Use of spent foundry sand and fly ash for the development of green self-consolidating concrete

In the United States alone, the foundry industry discards up to 10 million tons of sand each year, offering up a plentiful potential resource to replace sand in concrete products. However, because the use of spent foundry sand (SFS) is currently very limited in the concrete industry, this study investigates whether SFS can successfully be used as a sand replacement material in cost-effective, green, self-consolidating concrete (SCC). In the study, SCC mixtures were developed to be even more inexpensive and environmentally friendly by incorporating Portland cement with fly ash (FA). Tests done on SCC mixtures to determine fresh properties (slump flow diameter, slump flow time, V-funnel flow time, yield stress, and relative viscosity), compressive strength, drying shrinkage and transport properties (rapid chloride permeability and volume of permeable pores) show that replacing up to 100% of sand with SFS and up to 70% Portland cement with FA enables the manufacture of green, lower cost SCC mixtures with proper fresh, mechanical and durability properties. The beneficial effects of FA compensate for some possible detrimental effects of SFS.     

Durability performance potential and strength of blended Portland limestone cement concrete

This paper describes a study on the durability potential and strength of composite Portland-limestone cement (PLC) concrete mixtures blended with ground granulated blast furnace slag (GGBS) and/or fly ash (FA). Their performance was compared against ordinary Portland cement, plain PLC and Portland-slag cement concrete mixtures. Using the South African Durability Index approach, results indicate reductions in the penetrability of the composite PLC blends compared to the other mixtures. The durability indicators are chloride conductivity, gas (oxygen) permeability and water sorptivity. Compressive strength of the composite PLC mixtures containing both GGBS and FA showed competitive performance with the comparative mixtures, but FA blended PLC mixtures had diminished compressive strength values. The paper also presents considerations on the practical implications of using blended PLC concrete mixtures.     

Effect of metakaolin and silica fume on the durability of self-consolidating concrete

Metakaolin (MK) is a valuable admixture for concrete/cement applications that can enhance the performance of cementitious composites through high pozzolanic reactivity, much like silica fume (SF). While SF concrete is characterized by superior mechanical and durability performance, concrete containing MK achieves comparable properties at a lower price and with better workability. The objective of this study is to investigate the effect of cement replacement by MK on the durability of self-consolidating concrete (SCC); the effect of SF at similar levels of MK replacement has also been included for comparison. The durability performance of SCC was evaluated based on the results of drying shrinkage, freezing and thawing, salt scaling, and rapid chloride permeability tests. The results of these tests indicate that highly durable SCC mixtures can be produced using a high MK content with an optimum percentage of around 20%. The results also show that the durability of SCC, especially with high MK content, is higher than that of SCC containing SF.     

Studies on the corrosion resistance of reinforced steel in concrete with ground granulated blast-furnace slag--An overview

The partial replacement of clinker, the main constituent of ordinary Portland cement by pozzolanic or latent hydraulic industrial by-products such as ground granulated blast furnace slag (GGBFS), effectively lowers the cost of cement by saving energy in the production process. It also reduces CO2 emissions from the cement plant and offers a low priced solution to the environmental problem of depositing industrial wastes. The utilization of GGBFS as partial replacement of Portland cement takes advantage of economic, technical and environmental benefits of this material. Recently offshore, coastal and marine concrete structures were constructed using GGBFS concrete because high volume of GGBFS can contribute to the reduction of chloride ingress. In this paper, the influence of using GGBFS in reinforced concrete structures from the durability aspects such as chloride ingress and corrosion resistance, long term durability, microstructure and porosity of GGBFS concrete has been reviewed and discussed.     

The relationship between chloride migration rate for concrete and electrical current in steady state using the accelerated chloride migration test

In this study the electrochemical technique is applied to accelerate chloride ion migration in concrete to determine the chloride ions in anode cell. This paper presents a new method for determining the chloride migration rate in concrete from steady state migration test by measuring the electrical current. The plain ordinary Portland cement concrete and concrete containing different type of mineral admixtures (fly ash and slag) with w/b ratios of 0.35, 0.45, 0.55, and 0.65 were used.For a given charge passed in steady state, the current corresponding to the given charge passed was correlated with the chloride migration rate. The results for all mixtures show that the chloride migration rate and the current corresponding to a given charge passed in steady state is linearly correlated.     

