Effects of partial oxidation of crumb rubber on properties of rubberized mortar

It is a generally accepted result that the inclusion of rubber particles causes the concrete to degrade physical properties of the concrete. In this study the crumb rubber was partially oxidized and used as additives of mortars. Dramatically, the compressive strength of the rubberized mortars (with 6 wt.%) was greater than that of mortars without the crumb rubber. To understand these phenomena, Fourier Transform Infrared Spectroscopy (FTIR) was used to explore the functional groups on surfaces of the crumb rubber, and Scanning Electron Microscopy (SEM) was used to observe microstructures of mortars. With the partial oxidation treatment, rubber surfaces produced hydrophilic functional groups as indicated by IR spectra and the hydration of the cement near the crumb rubber was enhanced as shown by SEM, leading to stronger mortars.     

橡胶对水泥基材料干燥收缩性能的影响

为了考察橡胶增加水泥基材料干燥收缩量的机制,以橡胶水泥砂浆作为研究对象,采用毛细管张力理论分析了造成水泥砂浆干燥收缩的因素。使用压汞试验研究橡胶/水泥砂浆的孔结构,并进行了弹性模量和干燥收缩试验。研究结果表明,橡胶掺入会降低水泥砂浆的弹性模量,增加其孔隙率和干燥收缩量,且相同掺量条件下,小粒径橡胶的作用效果更明显。基于试验数据,考虑橡胶掺入对砂浆弹性模量的折减系数KE和橡胶掺入对毛细孔(孔径50nm的孔隙)数量的增加系数K_h,拟合了橡胶对水泥砂浆干燥收缩的影响参数δ_(mr)。     

FRP tube encased rubberized concrete cylinders

In this study, FRP tube encased waste tire rubber modified concrete cylinders were investigated. Four batches of confined and unconfined concrete cylinders with a diameter of 101.6 mm and a height of 304.8 mm were prepared. Each batch contained three confined cylinders and three unconfined cylinders. The total number of effective cylinders prepared was 24. Batches 1–3 were made of rubberized concrete. In Batch 1, 15% by volume of coarse aggregate was replaced by waste tire fibers or stripes; in Batches 2 and 3, 15% by volume of sand and 30% by volume of sand were replaced by crumb rubbers, respectively. Batch 4 was a control batch with conventional plain concrete. Uniaxial compression tests were conducted on all the cylinders per ASTM C39. Strain gages were installed on the encased rubberized concrete cylinders to obtain local strain distributions. The compressive strength and strain at the peak load were compared with the predictions by available design-oriented confinement models. It is found that the FRP tube encased rubberized concrete cylinders have higher confinement effectiveness, ductility, and elastic regions than FRP tube encased conventional concrete cylinders. Waste tire fiber modified concrete performs better than crumb rubber modified concrete with a lower cost. Instead of volume contraction, FRP encased rubberized concrete cylinders experience volumetric expansion. The current design-oriented confinement models cannot consistently predict the compressive strength and strain of the encased cylinders. An 1-D coupon test cannot uniquely determine the hoop tensile strength of the FRP tubes which are subjected to a 2-D stress condition.     

Characteristics of cement-soil mortars

Cement-soil mortars are commonly used for the construction of soil-cement block masonry. The paper focuses on an experimental study in understanding the various characteristics of cement soil mortars in fresh and hardened state. Workability, strength, water retentivity, shrinkage and stress-strain characteristics of cement soil mortars and bond strength of soil-cement block couplets using such mortars are examined. Characteristics of 1:6 cement mortar and 1:1:6 cement lime mortar are also examined for the purposes of comparison. Workability of mortars has been quantified by conducting flow table tests. Results of flow values obtained for mortars from various construction sites are reported. There is a linear relationship between flow and water cement ratio of the mortars. Flow increases with increase in water-cement ratio. Very high flow value of 130% can be achieved for cement soil mortars and cement lime mortars. Reduction in flow value from 100% to 80% leads to increase in strength and modulus of mortars. Clay fraction of the mortar mix controls the flow, strength, density, shrinkage value and modulus of cement soil mortars. Cement-soil mortars lead to better tensile bond strength for soilcement block couplets when compared to the cement mortar and cement lime mortar.     

