Elsevier

Cement and Concrete Composites
水泥与混凝土复合材料

Volume 43, October 2013, Pages 39-47
第 43 卷，2013 年 10 月，第 39-47 页
Cement and Concrete Composites

Flexural performance of fiber-reinforced ultra lightweight cement composites with low fiber content
纤维增强超轻质水泥复合材料在低纤维含量下的弯曲性能

https://doi.org/10.1016/j.cemconcomp.2013.06.006Get rights and content  获取权利和内容
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Abstract  摘要

This paper presents an experimental study on flexural performance of ultra lightweight cement composites (ULCC) with 0.5 vol% fibers. Low density of the ULCC is achieved by using cenospheres from coal-fired power plants as micro aggregates. Effects of shrinkage reducing admixture (SRA) and fiber types on compressive strength and flexural performance of the ULCC are investigated. ULCC with density of 1474 kg/m3, compressive strengths of 68.2 MPa, flexural strength of 8 MPa, and deflection hardening behavior can be produced. Such good performance could be attributed primarily to the SRA which reduced entrapped air in paste matrix and densified fiber–matrix interface. The improvement on the flexural performance of the ULCC depends on fibers used and bond between fibers and matrix. Improvement of the flexural performance of the steel fiber (coated with brass) reinforced ULCC due to the densification effect by SRA was more significant than that of the PE fiber reinforced ULCC.
本文研究了含 0.5 vol%纤维的超轻质水泥复合材料（ULCC）的弯曲性能。通过使用燃煤电厂的微珠作为细骨料，实现了 ULCC 的低密度。研究了减缩剂（SRA）和纤维类型对 ULCC 抗压强度和弯曲性能的影响。密度为 1474 kg/m³的 ULCC，抗压强度为 68.2 MPa，弯曲强度为 8 MPa，并表现出挠度硬化行为。这种优异的性能主要归因于 SRA，它减少了浆体基体中的夹杂物并密实了纤维-基体界面。ULCC 弯曲性能的提高取决于所使用的纤维和纤维与基体的结合。由于 SRA 的密实效应，钢纤维（镀黄铜）增强 ULCC 的弯曲性能提高比 PE 纤维增强 ULCC 更为显著。

Keywords  关键词

Flexural performance
Polyethylene fiber
Steel fiber
Shrinkage reducing admixture
Ultra lightweight cement composite

弯曲性能聚乙烯纤维钢纤维减缩剂超轻质水泥复合材料

1. Introduction  1. 引言

Lightweight concretes are produced by introducing air voids into the concretes. There are generally three types of lightweight concretes: (1) lightweight aggregate concrete (voids are mainly in aggregates), (2) cellular concrete and foam concrete (voids are in cement paste), and (3) no fines concrete (sand is eliminated and voids are between coarse aggregate particles). Among them, the first type of lightweight concrete is typically used for structural applications with low permeability requirement. Structural lightweight aggregate concretes (LWAC) typically have density between 1400 and 2000 kg/m3 compare to about 2300–2400 kg/m3 for normal weight concrete. (Unless otherwise stated, density given in this paper is based on wet specimens while the strength is based on 28 days.) In the conventional LWAC, lightweight aggregates such as expanded clay, expanded shale, or sintered fly ash aggregates [1], [2], [3] are commonly used.
轻质混凝土是通过在混凝土中引入气孔来生产的。轻质混凝土通常分为三种类型：（1）轻骨料混凝土（气孔主要存在于骨料中），（2）泡沫混凝土和发泡混凝土（气孔存在于水泥浆中），以及（3）无砂混凝土（去除砂，气孔存在于粗骨料颗粒之间）。其中，第一类轻质混凝土通常用于低渗透性要求的结构应用。结构轻骨料混凝土（LWAC）的密度通常在 1400 至 2000 kg/m³之间，而普通混凝土的密度约为 2300–2400 kg/m³。（除非另有说明，本文中给出的密度基于湿试件，而强度基于 28 天。）在传统 LWAC 中，通常使用膨胀粘土、膨胀页岩或烧结粉煤灰骨料[1]、[2]、[3]。
Since late 20th century, high strength LWAC with low permeability has been successfully developed with strength up to 102 MPa and density ranging from 1595 to 1880 kg/m3 [4]. For these LWAC, specific strength (strength-to-density ratio) varies from about 36 kPa/kg m−3 to about 54 kPa/kg m−3. For LWAC of lower density between 1400 and 1700 kg/m3, strengths ranging from 30 to 60 MPa were reported [5]. According to a state-of-the-art review on developments of high strength lightweight concrete by Wee [6], the specific strength of lightweight concrete typically decreases with the reduction of density, and it is a challenge to make LWAC with density below 1500 kg/m3 and strength above 50 MPa.
自 20 世纪末以来，成功开发出低渗透性高强度轻质水泥复合材料（LWAC），其强度高达 102 MPa，密度范围为 1595 至 1880 kg/m 3 [4]。对于这些 LWAC，其比强度（强度与密度的比值）介于约 36 kPa/kg m −3 至约 54 kPa/kg m −3 之间。对于密度在 1400 至 1700 kg/m 3 之间的低密度 LWAC，报道的强度范围为 30 至 60 MPa [5]。根据 Wee 对高强度轻质混凝土发展现状的综述[6]，轻质混凝土的比强度通常随着密度的降低而下降，而制造密度低于 1500 kg/m 3 且强度高于 50 MPa 的 LWAC 是一项挑战。
Ultra lightweight cement composite (ULCC) [7] is a type of novel composites characterized by combinations of low densities <1500 kg/m3, high compressive strengths ⩾60 MPa with specific strength of up to 47 kPa/kg m−3. The ULCC was originally designed for potential structural applications in steel–concrete composites and sandwich structures [8], [9]. In addition, due to their high specific strength (low density and high strength) and low permeability, the ULCC may be used potentially in structures where weight of the material is critical, e.g. floating structures.
超轻水泥复合材料（ULCC）[7]是一种新型复合材料，其特点是低密度<1500 kg/m 3 ，高抗压强度⩾60 MPa，比强度高达 47 kPa/kg m −3 。ULCC 最初是为钢-混凝土复合材和夹层结构中的潜在结构应用而设计的[8], [9]。此外，由于其高比强度（低密度和高强度）和低渗透性，ULCC 有可能用于材料重量至关重要的结构中，例如浮体结构。
Low density of the ULCC is achieved by using cenospheres obtained from coal-fired thermal power plants [10], [11], [12] as micro-lightweight aggregates. The cenospheres consist of hollow interior covered by thin shell with typical thicknesses of 2.5–10.5% of its diameter [13]. Their typical particle sizes are between 10 and 300 μm [14]. An optical microscope image of a typical cross-section of the ULCC is shown in Fig. 1.
低密度超轻水泥复合材料（ULCC）是通过使用从燃煤热电厂获得的微轻骨料——泡沫球[10], [11], [12]——来实现的。泡沫球具有中空的内部和薄壳，壳的厚度通常为其直径的 2.5–10.5%[13]。它们的典型粒径在 10 至 300 μm 之间[14]。图 1 显示了 ULCC 典型横截面的光学显微镜图像。
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Fig. 1. Optical microscope image (approximately 1.1 mm by 0.8 mm) of ultra lightweight cement composite with cenospheres.
图 1. 膨胀球超轻水泥复合材料的显微镜图像（约 1.1 毫米×0.8 毫米）。