核电厂预应力混凝土安全壳中氯离子扩散系数的预测方法EI北大核心CSCD

氯离子扩散系数是评价沿海环境中核电厂预应力混凝土安全壳耐久性的重要参数。该文基于两尺度方法,通过分析双轴等压下水泥浆基体、界面和混凝土细观结构,建立了水泥浆基体和界面毛细孔隙率与预应力之间的定量关系。为了量化预应力对微裂纹闭合、产生和扩展的影响,提出了临界毛细孔隙率与预应力之间的经验公式。将混凝土模拟成由骨料、界面和水泥浆基体组成的三相复合材料,获得预应力混凝土氯离子扩散系数比。通过与试验结果比较,校正了经验公式中的两个参数。用三组试验数据初步验证了该预测方法的有效性。     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

Effects of nano-kaolinite clay on the freeze–thaw resistance of concrete

This paper investigates the effects of nano-kaolinite clay (NKC) on the freezing and thawing (F–T) behavior of concrete. In our experiments, we substituted NKC for 0%, 1%, 3%, and 5% of mixtures of ordinary Portland, cement, by weight. The blended concrete was prepared using w/c ratio as 0.5. A rapid freeze–thaw Cabinet was then used to measure the resistance of ordinary Portland cement concrete, as opposed to the concrete/NKC mixture, to examine deterioration caused by repeated F–T actions. We regularly measured the properties of the concrete specimens, including the pore structure, mass, electrical resistivity, chloride diffusion coefficient, compressive strength and dynamic modulus of elasticity. A computed tomography scan test evaluated the porosity characteristics of the concrete. This paper also applied scanning electron microscopy and X-ray diffraction tests in order to investigate the micro morphology and chemical element distributions inside of the concrete. The experimental results and visual comparisons revealed that the introduction of NKC improves the F–T resistivity values, as compared to the control concrete. The samples with 5% NKC exhibited the highest compressive strength, chloride diffusion resistivity, relative dynamic modulus of elasticity, and the most electrical resistivity after 125 F–T cycles. We designated the anti-freezing durability coefficient (DF) as the index to assess the F–T resistivity of concrete. The following research discusses the relationship between the concrete’s DF and the number of F–T cycles, compressive strength, chloride diffusion coefficient, and the electrical resistivity of the concrete samples.     

Gas permeability and capillary porosity of self-compacting concrete

Self-compacting concrete (SCC) can be placed without any external compaction avoiding some health risks as well as environmental problems. In order to obtain its key properties, a large amount of fine particles and a new generation of superplasticizers can be used. Earlier research by means of mercury intrusion porosimetry, already pointed out an important difference in pore structure between SCC and traditional concrete (TC). Since the transport properties of concrete are strongly depending on its pore structure, the question rises to what extent the gas permeability of SCC gets affected by the change in mixture design. In this paper the gas permeability of 16 mixtures SCC and 4 mixtures TC are being evaluated with special attention to the difference between SCC and TC, and the influence of the following parameters: water/cement ratio, powder content, type of filler, fineness of the filler, type of aggregate and cement/powder ratio. It was concluded that the gas permeability of SCC is about 5 times lower than the gas permeability of TC. The parameter with the largest impact on the gas permeability seems to be the water content and secondly the powder content. The capillary porosity has been estimated and a rather good correlation has been found with the gas permeability.      

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.     

