Key Engineering Materials Vols. 629-630

Abstract: Chloride resistant HPC and protective cover are two basic measurements to improve the durability of concrete in chloride environment. Though it provides crucial cover for concrete to resist chloride ions, spacer has limited chloride resistant ability, which is overlooked by past researchers. Cementitious spacers are easy access for chloride ions to penetrate into concrete resulting in reduction of structural durability. To improve cementitious spacers’ performance, a systematic study was conducted. Test results showed that there was major difference between mortar and concrete in terms of chloride coulomb electric flux but minor difference in terms of chloride ion diffusion coefficient, which implied using chloride ion diffusion coefficient as spacer’s durability indicator was preferable; parameters of mix design had a similar influence on mortar and concrete and, with the same mixing parameters, the strength and chloride resistant ability of mortar were weaker than concrete’s; it was feasible to develop the mix design of chloride resistant cementitious spacers based on concrete’s design method with certain adjustments, such as using stricter mix proportion, adding small-size coarse aggregate, lowering water-binder ratio and optimizing the binder proportion, to achieve higher strength and durability.

Abstract: High performance self-compacting mortar has the ability to push out air bubbles under its own weight. Consequently, the resistance against freeze-thaw cycles with or without deicing salts can decrease due to the total air content reduction. In order to assure the necessary expansion zones^1,2 air entraining agents (AEA) are commonly used to increase the amount of stable air bubbles. Depending on the mixture, the workability and rheology decrease or increase by the entrained air bubbles³. This will depend on the ratio between the surface tension and the shear stress applied during the test. If the latter can overcome the first, the bubbles will deform and increase the fluidity of the mixture. Besides the influence on the durability and the fresh concrete, air entraining agents also alter the pore structure and the mechanical performance of the mortar. The effect of AEA on these properties is the subject of this paper. The pore structure is examined on two different levels. On the one hand, mercury intrusion porosimetry is used to investigate the capillary porosity, ranging from 10 nm to 10 μm. On the other hand air void analysis and fluorescence microscopy is performed to evaluate the larger air bubbles ranging from 0.1 mm to 1 mm⁴. Both techniques showed an overlap in their measuring range. Consequently it was possible to compare the techniques critically. Similar as in literature, mercury intrusion porosimetry underestimates the amount of larger air bubbles in mortar, due to its measuring principle⁵. Furthermore, the bubbles with a diameter of 80 μm increase significantly by the addition of AEA. This confirms the average air bubble size often used in literature to explain the influence of AEA on the workability and rheology³. The influence of air entraining agent on the mechanical performance was tested by the compressive and bending tensile strength. In conclusion, a good balance is necessary between the air content necessary for a proper freeze-thaw resistance without changing the mechanical performance drastically.

Abstract: Properties of cement paste were affected significently by fineness of cement which is one of the most important influence factors. Effect of fineness on the rheology properties, strength, hydration and pore structure of cement was studied. The results showed that: plastic viscosity of the cement paste can be reduced by the increase of cement fineness to some extent. The strength of cement, especially the early strength significantly increased; heat of hydration increased, the peak of hydration heat brought forward and induction period shortened; porosity of cement paste reduced with the increase of cement fineness. There is a good negative linear correlation between characteristics particle size D (0.1) and 1 day hydration heat of three different kinds of cement. Keywords: cement; fineness; hydration; porosity

Abstract: Fly ash (FA) and granular blast-furnace slag (GBFS) are usual mineral admixtures to conventional concrete, and their contents substituted for Portland cement definitely affect development rate of strength of concrete. C30 and C60 concrete samples with FA and/or GBFS were prepared to study the influence of substitution content of the mineral admixtures on 3 d, 7 d and 28 d strength. The results reveal that the development rate of strength in period from 3 d to 7 d gets slow with increasing content of mineral admixtures except for concrete with only GBFS less than 20%. In the case of substituting FA as the only mineral admixture for part of cement, the development rate of strength of C30 concrete in period from 7 d to 28 d keeps roughly constant even that of C60 concrete increases. When substituting mineral admixtures in the presence of GBFS for cement within experimental range, the development rate of strength in period from 7 d to 28 d gets fast with increasing substitution content. The enhancing effect of combining FA and GBFS occurs in period from 7 d to 28 d for both C30 and C60 concretes (FA+GBFS≤40%), even occurs in period from 3 d to 7 d for C60 concrete. Based on 7 d strength and the development rate, 28 d strength of concrete can be predicted accurately.

Abstract: The influence of slag dosage on the hydration kinetics of slag-cement composite cementing material system is studied. The results show that the exothermic rate of hydration decreases as the slag dosage increases or the temperature of hydration decreases. The non-evaporable water content increases at first and then decreases with the increasing of slag dosage, and reaches the maximum as slag dosage is 30%. The compressive strength decreases as slag dosage increases, the increasing rate of which is evidently before 28 days, after that it is flat or become slow. It can be seen from SEM (scanning electron microscope), in the system of slag dosage below 30%, the gel-like structures bind with crystals which make the system very compact.

