Bond-slip behavior of self-compacting concrete and vibrated concrete using pull-out and beam tests

This paper analyzes the bond behavior of an innovative construction material, self-compacting concrete (SCC), in comparison to vibrated concrete (VC), using pull-out and beam tests according to the Rilem procedures. The main analyzed parameters were the concrete compressive strength, the steel bar diameter, the concrete type and the test adequacy to provide a value for bond strength. Also, a comparison between Code provisions and empirical equations was done. According to the results, SCC and VC specimens presented similar behavior. The equations results showed a satisfactory approach compared to the experimental ones, but as expected Code provisions were very conservative. Based on the obtained results, it could be concluded that the same parameters adopted for VC can be extended for SCC.     

Advantages of mortar-based design for coloured self-compacting concrete

The development of coloured self-compacting concrete (C-SCC) opens new fields of application, since it adds attractive alternatives for challenging architectural designs in terms of shapes and colour, to the already versatile aesthetic characteristics of traditional coloured concrete. The benefits of using cement paste or mortar tests as a previous step in SCC design to optimize mixture proportions have been recognized. This paper shows the advantages of using a mortar-based mix design methodology for C-SCC. In addition to rapid and easy determination of the proportions of the mixtures including different types of pigments, cements, mineral additions, and chemical admixtures, the mortar approach enables observation of the effect of pigments on the viscosity of C-SCC and evaluation of specific aspects as the colorimetric parameters, the colour homogeneity and the surface finishing.     

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.      

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.     

Shrinkage and restrained shrinkage cracking of self-compacting concrete compared to conventionally vibrated concrete

Self-compacting concrete (SCC) used in Switzerland contains about 80 l/m³ more volume of paste than conventionally vibrated concrete (CVC). Consequently, there are some systematic differences in the properties of the hardened concrete. Normally, shrinkage of SCC is higher than shrinkage of CVC. Therefore, risk of cracking in case of restrained deformations can be increased for SCC. In this study shrinkage of thirteen different SCC mixtures using volume of paste, water content, type of binder, grain size distribution or content of shrinkage reducing admixture (SRA) as variables was compared with shrinkage of three different CVC mixtures with constant volume of paste but variable w/b. Furthermore, the risk of cracking of the different SCC- and CVC-mixtures in restrained conditions was studied under constant and varying curing conditions. The results show that shrinkage is mainly depending on volume of paste. Due to the higher volume of paste, SCC displayed higher shrinkage than CVC. Adding an SRA was the only measure to reduce shrinkage of SCC to values of CVC. Restrained shrinkage cracking is depending on shrinkage rate, mechanical properties and drying velocity. For slow shrinkage stress development, cracking risk of SCC can be lower compared to CVC despite the higher shrinkage rate.     

C40自密实微膨胀钢管柱混凝土配合比设计及泵送顶升浇注施工技术

自密实混凝土用于钢管混凝土结构中,对方便钢管混凝土的施工,确保工程质量具有重要意义。通过应用高性能混凝土配制技术,提出适用于C40自密实钢管混凝土的最佳配合比,应用泵送顶升浇注施工技术、施工工艺流程中注意的一些问题。     

Shape factors of self-compacting concrete specimens subjected to uniaxial loading

One of the problems encountered when comparing the mechanical properties of self-compacting concrete (SCC) is the use of different specimen sizes all over the world. For vibrated concrete (VC), conversion factors are defined to convert the obtained compressive strength on one specimen type to another. In order to investigate the applicability of these factors for SCC, a total of 2 VC and 10 SCC mixtures were selected varying in cement type, cement content, water-to-cement ratio and water-to-powder ratio. Beside cubes with sides of 100 mm, 150 mm and 200 mm, cylinders with a diameter of 100 mm and diameter 150 mm were cast and cores with a diameter of 100 mm, 80 mm and 50 mm were drilled. A significant difference of about 10% in the shape factors between SCC and VC has been found. Mix design parameters, such as the fraction of powder, the cement-to-powder ratio and the water-to-cement ratio, seem to have little influence on the obtained shape factors.     

Deformation behaviour of self-compacting concrete under tensile loading

The deformation behaviour of self-compacting concretes of the compressive strength classes C30/37, C45/55 and C60/75 according to the European Standard EN 206-1 under sustained tensile loading was investigated up to 2.5 years. The long-term tensile strength was estimated to be 69% of the short-term tensile strength, determined at an age of 28 days. Load-free shrinkage has been measured on companion specimens. The usual way to determine creep is to subtract the measured shrinkage strain and the elastic or initial strain from the measured total strain. In doing so, a phenomenon was discovered which is called stress-induced shrinkage. It turned out that the drying shrinkage was larger for loaded specimens than for load-free specimens. Similar results have been found earlier. It seemed that the stress-induced shrinkage tends to increase with decreasing compressive strength. An important practical consequence is arising from stress-induced shrinkage: structural elements subjected to tensile␣load, tend to dry and to shrink faster than one␣would expect based on the assumption of load-free shrinkage. In the case of sustained deformation, this would raise the risk of cracking and would have a negative effect on the durability of a concrete structure. An erratum to this article can be found at     

Effect of the mix design on the robustness of fresh self-compacting concrete

Self-compacting concrete (SCC) has many advantages compared to vibrated concrete. A disadvantage is the lower robustness of fresh SCC. SCC is more sensitive to small changes in the mix design, material properties, and the applied production methods. In an experimental program, the influence of important mix design parameters on the robustness of SCC was studied. First, the influence of the paste volume and the water-to-powder volumetric ratio was investigated. Depending on the mechanisms providing stability in the mixture, different levels of impact were observed. When the yield stress is the main factor providing stability in the mixture, a change in the water content will mainly affect the yield stress, making the stability of the yield stress the most important factor determining the robustness of the mixture and can be improved by lowering the paste volume. Analogue, the sensitivity of the plastic viscosity is determining the robustness of mixtures in which mainly the plastic viscosity is providing stability. The robustness of such a mixture can be improved by increasing the water-to-powder volumetric ratio. The influence of two types of viscosity modifying agents (VMA's) on the robustness of fresh SCC was examined in a second stage. The two used VMA's (diutan gum and attapulgite clay) were especially effective in SCC mixtures having a high yield stress and a low plastic viscosity. In mixtures having a low yield stress and a high plastic viscosity, the inclusion of a VMA in the mix design resulted in a decrease of the robustness.     

Assessment of factors influencing mechanical properties of steel fiber reinforced self-compacting concrete

In the last decade the steel fiber reinforced self-compacting concrete (SFRSCC) has been used in several partially and fully structural applications. This study investigates how the inclusion of steel fibers affects the properties of SFRSCC. For this purpose, an extensive experimental program including different cement contents of 400, 450 and 500 kg/m³, two maximum aggregate sizes of 10 and 20 mm along with steel fiber volume fractions of 0%, 0.38%, 0.64% and 1% was conducted. The water/cement ratio was kept constant at 0.45 for all the mixes studied. Mechanical properties were tested for compressive, splitting tensile and flexural strengths and modulus of elasticity. The results showed that mixture characteristics and volume fraction of steel fibers can significantly affect these major properties. Furthermore, this study represents extensive comparisons using database that have been gathered from a wide variety of international sources reported by many researchers and data obtained experimentally, which came up with about some discrepancies in the results.     

Development of self-compacting high and ultra high performance concretes with and without steel fibres

This paper describes the steps taken to develop self-compacting high and ultra high-performance concretes with and without steel fibres. For the self-compacting concrete mixes without steel fibres the fulfilment of flow and cohesiveness criteria are sufficient for the mix design. However, for the design of self-compacting concrete mixes with steel fibres it is found, as expected, that they must additionally meet the passing ability criterion. The plastic viscosity of the mixes with and without steel fibres has been estimated from the known plastic viscosity of the cement paste using simple micromechanical relations.     