氨化与磺化改性橡胶混凝土机理及强度研究

为改善橡胶混凝土的强度,本工作选用尿素和NaHSO3两种改性剂对橡胶颗粒进行氨化与磺化改性。借助傅里叶红外光谱仪验证引入的极性亲水基团,利用水接触角试验仪分析改性橡胶的亲水性能,通过粘接强度试验和抗压强度试验研究改性橡胶对橡胶混凝土强度的提升效果,用扫描电子显微镜(SEM)表征橡胶混凝土的微观破坏形貌。结果表明:氨化改性在橡胶颗粒表面引入羰基和氨基等,磺化改性在橡胶颗粒表面引入羟基和磺酸基等;与普通组相比,氨化与磺化改性后,改性橡胶-水接触角分别降低31°、35°,改性橡胶与水泥净浆的粘接强度分别提升44%、53%,改性橡胶混凝土的抗压强度在不同橡胶粒径与掺量条件下均得到提升;SEM结果表明极性亲水基团的引入能较好地提升橡胶颗粒与胶凝材料的粘接性能,有利于改善两者结合界面薄弱的问题,增强橡胶混凝土的整体强度。     

Si-69偶联剂处理的橡胶水泥砂浆抗压强度研究

通过正交实验选出Si-69偶联剂的最佳工艺条件,然后测试将最优条件处理下的橡胶作为一种添加成分取代部分粗骨料的水泥砂浆的抗压强度.试样分别为橡胶粉和橡胶块,掺量为水泥质量的3%、5%、10%,以考察橡胶粒度和掺量变化对抗压强度的影响.实验结果表明,掺有Si-69偶联剂处理橡胶的水泥砂浆的抗压强度比掺加未处理橡胶的水泥砂浆3%和10%时均有一定的提高,掺5%粗橡胶时也有一定提高,但掺5%细橡胶时却有一定的降低.     

Strength development and chloride penetration in rubberized concretes with and without silica fume

The study presented herein has been carried out in order to investigate the strength development and chloride permeability characteristics of plain and rubberized concretes with and without silica fume. For this purpose, two types of tire rubber, namely crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Two water-cementitious material (w/cm) ratios (0.60 and 0.40), three moist curing periods (3, 7, and 28 days), four designated rubber contents (0, 5, 15, and 25 by total aggregate volume), two silica fume content (0 and 10% by weight of cement), and five different testing ages (3, 7, 28, 56, and 90 days) were considered as experimental parameters. The results indicated that for a given w/cm ratio and moist curing period, the use of rubber significantly aggravated the chloride ion penetration through concrete but the degree of the rate of the increment of the chloride permeability depended on the amount of the rubber used. When the curing period was extended from 3 to 28 days, the reduction in the magnitude of chloride penetration depth was notably higher for the rubberized concretes, even at a rubber content of as high as 25%. It was also observed that silica fume may be considered as a remedy to enhance the chloride penetration resistance of the rubberized concretes.     

The material properties of sulphonated phenolic resin reinforced cement mortars

The material properties of new sulphonated phenolic resin (SP) reinforced cement mortars have been investigated. SP was found to promote the dispersion of cement particles and to interact with Ca(OH)₂. As a result, the resulting mortars exhibit better workability, more compact structure and higher compressive strength than plain mortars. The mortar with 1 wt% SP present after 28 days curing exhibits a compressive strength of 66MPa, which is about 18% higher than that of plain mortar.     

Effects of polymer–cement ratio and accelerated curing on flexural behavior of hardener-free epoxy-modified mortar panels

This experimental study reports the applicability of hardener-free epoxy-modified mortar panels to permanent forms as precast concrete products. Hardener-free epoxy-modified mortars are mixed using a Bisphenoal A-type epoxy resin without any hardener with various polymer–cement ratios and steel fiber reinforcement, and subjected to different curings. Hardener-free epoxy-modified mortar panels are prepared with same polymer–cement ratios and steel fiber reinforcement on trial, and tested for flexural behavior under four-point (third-point) loading. The effects of polymer–cement ratios and curings on strength properties of hardener-free epoxy-modified mortars, and on the flexural strength, flexural stress-extreme tension fiber strain relation, flexural load–deflection relation of hardener-free epoxy-modified mortar panels were examined. The adhesion in tension (to placed concrete) of the hardener-free epoxy-modified mortar panels was also tested. As a result, the hardener-free epoxy-modified mortar panels develop a high flexural strength, large extensibility and good adhesion to the placed concrete. The epoxy-modified mortar panels are more ductile and have high load-bearing capacity than unmodified mortar panels and can be used as precast concrete permanent forms in practical applications.     