Literatures review revealed limited numbers of studies involving the use of cenospheres or similar to produce lightweight cementitious composites [14], [15], [16], [17], [18], [19], [20]. By reference to the compressive strength and specific strength of the lightweight mixtures from these studies, none of them had combination of high strength and low density of the ULCC achieved in this study. The earliest patent on the use of microspheres was filed in 1999, in which the described cement-based invention could achieve 28-day compressive strength of 28 MPa after curing [16]. Several studies on lightweight composites with microspheres (or cenospheres) were published between 2000 and 2003 [17], [18], [19]. The maximum compressive and flexural strengths reported from these studies were about 35 and 6 MPa, respectively, with densities ranging from 1510 to 1840 kg/m3. These lightweight composites did not contain fibers. Another patent filed in 2003 [20] presented properties of ten fiber-reinforced lightweight composites using microspheres. The composites developed had strength below 50 MPa with densities ranging from 930 to 1780 kg/m3 and specific strength of 21.6–28.8 kPa/kg m−3.
文献综述表明，关于使用微珠或类似材料生产轻质水泥基复合材料的研究有限[14], [15], [16], [17], [18], [19], [20]。参考这些研究中轻质混合料的抗压强度和比强度，它们均未实现本研究中 ULCC 的高强度和低密度组合。关于使用微珠的最早专利于 1999 年申请，其中描述的水泥基发明在固化后可达到 28 天抗压强度 28 MPa[16]。2000 年至 2003 年间发表了数项关于含微珠（或微珠）的轻质复合材料的研究[17], [18], [19]。这些研究报道的最大抗压强度和抗折强度分别为约 35 MPa 和 6 MPa，密度范围在 1510 至 1840 kg/m³。这些轻质复合材料不含纤维。2003 年申请的另一项专利[20]展示了使用微珠的十种纤维增强轻质复合材料的性能。 所开发的复合材料强度低于 50 MPa，密度范围为 930 至 1780 kg/m 3 ，比强度为 21.6–28.8 kPa/kg m −3 。
To achieve high strength, a water/binder ratio of 0.35 and silica fume dosage of 8% by mass of total binder were used. Fibers were incorporated in the ULCC to improve its flexural toughness and energy absorption capacity. Addition of fibers can improve mechanical properties of mortars and concretes, especially flexural performance after matrix cracking [21], [22], including flexural toughness and residual strength. Higher flexural performance of the fiber-reinforced mortars or concretes can be achieved by increasing pull-out resistance of the fibers from mortar or concrete matrices, provided the fibers do pull out instead of rupture. Conventional methods to improve the pull-out resistance include changing the geometry of fibers [23] and using silica fume in the concrete matrix [24]. Recently, plasma treatment [25], ozone treatment [26], [27] were used to reduce the contact angle between polymer fibers and water to improve the bonding with cement paste matrices.
为达到高强度，采用水胶比为 0.35，并使用占总量 8%的硅灰。纤维被掺入超轻水泥混凝土（ULCC）中以提高其弯曲韧性和能量吸收能力。纤维的添加可以改善砂浆和混凝土的力学性能，特别是基体开裂后的弯曲性能[21], [22]，包括弯曲韧性和残余强度。通过增加纤维从砂浆或混凝土基体中的拔出阻力，可以实现纤维增强砂浆或混凝土更高的弯曲性能，前提是纤维能够拔出而不是断裂。提高拔出阻力的传统方法包括改变纤维的几何形状[23]以及在混凝土基体中使用硅灰[24]。最近，等离子体处理[25]、臭氧处理[26], [27]被用于降低聚合物纤维与水之间的接触角，以改善与水泥基体的粘结。
Due to high amount of fine particles of cenospheres, cement, silica fume plus fibers, the ULCC may contain high volumes of entrapped air. This leads to poor contact between the fibers and cement paste or mortar matrix and weak bond between them, which influences the flexural performance of the composites after matrix cracking. Recent research [28] shows that the use of shrinkage reducing admixture (SRA) can reduce surface tension of pore solution, air content of mortar matrix, and enhance the wettability of fiber in the fresh cement mortar, thus significantly improve the flexural toughness of fiber reinforced mortars due to the densification of transition zone between the fiber and matrix. This suggests that the SRA may be useful in improving flexural performance of fiber reinforced ULCC. If the addition of SRA can improve the flexural performance of fiber reinforced ULCC, lower fiber content may be used to satisfy structural design requirements, which means reduced cost and more workable mixtures.
由于微珠、水泥、硅灰和纤维的细颗粒含量高，超轻质水泥复合材料（ULCC）可能含有大量封闭空气。这导致纤维与水泥浆或砂浆基体之间的接触不良，以及它们之间的粘结强度弱，从而影响基体开裂后复合材料的弯曲性能。最近的研究[28]表明，使用减缩剂（SRA）可以降低孔隙溶液的表面张力、降低砂浆基体的含气量，并提高纤维在新鲜水泥砂浆中的润湿性，从而由于纤维与基体之间过渡区的致密化，显著提高纤维增强砂浆的弯曲韧性。这表明 SRA 可能有助于提高纤维增强 ULCC 的弯曲性能。如果 SRA 的添加能够提高纤维增强 ULCC 的弯曲性能，则可以使用更低的纤维含量来满足结构设计要求，这意味着成本降低和更易施工的混合物。
This paper presents an experimental study on the development of fiber-reinforced ultra lightweight cement composites (ULCC) at a low fiber volume fraction of 0.5% with a focus on flexural performance of the ULCC. Effects of SRA and types of fibers (straight steel fibers with brass coating and polyethylene (PE) fibers) and synergy of the steel and PE fibers on compressive strength and flexural performance of the ULCC were investigated. In addition, a comparison is made on flexural performance between fiber-reinforced ULCC and fiber reinforced high strength cement mortar.
本文研究了纤维增强超轻质水泥复合材料（ULCC）在低纤维体积含量 0.5%下的性能发展，重点关注 ULCC 的弯曲性能。研究了 SRA 的影响以及纤维类型（镀黄铜直钢纤维和聚乙烯（PE）纤维）对 ULCC 抗压强度和弯曲性能的作用，并探讨了钢纤维和 PE 纤维的协同效应。此外，还对比了纤维增强 ULCC 与纤维增强高强度水泥砂浆的弯曲性能。

2. Experimental details  2. 实验细节

2.1. Materials  2.1. 材料

The shrinkage reducing admixture used in this study was a commercially available product in colorless liquid form without water.
本研究中使用的减缩剂是一种市售的无色液体形态产品，不含水。
Cenospheres used in the ULCC had an average particle density of approximately 870 kg/m3. Particle size distribution of the cenospheres is given in Fig. 2, and most of the particles had sizes from 10 to 300 μm. The cenospheres had low CaO content of less than 1% but high combined SiO2 and Al2O3 content of approximately 90%. X-ray diffraction analysis of the cenosphere material indicated that it contained a large amount of amorphous material and small amounts of quartz and mullite crystals. ASTM C227 and C1260 test results indicate that the cenospheres used in the ULCC are not potentially deleterious due to alkali–silica reaction [29].
ULCC 中使用的微珠平均颗粒密度约为 870 kg/m³。微珠的粒径分布如图 2 所示，大部分颗粒的尺寸在 10 至 300 μm 之间。微珠的 CaO 含量较低，小于 1%，但 SiO₂和 Al₂O₃含量较高，约为 90%。对微珠材料的 X 射线衍射分析表明，其中含有大量的非晶质材料和少量的石英及莫来石晶体。ASTM C227 和 C1260 测试结果表明，ULCC 中使用的微珠不会因碱-硅酸反应而具有潜在危害性[29]。
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Fig. 2. Particle size distribution of cenospheres.
图 2. 珍珠陶土的粒径分布

Properties of steel fibers (ST) coated with brass and polyethylene (PE) fibers used in this study are given in Table 1. The PE fiber had slightly higher tensile strength but lower elastic modulus than the steel fiber. According to the information from the manufacturer, the PE fibers were not surface treated, and the fibers were used in the ULCC as-received. ASTM Type I Portland cement (also EN ‘CEM I 52.5N’) and silica fume were used in all mixtures, and their compositions are given in Table 2. A polycarboxylate based superplasticizer (SP) was used to obtain comparable flow at 200 ± 10 mm for all mixtures according to BS EN 1015-3 [30].
本研究中使用的包覆黄铜和聚乙烯（PE）纤维的钢纤维（ST）性能见表 1。PE 纤维的拉伸强度略高于钢纤维，但弹性模量较低。根据制造商提供的信息，PE 纤维未经表面处理，在 ULCC 中使用了原状纤维。所有混合物均使用 ASTM I 型波特兰水泥（也符合 EN 'CEM I 52.5N'）和硅灰，其组成见表 2。为使所有混合物在 200 ± 10 mm 处获得可比的流动性，根据 BS EN 1015-3 [30]使用了基于聚羧酸的高效减水剂（SP）。

