不同龄期钢纤维增强水泥砂浆纤维拉拔试验与模拟研究

钢纤维增强水泥基复合材料作为一种多相复合材料,其增强增韧效果的发挥依赖于钢纤维与基体之间的界面粘结性能。通过开展不同龄期的钢纤维增强水泥基复合材料单根纤维拉拔试验及数值模拟研究,分析了龄期对钢纤维增强水泥砂浆界面粘结性能的影响,建立了不同龄期的单根纤维拉拔细观模型,通过将模拟结果与试验结果进行对比验证模型的有效性。根据所建立的细观模型分别对不同龄期钢纤维增强水泥砂浆纤维-基体间的界面粘结作用机理及纤维-基体间粘结表面在纤维拔出过程中的应力变化进行了分析。结果表明:所建立细观模型模拟得到的纤维最大拉拔力及荷载-滑移曲线与试验结果吻合较好,钢纤维的最大拉拔力及钢纤维-水泥砂浆基体的界面粘结强度均随着龄期的增加而增加;在7 d龄期内界面粘结强度的增长速度较快,7 d龄期后增长速度放缓;随着龄期的增加,不同龄期段的界面粘结强度的增长率逐渐减小并趋于稳定。采用拟合得到的粘结表面材料参数能够有效模拟各龄期下单根钢纤维从水泥砂浆中的拔出过程。     

Effects of styrene–acrylic emulsion on bond performance and interface microstructure of repair mortar of ternary cementitious system

This study investigates the effects of styrene–acrylic emulsion (SAE) as a modifier and interfacial agent on the interfacial bond performance of ordinary Portland cement–aluminate cement–gypsum (PAG) repair mortar. The hydration products and interfacial microstructure are analyzed via x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The results demonstrate that the addition of SAE can effectively enhance the tensile bond strength and flexural bond strength of the PAG repair mortar, and the optimal addition amount of SAE is 10 wt%. The tensile and flexural bond strengths of the PAG repair mortar with SAE interfacial agent at 40% concentration are 1.38 and 1.34 times than those of the mortar without the interfacial agent, respectively. XPS and FTIR analyses reveal that the carboxyl groups in SAE and Ca²⁺ generated from cement hydration form Ca²⁺–carboxyl complexes. The SEM and XRD analyses indicate that SAE can alter the distribution and size of crystals at the bond interface and considerably reduce the thickness of the bond interface; however, SAE cannot change the type of hydration products at the bond interface.     

Bonding behavior of concrete matrix and alkali-activated mortar incorporating nano-SiO2 and polyvinyl alcohol fiber: Theoretical analysis and prediction model

As an emerging construction material, alkali-activated mortar is considered as a sustainable alternative to cementitious composites for the repairing and reinforcement of existing defective buildings. Furthermore, the bonding performance of alkali-activated mortar and concrete matrix can be promoted by adding polyvinyl alcohol (PVA) fiber and nano-SiO₂ (NS). In this study, the effects of PVA fiber and NS contents, alkali-activated mortar type, concrete strength grade, and interfacial roughness on the bonding behavior of two-interfaced shear samples were explored. Based on the experimental results, the grey relation analysis was applied to evaluate the significance of each factor on the bond properties of the alkali-activated mortar and concrete matrix. A prediction model of artificial neural network (ANN) was established considering the effects of alkali-activated mortar type, concrete strength grade, and interfacial type on the bond strength of the samples. The relevant factors affecting bond strength derived by grey relation analysis and weight contribution algorithm was compared and analyzed. Results of the two-interfaced shear test showed that the addition of PVA fiber and NS can significantly boost the bonding property of the samples, and the bond strength increases with the increase of concrete strength grade, alkali-activated mortar strength, and interfacial roughness. Grey relation analysis results indicate that the interfacial type has the most noticeable effect on the bond strength of the samples, followed by the concrete strength grades and the alkali-activated mortar types. The optimum bond strength is derived from PN-C40-III, which is alkali-activated mortar with 0.6% PVA fiber and 1.0% NS contents, concrete strength grade of C40, and interface of type III. The prediction results of the ANN indicate that the predicted values of the bond strengths of the samples are consistent with the experimental values (R = 0.982), and the importance of each factor towards the bond behavior derived by the grey relation analysis and weight contribution algorithm is ultimately consistent after normalization.     

