Abstract
Self-consolidating concrete SCC is highly workable concrete that easily flows through congested and complex reinforced concrete formwork filling all the gaps and without any form of segregation. SCC is an attractive alternative to conventional concrete, which requires skilled labor to perform proper consolidation on site, eliminating the need for additional consolidation. Many solutions to prevent segregation using different types of admixtures are available to obtain SCC, but they increase the cost of produced concrete. Nevertheless, high cost of SCC (25-50% more expensive than conventional concrete) and lack of knowledge of SCC properties in aggressive environment, are two major obstacles preventing the wide spread usage of SCC, and Due to the competitive nature of construction industry, the contractors shy away from high cost solutions to segregation problem. Therefore, the aim is to find the best solutions that offer optimized admixtures percentage in order to obtain the aspired results with maximum efficiency and at minimum cost.
Introduction
Segregation in concrete is the separation of concrete mixture ingredients, during pouring procedure, due to several factors and it is a wide spread problem which yields a sub-standard concrete structure. In concrete technology, the term “segregation” is used to describe three types of ingredients separation from concrete mass; the first form is separation of coarse aggregate from the mixture in its plastic state due to its greater weight, the second form of segregation occurs when the cement paste (or grout) separates due to high water content and the last form involves water separation from the rest of material due to its lighter weight know by “water bleeding”. The causes of the segregation phenomena are related to the different size and weight properties of concrete mixture which consists of cement, water, aggregate and admixture.
The problems associated with segregation and subsequent dangers are very costly to repair, besides causing delays of work in order to repair the same structure. The best approach to solve the problem is prevention and the use of self-consolidating concrete SCC offers that.
Self-consolidating or self-compacting concrete (SCC) is a recent and promising development in concrete technology offering solution to many inherent problems associated with it and offers an innovative solution to segregation. Self-consolidating concrete (SCC) is a highly workable concrete that flows under its own weight and fills the formwork without the need for vibration and additional consolidation. SCC has three essential property requirements: flowing ability, passing ability and segregation resistance both statically (the separation after concrete is cast into place) and dynamically (the separation during the process of placement).
To obtain self-consolidating concrete SCC, high cost chemical admixtures such as high-range water-reducing admixtures (HRWRA) and viscosity-modifying admixtures (VMA) are used, besides other types of admixtures.
Replacing Portland cement with natural Zeolite (NZ)
SCC contains a high percentage of Portland cement which is one of the main challenges from an environmental point of view in optimization of its mixture proportions, due to large amounts of CO2 emissions during Portland cement production. Two techniques were used to decrease the Portland cement content in SCC mixtures; adopting a dense aggregate grading technique, to lower the binding effect, and partially replacing Portland cement with natural zeolite. It was concluded that using higher replacement levels of natural zeolite (up to 30%) has significantly improved segregation resistance and lowered the required dosage of high-range water-reducer agent. [1]
Self-consolidating concrete with supplementary cementitious materials
To achieve a workable SCC mixture, one can simply increase the water-to-cementitious materials ratio (W/CM). However, such practice may cause segregation and bleeding, negatively affect the mechanical properties of concrete, and most importantly inflict destructive effects on concrete durability. To alleviate such effects, the development of SCC usually requires the use of expensive chemical admixtures such as high-range water-reducing admixtures (HRWRA) and viscosity-modifying admixtures (VMA). Using high contents of fly ash as partial replacement of cement enhances concrete workability as well as the long-term compressive strength. However, binary SCC mixtures made with 60% class F fly ash developed low early-age compressive strength but are still acceptable as normal concrete. [2]
Self-consolidating high-strength concretes, including palm oil fuel ash (POFA)
Self-consolidating high-strength concretes SCHSC, including palm oil fuel ash (POFA), were produced based on the water/binder (W/B) mixing ratio. POFA was used in the concrete mixtures by replacing 0–30% of normal Portland cement by weight. The freshly mixed concrete mixtures were tested to determine its properties in segregation resistance. The W/B ratio, POFA content, and HRWR dosage affected the segregation resistance of concretes, but maintained the acceptable criteria. The higher dosage of HRWR was needed for a lower W/B ratio and higher POFA content to achieve the sufficient flow in concrete. [5]
Conclusion
Self-consolidating concrete SCC has increasingly became an alternative to normal concrete, but there are still many technical and economic challenges related to its development. SCC can be produced with low Portland cement PC by partial replacing of PC with natural Zeolite NZ and application of a dense-packing concept, which leads to increased viscosity providing the needed stability and high segregation resistance required for SCC mixtures.
High contents of admixtures in SCC mixtures significantly lowered concrete permeability compared to that of the normal mixture and it also reduces the shrinkage of such mixtures.
The adequate application of SCC depends highly on its proper segregation resistance, and in order design a SCC mixture, which remains stable with small variations in raw materials content, it is important to understand the mechanism of how mix proportions affect robustness of the concrete mixture.