Table 1. Properties of fibers.
表 1. 纤维性能

Fiber types  纤维类型Tensile strength (MPa)  抗拉强度（MPa）Elastic modulus (GPa)  弹性模量（GPa）Length (mm)  长度（mm）Diameter (μm)  直径 (μm)Aspect ratio  长径比Density (kg/m3)  密度 (kg/m 3 )
Polyethylene (PE)a  聚乙烯 (PE) a 26107912393080.97
Steel (ST)b  钢 (ST) b 250020013160817.85
a
Spectra® fiber 900.  Spectra® 纤维 900.
b
Dramix® OL13/.16, straight with brass coating.
Dramix® OL13/.16, 直径带黄铜镀层。

Table 2. Chemical and mineral compositions of cement and silica fume (% by mass).
表 2. 水泥和硅灰的化学及矿物成分（质量百分比）。

Composition  成分CaOSiO2  SiO₂Al2O3Fe2O3MgONa2OK2OSO3LOIC3SC2SC3AC4AF
Cement  水泥63.621.64.23.02.40.190.52.72.254.124.87.57.5
Silica fume  硅灰0.296.00.30.30.40.050.60.21.5N/AN/AN/AN/A

2.2. Mix proportions and specimen preparations
2.2. 混合比例和试样制备

Thirteen ULCC mixtures were included in this study with two plain ULCC and eleven fiber-reinforced ULCC. Mix proportions of the mixtures are given in Table 3, in which “ST” denotes steel fibers, “PE” denotes polyethylene fibers, and “HY” denotes a combination of ST and PE fibers. Ten of the fiber-reinforced ULCC mixtures contained 0.5% of steel fibers, PE fibers, or a combination of these by volume of the ULCC. Half of these mixtures contained 2.5% SRA by mass of binder (cement + silica fume), whereas the other half of the mixtures were control without the SRA. According to manufacturer’s data sheet, the SRA used in this research does not contain water, and it is recommended that equal amount of water be replaced when the SRA is incorporated in mortar or concrete mixtures. The SRA dosage of 2.5% was selected based on preliminary tests that air content in the ULCC would not be reduced significantly beyond this dosage.
本研究包括 13 种 ULCC 混合物，其中包含 2 种普通 ULCC 和 11 种纤维增强 ULCC。混合物的配合比见表 3，其中“ST”表示钢纤维，“PE”表示聚乙烯纤维，“HY”表示钢纤维和聚乙烯纤维的组合。10 种纤维增强 ULCC 混合物中包含 0.5%体积的钢纤维、聚乙烯纤维或这两种纤维的组合。其中一半混合物按胶凝材料（水泥+硅灰）质量的 2.5%掺入 SRA，另一半混合物为不加 SRA 的对照组。根据制造商的数据表，本研究中使用的 SRA 不含水，建议在将 SRA 掺入砂浆或混凝土混合物时，用等量的水进行替代。2.5%的 SRA 掺量是基于初步试验结果选定的，该结果表明，当 ULCC 中的空气含量超过此掺量时，不会显著降低。

Table 3. Mixture proportions of ULCC.
表 3. ULCC 的混合比例

Mix ID  混合物 IDFiber (vol%)  纤维（体积%）Mixture proportion of matrix by mass of total binder
胶凝材料总质量的混合比例
Steel  钢PEWater/binder  水/胶凝材料Binder  胶凝材料Cenosphere/binder  气孔/胶凝材料SRA/binder  SRA/胶凝材料
Cement  水泥SFa
Plain  普通000.350.920.080.420
ST-500.50000.350.920.080.420
HY-10.3750.1250.350.920.080.420
HY-20.2500.2500.350.920.080.420
HY-30.1250.3750.350.920.080.420
PE-5000.5000.350.920.080.420

Plain_SRA000.3250.920.080.420.025
ST-50_SRA0.50000.3250.920.080.420.025
HY-1 _SRA0.3750.1250.3250.920.080.420.025
HY-2_SRA0.2500.2500.3250.920.080.420.025
HY-3_SRA0.1250.3750.3250.920.080.420.025
PE-50_SRA00.50.3250.920.080.420.025
ST-37.5_SRA0.37500.3250.920.080.420.025
a
Silica fume.  硅灰
An additional fiber reinforced ULCC mixture with 0.375% of steel fiber (ST-37.5_SRA) by volume was prepared to compare with ULCC mixture ST-50_SRA, and to support the assumption that the flexural toughness is increased linearly with the fiber dosage up to 0.5% discussed in Section 3.2.3.
额外制备了一种含 0.375%体积钢纤维（ST-37.5_SRA）的纤维增强超轻水泥复合材料混合物，用于与 ST-50_SRA ULCC 混合物进行比较，并支持第 3.2.3 节中讨论的弯曲韧性随纤维用量增加至 0.5%而线性增加的假设。
For comparison, a high strength mortar (HSM) with sand/binder ratio of 2.5 was made. The HSM had the same water/binder ratio, SRA dosage, silica fume %, and fiber type and dosage as ULCC mixture ST-50_SRA given in Table 3. Natural sand with a fineness modulus of 2.96 and maximum size of 4.75 mm was used in the HSM.
为了对比，制作了一种高强砂浆（HSM），其砂/胶比为 2.5。HSM 与表 3 中给出的 ULCC 混合物 ST-50_SRA 具有相同的水/胶比、SRA 掺量、硅灰百分比和纤维类型及掺量。HSM 中使用了细度模数为 2.96、最大粒径为 4.75 mm 的天然砂。
For each mixture, four 100 × 100 × 400-mm prisms and six 100-mm cubes were cast for flexural performance and compressive strength test, respectively. The fresh ULCC was filled to various molds in two layers and compacted on a vibration table with a total vibration time of 30 s. The specimens were covered by plastic sheets and demolded within 48 h after casting. They were then stored in a fog room at temperatures of 28–30 °C (simulating tropical environment) until testing at 7 and 28 days.
对于每种混合料，分别浇筑了四个 100 × 100 × 400 毫米的棱柱体和六个 100 毫米的立方体，用于弯曲性能和抗压强度测试。新鲜超轻水泥混凝土（ULCC）分两层填入不同模具中，并在振动台上振捣 30 秒进行压实。浇筑后，试样用塑料薄膜覆盖，并在 48 小时内脱模。随后，它们被存放在温度为 28–30°C 的雾室中（模拟热带环境），直至在 7 天和 28 天时进行测试。

2.3. Test methods and procedures
2.3. 测试方法和程序

2.3.1. Density, air content, and compressive strength of ULCC
2.3.1. ULCC 的密度、含气量和抗压强度

Density of all the ULCC specimens was determined after demolding using water displacement method. Air content of the specimens was calculated by gravimetric method according to ASTM C138 [31]. Density of the cement and cenospheres used for the calculation of the air content was measured by AccuPyc 1330 Pycnometer based on gas displacement principle. Compressive strength was determined at 28 days using 100-mm cubes according to BS EN 12390-3:2002 [32].
所有 ULCC 试件的密度在脱模后采用排水法测定。试件的含气量根据 ASTM C138 [31]采用重量法计算。用于计算含气量的水泥和微珠的密度采用基于气体置换原理的 AccuPyc 1330 Pycnometer 测量。抗压强度根据 BS EN 12390-3:2002 [32]在 28 天时采用 100mm 立方体测定。

