Mix design method for self compacting metakaolin concrete with different properties of coarse aggregate

This study deals with a proposed mix design method for SCC utilizing different properties of coarse aggregate. The work was conducted in three phases, i.e. paste, mortar and concrete to facilitate the mix design process. Initial investigation on cement paste determined the basis for water cement ratio and superplasticizer dosage for the concrete. For the study on mortar, metakaolin (MK) as pozzolan was used at replacement levels of 5%, 10%, 15%, and 20% by weight of cement. Self compactability of mortars was obtained by adding suitable materials such as mineral admixtures and superplasticizer which provided a sufficient balance between flowability and viscosity of the mix. The optimum MK replacement level for cement was 10% from the viewpoint of workability and strength. Flowability of mortar decreased with the use of metakaolin. Moreover, strength of mortar increased when the optimum replacement level of pozzolan was used. Different fresh concrete tests were adopted. The results obtained for fresh concrete properties showed that flowability of concrete increased with increase flowability of mortar. The mixes which contained coarse aggregate with lower volume, small size, and continuous grading affected positively the fresh properties of SCC. Finally, the mix design method used was successful in producing SCC with different coarse aggregate properties.     

Durability of recycled aggregate concrete designed with equivalent mortar volume method

Results of a comprehensive investigation about the durability of structural-grade concrete made with recycled concrete aggregate (RCA) are presented. The RCA-concrete mixes were proportioned using a new concrete mix design method, termed the equivalent mortar volume (EMV) method. The EMV method is based on the hypothesis that RCA is a composite material comprising mortar and natural aggregate; therefore, when proportioning a concrete mixture containing RCA, one must account for the relative amount and properties of each the two components and adjust both the fresh coarse aggregate and fresh paste content of the mix accordingly. Tests were conducted to study the freeze–thaw, chloride penetration and carbonation resistances of the mixes proportioned by the EMV method and by the conventional method. Results of the test showed that RCA-concrete mixes proportioned by the EMV method have higher resistance to freeze–thaw action, chloride penetration and carbonation than those designed with the conventional method, and they satisfy the current requirements for concrete exposed to severe environments.     

Influence of steam curing on the pore structures and mechanical properties of fly-ash high performance concrete prepared with recycled aggregates

In this research work, High Performance Concrete (HPC) was produced employing 30% of fly ash and 70% of Portland cement as binder materials. Three types of coarse recycled concrete aggregates (RCA) sourced from medium to high strength concretes were employed as 100% replacement of natural aggregates for recycled aggregate concrete (RAC) production. The specimens of four types of concretes (natural aggregate concrete (NAC) and three RACs) were subjected to initial steam curing besides the conventional curing process. The use of high quality RCA (>100 MPa) in HPC produced RAC with similar or improved pore structures, compressive and splitting tensile strengths, and modulus of elasticity to those of NAC. It was determined that the mechanical and physical behaviour of HPC decreased with the reduction of RCA quality. Nonetheless steam-cured RACs had greater reductions of porosity up to 90 days than NAC, which led to lower capillary pore volume.     

Effect of coarse aggregate blending on short-term mechanical properties of self compacting concrete

This investigation is mainly focused on finding the unit weight, compressive strength, modulus of elasticity (MOE) and splitting tensile strength (STS) of SCC mixes with different coarse aggregate blending (60:40 and 40:60) (20 mm and 10 mm) and coarse aggregate content (28% and 32%) and these properties were compared to a conventional concrete (CC). All SCC mixes had 35% replacement of cement with class F fly ash. The coarse aggregate blending did not affect the compressive strength of SCC mixes, but it affected the unit weight, MOE and STS of SCC mixes. A new parameter called coarse aggregate points (CAPs) has been introduced to study the effect of coarse aggregate blending in a particular coarse aggregate content on mechanical properties of SCC mixes. It is observed that for the given strength, SCC mixes with the same CAP value have shown similar mechanical properties. The measured MOE of all mixes were compared with ACI 363R and AASHTO LRFD/ACI 318 predicted equations. The measured STS of all mixes were compared with ACI 363R and CEB-FIP predicted equations.     

Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates

In this study, the fresh and hardened properties of self-compacting concrete (SCC) using recycled concrete aggregate as both coarse and fine aggregates were evaluated. Three series of SCC mixtures were prepared with 100% coarse recycled aggregates, and different levels of fine recycled aggregates were used to replace river sand. The cement content was kept constant for all concrete mixtures. The SCC mixtures were prepared with 0, 25, 50, 75 and 100% fine recycled aggregates, the corresponding water-to-binder ratios (W/B) were 0.53 and 0.44 for the SCC mixtures in Series I and II, respectively. The SCC mixtures in Series III were prepared with 100% recycled concrete aggregates (both coarse and fine) but three different W/B ratios of 0.44, 0.40 and 0.35 were used. Different tests covering fresh, hardened and durability properties of these SCC mixtures were executed. The results indicate that the properties of the SCCs made from river sand and crushed fine recycled aggregates showed only slight differences. The feasibility of utilizing fine and coarse recycled aggregates with rejected fly ash and Class F fly ash for self-compacting concrete has been demonstrated.     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Compressive strength and durability properties of ceramic wastes based concrete

This paper presents an experimental study on the properties and on the durability of concrete containing ceramic wastes. Several concrete mixes possessing a target mean compressive strength of 30 MPa were prepared with 20% cement replacement by ceramic powder (W/B = 0.6). A concrete mix with ceramic sand and granite aggregates were also prepared as well as a concrete mix with natural sand and coarse ceramic aggregates (W/B = 0.5). The mechanical and durability performance of ceramic waste based concrete are assessed by means of mechanical tests, water performance, permeability, chloride diffusion and also accelerated aging tests. Results show that concrete with partial cement replacement by ceramic powder although it has minor strength loss possess increase durability performance. Results also shows that concrete mixtures with ceramic aggregates perform better than the control concrete mixtures concerning compressive strength, capillarity water absorption, oxygen permeability and chloride diffusion. The replacement of cement and aggregates in concrete by ceramic wastes will have major environmental benefits.     

Mechanical behavior of recycled coarse aggregate concrete reinforced with steel fibers under direct shear

Recycled coarse aggregate (RCA) concrete has attracted more and more attention worldwide in recent years due to the exhaustion of natural coarse aggregate and environmental pollution from construction and demolition waste in civil engineering. In this study, experiments were carried out on over 100 specimens to investigate the mechanical properties and failure mode of concrete with different volume content of steel fibers (0%, 0.5%, 1%, 1.5%, and 2%) and different RCA replacement ratio (0%, 30%, 50% and 100%) under direct shear load. The results show that addition of steel fibers can effectively improve the shear strength and shear toughness of RCA concrete. For a given compressive strength, the RCA replacement ratio has negligible impact on shear strength, but shear deformation and toughness increase as RCA replacement ratio reaches above a ‘limiting value’. A shear strength formula for steel fiber reinforced RCA concrete (SFRCAC) based on compressive strength and characteristic coefficient of steel fiber has been put forward.     

Performance evaluation of asphalt mixtures containing recycled concrete aggregates

Abstract

To evaluate the feasibility of using Recycled Concrete Aggregates (RCA) in asphalt mixtures, the coarse RCA and fine RCA were prepared as a partial replacement of the natural aggregates (NA). Different amounts of replacement of NA with RCA were investigated, and the mechanical properties and pavement performance of asphalt mixtures containing different proportions of RCA were analysed based on laboratory tests. The results indicated that with increasing the RCA percentage, the optimum asphalt content increased and the bulk density of mixtures decreased as well. Mixtures containing 40% coarse RCA or 20% fine RCA both showed satisfactory performance. Besides, the mixture containing 40% fine RCA had the highest asphalt content, but gave much better performance compared to the virgin mix except for its bad resistance to permanent deformation. Finally, the pavement performance of mixtures containing 60% coarse RCA and 50% coarse RCA were unacceptable.     

Influence of recycled aggregates and high contents of fly ash on concrete fresh properties

This study presents the fresh properties of concrete with supplementary cementitious materials (SCM) and recycled concrete aggregates (RCA), with emphasis on the feasibility of using high volumes of fly ash (FA) in RCA concrete. For this purpose, two mix families (0% coarse RCA and 100% coarse RCA) were produced, both with and without superplasticizers (SP). The coarse natural aggregates (NA) were replaced with coarse RCA at 0% and 100%, respectively. For each of the mentioned families, three incorporation levels (0%, 50% and 100%) of fine RCA were used with 0%, 30% and 60% of FA, resulting in 28 compositions. Each mix was tested in the fresh state by means of slump, density and air content. The results of this study show that RCA decreased the slump of concrete mixes, but the required water content can be minimized by incorporation FA. Regardless of the water absorption of the aggregates, for a given fine RCA incorporation ratio and the same ratio of FA, no increase in water content is required to obtain the same target slump as in the reference concrete. On the other hand, for a given coarse RCA incorporation ratio, a five times lower FA ratio is enough to obtain the same target slump as in the reference concrete. Air voids in concrete mixes were more affected by the shape of the aggregates than by their water absorption. The air content of concrete mixes increased as the incorporation levels of FA and RCA increased. However, in comparison with the individual effects, the air content decreased by combining the incorporation of both FA and RCA. Moreover, the rate of reduction in fresh density by increasing the incorporation of RCA and FA was similar in concrete mixes with and without SP.     

