required.
1.5.0 ADVANTAGES OF SELF-COMPACTING CONCRETE:
Simple inclusion even in complicated formwork and tight reinforcement:
- Higher installation performance since no compaction work is necessary which leads to reduced construction times, especially at large construction sites
- Reduced noise pollution since vibrators are not necessary
- Higher and more homogenous concrete quality across the entire concrete cross-section, especially around the reinforcement
- Improved concrete surfaces (visible concrete quality)
- Typically higher early strength of the concrete so that formwork removal can be performed more quickly.
1.5.1 DISADVANTAGES OF SELF-COMPACTING CONCRETE
- SCC requires higher powder and admixture (particularly super-plasticizers) contents than normally compacted concrete and so the material cost is higher.
- The increased content of powder and admixture also leads to higher sensitivity of SCC to material variation than that of normally vibrated concrete; thus greater care with quality control is required.
1.5.2 CONSTITUENTS OF SCC
With regard to its composition, SCC consists of the same components as conventionally vibrated concrete, which are
- Chemical Admixtures i.e. Superplasticizers and Viscosity Modifying Agents Mineral Admixtures i.e., fly ash, Silica Fume, etc.
1.5.3 PROPERTIES OF SCC
Following are the properties of hardened self-compacting concrete:
1. Compressive strength
SCC compressive strengths are comparable to those of normally compacted concrete made with similar proportions and water cement ratio. There is no difficulty in producing SCC with compressive strength up to 60N/mm2. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681).
2. Tensile strength
Tensile strengths are assessed indirectly by the splitting test on cylinders. For SCC, both the tensile strengths themselves, and the relationships between tensile and compressive strengths are of similar order to those of normally compacted concrete. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681).
3. Bond strength
The strength of the bond between concrete and reinforcement are assessed by pullout tests, using deformed reinforcing steel of two different diameters, embedded in concrete prisms. For both civil engineering and housing categories, the SCC bond strengths, related to the standard compressive strengths, were higher than those of the reference concrete were. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681).
4. Modulus of elasticity
Previous results available indicate that the relationships between static modulus of elasticity and compressive strengths were similar for SCC and the reference mixes. A relationship in the form of E/ (fcu) 0.5 has been widely reported, and all values of this ratio were close to the one recommended by ACT for structural calculations for normal weight compacted concrete. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681).
5. Shrinkage and creep
None of the results obtained indicates that the shrinkage and the creep of the SCC mixes were significantly greater than those of traditional vibrated concrete. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681)
6. Some aspects of durability
Elements of all types of concrete have been left exposed for future assessment of durability but some preliminary tests have been carried out.
The permeability of the concrete, a recognized indicator of likely durability, has been examined by measuring the water absorption of near surface concrete. The results suggest that in the SCC mixes, the near surface concrete was denser and more resistant to water ingress than in the reference mixes. Carbonation depths have been measured at one year. (http://theconstructor.org/concrete/properties-of-hardened-self-compacting-concrete/7681)
1.6.0 FUNCTIONAL REQUIREMENT OF SCC
According to EFNARC specification (European Federation of National Association Representing for Concrete), SCC must be designed to fulfil the requirements of EN 206 regarding density, strength development, final strength and durability which obtains the following requirements.
- Filling Ability – The ability of SCC to flow under its own weight into and fill completely all spaces within intricate formwork, containing obstacles, such as reinforcement.
- Passing Ability – The ability of SCC to flow through openings approaching the size of the mix coarse aggregate, such as the spaces between steel reinforcing bars, without segregation.
- Resistance to Segregation – The ability of SCC to remain homogeneous during transport, placing, and after placement.
1.7.0 PHYSICAL AND CHEMICAL PROCESS OF SCC
- The physical process is due to the particles fineness of the supplementary cementing materials that are much smaller than that of the cement, thereby providing densely packed particles between fine aggregates and cement grains, and, hence, the reduction in porosity.
- The chemical process is due to the activation of the non-crystalline silica, by the calcium hydroxide produced from the hydrating cement to form secondary calcium silicate hydrate that also fills the pore spaces and further reduces the porosity
1.8.0 PROBLEM STATEMENT
There are currently not universally accepted design, proportioning or acceptance criteria for the use of SCC in prestressed girders. Although SCC has been used successfully in several precast and cast-in-place applications and many of the properties of SCC have Been established, several issues must still be resolved in order to successfully use SCC in The production of prestressed bridge elements. Many of these concerns are related to long term behaviour of the element in service.
SCC is similar to conventional concrete in terms of compressive strength. Due to the lower content of coarse aggregate, however, there is some concern that:
(1) SCC may have a lower modulus of elasticity, which may affect deformation characteristics of prestressed concrete members
(2) Creep and shrinkage will be higher, affecting prestress loss and long term deflection.
1.8.1 SCC POTENTIALS BEYOND CONVENTIONAL CONCRETE
- Improved efficiency
- Use with close meshed reinforcement
- For slender Component
- For complex geometric shapes
- Generally where compaction is difficult
- Fast installation rates
- Reduced damage to health
1.8.2 SCC AND MEASUREMENT OF ITS FLOW PROPERTIES
Conventional workability tests, devised for normal ranges of concrete mixtures are not adequate for self-compacting concrete, because they are not sensitive enough to detect the tendency to segregation. For example, a slump test may show collapse, (a slump of say 280 mm) and yet in one case the mixture may be stable and in other cases either the aggregate may settle down or the slurry may tend to “run”. Therefore test equipment was fabricated for judging the following characteristics.
(1) Self-compatibility: The U-tube test gives an indication of the resistance of the mixture to flow round obstructions in a U-type mould. 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.
(2) Deformability: The slump flow test as specified by the Japan Society of Civil Engineers (JSCE) judges the ability of concrete to deform under its own weight against the friction of the base, this test, however, cannot evaluate whether the concrete will pass through the space between the reinforcement bars. This test is useful also as a routine control test, to detect the tendency for slurry to separate from the mixture.
(3) Viscosity: Viscosity of the mortar phase is obtained by a V-funnel apparatus, this is useful for adjusting the powder content, water content and admixture dosage.
(4) Filling ability test: It is also used to determine the ability of the concrete to deform readily through closely spaced obstacles.
Many different methods have been developed to characterise the properties of SCC. No single method has been found till date which characterises all the relevant workability aspects and hence, each mixed has been tested by more than one test method for the different workability parameters.