2.3.2. Flexural performance
2.3.2. 弯曲性能

Flexural performance of 100 × 100 × 400-mm prisms was determined at 28 days with third-point loading (4-point bending, span length 300 mm) using an Instron closed-loop, servo-controlled testing system as per ASTM C1609 [33]. A schematic diagram of the ASTM C1609 test setup is shown in Fig. 3. During the test, both applied load and mid-span deflection of the specimens in the direction of the applied load were recorded. The deflections were measured by two linear variable displacement transducers (LVDTs) placed on both sides of the specimen. The output from this test was a load–deflection curve from which flexural performance parameters were derived using absolute values of load or strength at specific deflections. Four specimens were tested for each mixture, and the flexural load–deflection curves were averaged by using Average Multiple Curves (AMC) function from Origin 8.5 software. An example of load–deflection curves from four test specimens of HY-2_SRA and their average curve is shown in Fig. 4. The repeatability of the test results appears reasonable.
根据 ASTM C1609 [33]标准，使用 Instron 闭环伺服控制测试系统，在 28 天时对 100 × 100 × 400 毫米的棱柱体进行三点加载（四点弯曲，跨度长度 300 毫米）以确定其弯曲性能。ASTM C1609 测试装置的示意图如图 3 所示。测试过程中，记录了施加的载荷以及试样在加载方向上的中跨挠度。通过放置在试样两侧的两个线性可变位移传感器（LVDT）测量挠度。该测试的输出为荷载-挠度曲线，通过使用特定挠度时的荷载或强度的绝对值来推导弯曲性能参数。每种混合料测试了四个试样，并使用 Origin 8.5 软件的多个曲线平均（AMC）功能对弯曲荷载-挠度曲线进行平均。HY-2_SRA 四个测试试样的荷载-挠度曲线及其平均曲线的示例如图 4 所示。测试结果的重复性看起来是合理的。
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Fig. 3. Schematic diagram of the ASTM C1609 test setup.
图 3. ASTM C1609 测试装置示意图

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Fig. 4. Average curve of the load–deflection curves by using Origin 8.5 software.
图 4. 使用 Origin 8.5 软件得到的平均荷载-变形曲线

2.3.3. Measurement of surface tension of synthetic pore solution and contact angle between fibers and the pore solution
2.3.3. 合成孔隙溶液表面张力的测量及纤维与孔隙溶液之间的接触角

Solutions that simulate pore solutions of the cement pastes in the ULCC with and without SRA were prepared for measurements of their surface tensions. The solutions contained 0.35 M KOH and 0.05 M NaOH in de-ionized water, and the dosage of SRA was equivalent to 2.5% by mass of (cement + silica fume) or 7.14% by mass of water in the ULCC mixtures. Surface tension of the solutions was measured using Wilhelmy Plate Method with a K14 Krüss Tensiometer.
为测量其表面张力，制备了模拟 ULCC 中水泥浆体孔隙溶液（含 SRA 和不含 SRA）的溶液。这些溶液含有 0.35 M KOH 和 0.05 M NaOH，去离子水，SRA 的用量相当于（水泥+硅灰）质量的 2.5%或 ULCC 混合物中水的质量的 7.14%。使用 K14 Krüss 张力计，采用 Wilhelmy 板法测量溶液的表面张力。
Contact angles between the fibers and the above solutions were measured by the same equipment. For these measurements, the tension metric method (Micro-Wilhelmy technique) was used [34]. Details of the method can be found in reference [28]. At least six samples under each condition were tested for each type of fibers.
纤维与上述溶液之间的接触角使用相同设备进行测量。对于这些测量，采用了张力指标法（Micro-Wilhelmy 技术）[34]。该方法的详细信息可在参考文献[28]中找到。每种纤维类型在每个条件下至少测试了六个样品。

3. Results and discussion
3. 结果与讨论

3.1. Density and compressive strength of ULCC
3.1. 密度和抗压强度

Table 4 presents the average density, compressive strength, specific strength, and air content of the ULCC. The specific strength is defined as strength-to-density ratio. For the ULCC without SRA, the addition of the fibers resulted in decreases in the density from 1295 kg/m3 to 1125–1230 kg/m3 and increases in the air content from 15% to 20–26%. Compressive strength of the ULCC was also decreased from 47 MPa to 27–36 MPa due to the incorporation of the fibers. This was probably due to the effect of fibers on air void content in the ULCC which led to reductions in their densities, compressive strengths, and specific strengths due to increased entrapped air in the mixtures.
表 4 展示了超轻水泥复合材料（ULCC）的平均密度、抗压强度、比强度和含气量。比强度定义为强度与密度的比值。对于不含 SRA 的 ULCC，纤维的添加导致密度从 1295 kg/m³降低到 1125–1230 kg/m³，含气量从 15%增加到 20–26%。由于纤维的掺入，ULCC 的抗压强度也从 47 MPa 降低到 27–36 MPa。这可能是由于纤维对 ULCC 中空气孔隙含量的影响，导致混合物中 entrapped air 增加而使它们的密度、抗压强度和比强度降低。

Table 4. Average density, compressive strength, specific strength and air content of ULCC.
表 4. ULCC 的平均密度、抗压强度、比强度和含气量。

Mix ID  混合物 IDDensity (kg/m3)  密度 (kg/m 3 )Compressive strength (MPa)
抗压强度（MPa）		Specific strength (kPa/kg m−3)
比强度（kPa/kg m −3 ）		Air contenta (%)  含气量 a (%)
Plain  普通129546.936.215.3
ST-50119529.524.723.5
HY-1123236.329.420.2
HY-2113927.123.825.7
HY-3112528.525.326.2
PE-50118629.124.521.8

Plain_SRA142467.047.06.8
ST-50_SRA147468.246.35.1
HY-1 _SRA146868.046.35.0
HY-2_SRA147467.745.94.0
HY-3_SRA144163.143.85.6
PE-50_SRA142163.344.56.4
ST-37.5_SRA146967.846.25.2
a
Air content is the volume of voids in the composite, but does not include the enclosed voids inside cenospheres.
空气含量是复合材料中空隙的体积，但不包括空心球内部的封闭空隙。
The incorporation of the SRA in the ULCC reduced entrapped air, increased density, compressive strength, and specific strength of the ULCC substantially (Table 4). The effect of SRA on the density and compressive strength of the ULCC is clearly illustrated in Fig. 5, Fig. 6, respectively. It was observed that the density, compressive strength, and specific strength of the ULCC with SRA were reasonably consistent, and were less affected by the type and content of the steel and PE fibers incorporated. The air content, density, and compressive strength of the ULCC with SRA ranged from 4.0 to 6.8%, 1421–1474 kg/m3, and 63.1–68.2 MPa, respectively.
将 SRA 掺入 ULCC 中显著减少了内部空隙，提高了 ULCC 的密度、抗压强度和比强度（表 4）。SRA 对 ULCC 密度和抗压强度的影响分别如图 5 和图 6 所示。观察到掺有 SRA 的 ULCC 的密度、抗压强度和比强度较为一致，且受掺入的钢纤维和 PE 纤维类型及含量的影响较小。掺有 SRA 的 ULCC 的空气含量、密度和抗压强度分别介于 4.0%至 6.8%、1421–1474 kg/m³和 63.1–68.2 MPa 之间。
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Fig. 5. Effect of SRA on the density of the ULCC.
图 5. SRA 对 ULCC 密度的影响。

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Fig. 6. Effect of SRA on the compressive strength of the ULCC.
图 6. SRA 对 ULCC 抗压强度的影响