涂层对速凝3D打印水泥砂浆力学性能的影响

3D打印砂浆普遍存在层间界面粘结较弱、抗折强度较低的问题,而膨胀水泥浆涂层涂覆于砂浆表面可产生表层压应力,进而提升其抗折强度,作为界面剂涂覆于层间可同时增强其界面粘结强度。本文通过将硫铝酸盐水泥与膨胀剂混合而成的涂层涂覆于速凝3D打印砂浆的表面与层间,研究了该涂层对不掺纤维及掺0.5%(体积分数)玄武岩纤维3D打印砂浆试件力学性能的影响规律。结果表明,涂覆层间涂层对无纤维与掺纤维速凝3D打印砂浆的层间界面粘结强度提升率分别为21.4%、12.2%,同时对其抗压强度也有一定提升作用。仅涂覆表面涂层与仅涂覆层间涂层对3D打印砂浆的抗折强度提升效果相当;同时涂覆表面及层间涂层对3D打印砂浆的抗折强度提升效果最显著,与无纤维、掺纤维的无涂层试件相比,提升率最高分别可达44.2%、23.2%。涂层对于无纤维3D打印试件层间粘结强度及抗折强度的提升效果优于掺纤维试件。     

The effect of matrix composition on fibre/matrix interfacial bond shear strength in fibre-reinforced mortar

The effects of factors associated with the composition of the matrix, i.e. curing conditions and time and mix proportions, on the shear strength of the interfacial bond between steel fibres and a cementitious mortar matrix have been examined experimentally using a single-fibre pull-out test technique. The experimental results indicate that bond shear strength increases significantly with an increase in matrix curing time and, for specimens with the fibre axis perpendicular to the direction of casting and compaction of the matrix, with a decrease in the proportion of water by weight in the matrix mortar. This latter effect is attributed to bleed water gain under the embedded fibre, as it is not observed in specimens with the fibre axis parallel to the direction of casting and compaction of the matrix. Furthermore, the results indicate that there is no correlation between interfacial bond shear strength and matrix mortar compressive strength.     

Application of nanoindentation testing to study of the interfacial transition zone in steel fiber reinforced mortar

The characteristics of the profiles of elastic modulus and hardness of the steel fiber-matrix and fiber-matrix-aggregate interfacial zones in steel fiber reinforced mortars have been investigated by using nanoindentation and Scanning Electron Microscopy (SEM), where two sets of parameters, i.e. water/binder ratio and content of silica fume were considered. Different interfacial bond conditions in the interfacial transition zones (ITZ) are discussed. For sample without silica fume, efficient interfacial bonds across the steel fiber-matrix and fiber-matrix-aggregate interfaces are shown in low water/binder ratio mortar; while in high water/binder ratio mortar, due to the discontinuous bleeding voids underneath the fiber, the fiber-matrix bond is not very good. On the other hand, for sample with silica fume, the addition of 10% silica fume leads to no distinct presence of weak ITZ in the steel fiber-matrix interface; but the effect of the silica fume on the steel fiber-matrix-aggregate interfacial zone is not obvious due to voids in the vicinity of steel fiber.     

减缩剂改性新老混凝土修补界面层的细观结构与粘结强度

针对既有界面剂在新老混凝土修补中的缺点,以国产减缩剂对水泥砂浆界面剂进行改性,配制不同界面剂,分别对花岗岩石板和老砂浆界面进行修补,测试粘结强度,并观察细观结构。对比试验表明,减缩剂可有效改善界面层的宏观性能与细观结构,界面粘结强度显著提高。     

Properties of fly ash-modified cement mortar-aggregate interfaces

This paper investigates the effect of fly ash on strength and fracture properties of the interfaces between the cement mortar and aggregates. The mortars were prepared at a water-to-binder ratio of 0.3, with fly ash replacements from 15 to 55%. Notched mortar beams were tested to determine the flexural strength, fracture toughness, and fracture energy of the plain cement and fly-ash modified cement mortars. Another set of notched beams with mortar-aggregate interface above the notch was tested to determine the flexural strength, fracture toughness, and fracture energy of the interface. Mortar-aggregate interface cubes were tested to determine the splitting strength of the interface. It was found that a 15% fly ash replacement increased the interfacial bond strength and fracture toughness. Fly ash replacements at the levels of 45 and 55% reduced the interfacial bond strength and fracture toughness at 28 days, but recovered almost all the reduction at 90 days. Fly ash replacement at all levels studied increased the interfacial fracture energy. Fly ash contributed to the interfacial properties mainly through the pozzolanic effect. For higher percentages of replacement, the development of interfacial bond strength initially fell behind the development of compressive strength. But at later ages, the former surpassed the latter. Strengthening of the interfaces leads to higher long-term strength increases and excellent durability for high-volume fly ash concrete.     