Mechanical and thermal properties of prepacked aggregate concrete incorporating palm oil fuel ash

Prepacked aggregate concrete (PAC) is a special type of concrete which is made by placing coarse aggregate in a formwork and injecting a grout either by pump or under the gravity force to fill the voids. Use of pozzolanic materials in conventional concrete has become increasingly extensive, and this trend is expected to continue in PAC as well. Palm oil fuel ash (POFA) is one of these pozzolanic ash, which has been recognized as a good pozzolanic material. This paper presents the experimental results of the performance behaviour of POFA in developing physical and mechanical properties of prepacked aggregate concrete. Four concrete mixes namely, prepacked concrete with 100% OPC as a control, and PAC with 10, 20 and 30% POFA were cast, and the temperature growth due to heat of hydration and heat transfer in all the mixtures was recorded. It has been found that POFA significantly reduces the temperature rise in prepacked aggregate concrete and delay the transfer of heat to the concrete body. The compressive and tensile strengths, however, increased with replacement up to 20% POFA. The results obtained and the observation made in this study suggest that the replacement of OPC by POFA is beneficial, particularly for prepacked mass concrete where thermal cracking due to extreme heat rise is of great concern.     

Laboratory study of larger sized aggregate in Portland cement concrete

A laboratory examination of the effects of coarse aggregate type and size on the mechanical properties of concrete is presented, in an effort to develop more cost-efficient mixes for pavements and other highway structures. Aggregate blending is used to generate the required coarse aggregate gradations. Six different concrete mixes are prepared, using three different coarse aggregate gradations, along with two different aggregate types, natural and crushed. Test results show that coarse aggregate properties often do not have a significant effect on the mechanical properties of concrete. When significant differences are observed, these are confounded by variability issues related to the testing protocols themselves, and by mineralogical distinctions among the various aggregate blends.     

Investigations on Alkali-Activated Slag/Fly Ash Concrete with steel slag coarse aggregate for pavement structures

The present investigation is conducted to evaluate the effect of steel slag coarse aggregates on mechanical properties and fatigue behaviour of Alkali-Activated Slag Fly Ash Concrete (AASFC) mixes. AASFC mixes were prepared with steel slag coarse aggregates by replacing natural coarse aggregates at various replacement levels (0, 25, 50, 75 and 100% by volume). Various mechanical properties and fatigue performance were tested and compared with conventional Portland concrete. The incorporation of steel slag aggregates resulted in decrease in mechanical strength of AASFC mixes. The fatigue lives of AASFC mixes containing steel slag were found to be lower than AASFC with natural coarse aggregates. Two-parameter Weibull distribution was used for statistical analysis of fatigue data and it was observed that the fatigue data of concrete mixes can be approximately modelled using Weibull distribution. Steel slag aggregates reported acceptable performance in AASFC mixes for its use in pavement quality concrete.     

基于CT图像的再生混凝土细观破坏裂纹分形特征

为深入研究再生混凝土的破坏形态和内部裂纹扩展情况与普通混凝土之间的差异,以不同再生粗骨料(RCA)取代率的再生混凝土为研究对象,利用Phoenix v | tome | x s240微焦点工业CT获取再生混凝土加载到90%预估破坏荷载的二维扫描图像,借助Photoshop CS6图像处理软件,对材料内部破坏裂纹进行提取,进而基于分形几何理论,以分形维数及多重分形谱表征裂纹的分形扩展规律,建立分形维数和多重分形谱特征参数与RCA取代率和再生混凝土抗压强度的关系。结果表明:再生混凝土的细观受力破坏模式与普通混凝土不同,其受力破坏形态不仅取决于粗骨料与水泥浆体的界面黏结强度,还取决于RCA自身性能,当裂纹发展至天然粗骨料或强度较高的RCA时会绕过骨料表面继续发展,发展至强度较低的RCA时会贯穿骨料;分形维数可定量描述混凝土材料内部细观裂纹的整体扩展情况,即裂纹越丰富,分形维数越大;多重分形谱可反映从局部到整体不同层次的细观裂纹特征,裂纹分形维数和多重分形谱特征参数均与RCA取代率呈线性下降关系,与抗压强度呈线性增长关系;本研究可为再生混凝土在大型结构工程中的广泛应用奠定理论和实验基础。      

A comprehensive investigation into the effect of aging and coarse aggregate size and volume on mechanical properties of self-compacting concrete

The popularity of self-compacting concrete (SCC), as an innovative construction materials in concrete industry, has increased all over the world in recent decades. SCC offers a safer construction process and durable concrete structure due to its typical fresh concrete behavior which is achieved by SCC’s significantly different mixture composition. This modification of mix composition may have significant effect on the hardened mechanical properties of SCC as compared to normal vibrated concrete (NVC). Therefore, it is necessary to know whether the use of all rules and relations that have been formulated for NVC in current design codes based on years of experience are also valid for SCC. Furthermore, this study represents an extensive evaluation and comparison between mechanical properties of SCC using current international codes and prediction equations proposed by other researchers. Thus, in this experimental study, major mechanical properties of SCC are investigated for twelve SCC mixes with wide spectrum of different variables i.e. maximum coarse aggregate size, coarse aggregate volume and aging. In the present study, an extensive body of data reported by many researchers for SCC and NVC has been used to validate the obtained results.     