Abstract: The traditional mix proportion for concrete used in building field could not meet the requirements for industrialization production any more, and the optimization needs developing to obtain an ideal mix proportion for precast concrete with high early strength and good workability, as well as economy. To achieve the high early strength, Low-Alkalinity Sulphoaluminate Cement was selected, accompanied by Ordinary Portland Cement. The properties of other materials, namely aggregates (medium sand and gravel) and admixtures (polycarboxylate-based superplasticizer and sodium sulfate), were determined through literature review. The parameters of materials, such as the fineness modulus, mud-content of aggregates and cement strength, were determined. Besides, the range of water-cement ratio, admixture content and the ratio of two kinds of cements etc. in a total of 13 preliminary concrete mixes are analyzed. With further analysis, the four dominant factors were chosen, that is, water-cement ratio, cement content, superplasticizer content and sand ratio, leading to a series of orthogonal experiments with 3 levels of each factor. By analysis of test result data and consideration of cost, the optimized mix for precast concrete in building industrialization was carried out, of which the compressive strength in 10 hours reached the required strength and other properties reasonable.

Abstract: Contemporary knowledge of SCC concretes, that brought about of worthy endeavors of numerous researcher and engineers have come in handy now.Taking advantage of such material makes compound construction more efficient, easier and less expensive. Upper Gotvand , a 180 meters height rock fill dam located southwestern Iran have several affiliated projects using SCC concrete for their high reinforced or hard to access segments such as bottom outlet structure, pressure tunnel blocks, concrete coating for penstock steel lining, upper and lower bends, powerhouse structures and etc. Particular structural peculiarities for each individual projects have been detected as well and SCC-related conclusions were made. To satisfy essential needs in order to make this concrete more ideal we are about to study mix design components to select the most consistent ingredients. Different available cements, third generation admixtures, and aggregate sources interaction were explored to elicit the most optimized mix design, native to region. In this article, accomplished quality control tests have been introduced. Test results have led to satisfactory mix manipulation and to ensure optimized attributes for required practical applications. It was revealed among Gotvand SCC ingredients, Binders and fines are the most ruling components, capable of governing practical traits. River aggregates are highly beneficial and Gelenium51P acts consistently with Ilam cement. Assigned Q.C. tests have led us to favorite mix named, NA45. Because of higher lateral pressure of fresh SCC than ordinary concrete, it was allocated to heading and upper lifts of walls. More water proof forms and more support requirement as well as longer setting time were concluded as relative constraints of SCC at practice.

Abstract: This study deals mainly deals with the rheological and strength properties of self-compacting concrete (SCC) incorporating high volume replacements of slag. Plastic viscosity and yield stress were evaluated using ICAR rheometer. The effect of high range water reducing admixture (HRWR) dosage, the time to reach 500 mm diameter (T50), the final slump flow, V-funnel flow times and L-box blocking ratio were also investigated and studied in this research program. The results show that the plastic viscosity decreases with the increase of the percentage of slag and the yield stress was nearly zero for all the replacements studied. The compressive strengths determined for the SCCs demonstrate that high strength SCC of more than 100 MPa can be realized by using slag appropriately.

Abstract: Concrete consists of many kinds of raw materials, the safety and quality of construction are greatly influenced by the stability of concrete. Thus, how to improve the performance of concrete and construction method, i.e. avoid the bleeding, segregation and honeycomb, etc, becomes a very important issue and imminent mission. This study discusses the different mixing designs of SCC with the amounts of cementitious materials ( OPC, GGBFS, and FA) ranged from 400 - 500 kg/m³. Meanwhile, the fresh and hardened properties of SCC, such as the slump flow, setting time, heat of hydration, and compressive strength were also determined. Test results indicate SCC binding materials can be classified by its compacting ratio. For instance, to pass the boxing test of R1/R2 within and without hindrance, the minimum binding materials should be 500kg/m³, 450kg/m³ and 400kg/m³. The optimum binding materials research shows if use cement, GGBFS and FA at the same time , it can increase the initial and final setting time and decrease the heat of hydration. The SCC binding materials’ concept will meet the requirement of " Safety, Durability, Workability and Ecology " for the new era. Thus, the amount of cement was at least 250kg/m3 or more for SCC mix design and the adding of GGBFS and FA was recommended to maintain 20 - 40 % of all cementitious materials based on the test results.

Abstract: Self-Compacting Concrete (SCC) incorporating a blend of normal aggregate and waste rubber or expanded clay aggregate were prepared in present paper. And then the properties of SCC incorporating combined aggregates in fresh state and hardened state, including workability, dynamic elastic modulus, compressive strength and chloride ion permeability were investigated by experiments. Results indicate that utilization of rubber particles as a fine aggregate or expanded clay particles as a coarse aggregate by partially replacing sand or coarse aggregate of the same volume can successfully produce the SCC with suitable workability. The addition of rubber or expanded clay particles results in a remarkable reduction of mechanical strength and dynamic elastic modulus of SCC. And the compressive strength and dynamic elastic modulus of SCC with expanded clay aggregate replacing coarse aggregate is higher than that of SCC with the same volume rubber replacing sand. The incorporation of rubber or expanded clay aggregate in concrete exhibits a reduction in chloride ion permeability as compared to normal concrete. It is of great importance for further understanding the effect of waste rubber and expanded clay aggregate on workability, mechanical properties and durability of SCC.