Transport properties of self compacting concrete with limestone filler or fly ash

The durability of a cementitious material is greatly influenced by the permeability of the material for potentially aggressive substances. As the pore structure of self compacting concrete (SCC) might be different in comparison with traditional concrete (TC), some changes in durability behaviour may occur. At this moment however, it is unclear how significant these differences will be with regard to the concrete practice. In this paper, the gas and water transport in SCC with limestone filler or fly ash is investigated experimentally. Nine different concrete compositions are considered: one TC and eight SCC mixtures. Some important parameters like the water/cement (W/C) and cement/powder ratio (C/P), type of filler (limestone filler and fly ash), type of aggregate and type of cement are considered. The results of the gas and water transport are discussed and linked to experimental data concerning pore volume. Lower transport properties can be obtained by using fly ash instead of limestone as filler material, by lowering the W/C ratio, decreasing the C/P ratio at a constant W/C ratio or using blast furnace slag cement instead of portland cement. The effect of changing from gravel to crushed limestone is small. SCC is differing strongly of TC with respect to the apparent gas permeability. This difference is probably due to the differences in pore volume, as seen from MIP results.     

Effect of ground fly ash and ground bagasse ash on the durability of recycled aggregate concrete

This research aims to study the effect of ground fly ash (GFA) and ground bagasse ash (GBA) on the durability of recycled aggregate concrete. Recycled aggregate concrete was produced with recycled aggregate to fully replace crushed limestone in the mix proportion of conventional concrete (CON) and GFA and GBA were used to partially replace Portland cement type I at the rate of 20%, 35%, and 50% by weight of binder. Compressive strength, water permeability, chloride penetration depth, and expansion by sulfate attack on concretes were investigated.The results reveal that the use of GFA and GBA to partially replace cement in recycled aggregate concrete was highly effective in improving the durability of recycled aggregate concrete. The suitable replacement of GFA or GBA in recycled aggregate concrete to obtain the suitable compressive strength, low water permeability, high chloride penetration resistance, and high sulfate resistance is 20% by weight of binder.     

Durability characteristics of self-consolidating concrete designated for repair applications

This paper presents test results carried out to study the influence of key mixture parameters on frost durability, scaling resistance, and transport properties of self-consolidating concrete (SCC) that can be used in structural repair. Regardless of the w/cm, binder type, or admixture combination, properly designed SCC can develop high resistance to freezing and thawing with frost durability factor greater than 80%. The optimized mixtures had 56-day rapid chloride-ion permeability (RCP) values of 200–900 Coulomb. On the average, SCC made with 0.42 w/cm developed 20% higher capillary porosity, 20% lower compressive strength, and 30% greater RCP value compared to similar SCC prepared with 0.35 w/cm. The type of blended cement in use had considerable influence on transport properties. For a given mix design, concrete with 180 mm slump consistency exhibited similar RCP value of 505 Coulomb compared to 630 Coulomb for the same concrete with greater dosage of HRWRA to secure a slump flow consistency of 670 mm.     

A model predicting carbonation depth of concrete containing silica fume

Silica fume (SF) has been used since long as a mineral admixture to improve durability and produce high strength and high performance concrete. Due to the pozzolanic reaction between calcium hydroxide and silica fume, compared with ordinary Portland cement, the carbonation of concrete containing silica fume is much more complex. In this paper, based on a multi-component concept, a numerical model is built which can predict the carbonation of concrete containing silica fume. The proposed model starts with the mix proportions of concrete and considers both Portland cement hydration reaction and pozzolanic reaction. The amount of hydration products which are susceptible to carbonate, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), as well as porosity can be obtained as associated results of the proposed model during the hydration period. The influence of water-binder ratio and silica fume content on carbonation is considered. The predicted results agree well with experimental results.     

Comparative study of different test methods for reinforced concrete durability assessment in marine environment

There are many different test methods to assess reinforced concrete durability. As in marine environment reinforcement corrosion due to chloride attack is the most important degradation process, chloride penetration rate has been compared with different durability tests results (concrete strength, porosity, water absorption, water penetration depth under pressure, capillarity, water and oxygen permeability) carried out on concrete cores obtained from the caissons of seven Spanish wharves. Data have been studied separately, depending on concrete location (chloride penetration rate is faster in submerged concretes than in tidal zone concretes) and cement type (mineral admixtures reduce permeation rate due to pore size refinement). Results show that it is advisable to control concrete water tightness through water penetration under pressure test; additionally, in order to make sure a slow corrosion rate, it should be advisable to control oxygen permeability in tidal zone concretes.     