Application of a weighted Grey-Taguchi method for optimizing recycled aggregate concrete mixtures

Assessment of the optimal mixture is an important issue to obtain desired quality. This paper integrates grey relational analysis and an objective weighting technique into the Taguchi method to propose the weighted Grey-Taguchi method. This method can be employed to assess the optimal mixture with multiple responses. In the application of this method, water/cement ratio, volume ratio of recycled coarse aggregate, replacement by river sand, content of crushed brick, and cleanliness of aggregate are selected as control factors with responses of slump, slump-flow, resistivity (7-day, 14-day, 28-day), ultrasonic pulse velocity (7-day, 14-day, 28-day), and compressive strength (7-day, 14-day, 28-day) to assess the optimal mixture of recycled aggregate concrete. Results demonstrate and verify that the optimal mixture has a water/cement ratio of 0.5, a volume fraction of recycled coarse aggregate of 42.0%, 100% replacement of river sand, 0% crushed brick, and water-washed aggregates.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Valorization of co-combustion fly ash in concrete production

The purpose of this paper is to compare the effects of two different Supplementary Cementing Materials (SCMs) on mechanical and durability-related properties of structural concrete. Three mixes were produced, where coal and co-combustion fly ashes were used as partial substitute of cement (20% in volume) and compared with a control/reference concrete. Performances investigated included fresh concrete properties, compressive and tensile strength, elastic modulus, permeability, capillarity and drying/wetting resistance. Results indicate that both the SCMs can be classified as low-carbon fly ashes, and their use in concrete improves the workability of the mixes. A slight reduction of mechanical strength was observed for the concretes including both the SCMs. In addition, concrete transport properties were also slightly reduced when co-combustion fly ash was used. Wetting-drying cycles affected significantly the durability of all the mixes: compressive strength after these cycles was significantly lowered, and the cracks occurred due to the thermal stress applied, appeared to be filled by needle-shape crystals of ettringite.     

Investigation on the scattering-filling coarse aggregate self-consolidating concrete

To secure good flowability and workability of SCC, the volume fraction of coarse aggregate keep at an extremely low level. A new kind of SCC pouring method named scattering-filling coarse aggregate process was invented: it was method to scatter 20% (volume fraction to the finished concrete) of extra coarse aggregate into the fresh SCC mixture to replace the fresh concrete mixture while the concrete was pouring. A high strength (82 MPa) SCC just composing 360 kg/m³ cement and 120 kg/m³ class F fly ash was prepared with this process. With an increase of the extra coarse aggregate replacing ratio from 0 to 30%, the compressive strength of SCC increased steadily and reached a peak value when this ratio is 20%, then the strength dropped sharply. The drying shrinkage ratio and the chloride ion permeability decreased with the increase of that ratio. The scattering-filling coarse aggregate process can cast high strength SCC with lower cementitious materials content and produce concrete with better performance than the ordinary process.     

Mix design of concrete for prestressed concrete sleepers using blast furnace slag and steel fibers

The application of ground granulated blast furnace slag (GGBFS) and steel fibers in prestressed concrete railway sleepers was investigated in this study. The use of GGBFS was considered as an eco-friendly material aimed at reducing CO₂ emissions and energy consumption as well as to enhance the durability performance of railway sleepers. Steel fibers improves the durability and structural performance in terms of crack control and reduction of spalling and can replace shear reinforcement. The mix proportions of the concrete incorporating GGBFS (56% GGBFS) and GGBFS with steel fibers (56% GGBFS and 0.75% steel fibers) were determined through a series laboratory tests and a life cycle assessment. These mixes satisfied the requirements of the Korean Railway Standard and resulted in improved flexural capacity as well as less CO₂ emissions compared with current railway sleepers. Using these mixes, a total of ninety prestressed concrete sleepers were produced in a factory under the same manufacturing process as current railway sleepers, and their mechanical properties as well as durability performance were evaluated. The mix with partial replacement of Type III Portland cement by GGBFS showed an improved resistance to chloride ion penetration and freeze-thaw cycles compared with the concrete used for current railway sleepers. However, these mixes were more vulnerable to carbonation. The mix with GGBFS and steel fibers (mix BSF) showed a slightly better durability performance than the mix with GGBFS only (mix BS), including better carbonation and freeze-thaw resistances. The mix BSF showed decreased chloride ion penetration depth than mix BS but showed a slightly higher chloride ion diffusion coefficient.