The addition of SRA significantly reduced the air content from 20.2–26.2% to 4.0–6.4% in the fiber-reinforced ULCC. The difference on the air content of the ULCC with and without SRA is clearly shown in optical microscope pictures in Fig. 7. More large air voids were observed in the ULCC without SRA (Mix ST-50) than in the ULCC with SRA (Mix ST-50_SRA).
SRA 的添加显著降低了纤维增强超轻水泥复合材料中的空气含量，从 20.2–26.2%减少到 4.0–6.4%。ULCC 中添加 SRA 与不添加 SRA 的空气含量差异在图 7 的光学显微镜图片中清晰显示。与添加 SRA 的 ULCC（混合物 ST-50_SRA）相比，未添加 SRA 的 ULCC（混合物 ST-50）中观察到更多的大气孔。
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Fig. 7. Optical microscope images of the ULCC (a) without SRA (ST-50) and (b) with SRA (ST-50_SRA), showing lower large air void content of the ULCC with SRA.
图 7. ULCC 的光学显微镜图像（a）不含 SRA（ST-50）和（b）含 SRA（ST-50_SRA），显示含 SRA 的 ULCC 具有较低的大气泡含量。

The reduced air content and increased density of the ULCC with SRA might be related to two functions of the SRA: (1) reduce the surface tension of the solution in the fresh ULCC mixtures and (2) enhance the wettability of the fibers by reducing the contact angle with the solution.
SRA 的减少空气含量和增加密度可能与 SRA 的两个功能有关：(1)降低新鲜 ULCC 混合物中溶液的表面张力，(2)通过减少溶液的接触角来增强纤维的润湿性。
As shown in Table 5, the incorporation of SRA reduced the surface tension of a solution that simulates concrete pore solution from 63.8 to 27.5 dynes/cm. The reduction of the surface tension can destabilize air voids which resulted in reduced air content and increased density of the ULCC. Increased density of foam lightweight concrete by SRA was also mentioned by Chindaprasirt and Rattanasak [35].
如表 5 所示，SRA 的加入将模拟混凝土孔隙溶液的溶液表面张力从 63.8 降低到 27.5 达因/厘米。表面张力的降低会破坏气孔的稳定性，从而导致 ULCC 的空气含量减少和密度增加。Chindaprasirt 和 Rattanasak [35]也提到了 SRA 通过增加泡沫轻质混凝土的密度。

Table 5. Effect of SRA on surface tension of the solutions and contact angle between the fibers and the solutions with and without SRA.
表 5. SRA 对溶液表面张力的影响以及纤维与有、无 SRA 溶液之间的接触角

Solutions that simulate pore solutions
模拟孔隙溶液的溶液		Surface tension (dynes/cm)
表面张力（达因/厘米）		Advancing contact angle (°)
接触角（度）
Steel fiber  钢纤维PE fiber  聚乙烯纤维PP fiber [28]  聚丙烯纤维 [28]
Without SRA  没有 SRA63.885.675.085.1
With SRAa  配筋率 0#27.533.123.439.9
a
Dosage of SRA is 7.14% by mass of water.
SRA 的用量为水的 7.14%。
The contact angle is directly proportional to the surface tension according to Young’s equation [36]. As shown in Table 5, the SRA reduced the contact angles between the fibers and the solution that simulates concrete pore solution substantially. The reductions in the contact angles indicate better wettability of the fibers in the fresh mixtures, which would result in better contact between the fibers and hardened matrices. This may contribute to the increased density of the interfacial transition zone between the fibers and surrounding matrices.
根据杨氏方程[36]，接触角与表面张力成正比。如表 5 所示，SRA 显著降低了纤维与模拟混凝土孔隙溶液之间的接触角。接触角的减小表明纤维在新鲜混合物中的润湿性更好，这将导致纤维与硬化基体之间接触更紧密。这可能有助于增加纤维与周围基体之间界面过渡区的密度。

3.2. Flexural performance
3.2. 弯曲性能

Load–deflection curves of the ULCC with 0.5% fibers without and with SRA are shown in Fig. 8, Fig. 9, respectively. All the curves are plotted with dual-Y axis (load and flexural strength). Flexural parameters derived from these curves according to ASTM C1609 are summarized in Table 6. Flexural performance of the fiber-reinforced ULCC was characterized by first-peak strength (f1) prior to the formation of matrix crack (i.e. pre-crack behavior) and peak strength (fp) in the post-crack stage if deflection-hardening was observed. Deflection hardening was defined for mixtures with fp greater than f1, and was a phenomenon where the load required to overcome pull-out resistance of the fibers was greater than the flexural capacity of the matrix. Flexural toughness T150100 was obtained from the area under the load–deflection curve up to 2 mm in deflection to characterize the fiber-reinforced ULCC. Residual strengths at deflections of 0.5 mm and 2 mm are also given in Table 6.
含有 0.5%纤维的 ULCC 在无 SRA 和有 SRA 情况下的荷载-变形曲线分别如图 8 和图 9 所示。所有曲线均采用双 Y 轴（荷载和弯曲强度）绘制。根据 ASTM C1609 标准，从这些曲线中推导出的弯曲参数汇总于表 6。纤维增强 ULCC 的弯曲性能由形成基体裂缝前的首次峰值强度（f 1 ，即预裂缝行为）和观察到变形硬化时的峰值强度（f p ）来表征。变形硬化是指当 f p 大于 f 1 的混合料中，克服纤维拔出阻力所需的荷载大于基体的弯曲承载力。弯曲韧性 T150100 是通过荷载-变形曲线在变形量为 2 mm 时的面积来获得，以表征纤维增强 ULCC。表 6 中也给出了 0.5 mm 和 2 mm 变形时的残余强度。
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Fig. 8. Average load–deflection curves of fiber reinforced ULCC without SRA.
图 8. 无 SRA 的纤维增强超轻水泥基复合材料平均荷载-变形曲线

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Fig. 9. Average load–deflection curves of fiber reinforced ULCC with SRA.
图 9. 纤维增强超轻水泥复合材料（SRA）的平均荷载-变形曲线。

Table 6. Flexural parameters (according to ASTM C1609).
表 6. 弯曲参数（根据 ASTM C1609）。

Mix ID  混合物 IDP1 (kN)Pp (kN)δ1 (mm)δp (mm)f1 (MPa)fp (MPa)P600100 (kN)f600100 (MPa)P150100 (kN)f150100 (MPa)T150100 (J)
Plain  普通18.26–0.08–5.48–––––0.75
ST-5013.10–0.07–3.93–11.263.388.782.6318.81
HY-114.58–0.07–4.37–10.873.268.852.6617.50
HY-214.17–0.08–4.25–9.352.807.182.1516.18
HY-312.67–0.06–3.80–8.562.576.672.0016.01
PE-5015.10–0.08–4.53–11.643.498.942.6820.95

Plain_SRA20.38–0.07–6.11–––––0.85
ST-50_SRA20.7226.620.070.996.228.0025.517.6523.757.1248.78
HY-1 _SRA19.8722.600.060.705.966.8021.726.5220.076.0241.71
HY-2_SRA21.82–0.07–6.55–20.066.0216.965.0937.42
HY-3_SRA19.81–0.07–5.94–17.945.3814.984.4932.68
PE-50_SRA20.04–0.07–6.01–15.854.7511.623.4928.11
ST-37.5_SRA19.90–0.07–5.97–18.745.3917.004.8936.51
P1 – first-peak load; Pp – peak load; δ1 – net deflection at first-peak load; δp – net deflection at peak load; f1 – first-peak flexural strength; fp – peak flexural strength; P600100, P150100 – residual loads at net deflections of L/600 (0.5 mm in this study) or L/150 (2 mm in this study), respectively; L – Span length (300 mm in this study); f600100, f150100 – residual strength at net deflections of L/600 or L/150, respectively; T150100 – flexural toughness (area under load–deflection curve up to a deflection at 2 mm).
P 1 – 首次峰值荷载；P p – 峰值荷载；δ 1 – 首次峰值荷载下的净变形；δ p – 峰值荷载下的净变形；f 1 – 首次峰值弯曲强度；f p – 峰值弯曲强度； P600100 , P150100 – 分别为 L/600（本研究中为 0.5 mm）或 L/150（本研究中为 2 mm）净变形时的残余荷载；L – 跨度长度（本研究中为 300 mm）； f600100 , f150100 – 分别为 L/600 或 L/150 净变形时的残余强度； T150100 – 弯曲韧性（荷载-变形曲线在 2 mm 变形时的面积）。

3.2.1. Effect of shrinkage reducing admixture
3.2.1. 减少收缩剂的影响

For the ULCC without SRA, the plain mixture without fiber had the highest first-peak strength f1 of 5.5 MPa compared with those with fibers. This was probably also related to the increase in the entrapped air in the fiber-reinforced ULCC discussed in Section 3.1. The plain ULCC failed after the f1 was reached, whereas the fiber-reinforced ULCC exhibited load-carrying capacities after the matrix crack due to crack-bridging by the fibers. However, the post-crack behavior among the five mixtures with different types and combinations of the fibers was not significantly different (Fig. 8).
对于不含 SRA 的 ULCC，不含纤维的普通混合料在纤维混合料中具有最高的第一峰值强度 f 1 ，为 5.5 MPa。这可能与第 3.1 节中讨论的纤维增强 ULCC 中夹带的空气增加有关。普通 ULCC 在达到 f 1 后失效，而纤维增强 ULCC 由于纤维的裂缝桥接作用，在基体裂缝后仍表现出承载能力。然而，五种不同纤维类型和组合混合料的裂缝后行为没有显著差异（图 8）。
For the ULCC with SRA, however, all the mixtures had similar first peak strength f1 of 6.25 ± 0.30 MPa (Table 6) regardless of the fiber types and combinations. The performance of the fiber-reinforced mixtures after the first crack seems to be affected by the types and combinations of the fibers substantially (Fig. 9), and two mixtures with high steel fiber contents (ST-50_SRA and HY-1_SRA) showed deflection-hardening behavior. The effect of the fibers will be discussed in more details in Section 3.2.2.
然而，对于添加了 SRA 的 ULCC，所有混合料的初始峰值强度 f 1 均为 6.25 ± 0.30 MPa（表 6），无论纤维类型和组合如何。纤维增强混合料在出现第一条裂缝后的性能似乎受到纤维类型和组合的显著影响（图 9），其中两种高钢纤维含量的混合料（ST-50_SRA 和 HY-1_SRA）表现出挠度硬化行为。纤维的影响将在 3.2.2 节中详细讨论。
The effect of SRA on the flexural performance of the ULCC is more clearly illustrated in Fig. 10. From each figure, it is apparent that the mixtures with SRA had higher first peak strength and better post crack performance than that without SRA. The incorporation of the SRA improved the flexural toughness of the fiber-reinforced ULCC substantially compared with that of the control mixtures without SRA. The higher first peak strength of the ULCC with SRA might be attributed mainly to the increased density of the matrix due to the reduced surface tension of the solution and reduced air content. However, their better post crack performance might be attributed to the reduced contact angle between the fibers and solution which led to denser transition zone at the fiber–matrix interface and increased pull-out resistance of the fibers. The incorporation of the SRA also increased residual strength of the ULCC which indicates better load-carrying capacity of the composites after cracking. However, the extent of increase in the residual strength at a given deflection was affected by the type and combination of the fibers used. The reason will be discussed in the next section.
SRA 对 ULCC 弯曲性能的影响在图 10 中更为明显。从每张图中可以看出，添加 SRA 的混合料具有更高的首次峰值强度和更好的裂缝后性能，而没有添加 SRA 的混合料则相对较差。与不添加 SRA 的对照组相比，SRA 的加入显著提高了纤维增强 ULCC 的弯曲韧性。ULCC 添加 SRA 后更高的首次峰值强度可能主要归因于溶液表面张力降低和空气含量减少导致的基体密度增加。然而，它们更好的裂缝后性能可能归因于纤维与溶液之间接触角的减小，这导致了纤维-基体界面过渡区更致密，并增加了纤维的拔出阻力。SRA 的加入还提高了 ULCC 的残余强度，这表明裂缝后复合材料具有更好的承载能力。然而，在给定变形下残余强度的增加程度受所用纤维类型和组合的影响。这一点将在下一节中讨论。
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Fig. 10. Comparisons of load–deflection curves of the ULCC with and without SRA.
图 10. 带有和没有 SRA 的 ULCC 的荷载-变形曲线比较。