The interfacial bond stress in steel fiber cement composites

In fiber cement composites most fibers are in a state of partial bond due to internal stresses arising from moisture migration during fabrication and subsequent volume changes in the matrix. A wide variation in the computed interfacial bond strength therefore occurs depending upon the type of test or when derived from phenomena such as crack spacing. In practice debonding of the fibers occurs in flexural tension in the presence of a strain gradient. This paper presents further data on steel fiber mortar and concrete to confirm the validity of the composite mechanics approach to predict the composite flexural strength. It is shown that the composition of the matrix and its strength properties influence the fiber-matrix interfacial bond stress and the relative contributions of the matrix and the fibers to the composite flexural strength.     

A way for improving interfacial transition zone between concrete substrate and repair materials

An attempt to modify the repair interfacial transition zone by introducing fly ash into a primer between concrete substrate and repair materials was proposed. A comparison test was carried out for five different bond interfaces coated with five kinds of primers, namely neat cement paste, expansive paste, cement mortar, water-dispersible epoxy resin, and fly ash-modified mortar. The test results showed that the fly ash-modified primer made the microstructure of the repaired interface zone more dense and uniform. As a result, the splitting bond strength of the interface coated with the fly ash-modified primer was significantly higher than those coated with the other kinds of primers.     

Effect of pumice and fly ash incorporation on high temperature resistance of cement based mortars

The effects of high temperature on the mechanical properties of cement based mortars containing pumice and fly ash were investigated in this research. Four different mortar mixtures with varying amounts of fly ash were exposed to high temperatures of 300, 600, and 900 °C for 3 h. The residual strength of these specimens was determined after cooling by water soaking or by air cooling. Also, microstructure formations were investigated by X-ray and SEM analyses.Test results showed that the pumice mortar incorporating 60% fly ash revealed the best performance particularly at 900 °C. This mixture did not show any loss in compressive strength at all test temperatures when cooled in air. The superior performance of 60% FA mortar may be attributed to the strong aggregate-cement paste interfacial transition zone (ITZ) and ceramic bond formation at 900 °C. However, all mortar specimens showed severe losses in terms of flexural strength. Furthermore, specimens cooled in water showed greater strength loss than the air cooled specimens. Nevertheless, the developed pumice, fly ash and cement based mortars seemed to be a promising material in preventing high temperature hazards.     

矿渣掺量对胶砂和混凝土强度的影响

本文研究了矿渣掺量对胶砂强度、混凝土强度和混凝土与钢筋的粘结强度的影响.在水泥胶砂或粉煤灰-水泥胶砂中,用矿渣取代部分水泥后,胶砂3d强度会降低,且随取代量增加,胶砂3d强度逐渐降低.在水泥胶砂或粉煤灰-水泥胶砂中,当矿渣取代水泥量≤55％时胶砂28 d强度会增加,但矿渣取代水泥量至60％时胶砂28 d强度会下降.在水泥混凝土或粉煤灰-水泥混凝土中,当矿渣取代水泥量≦50％时混凝土28 d强度及其与钢筋的粘结强度会提高,但矿渣取代水泥量≥60％时混凝士强度和粘结强度会降低.     

处理刚玉集料对砂浆界面结构的改进

从微观形貌和水化产物、界面电导特性参数以及孔结构等三方面,研究了处理刚玉集料砂浆中界面区的结构特征,并与硅酸盐水泥熱料集料砂浆、标准砂集料砂浆等界面区的结构进行了比较。结果表明,处理刚玉集料在砂浆中发生了水化反应,水化产物对界面区产生致密作用,使得界面区成为砂浆中一个比较致密的区域。     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

Influence of the roughness of aggregate surface on the interface bond strength

An experimental investigation on the bond strength of the interface between mortar and aggregate is reported. Composite compact specimens were used for applying Mode I and Mode II loading effects. The influence of the type of mortar and type of aggregate and its roughness on the bond strength of the interface has been studied. It has been observed that the bond strength of the interface in tension is significantly low, though the mortars exhibited higher strength. The highest tensile bond strength values have been observed with rough concrete surface with M-13 mortar. The bond strength of the interface in Mode I load depends on the type of aggregate surface and its roughness, and the type of mortar. The bond strength of the interface between mortar M-13 cast against rough concrete in direct tension seems to be about one third of the strength of the mortar. However, it is about 1/20th to 1/10th with the mortar M-12 in sandwiched composite specimens. The bond strength of the interface in shear (Mode II) significantly increases as the roughness and the phase angle of the aggregate surface increase. The strength of mortar on the interface bond strength has been very significant. The sandwiched composite specimens show relatively low bond strength in Mode I loading. The behavior of the interface in both Mode I and Mode II loading effects has been brittle, indicating catastrophic failure.     

