轻骨料对混凝土自养护减缩效率的影响

通过测试不同配合比轻骨料混凝土的早期自收缩与长期干缩行为,研究轻骨料性能对混凝土自养护减缩效率的影响.结果表明:陶粒的预湿程度与释水能力对于混凝土自养护减缩效率的影响显著;随着预湿程度的增加早期自收缩减小,自养护减缩效率提高;饱和预湿的宾县页岩陶粒自养护减缩效率为39.1%,陶砂影响规律与其相似,但效率更高.实验中采用130kg/m3掺量的饱和预湿陶砂,自养护减缩效率可达95.6%.     

Integrated shrinkage,relative humidity,strength development,and cracking potential of internally cured concrete exposed to different drying conditions

Internal curing (IC) technology using prewetted lightweight fine aggregates (LWFAs) as additives has been proved an effective means for mitigating both autogenous shrinkage and early-age cracking under sealed curing conditions. However, for practical structures in the field, concrete experiences more complex environmental conditions compared to under sealed curing. To better utilize internal curing technology for durable concretes, this study conducts comprehensive investigations on both control and internally cured concretes at water/cement (w/c) ratio of 0.3 and 0.4 in terms of the influence of age when concrete is exposed to drying and the drying duration on the developments of integrated shrinkage, internal relative humidity, compressive and flexural strengths, mass loss, and cracking potential. The results are beneficial for producing internally cured concrete with increased resistance to early-age cracking and enhanced durability for infrastructure applications.     

Integrative Study on the Effect of Internal Curing on Autogenous and Drying Shrinkage of High-Strength Concrete

High-strength concrete generally has a low water-to-cement ratio, which in turn increases the possibility of early-age cracking due to its high autogenous shrinkage. In this article, the effect of internal curing using presoaked lightweight aggregate (PSLWA) in high-strength concrete on shrinkage and interior humidity is investigated by continuously measuring the deformation and interior humidity of the test specimen under plastic film sealing and surface drying conditions since specimen cast. Four mixture proportions with an induced curing water-to-cement ratio (W _IC/C) of 0, 0.04, 0.08, and 0.12 were used in experiments. The experimental results show that the decrease in interior humidity was gradually reduced with an increase in W _IC/C. Accordingly, both autogenous and drying shrinkage of concrete gradually decreased with an increase in W _IC/C. Internal curing cannot completely eliminate autogenous shrinkage because part of the autogenous shrinkage is developed within the humidity-saturated stage in early-age concrete.     

矿物掺合料对UHPC性能的影响

传统超高性能混凝土(UHPC)的硅灰用量一般都比较高,导致其制作成本较高,而且自收缩比较大,对实际工程应用造成了一定的影响。本文用粉煤灰和矿粉部分或全部替代硅灰制备UHPC,并对其工作性能、力学性能、自收缩及孔结构特征进行了试验研究。结果表明:采用粉煤灰或矿粉替代硅灰可以改善UHPC拌合物的流动性,替代率越高,拌合物的流动度越大;当采用粉煤灰或矿粉替代50%(质量分数)硅灰时,在标准养护下,对28 d抗压强度的影响较小,而在高温蒸养下,则会导致28 d抗压强度下降,当替代率达到100%(质量分数)时,无论是标准养护还是高温蒸养,都会显著降低28 d抗压强度;采用粉煤灰或矿粉替代硅灰能降低细孔的占比,增大孔径,减少自收缩,且粉煤灰对于自收缩的抑制效果优于矿粉。     

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.     

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.     

Relation between the molecular structure and the efficiency of superabsorbent polymers (SAP) as concrete admixture to mitigate autogenous shrinkage

Superabsorbent polymers (SAP) were studied as admixtures for mitigating the autogenous shrinkage of a high-strength concrete. The presence of Ca²⁺ ions in the alkaline solution modified the kinetics of the liquid uptake and release when compared to that in other saline solutions and distilled water. SAP with high density of anionic functional groups took up the cement pore solution quickly, but greatly released it subsequently. The cross-linking density had no pronounced influence on the behaviour of such SAP. SAP with lower density of anionic groups did not release the liquid over the time of experiment. All SAP counteracted autogenous shrinkage during the acceleration period of cement hydration. For the materials which released the absorbed pore solution no effect on autogenous shrinkage was found beyond the initial period. SAP materials which did not release the absorbed solution in the experiments with liquids continued the mitigation of autogenous shrinkage during the deceleration period. The internal curing had no negative effect on the compressive strength of the mortar.     

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.     

Internal curing by superabsorbent polymers in ultra-high performance concrete

To limit self-desiccation and autogenous shrinkage that may lead to early-age cracking of ultra-high performance concrete (UHPC), internal curing by means of superabsorbent polymers (SAP) may be employed. Cement pastes and UHPC with water-to-cement ratio below 0.25, with or without SAP, were studied. The absorption capacity of a solution-polymerized SAP was first determined on hardened cement pastes by SEM image analysis. It was observed that the SAP cavities become partially filled with portlandite during cement hydration. Isothermal calorimetry showed that water entrainment with SAP delays the main hydration peak, while after a couple of days it increases the degree of hydration in a manner similar to increasing the water-to-cement ratio. Internal curing by SAP is effective in reducing the internal relative humidity decrease and the autogenous shrinkage. Although the mechanical properties are affected by SAP addition, it is possible to reach compressive strengths of almost 150 MPa at 28 days.     

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.     

预湿水泥浆颗粒内养护材料对混凝土早期自收缩及抗压强度的影响

制备不同粒径和水灰比的水泥浆颗粒作为低水灰比混凝土内养护材料.以最佳内养护水灰比原则,设计了使用三种水灰比分别为0.6、0.7和0.8的同粒径水泥浆颗粒等体积取代砂子的混凝土.研究了不同水灰比水泥浆颗粒对混凝土早期自收缩、抗压强度和内部微结构的影响.结果表明:颗粒吸水率与其水灰比正相关、与其粒径负相关;预湿水泥浆颗粒可显著降低混凝土早期的自收缩,颗粒水灰比越大,自收缩降低效果越明显;但是掺入水泥浆颗粒也会降低混凝土的抗压强度,颗粒水灰比越高抗压强度降低越多,应用中应优化选择预湿颗粒的水灰比;水泥浆颗粒作为高性能混凝土内养护材料,可改善微观界面的孔隙结构,提高界面的密实性,减少混凝土早期的收缩和开裂.     

SAP内养护对碱激发矿渣胶凝材料性能的影响

碱激发矿渣(AAS)胶凝材料存在早期收缩大、开裂风险高的问题,限制了其工程应用。本文采用TAM、TGA、MIP等方法研究了高吸水性树脂(SAP)内养护对AAS胶凝材料水化热、水化产物及孔结构的影响,同时研究了SAP对AAS胶凝材料抗压强度及自收缩的影响规律。结果表明,SAP的加入会增加基体的孔隙率,降低AAS浆体的抗压强度,但是随着水化时间的延长,SAP的内养护作用可以促进矿渣水化,抗压强度的降低幅度逐渐减小。SAP的加入对AAS胶凝材料的水化放热过程有一定的延迟作用,表现为诱导期延长,第二放热峰滞后。SAP的加入使AAS胶凝材料水化产物总量增加,增加程度随着模数的增加而提高。此外,SAP抑制AAS浆体自收缩效果明显,添加SAP之后自收缩降低率最高可达81%。     

沸石对海水拌合超高性能混凝土性能的影响

利用沸石部分或完全(体积分数为0%、50%、100%)取代海水拌合超高性能混凝土(UHPC)中的河砂,对比研究了沸石对海水拌合UHPC宏观和微观性能的影响。结果表明,海水拌合促进UHPC的水化,从而缩短UHPC的凝结时间,提高UHPC的抗压强度和自收缩,而沸石吸收氯离子会抑制氯离子的加速水化作用,使海水拌合UHPC的凝结时间延长,抗压强度降低。此外,沸石的内养护作用可以有效改善海水拌合UHPC的自收缩,且效果比在淡水拌合UHPC中更好,这主要是因为沸石在海水拌合UHPC中释水的时间更早且更长。由于多孔沸石的强度低于河砂,所以UHPC的早期抗压强度随着沸石含量的增多而逐渐降低,养护至28 d后,沸石的内养护作用优化了界面过渡区,从而促使后期硬化浆体进一步密实。     

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.     

内养护对不同细度水泥制备的砂浆性能的影响

研究了饱和轻骨料内养护对不同细度水泥配制的砂浆自收缩、强度、水化程度、显微硬度以及界面过渡区形貌等的影响。结果发现：内养护可显著降低不同细度水泥配制的砂浆的早期自收缩,但减缩效果随着水泥比表面积增大而降低;内养护的砂浆后期自收缩仍持续增加,水泥越粗,自收缩后期增长越大;内养护能够显著促进水泥早期水化,这种促进作用在细水泥中最显著。在相同条件下,轻骨料的引入对砂浆强度的影响作用与水泥细度有关;显微硬度以及界面过渡区微观形貌结果显示,轻骨料内养护能显著改善粗水泥体系微观结构,对细水泥体系微观结构的改善则无显著贡献。     

Revisiting the protected paste volume concept for internal curing of high-strength concretes

Internal curing of high-strength concrete has been the subject of extensive research for the last decade. The concept of protected paste volume has been one of the most significant theoretical approaches to internal curing. In this paper, the applicability of the protected paste volume concept to internal curing is re-evaluated in view of recent experimental evidence. It is shown, that the concept of protected paste volume and recommendation to limit the spacing factor to approximately 200 μm, cannot be extended to internal curing of high-strength concrete, since the distance of penetration of the internal curing water into the surrounding matrix depends mainly on the availability of internal curing water to the surrounding cementitious matrix. The pore structure of LWA and the size of SAP particles seem to have a marked influence on the availability of internal curing water and thus are factors of greater importance than the spacing factor.     

戈壁地区不同养护方式下混凝土箱梁养护效果试验研究

为研究戈壁地区混凝土箱梁在不同养护方式下的各项性能及在这种特殊环境下的实用性,对实验原材料在现场进行验证试验研究其养护效果.结果表明:与塑料薄膜和土工布养护相比,橡塑板养护早期强度增长更快;不同养护方式下混凝土水化热均在20 h后达到峰值,并呈现正弦规律的周期性变化,给出了箱梁的水化热温度分布规律;箱梁应变的产生是由混凝土自收缩和温度共同作用的结果,通过曲线规律分析出了收缩应变和温度应变的作用阶段;通过微观结构测试,分析出了不同养护方式下混凝土孔结构的分布特征,具有较小孔间距系数的橡塑板养护力学性能优于其他两种养护方式.     

Effect of Internal Curing on Moisture Gradient Distribution and Deformation of a Concrete Pavement Slab Containing Pre-Wetted Lightweight Fine Aggregates

Internal curing technology has been proven effective in improving many properties of concrete. The application of this technology to the transportation infrastructure holds promise with increasing resistance to cracking and enhanced durability. On the other hand, an investigation of moisture distribution in concrete slabs is of significance for predicting slab deformation and the associated stress and failure. This study is performed to investigate the characteristics of the pre-wetted sintered fly ash lightweight fine aggregate (LWFA) used as an internal curing agent and the influence of internal curing on concretes exposed to sealed curing, external drying, and surface wetting conditions. The internal curing effects on the moisture distributions in concrete pavement slabs at w/c = 0.3 and 0.4 were evaluated through experimental measurements, and the effects on slab deformations were quantified using an equivalent temperature difference. The findings of this study will benefit the further application of internal curing technology to the transportation infrastructure's construction.     

超细矿渣粉和偏高岭土对硫铝酸盐水泥早期收缩性能的影响

通过开展化学收缩、自收缩与干燥收缩试验,研究了超细矿渣粉和偏高岭土对硫铝酸盐水泥早期收缩性能的影响。结果表明,掺入超细矿渣粉与偏高岭土会增大水泥浆体的内部相对湿度,能有效抑制水泥浆体的化学收缩、自收缩与干燥收缩,且掺量越大,抑制效果越明显,根据水泥浆体的内部相对湿度能够大致判断其自收缩的变化规律。掺入超细矿渣粉与偏高岭土会加快硫铝酸盐水泥的早期水化,使化学收缩变化速率达到峰值的时间提前。当超细矿渣粉的掺量为20%(质量分数,下同)或偏高岭土的掺量为10%、20%时,与空白组相比水泥浆体的7 d自收缩分别减小了42.21%、35.89%和63.73%,7 d干燥收缩分别减小了24.89%、16.42%和30.87%。在相同掺量条件下,掺入偏高岭土的水泥浆体化学收缩、自收缩与干燥收缩显著小于掺入超细矿渣粉的水泥浆体。自收缩与线性化学收缩的比值随龄期的增长而减小,掺入超细矿渣粉与偏高岭土后,自收缩与线性化学收缩的比值进一步减小。     

Sulphate resistance of self-compacting concrete

This article outlines a laboratory study on sulphate resistance of self-compacting concrete (SCC). For this purpose, more than 40 cylinders of concrete were subjected to a solution with sodium sulphate, sea or distilled water during 900 days. Age at start of testing was either 28 or 90 days. Weight and internal fundamental frequency (IFF) were measured. Comparison was done with the corresponding properties of vibrated concrete (VC). When cured in a solution with sodium sulphate, the results show larger loss of mass of SCC than that of VC probably due to the limestone filler content in SCC. After curing in water, sea or distilled, no such weight difference between the curing types was observe. IFF did not decrease or differ between the two types of concrete, i.e. no internal deterioration took place due to thaumasite sulphate attack (TSA) during the 900 days of exposure. The project was carried out from 1999 to 2002.