3.2.2. Effect of fiber types and combinations
3.2.2. 纤维类型和组合的影响

In the ULCC without SRA, there was no clear trend on the effect of types and combinations of the fibers on their flexural performance. This was due to the fact that the performance of the lightweight composites is affected by air void content of the composites. The ULCC with SRA, however, had comparable density, compressive strength, and first-peak flexural strength (f1) regardless of the type and combination of the fibers used. This indicates that the fiber type and combination did not have significant effect on the matrix of the ULCC with SRA. However, the flexural behavior of the ULCC with SRA after the first peak seems to be affected by the fibers significantly (Fig. 9). The results indicate that the steel fibers were more efficient in enhancing the flexural performance of the ULCC than the PE fibers. As shown in Fig. 9, the load-carrying capacity of the ULCC after the first peak seems to be increased with the increase in the steel fiber content. The flexural toughness T150100 of the ULCC was also increased with the increase in steel fiber content (Table 6).
在不含 SRA 的 ULCC 中，纤维类型和组合对其弯曲性能的影响没有明显趋势。这是因为轻质复合材料的性能受复合材料中气孔含量的影响。然而，含 SRA 的 ULCC 无论使用何种类型的纤维组合，其密度、抗压强度和首次峰值弯曲强度（f 1 ）均相当。这表明纤维类型和组合对含 SRA 的 ULCC 基体没有显著影响。然而，含 SRA 的 ULCC 在首次峰值后的弯曲行为似乎受纤维影响显著（图 9）。结果表明，钢纤维比 PE 纤维更有效地提高了 ULCC 的弯曲性能。如图 9 所示，ULCC 在首次峰值后的承载能力似乎随着钢纤维含量的增加而提高。ULCC 的弯曲韧性 T150100 也随着钢纤维含量的增加而提高（表 6）。
Two mixtures with higher steel fiber contents (ST-50_SRA and HY-1_SRA) exhibited deflection-hardening behavior, although the total fiber content was only 0.5% by volume of the ULCC. Furthermore, specimens of the mixture ST-50_SRA displayed multiple cracks in the flexural test, which indicates higher energy absorption capacity than those with a single crack. These might be related to the bond between the fibers and matrix as the matrix of the various ULCC mixtures with SRA was similar.
两种钢纤维含量较高的混合物（ST-50_SRA 和 HY-1_SRA）表现出挠曲硬化行为，尽管总纤维含量仅为 ULCC 体积的 0.5%。此外，ST-50_SRA 混合物的试件在挠曲试验中显示出多条裂缝，这表明其能量吸收能力高于只有单条裂缝的试件。这可能与其纤维与基体的粘结有关，因为所有含有 SRA 的 ULCC 混合物的基体相似。
Table 5 shows that the SRA reduced contact angle between the steel fibers and solution from 85.6° to 33.1°, and that between the PE fibers and solution from 75.0° to 23.4°. For polypropylene (PP) fiber investigated in a previous study [28], the same type and dosage of SRA reduced the contact angle from 85.1° to 39.9°. With SRA, the contact angle between the fibers and pore solution is in an order of (PE fiber) < (steel fiber) < (PP fiber). The lower contact angle would increase wettability of the fibers and increase the density of the transition zone between the fiber and matrix. From this point of view, the interface transition zone between the PE fibers and matrix would be better than that between the steel fibers and matrix due to the addition of SRA. However, it was observed from Fig. 10 that the difference between the flexural performance of the ULCC with and without SRA after the first peak was increased with the increase in the steel fiber content. For example, T150100 of ST-50_SRA was almost 2.6 times that of ST-50, whereas T150100 of PE-50_SRA was only 1.3 times that of PE-50. This might be related to brass coating on the steel fibers which affected the bond strength between the fibers and cement paste matrix.
表 5 显示，SRA 将钢纤维与溶液的接触角从 85.6°降低至 33.1°，将聚乙烯（PE）纤维与溶液的接触角从 75.0°降低至 23.4°。对于先前研究中研究的聚丙烯（PP）纤维[28]，相同类型和剂量的 SRA 将接触角从 85.1°降低至 39.9°。在 SRA 的作用下，纤维与孔溶液的接触角顺序为（PE 纤维）<（钢纤维）<（PP 纤维）。较低的接触角会增加纤维的润湿性，并增加纤维与基体之间的过渡区密度。从这个角度来看，由于添加了 SRA，PE 纤维与基体之间的界面过渡区会优于钢纤维与基体之间的界面过渡区。然而，从图 10 可以看出，随着钢纤维含量的增加，ULCC 在第一个峰值后与不添加 SRA 的 ULCC 的弯曲性能差异增大。例如，ST-50_SRA 的 T150100 值几乎是 ST-50 的 2.6 倍，而 PE-50_SRA 的 T150100 值仅为 PE-50 的 1.3 倍。 这可能与此有关，即钢纤维上的黄铜涂层影响了纤维与水泥基体之间的结合强度。
Chan and Li [37] conducted single fiber pull-out tests, and found out that the bond strength between brass fibers and cement paste is roughly 7 times of that between PE fibers and the same cement paste. The PE fibers they used were the same as those used in this study. They attributed the significant different bond strengths to different bond failure modes, and suggested “cohesive” bond failure for brass-cement paste system (torturous failure path through transition zone) and “adhesive” bond failure for PE-cement paste system (smooth failure path through fiber–matrix interface). According to Khalaf and Page [38], the bond of brass with cement paste is mainly due to the chemicophysical properties of brass that allows chemical reactions to occur while in contact with cement material. Chan and Li [37] further suggest that the densification of the transition zone may not be effective in improving bond strength if the bond failure is governed by adhesive failure at the interface, whereas bond strength improvement can be achieved by densification of the transition zone if the bond failure is governed by cohesive failure in the transition zone. The results shown in Fig. 10 indicate that the effect of brass coating on the steel fibers in combination with the reduced contact angle due to the use of SRA contributed to the deflection hardening behavior of the ULCC with only 0.5% fibers (ST-50_SRA).
Chan 和 Li [37]进行了单纤维拔出试验，发现黄铜纤维与水泥浆之间的粘结强度大约是聚乙烯纤维与相同水泥浆之间粘结强度的 7 倍。他们所使用的聚乙烯纤维与本研究中使用的相同。他们将粘结强度的显著差异归因于不同的粘结破坏模式，并建议黄铜-水泥浆系统为“粘聚”型粘结破坏（通过过渡区的曲折破坏路径），而聚乙烯-水泥浆系统为“粘着”型粘结破坏（通过纤维-基体界面的平滑破坏路径）。根据 Khalaf 和 Page [38]的研究，黄铜与水泥浆的粘结主要是由于黄铜的理化性质，使其在与水泥材料接触时能够发生化学反应。Chan 和 Li [37]进一步指出，如果粘结破坏是由界面处的粘着破坏控制的，那么过渡区的致密化可能不会有效提高粘结强度；而如果粘结破坏是由过渡区内的粘聚破坏控制的，那么通过过渡区的致密化可以提高粘结强度。图中的结果表明... 10 表明黄铜涂层对钢纤维的影响，以及由于使用 SRA 而降低的接触角，共同促成了仅含 0.5%纤维的 ULCC（ST-50_SRA）的挠曲硬化行为。
From Fig. 10, it seems that the SRA was not as effective in enhancing the flexural performance of the ULCC with PE fibers after the matrix crack compared to that with the steel fibers. This might be due to the different failure modes between fibers and matrix as described by Chan and Li [37]. Lower effect of the SRA on the flexural performance after the matrix crack was also reported for normal weight mortar with polypropylene (PP) fibers in comparison to that with the steel fibers [28]. In that research, the addition of SRA improved the toughness of steel fiber reinforced mortar by 51%, whereas the improvement for the PP fiber reinforced mortar was only 16%.
从图 10 可以看出，在基体开裂后，SRA 对聚乙烯纤维增强超轻骨料混凝土的弯曲性能提升效果不如钢纤维增强的混凝土。这可能是由于纤维和基体之间不同的破坏模式，正如 Chan 和 Li[37]所描述的那样。与钢纤维相比，聚丙烯（PP）纤维增强普通重骨料混凝土在基体开裂后 SRA 对弯曲性能的影响也较低[28]。在该研究中，SRA 的添加使钢纤维增强砂浆的韧性提高了 51%，而聚丙烯纤维增强砂浆的提高仅为 16%。

3.2.3. Synergy between steel and PE fibers on flexural toughness of ULCC
3.2.3. 钢纤维和聚乙烯纤维对超轻骨料混凝土弯曲韧性的协同作用

To investigate the synergy between the steel and PE fibers in the fiber-reinforced ULCC, flexural toughness was calculated for the mixtures HY-1_SRA, HY-2_SRA and HY-3_SRA, and compared with that determined from the experiments (Table 7). The flexural toughness was calculated according to the relative contributions of the steel and PE fibers in each mixture based on T150100 of the mixtures with 0.5% steel fibers (ST-50_SRA) and 0.5% PE fibers (PE-50_SRA). The calculation was based on the assumptions that these fibers contribute independently to the toughness which is increased linearly with the fiber dosage up to 0.5% by volume. For example, for the HY-1_SRA mixture with 0.375% steel fibers and 0.125% PE fibers, the flexural toughness was calculated as 48.78*(3/4) + 28.11*(1/4) = 43.61 J. The values of 48.78 and 28.11 J are the flexural toughness (T150100) of ST-50_SRA and PE-50_SRA, respectively, from Table 6. The assumption that the flexural toughness is increased linearly with the fibers is partially verified by the mixture containing 0.375% steel fibers (ST-37.5_SRA). As shown in Table 7, the experimentally determined and calculated flexural toughness of the mixture ST-37.5_SRA are almost the same.
为研究纤维增强超轻水泥混凝土（ULCC）中钢纤维和聚乙烯纤维的协同作用，计算了混合物 HY-1_SRA、HY-2_SRA 和 HY-3_SRA 的弯曲韧性，并与实验测定值进行了比较（表 7）。弯曲韧性是根据每种混合物中钢纤维和聚乙烯纤维的相对贡献计算的，基于 0.5%钢纤维（ST-50_SRA）和 0.5%聚乙烯纤维（PE-50_SRA）的混合物 T150100 。计算基于以下假设：这些纤维独立地贡献于韧性，并且随着纤维用量的增加（体积分数）至 0.5%，韧性线性增加。例如，对于含有 0.375%钢纤维和 0.125%聚乙烯纤维的 HY-1_SRA 混合物，其弯曲韧性计算为 48.78 * (3/4) + 28.11 * (1/4) = 43.61 J。48.78 和 28.11 J 分别是表 6 中 ST-50_SRA 和 PE-50_SRA 的弯曲韧性（ T150100 ）。 认为弯曲韧性随纤维含量线性增加的假设在含有 0.375%钢纤维（ST-37.5_SRA）的混合物中得到了部分验证。如表 7 所示，混合物 ST-37.5_SRA 的实验测定和计算弯曲韧性几乎相同。

Table 7. Flexural toughness calculation.
表 7. 弯曲韧性计算

Mix ID  混合物 IDFiber (vol%)  纤维（体积%）Experimentally determined flexural toughness (J)
实验确定的弯曲韧性（J）		Calculated flexural toughness (J)
计算得到的弯曲韧性（J）
Steel  钢PE
ST-50_SRA0.5048.78–
PE-50_SRA00.528.11–
HY-1 _SRA0.3750.12541.7143.61
HY-2_SRA0.250.2537.4238.45
HY-3_SRA0.1250.37532.6833.28
ST-37.5_SRA0.375036.5136.59
The calculated flexural toughness of the HY-1_SRA, HY-2_SRA and HY-3_SRA are slightly higher than the experimentally determined flexural toughness (T150100). This indicates that there is no synergy between steel fiber and PE fiber on the flexural toughness in the ULCC with 0.5% fiber.
HY-1_SRA、HY-2_SRA 和 HY-3_SRA 的计算弯曲韧性略高于实验测定的弯曲韧性（ T150100 ）。这表明在 0.5%纤维含量的超轻水泥复合材料中，钢纤维和聚乙烯纤维之间在弯曲韧性方面没有协同作用。

3.2.4. Ultra lightweight cement composites
3.2.4. 超轻质水泥复合材料

The results of this experimental study indicate that ultra lightweight cement composites with good flexural performance can be produced with only 0.5% fibers in combination with SRA. Specifically, with 0.5% steel fibers by ULCC volume and 2.5% SRA by mass of cementitious materials, an ULCC (ST-50_SRA) was obtained with low density of 1474 kg/m3, high compressive strength of 68.2 MPa, and good toughness T150100 of 48.78 J. This ULCC also showed deflection hardening behavior with peak flexural strength of 8 MPa and multiple cracking when tested in flexure.
这项实验研究的结果表明，通过结合 SRA，仅使用 0.5%的纤维即可生产出具有良好弯曲性能的超轻质水泥复合材料。具体来说，当 ULCC 体积中钢纤维含量为 0.5%，水泥基材料质量中 SRA 含量为 2.5%时，获得了密度为 1474 kg/m³的 ULCC（ST-50_SRA），其抗压强度高达 68.2 MPa，韧性良好，为 48.78 J。这种 ULCC 在弯曲测试中也表现出挠度硬化行为，其峰值弯曲强度为 8 MPa，并出现多重开裂。
Fig. 11 compares load–deflection curves of the ULCC (ST-50_SRA) with the HSM with natural sand. The HSM had 28-day compressive strength of 88 MPa and flexural strength of 9.4 MPa, which were higher than those of the ULCC. In the post-peak region, however, the ULCC had higher load-carrying capacity and showed deflection hardening behavior. The residual strength at 2-mm deflection of the ULCC was 7.12 MPa, whereas that of the HSM was only 4.10 MPa. These may be attributed to the distribution of fibers which is affected by the maximum aggregate size used. In fiber-reinforced composites, larger maximum aggregate size results in greater interaction among the fibers [39], [40]. The HSM had maximum aggregate size of 4.75 mm, much larger than 0.30 mm of the cenospheres used in the ULCC. Thus, the fibers can be distributed more homogeneously in the ULCC, which probably contributed to the better post-peak behavior in the flexural test. The higher 28-day compressive strength and flexural strength of the HSM may be attributed to the higher strength of the natural sand than cenospheres used in the ULCC. According to manufacturer’s datasheet, the crush strength of the cenospheres was only about 40 MPa.
图 11 比较了 ULCC（ST-50_SRA）与使用天然砂的 HSM 的荷载-变形曲线。HSM 的 28 天抗压强度为 88 MPa，抗折强度为 9.4 MPa，这些指标均高于 ULCC。然而，在峰值后区域，ULCC 具有更高的承载能力，并表现出变形硬化行为。ULCC 在 2 mm 变形时的残余强度为 7.12 MPa，而 HSM 的残余强度仅为 4.10 MPa。这可能是由于纤维分布受到所用最大骨料尺寸的影响。在纤维增强复合材料中，更大的最大骨料尺寸会导致纤维间相互作用增强[39][40]。HSM 的最大骨料尺寸为 4.75 mm，远大于 ULCC 中使用的 cenospheres 的 0.30 mm。因此，ULCC 中的纤维分布更加均匀，这可能是其弯曲试验中峰值后行为更好的原因。HSM 具有更高的 28 天抗压强度和抗折强度，这可能是由于天然砂的强度高于 ULCC 中使用的 cenospheres。 根据制造商的数据表，微球的抗压强度只有大约 40 MPa。
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Fig. 11. Comparisons of load–deflection curves of fiber reinforced ULCC and high strength mortar (HSM).
图 11. 纤维增强 ULCC 和高强度砂浆（HSM）的荷载-变形曲线比较

For structural applications, drying shrinkage strain and creep behavior of the ULCC are being investigated and will be presented in a future paper.
对于结构应用，正在研究 ULCC 的干燥收缩应变和蠕变行为，并将在一篇未来的论文中呈现。

4. Conclusions  4. 结论

Based on the experimental results, the following conclusions are drawn:
根据实验结果，得出以下结论：
  • (1)
    Ultra lightweight cement composite (ULCC) with low density of 1474 kg/m3, high compressive strength of 68.2 MPa, high flexural strength of 8 MPa, and deflection hardening behavior was developed by using only 0.5% steel fibers in combination with shrinkage reducing admixture (SRA).
    密度为 1474 kg/m³的超轻质水泥复合材料（ULCC），具有 68.2 MPa 的高抗压强度、8 MPa 的高弯曲强度，并表现出挠度硬化行为，这是通过仅使用 0.5%的钢纤维与收缩减少剂（SRA）结合而开发的。
  • (2)
    Such good performance of the ULCC could be attributed primarily to the use of SRA which reduced entrapped air content in the matrix, densified fiber–matrix transition zone, and increased pull-out resistance of the fibers in the ULCC.
    ULCC 的这种优良性能主要归因于 SRA 的使用，它减少了基体中的夹杂物含量，密实了纤维-基体过渡区，并增加了 ULCC 中纤维的拔出阻力。
  • (3)
    Although the SRA improved the fiber–matrix interface, the consequent improvement on the flexural performance of the ULCC depends on the type of the fibers used and bond between the fibers and matrix. Improvement of the flexural performance of the steel fiber (coated with brass) reinforced ULCC due to the densification effect by SRA was more significant than that of the PE fiber reinforced ULCC.
    尽管 SRA 改善了纤维-基体界面，但 ULCC 的弯曲性能的后续改善取决于所用纤维的类型以及纤维与基体之间的结合。由于 SRA 的致密化效应，钢纤维（镀黄铜）增强 ULCC 的弯曲性能改善比 PE 纤维增强 ULCC 更为显著。
  • (4)
    For different combinations of fibers, the PE fibers and steel fibers seem to contribute independently to the flexural toughness which is increased linearly with the fiber dosage up to 0.5% by volume. At a fiber dosage of 0.5%, no further improvement of the flexural performance was observed when combinations of the PE and steel fibers were used in the ULCC.
    对于不同的纤维组合，聚乙烯纤维和钢纤维似乎独立地对弯曲韧性做出贡献，其随着纤维体积含量的增加而线性增加，直到 0.5%。当纤维含量为 0.5%时，在 ULCC 中使用聚乙烯纤维和钢纤维的组合并未观察到弯曲性能的进一步改善。
  • (5)
    The ULCC had lower first peak strength but better post-peak behavior compared with natural sand mortar of comparable water/binder ratio, SRA and silica fume dosage, and fiber type and content.
    与具有相同水胶比、SRA 和硅灰掺量以及纤维类型和含量的天然砂砂浆相比，ULCC 的初始峰值强度较低，但峰值后的性能更好。

Acknowledgments  致谢

Grateful acknowledgement is made to A*STAR, Singapore Science and Engineering Research Council, Maritime and Port Authority of Singapore, American Bureau of Shipping, and National University of Singapore for funding this research. The authors would also like to thank undergraduate student Mr. Meas Sophea for the assistance of experimental work and laboratory technologist Mr. Ow Weng Moon for the ASTM C1609 tests.
对新加坡 A*STAR 机构、新加坡科学工程研究委员会、新加坡海事及港口管理局、美国船级社以及新加坡国立大学表示感谢，感谢它们为本研究提供了资金支持。作者们还感谢本科生 Meas Sophea 先生在实验工作中的协助，以及实验室技术员 Ow Weng Moon 先生进行的 ASTM C1609 测试。

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