Effects of polymer impregnation on mortar-aggregate bond strength

Polymer-impregnated concrete (PIC) is recognized as a superior construction material with higher durability and strength than plain concrete. The increase in strength has been thought to be a result of an increase in bond strength between mortar and aggregate phases and a reduction of the porosity of the mortar. However, there has been no direct evidence to support that an enhanced mortar-aggregate bond is achieved. In this study, modified briquet tension specimens were tested to determine effects of polymer-impregnation on tensile bond strength, and prismatic specimens with inclined aggregates were tested to determine effects on compression-shear bond strength. Plain mortar briquets were also tested. Results indicate that polymer impregnation does not significantly improve interface bond strength in PIC. However, mortar tensile strength is increased. A review of the failure process (microcracking) in plain concrete is presented. It is proposed that if the same microcracking process occurs in PIC, then high compressive strength is a result of high mortar tensile strength.     

纤维与水泥砂浆界面黏结性能研究

本文采取“8字”型试件进行单根粗纤维拉拔试验,通过混合正交试验研究了纤维类型、纤维直径、纤维埋置长度及砂浆基体水胶比对纤维-砂浆界面黏结强度显著性优先次序,以及纤维与砂浆间最佳组合的影响。同时得到了纤维最大拔出荷载及荷载-滑移曲线,系统分析了纤维与水泥砂浆的界面黏结性能。试验结果表明,直径为0.6 mm、埋置长度为20 mm的聚丙烯粗纤维,水胶比为0.51的砂浆基体的界面黏结强度最大,其中平均黏结强度为7.71 MPa,等效黏结强度达到13.25 MPa。直径为0.6 mm、埋置长度为10 mm的聚乙烯醇(PVA)纤维,水胶比为0.41的砂浆基体的界面黏结强度次之。四种因素对界面黏结强度的影响优先次序为纤维种类、纤维直径、砂浆水胶比、纤维埋置长度,其中纤维种类对界面平均黏结强度的影响较为显著,纤维直径对界面等效黏结强度的影响较为显著。     

聚丙烯酸酯乳液在水泥砂浆中的应用

为了提高水泥砂浆的韧性,选用了能形成柔性薄膜结构的聚合物--聚丙烯酸酯乳液对水泥砂浆进行改性.研究了聚丙烯酸酯乳液对水泥砂浆体积密度、抗压强度、抗折强度、韧性、动弹模量和黏结抗拉强度的影响.结果表明:聚丙烯酸酯乳液在一定程度上降低了水泥砂浆体积密度和抗压强度,对抗折强度影响较小,改善了水泥砂浆的韧性,并且可提高黏结抗拉强度.当乳液掺量(质量分数)大于5%时,28 d混合养护聚丙烯酸酯乳液水泥砂浆的压折比降低到3以下,只有当乳液掺量大于10%时,水泥砂浆的黏结抗拉强度才明显提高.     

Use of a Crack-Bridging Single-Fiber Pullout Test to Study Steel Fiber/Cementitious Matrix Composites

The fracture process of steel fiber/cementitious matrix composites has been studied using a single-fiber pullout test that permits detailed measurements of the load-crack opening displacement relationship during fiber debonding and unloading. Using a suitable analytical model, the interfacial fracture energy and interfacial sliding friction have been calculated for composites incorporating steel fibers with cement paste or mortar matrices. Comparison of theoretical debonding curves with the experimental data show that the model accurately represents the fiber debonding process, except for a decrease in interfacial sliding friction due to wear of matrix asperities at the interface. Differences between the calculated interfacial properties of several specimens are associated with changes in the interfacial microstructure.     

On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine

Recent work on the strength-enhancing mechanism of triisopropanolamine (TIPA) suggested that TIPA enhances the mechanical properties of mortar and concrete by acting on the interfacial transition zone (ITZ) between paste and sand or aggregate rather than improving the properties of the hydrated binder. This paper presents compressive strength data for 10 Portland cements tested as cement paste as well as two different kinds of mortar after 28 days hydration, so that these two mechanisms could be compared directly. The average strength improvement with TIPA was 10% in the hydrated portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism.