Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

Characterization of elastic and time-dependent deformations in normal strength and high performance concrete by image analysis

Deformations in normal strength concrete (NSC) and high performance concrete (HPC) were examined using image analysis to better understand the distribution of strain in these materials as related to their composition and microstructure. Elastic strain, creep and shrinkage were shown to occur non-uniformly throughout the NSC and HPC microstructure and creep and shrinkage strain both increased with time, as expected. The non-uniformity in the strain measurements also increased with time, as a consequence of dissimilarities in time-dependent behavior of the paste and the aggregate. However, compared with NSC, the time-dependent strains in the HPC were lower and showed less variation, suggesting a more uniform microstructure. In both NSC and HPC, high-strain sub-regions were evident in the vicinity of the interfacial transition zone (ITZ), likely as a consequence of the strain mismatch between aggregate and paste. The thickness of the high strain sub-regions along the ITZ in HPC were approximately one half of those in NSC.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

The effects of air content on permeability of lightweight concrete

Air entraining agent is used to control the floatation of lightweight aggregate (LWA) in lightweight aggregate concrete (LWAC), therefore reducing the segregation of LWAC. At the same time, using an air entraining agent will affect the water sorption of the concrete. In this paper, two lightweight concrete mixes of density 1000 kg/m³ and air content of 13.5% and 31.9% were compared and the effects of entrained air on the strength, surface sorptivity, and chloride permeability of LWAC are presented. Results show that the use of porous LWA would not lower the permeability resistance of concrete. Entrained air had little effect on sorptivity but a major effect on chloride permeability. The weaker pores' network in the cement paste is the basic cause for the high chloride permeability of concrete than the use of porous LWA. Although chloride permeability of low density LWAC concrete decreased with age of concrete, it was found that the concrete was not dense enough to stop the chloride ion to penetrate through the concrete before the concrete mature at 90 days.     

轻骨料的返水作用对粉煤灰二次水化的影响(英文)

研究了轻骨科掺量不同的混凝土当掺与不掺粉煤灰时不同龄期的抗氯离子扩散性能以及骨料附近水化产物中Ca(OH)2含量和水泥石孔结构的变化情况.结果表明:随着混凝土中轻骨料掺量的增加,掺加了粉煤灰的浆体28～90 d期间水化产物中Ca(OH)2含量的减少幅度大于未掺粉煤灰试样的,且水泥石孔隙率有所降低,孔径细化,掺入粉煤灰后轻骨科掺量越多的试样孔径细化越显著,可见轻骨料后期返出的水分起到了自养护作用,保证了粉煤灰的水化.在混凝土中掺入适量的轻骨料,其返水特性与粉煤灰二次水化反应的协同作用使得混凝土后期抗渗性能显著提高,优于普通骨料混凝土.     

钢管自密实轻骨料混凝土变形性能试验研究

采用堆积密度分别为500 kg/m³、800 kg/m³的页岩陶粒和堆积密度为1 600 kg/m³的普通碎石作为粗骨料配制自密实轻骨料混凝土和普通混凝土,并成型了钢管轻骨料混凝土与钢管普通混凝土,对比研究了二者的收缩应变、轴压应力-应变变化规律和温度-应变变化规律。结果表明:随着粗骨料堆积密度的降低,同配比轻骨料混凝土的密度、强度和弹性模量均逐渐降低;相同龄期时,钢管轻骨料混凝土及其核心轻骨料混凝土的收缩应变均小于钢管普通混凝土及其核心混凝土的收缩应变,钢管与核心轻骨料混凝土的密贴性更好;钢管轻骨料混凝土的轴压应力-应变变化规律与钢管普通混凝土的基本相同,与钢管普通混凝土相比,钢管轻骨料混凝土的弹性模量有所降低,但比非钢管约束状态下轻骨料混凝土弹性模量的降低幅值有所减小;钢管轻骨料混凝土与钢管普通混凝土的温度-应变相当,均为4.0 με/℃左右。     

大堆积密度骨料配制的混凝土的性能(英文)