Influences of superplasticizer modification and mixture composition on the performance of self-compacting concrete at varied ambient temperatures

The fresh behaviour of self-compacting concrete (SCC) at varying temperatures differs from that of normal vibrated concrete. This is because the rheology of SCC depends not only on degree of cement hydration, but also on the adsorption of superplasticizers – mostly polycarboxylate based polymers (PCE) -, which is affected by the time and hydration progress. Due to the variety of PCEs and mixture compositions for SCC a prediction of the rheology at varying temperatures is complicated. The charge densities of PCEs as well as the water to solid ratio in the paste are identified to be the main decisive parameters for robust fresh concrete properties.Rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the PCE were conducted. SCC which is rich in powder components showed robust performance at low temperatures while SCC with low powder content was favourable at high temperatures. High charge density PCE pointed out to be very robust at low temperatures but at high temperatures it significantly reduced the flow retention. Low charge density PCE could not generate self-compacting properties at low temperatures but retained the flow performance over sufficiently long time. Based on considerations about particle interactions and adsorption mechanisms of PCEs, the relevant processes are explained and options for the development of robust mixture compositions for individual temperature ranges are itemised.     

多角度肋片强化相变材料换热仿真分析

目的 针对太阳能热利用领域中相变材料的封装结构提出圆柱体相变蓄热棒,并设计多角度肋片以加快相变材料融化速率。方法 采用CFD仿真技术,分析不同形状肋片对蓄热棒中相变材料融化特性的影响,计算各模型相变材料的融化时间、温度响应速率和平均传热系数。结果 在800 W/m²的热流边界条件下,无肋片蓄热棒的相变材料完全融化需要2 813 s,设计的12组肋片中Tra–45模型性能最优,相变材料的融化时间比无肋片对照组的缩短了5.4%;Tra–45模型中相变材料温度分布集中,且最高温度上升了6 ℃,Tra–45模型的温度响应速率较对照组的提升了5%;Tra–45模型的平均换热系数达到9.97 W/(m².K),较对照组的提升了2.8%。结论 蓄热棒内增加梯形45°肋片后,相变材料融化速率加快,蓄热棒内温度分布均匀。同时,相变材料的温度响应速率提高,平均换热系数显著增加,可满足频繁充放热的需求。     

Influence of recycled glass content and curing conditions on the properties of self-compacting concrete after exposure to elevated temperatures

The aim of this paper is to assess the performance of self-compacting glass concrete (SCGC) after exposure to four elevated temperatures of 300 °C, 500 °C, 600 °C and 800 °C. The influence of curing conditions on the high temperature performance of SCGC was also investigated. For each curing regime, five SCGC mixtures were prepared with recycled glass (RG) which was used to replace natural fine aggregate at the level of 0%, 25%, 50%, 75% and 100%. After exposure to the elevated temperatures, concrete mass loss, density, water porosity, ultrasonic pulse velocity (UPV) and water sorptivity were determined and then a compressive strength test was conducted. The test results indicate that regardless of the exposure temperature, all the water cured specimens had higher residual strengths and mass losses while the water porosity and water sorptivity values were lower as compared to the corresponding air cured specimens. The incorporation of RG in the concrete mixes helped to maintain the concrete properties after the high temperature exposure due to the melting and resolidification of the recycled glass in the concrete matrix.     

Phase Change Materials for the Improvement of Heat Protection

The use of phase change materials (PCM) that absorb and store heat by an aggregate state change for applications in fire fighters' protective clothing was studied. PCM can help improve the heat protection of the clothing combination, and thus contribute to a reduction of the weight of the equipment and an improvement of the wearing comfort. It was found that PCM have a positive effect on heat protection, but the efficiency of the heat absorption depends on the location of the PCM layer and the incident heat intensity. Furthermore, as the PCM used was made of paraffin, its burning behavior has to be improved for a commercial use in fire fighters' protective clothing.     

Reduction of fire spalling in high-performance concrete by means of superabsorbent polymers and polypropylene fibers: Small scale fire tests of carbon fiber reinforced plastic-prestressed self-compacting concrete

High-performance concrete (HPC) is prone to explosive spalling when exposed to fire, which may lead to failure of the concrete elements. Polypropylene fibers (PP) are often added to HPC, as upon their melting they create channels through which water vapor is evacuated, preventing the build-up of high vapor pressures. In self-compacting HPC (HPSCC), the amount of PP fibers needs to be limited in order to keep the self-compacting properties, which may reduce the fire resistance.In this paper, a novel strategy to reduce fire spalling in HPSCC is illustrated, based on adding small particles of superabsorbent polymers (SAP) during mixing. The SAP end up as empty macropores, similar to air voids, in the HPSCC matrix. The PP fibers-SAP voids system percolates at a lower fiber loading than the fibers alone, allowing maintenance of the self-compacting properties while reducing substantially the fire spalling. In particular, in this paper it is shown how addition of SAP is able to reduce fire spalling in thin-walled HPSCC slabs prestressed with carbon fibre reinforced plastic reinforcement.     

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.     

Effects of granulated blast furnace slag and superplasticizer type on the fresh properties and compressive strength of self-compacting concrete

This paper presents the results of an experimental investigation carried out to study the effect of granulated blast furnace slag and two types of superplasticizers on the properties of self-compacting concrete (SCC). In control SCC, cement was replaced with 10%, 15%, 20%, and 25% of blast furnace slag. Two types of superplasticizers: polycarboxylate based superplasticizer and naphthalene sulphonate based superplasticizers were used. Tests were conducted for slump flow, the modified slump test, V-Funnel, J-Ring, U-Box, and compressive strength. The results showed that polycarboxylate based superplasticizer concrete mixes give more workability and higher compressive strength, at all ages, than those with naphthalene sulphonate based superplasticizer. Inclusion of blast furnace slag by substitution to cement was found to be very beneficial to fresh self-compacting concrete. An improvement of workability was observed up to 20% of slag content with an optimum content of 15%. Workability retention of about 45 min with 15% and 20% of slag content was obtained using a polycarboxylate based superplasticizer; compressive strength decreased with the increase in slag content, as occurs for vibrated concrete, although at later ages the differences were small.     

Evaluating and forecasting the initial and final setting times of self-compacting concretes containing mineral admixtures by neural network

An experimental study was conducted to investigate the effects of using binary, ternary, and quaternary cementitious blends of portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GBS), silica fume (SF), and metakaolin (MK) on initial and final setting times of self-compacting concretes (SCCs). For this purpose, a total of 65 SCC mixtures were prepared at two different water binder ratios. Furthermore, based on the experimental results, neural network (NN) model-based explicit formulations were developed to predict the initial and final setting times of SCCs in terms of the amount of concrete constituents, namely mixing water, PC, FA, GBS, SF, MK, fine (fa) and coarse (ca) aggregates, and superplasticizer (SP). The test results have revealed that the mineral admixtures were very effective on the initial and final setting times of SCCs. Besides, it was found that the model developed by using NN seemed to have a high prediction capability of initial and final setting times of SCCs.     

Modelling the influence of age of steel fibre reinforced self-compacting concrete on its compressive behaviour

Steel fibre reinforced self-compacting concrete (SFRSCC) can combine the benefits of self-consolidating concrete technology with those derived from adding steel fibres to quasi-brittle cement based materials. In a recent applied research project joining pre-casting industry, private and public research institutions, a method was developed to design cost-competitive SFRSCC of rheological and mechanical properties required for the prefabrication of SFRSCC façade panels. To assure safe demoulding process of the panels, the influence of the concrete age on the compression behaviour of the SFRSCC should be known. For this purpose, series of tests with specimens of 12 h to 28 days were tested in order to analyze the age influence on the compressive strength, strain at peak stress, Young’s modulus, and compressive volumetric fracture energy. The experimental program was divided in two groups of test series, one with SFRSCC of a volumetric fibre percentage of 0.38% and the other with 0.57%. To apply the obtained data in the design and numerical analysis framework, the influence of the age on these SFRSCC properties was modelled. This work describes the carried out experimental program, presents and analyzes the obtained results, and provides the derived analytical expressions.