High strength blended cement concrete incorporating volcanic ash: Performance at high temperatures

The strength and durability of high strength blended cement concretes incorporating up to 20% of volcanic ash (VA) subjected to high temperatures up to 800 °C are described. The strength was assessed by unstressed residual compressive strength, while durability was investigated by rapid chloride permeability (RCP), mercury intrusion porosimetry (MIP), differential scanning calorimetry (DSC), crack pattern observations and microhardness testing. High strength volcanic ash concrete (HSVAC) exhibited better performance showing higher residual strength, chloride resistance and resistance against deterioration at high temperatures compared to the control high strength OPC concrete. However, deterioration of both strength and durability of HSVACs increased with the increase of temperature up to 800 °C due to weakened interfacial transition zone (ITZ) between hardened cement paste (hcp) and aggregate and concurrent coarsening of the hcp pore structure. The serviceability assessment of HSVACs after a fire should therefore, be based on both strength and durability considerations.     

Experimental study of the interfacial transition zone (ITZ) of model rock-filled concrete (RFC)

Rock-filled concrete (RFC), a new type of concrete that was developed mainly for large scale concrete construction, has a different casting process than conventional concrete: large rocks are piled into the formwork first, then self-compacting concrete (SCC) is poured in and fill the voids of the rock skeleton under gravity due to its high flowability. One of the key issues about RFC lies in its large interfaces between the SCC and rocks. In this paper, laboratory-scale model RFC consisting of coarse aggregates (simulating rocks) and cement grout (simulating SCC) was cast to simulate RFC in construction. The effects of different factors (aggregate size, rheology of cement grout, etc.) on the properties of the interfacial transition zone (ITZ) between cement paste and aggregates of model RFC were investigated using Backscatter Electron (BSE) and nanoindentation techniques. Furthermore, by comparing the results of BSE and nanoindentation at identical regions, the relationship between porosity and elastic modulus was found to agree well with empirical formulas, bridging the microstructure with the mechanical properties of concrete.     

Durability assessment of alkali activated slag (AAS) concrete

The environmental impact from the production of cement has prompted research into the development of concretes using 100% replacement materials activated by alkali solutions. This paper reports research into the durability of AAS concrete. The durability properties of AAS have been studied for a range of sodium oxide dosages and activator modulus. Properties investigated have included measurements of workability, compressive strength, water sorptivity, depth of carbonation and rapid chloride permeability. Microstructure studies have been conducted using scanning electron microscopy and energy dispersive X-ray spectroscopy. It was concluded that an activator modulus of between 1.0 and 1.25 was identified as providing the optimum performance for a sodium oxide dosage of 5% and that AAS concretes can exhibit comparable strength to concrete currently produced using Portland cement (PC) and blended cements. However, with regards to the durability properties such as water sorptivity, chloride and carbonation resistance; the AAS concretes exhibited lower durability properties than PC and blended concretes. This, in part, can be attributed to surface microcracking in the AAS concretes.     

Prediction of the chloride diffusion coefficient of concrete

It has been experimentally verified that the structure of the interfacial transition zone (ITZ) in concrete differs from that of bulk cement paste. As such, concrete should be modeled as a three-phase material at a mesoscopic level. This paper presents a three-phase composite model for predicting the chloride diffusion coefficient of concrete. Taking the inclusion as aggregate and the matrix as cement paste, the composite circle model is established by adding an ITZ layer in between the inclusion and the matrix. Solving the asymmetrical problem analytically, a closed-form solution for the chloride diffusion coefficient of concrete is derived. After verifying this model with experimental results, the effects of the aggregate area fraction, the chloride diffusion coefficient of ITZ, the ITZ thickness, the maximum aggregate diameter and the aggregate gradation on the chloride diffusion coefficient of concrete are evaluated in a quantitative manner. It is found that the chloride diffusion coefficient of concrete decreases with the increase of the aggregate area fraction and the maximum aggregate diameter, but increases with the increase of the chloride diffusion coefficient and thickness of ITZ. It is also found that the aggregate gradation has a significant influence on the chloride diffusion coefficient of concrete.