当今世界对可持续发展的关注促进了低水泥用量混凝土建筑工程的产生。增加混凝土中骨料体系的堆积密度是降低水泥用量的一个策略。增加骨料体系的堆积密度可以降低填充骨料颗粒间隙所需要的水泥浆体的量。研究了大堆积密度骨料配制的混凝土。这些混凝土比普通的结构混凝土低15%～25%的水泥用量,当骨料体系级配合理时混凝土工作性好。水泥浆体体积的降低在明显改进混凝土干缩与徐变性能的同时,也会对其工作性产生不利影响。骨料含量不会显著影响不同配比混凝土的抗压强度和劈裂强度。     

轻骨料吸水和返水特性研究进展

近年来,高性能混凝土在各类建筑结构中得到了越来越多的应用,但是自收缩和自重成为了高性能混凝土应用过程中的主要问题,轻骨料(LWA)自身的多孔结构使其具有较普通骨料相比特有的属性—吸返水能力.因此,将预湿轻骨料掺入到混凝土能够对起到内养护的作用,从而改善高性能混凝土自收缩.基于国内外已有的研究成果,本文综述了轻骨料内养护的作用机理和吸返水性能的试验方法.然而,轻骨料的吸水和返水特性研究较少,轻骨料的吸水和返水性能对混凝土的内养护作用仍处于探索阶段.     

碳化作用下轻骨料混凝土干缩变形及影响规律

采用碳化试验和干燥收缩试验相结合,研究了轻骨料混凝土在碳化作用下的干缩变形发展规律及其影响因素。结果表明,加速碳化作用下,轻骨料混凝土的干缩变形显著增长,而随着矿物掺合料的掺入,干缩率得到一定程度的抑制,其中掺超细粉煤灰轻骨料混凝土碳化干缩率最小;骨料预湿完全条件下轻骨料混凝土水胶比越大,碳化干缩变形越大;轻骨料混凝土在高浓度CO2加速碳化作用下干缩变形显著高于自然碳化混凝土;而在相对湿度50%条件下碳化时,其干缩率要大于其它湿度条件下的试样;另外,轻骨料预湿后混凝土碳化干缩率要低于未预湿轻骨料混凝土。因此,在实际工程中,选择合适的矿物掺合料、水胶比、养护条件以及骨料的预湿工艺对轻骨料混凝土的减缩防裂具有较好的效果。     

Internal curing of engineered cementitious composites for prevention of early age autogenous shrinkage cracking

This investigation was carried out to study the effects of using a replacement percentage of saturated lightweight fine aggregate (LWA) as an internal curing agent on the shrinkage and mechanical behavior of Engineered Cementitious Composites (ECC). ECC is a micromechanically-based, designed high-performance, fiber-reinforced cementitious composite with high ductility and improved durability due to tight crack width. Standard ECC mixtures are typically produced with micro-silica sand (200 µm maximum aggregate size). Two replacement levels of silica sand with saturated LWA (fraction 0.59–4.76 mm) were adopted: the investigation used 10 and 20% by weight of total silica sand content, respectively. For each LWA replacement level, two different ECC mixtures with a fly ash-to-Portland cement ratio (FA/PC) of 1.2 and 2.2 were cast. In a control test series, two types of standard ECC mixtures with only silica sand were also studied. To investigate the effect of replacing a portion of the silica sand with saturated LWA on the mechanical properties of ECC, the study compared the results of uniaxial tensile, flexure and compressive strength tests, crack development, autogenous shrinkage and drying shrinkage. The test results showed that the autogenous shrinkage strains of the control ECCs with a low water-to-cementitious material ratio (W/CM) (0.27) and high volume FA developed rapidly, even at early ages. The results also showed that up to a 20% replacement of normal-weight silica sand with saturated LWA was very effective in reducing the autogenous shrinkage and drying shrinkage of ECC. On the other hand, the partial replacement of silica sand with saturated LWA with a nominal maximum aggregate size of 4.76 mm is shown to have a negative effect, especially on the ductility and strength properties of ECC. The test results also confirm that the autogenous shrinkage and drying shrinkage of ECC significantly decreases with increasing FA content. Moreover, increasing FA content is shown to have a positive effect on the ductility of ECC.     

轻骨料预湿程度对混合骨料混凝土抗冻性能影响

研究了轻骨料预湿程度对不同水胶比混合骨料混凝土28 d和90 d抗冻性能以及界面区微观结构的影响.结果表明:风干轻骨料自身的多孔结构在混凝土内部起到了"引气"的作用,同时其强烈的吸水作用使得界面层厚度约为预湿轻骨科的2倍,结构也较致密,故其配制的混凝土抗冻性最优.随着轻骨料预湿程度的增加,骨料引气及吸水作用均减弱,故混...     

Use of porous ceramic waste aggregates for internal curing of high-performance concrete

Internal curing has become extensively used to reduce autogenous shrinkage and consequently mitigate the high risk of early age cracking of high-performance concrete (HPC). This paper investigates the efficiency of internal wet curing provided by a new type of aggregate, "recycled waste porous ceramic coarse aggregates" (PCCA). Six different silica fume HPCs with and without the PCCA are examined with respect to measured physical and mechanical property development. Four different replacement proportions of normal weight coarse aggregate (NCA) by the PCCA have been evaluated. The results have shown a high effectiveness of the PCCA for internal curing purposes, to drastically reduce and even to completely eliminate autogenous shrinkage of HPC prepared with a very low water/binder ratio (w/b) of 0.15. It has been found that the incorporation of 40% of the PCCA leads to a non-shrinking HPC that results in an insignificant internal stress accompanied by a significant increase of the compressive strength. It should be noted that for the different proportions of the PCCA incorporated no decrease of the compressive strength has been observed at either early or later ages, as is the case with some conventional lightweight aggregates.     

Identification of microstructural characteristics in lightweight aggregate concretes by micromechanical modelling including the interfacial transition zone (ITZ)

An approach which combines both experimental techniques and micromechanical modelling is developed in order to characterise the elastic behaviour of lightweight aggregate concretes (LWAC). More than three hundred LWAC specimens with various lightweight aggregate types (5) of several volume ratios and three different mortar matrices (normal, HP, VHP) are tested. The modelling is based on iterative homogenisation process and includes the ITZ specificities experimentally observed with scanning electron microscopy (SEM). In agreement with experimental measurements, the effects of mix design parameters as well as of the interfacial transition zone (ITZ) on concrete mechanical performances are quantitatively analysed. Confrontations with experimental results allow identifying the elastic moduli of LWA which are difficult to determine experimentally. Whereas the traditional empirical formulas are not sufficiently precise, predictions of LWAC elastic behaviours computed with the micromechanical models appear in good agreement with experimental measurements.     

Bond to reinforcement of polymeric lightweight flowable concrete for structural applications

Limited studies investigated the effect of styrene-butadiene rubber (SBR) latexes on bond properties of structural lightweight self-consolidating concrete (LWSCC). The mixtures tested in this investigation were prepared using expanded kaolinic clay lightweight aggregate (LWA), while the water-to-binder ratio was adjusted to secure compressive strength of 40?±?3.5?MPa. Testing was realized using the beam-end specimen method, and the parameters under evaluation included the LWA content (up to 40% of coarse aggregate volume), SBR dosage (up to 15% of binder mass), and bar diameter. Test results have shown that the initial stiffness of load vs. slip curves and ultimate bond strength of LWSCC considerably improved with SBR inclusion. This was related to the coupled effect of the SBR polymers that help relaxing stresses during loading and presence of LWA that reduces bleeding and promotes creation of hydration compounds at the steel-concrete transition zone. The experimental data are compared with the design bond strengths determined by CEB-FIP Model Code 2010, ACI 318-14, and European Code EC-2.     

铁尾矿陶粒混凝土的制备与性能分析

用铁尾矿陶粒替代普通混凝土天然粗骨料,根据《轻骨料混凝土技术规程》(JGJ/T 12—2019)设计配合比,制备LC30~LC40的铁尾矿陶粒混凝土。测定和计算其力学性能、耐久性、经济性,并与普通混凝土作对比。结果表明,铁尾矿陶粒混凝土能够满足《轻骨料混凝土技术规程》(JGJ/T 12—2019)中LC30配制强度,其强度与水胶比的相关性较普通混凝土弱,抗折强度和弹性模量分别仅有普通混凝土的60%和65%左右,但抗氯离子渗透能力和抗冻性能显著优于普通混凝土。本文研究了铁尾矿陶粒用于制备轻骨料混凝土的可行性和其混凝土的综合性能,为下一步装配式构件设计提供数据支撑,间接解决了陕南地区铁尾矿循环利用的问题。     

粗骨料对高性能混凝土早期自收缩的影响(英文)

采用带数据采集系统的涡流位移传感器测量了砂率、针片状骨料含量和最大粒径不同的高性能混凝土的早期自收缩,同时采用热电偶测量了混凝土内部的温度变化.结果表明:骨料性能与含量的变化对高性能混凝土的早期自收缩有较大影响.高性能混凝土的早期自收缩随测试时间、砂率和针片状骨料含量的增加而增大,随骨料最大粒径的增大而减小.     

Mixture-proportioning of high-performance concrete

The paper presents a new approach to design concrete mixtures. It is based upon a set of models relating composition and engineering properties of concrete, to be implemented into software, linked with a material database. The principles underlying the various models are summarized, most of which focus on the granular structure of fresh/hardened concrete. A global approach to concrete is promoted, where performance specifications can be formulated in terms of fresh concrete (yield stress, plastic viscosity, slump and air content), hardening concrete (adiabatic temperature rise and autogenous shrinkage) and hardened concrete (compressive strength at any age, tensile strength, elastic modulus, creep and shrinkage). This approach is illustrated through the design of a special high-shrinkage high-performance concrete (HPC) for road application. To date, durability is lacking in the model and requires further research.     

Plastic shrinkage of mortars with shrinkage reducing admixture and lightweight aggregates studied by neutron tomography

Water transport in fresh, highly permeable concrete and rapid water evaporation from the concrete surface during the first few hours after placement are the key parameters influencing plastic shrinkage cracking. In this work, neutron tomography was used to determine both the water loss from the concrete surface due to evaporation and the redistribution of fluid that occurs in fresh mortars exposed to external drying. In addition to the reference mortar with a water to cement ratio (w/c) of 0.30, a mortar with the addition of pre-wetted lightweight aggregates (LWA) and a mortar with a shrinkage reducing admixture (SRA) were tested. The addition of SRA reduced the evaporation rate from the mortar at the initial stages of drying and reduced the total water loss. The pre-wetted LWA released a large part of the absorbed water as a consequence of capillary pressure developing in the fresh mortar due to evaporation.     

Application of a combination of municipal solid waste incineration fly ash and lightweight aggregate in concrete

This paper highlights significant findings from focusing on developing a sustainable lightweight aggregate (LWA) concrete, which replaced Portland cement partially with municipal solid waste incineration fly ash (MSWI FA) and all of the conventional coarse aggregate with LWA sintered by MSWI FA, shale, and sludge. A series of four experiments, differing in dosage of MSWI FA and aggregate, were conducted for this project. The results of this study generally showed that appropriate amount of MSWI FA substitution for cement has no significantly lowered the compressive strengths of LWA concrete, while it can lower the oven-dry density and the thermal conductivity. The optimum performance of LWA concrete (after 28 days of curing) is as follows: (1) slump flow of 700 mm, (2) compressive strength of 30.14 MPa, (3) dry apparent density of 1.66 g/cm³, (4) thermal conductivity of 0.73 W (m K)^?1; the mixture ratio of LWA, fly ash, cement, and fine sand is 3.0: 0.1: 0.9: 2.0 based on dry weight. Meanwhile, the results of leaching test are much lower than the concentration limits of hazardous constituents of hazardous waste identification standard (GB/T 5083.3-2007) and landfill standard (GB16889-2008).     

Effect of internal curing on durability-related properties of high performance concrete

Internal curing of high performance concrete (HPC) by pre-saturated lightweight aggregates is a well-established method of counteracting self-desiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of high-performance concretes as a function of water to cement ratio is reported.