Flexural bond strength of masonry using various blocks and mortars

This paper deals with an experimental study on flexural bond strength of masonry using various blocks in combination with different mortars. Flexural bond strength of masonry has been determined by testing stack-bonded prisms using a modified bond wrench test set-up. The effect of mortar composition and strength on the masonry's flexural bond strength using three types of masonry units (stabilized mud blocks, stabilized soil-sand blocks and burnt brick) has been examined. The effect of the masonry unit's moisture content on flexural bond strength has also been studied. Increases in mortar strength lead to increased flexural bond strength for cement mortar, irrespective of the type of masonry unit. It has been found that combination mortars, such as soil-cement mortar and cement-lime mortar, lead to better bond strength compared to cement mortars. The moisture content of the masonry unit at the time of casting has displayed significant influence on the flexural bond strength of the masonry. It has been found that for each type of masonry unit, an optimum moisture content exists, beyond which the flexural bond strength falls off quickly.     

Factorial design used to model the compressive strength of mortars containing recycled rubber

This paper presents results from an investigation on the potential use of waste vulcanized rubber scrap (WRS) particles as aggregate in construction mortars. The investigation was carried out using a 3² factorial design of experiments and the response surface methodology. Mortar mixtures were prepared using WRS as fine aggregate (10, 20 and 30 vol.%) with water/cement ratios of 0.52, 0.55 and 0.60. Fresh mortar consistency index and hardened mortar 28-day compressive strength were evaluated. The influence of the WRS content on the physical and mechanical properties was established, and the suitability of WRS use in a mortar application was demonstrated.     

Measurement and prediction of the strength of rubberized concrete

Illegally discarded piles of automobile tires are sources of potential hazards. Current disposal methods are wasteful and costly as they require either consumption of landfill space or continuous costly maintenance. A solution to the problem of scrap-tire disposal is the potential use of tire chips and crumb rubber as mineral aggregate substitutes in Portland cement concrete mixes. In this study, some of the engineering properties of rubberized concrete were examined and a neural network was developed to predict its compressive and tensile strengths. Rubberized concrete was found to possess good esthetics, acceptable workability, and a smaller unit weight than normal concrete. However, rubberized concrete did not perform as well as normal concrete under repeated freeze-thaw cycles. It exhibited lower compressive and tensile strength than that of normal concrete. Unlike normal concrete, rubberized concrete had the ability to absorb a large amount of plastic energy under compressive and tensile loads. It did not demonstrate the typical brittle failure, but rather a ductile, plastic failure mode. Test results were analyzed so that a model can be developed to predict the strength of rubberized concrete. Two neural network models were developed to predict the reduction in the compressive and tensile strength as a result of replacing mineral aggregate with rubber aggregate. A maximum difference of 9·2% between test results and model prediction was detected during the testing of the neural networks.     

Laboratory investigation of carbonated BOF slag used as partial replacement of natural aggregate in cement mortars

Direct mineral carbonation produces a material rich in carbonates and with reduced quantities of free oxides. The aim of this work was to show that such materials can be used in the construction domain. Basic Oxygen Furnace (BOF) slag from the steelmaking process has been traditionally seen as unfit for bounded applications due to its propensity to swelling, resulting from hydration of its high free lime content. Here, BOF slag was crushed to suitable particle sizes, carbonated in an aqueous solution of carbonic acid, and utilized to replace 50% of natural sand aggregate in cement mortars. The mechanical and chemical properties of these mortars were compared to mortars containing non-carbonated slags, and a standard cement mortar as a reference. Tests were conducted to determine mortar paste consistency and soundness, and cured mortar compressive strength and leaching tendencies. The results showed a satisfactory performance for all considered aspects (comparable with the reference) of the mortar sample containing 37.5 wt% (1.5 in 4 parts solids) carbonated BOF slag of <0.5 mm particle size.     

Physico-mechanical,microstructural and dynamic properties of newly developed artificial fly ash based lightweight aggregate – Rubber concrete composite

The use of industrial by-products in concrete would increase the sustainability of the construction industry. In this study, the potential use of scrap crumb rubber as fine aggregate in lightweight (Lytag) concrete was experimentally investigated. The effects of replacing natural sand by crumb rubber particles on the physico-mechanical, micro-structural and dynamic properties of the Lytag concrete were evaluated. When the rubber was introduced, the reduction in compressive strength of the Lytag concrete was experienced due to the less than perfect bond between the cement paste and the rubber as confirmed by the micro-structural observation. Additionally, there was flocculation of some of the crumb rubber particles and the packing of the rubber particles contributed to pockets of voids resulted in anisotropy in the concrete. The results also showed that the rubber not only meliorated the resistance of the cementitious Lytag composite to cracking from impact load but overall impact strength was also improved as the rubber particles acted as impedance to crack initiation and propagation.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

橡胶混凝土的抗裂性能和弯曲变形性能

通过净浆、砂浆和混凝土3 个层面的试验研究, 探讨了橡胶混凝土的抗裂性能和弯曲变形特性。根据圆环试件的开裂时间, 对3 种水灰比( 0. 3, 0. 4, 0. 5) 水泥净浆中掺入不同体积含量橡胶颗粒( 10%, 15%, 20%,30% , 50%) 后的开裂敏感性进行了对比分析。结果表明, 在水泥净浆中掺入橡胶颗粒可以显著延缓试件的开裂时间, 提高抗裂性, 延缓时间随着橡胶颗粒掺量的增加和水灰比的增大而延长。弯曲试验考察了4 个橡胶颗粒体积含量( 8%, 16%, 21. 4%, 32%) 砂浆试件和3 个橡胶颗粒体积含量( 10%, 12. 5%, 15%) 混凝土试件的弯曲变形性能。结果表明, 含橡胶颗粒的砂浆试件和混凝土试件在弯曲过程中会产生明显的塑性变形, 试件不会在承受最大荷载时产生脆性断裂, 而是经过较大的塑性变形后延性破坏。与基准试件相比, 砂浆试件和混凝土试件破坏时的极限变形值均有大幅度提高。      

磁场诱导定向碳纤维增强水泥砂浆的力学性能

首次利用磁场诱导定向技术,制备了具有明显择优取向的碳纤维增强水泥砂浆,表征与测试了不同水灰比、龄期和纤维掺量的水泥砂浆的碳纤维取向、抗压和劈裂抗拉强度,研究了碳纤维的取向性对力学性能提升效果的影响。结果表明:水灰比、纤维掺量对碳纤维的取向性有显著影响;相较于无择优取向的普通碳纤维增强水泥砂浆,经磁场诱导定向的碳纤维增强水泥砂浆的劈裂抗拉强度有显著增加,而抗压强度无明显变化;相同水灰比下,纤维取向和纤维掺量是影响定向碳纤维增强水泥砂浆劈裂抗拉强度的主要因素。其中,定向碳纤维增强水泥砂浆劈裂抗拉强度增强效率的最佳碳纤维掺量为水泥的0.50%。      

Physical and mechanical properties of foamed Portland cement composite containing crumb rubber from worn tires

The management of worn tires is a concern in industrialized countries. The application of crumb rubber as lightweight aggregate in cement based materials is a green alternative for reusing this material. High replacements of natural sand by crumb rubber were studied and an air-entraining agent was employed to ensure a cellular structure in the cement-based composite. The obtained results from tests in fresh state reveal an improvement in workability. The tests conducted on hardened composite show promise for constructive applications where thermal and acoustic properties are required. The minimum requirement of mechanical strength for masonry units was achieved, since compressive strengths varied between 1 and 10 MPa. Finally, potential applications as a construction material have been highlighted.     

Effect of shrinkage reducing admixture on flexural behaviors of fiber reinforced cementitious composites

The use of shrinkage reducing admixture (SRA) at various concentrations was investigated in fiber reinforced cementitious composites. Both mortar and high strength concrete (HSC) matrices were tested. Two types of fibers—steel and polypropylene—were assessed. The effect of SRA was measured on the fundamental properties such as surface tension of the bulk fluids and the contact angle developed between the fibers and the bulk fluids, on the fresh properties such as the air content and the density, and finally on the hardened mechanical properties, specially the flexural behaviors. It was noted that SRA enhances the wettability of fibers and reduces the air content of fiber reinforced cement mortars, while critical SRA concentrations are existing. SRA with critical concentration can significantly improve the flexural toughness and residual strength of steel fiber reinforced cement mortar. In the case of polypropylene fiber, SRA is not as effective in enhancing the flexural behaviors as it is in the case of steel fiber. SRA is generally ineffective in reducing the air content of HSC and the properties of steel fiber reinforced HSC with SRA are inferior to those without SRA.     

Enhancement of recycled aggregate properties by accelerated CO2 curing coupled with limewater soaking process

Strengthening the attached old cement mortar of recycled concrete aggregate (RCA) is a common approach to enhance the RCA properties. Accelerated CO₂ curing has been regarded as an alternative way to enhance the properties of RA. However, the improvement of the properties of RCA was limited by the shortage of reactive components in the old cement mortar available for the carbonation reactions. In this study, a CO₂ curing process associated with a limewater saturation method was performed cyclically on cement mortar samples, aiming to enhance the properties of cement mortars via artificially introducing additional calcium into the pores of the cement mortars. The results indicated that the adopted treatment method promoted the level of carbonation which was demonstrated by higher CO₂ uptake by the limewater saturated cement mortar when compared to that without limewater treatment. After 3-cycles of limewater-CO₂ treatment, the density of the cement mortar slightly increased by 5.7%, while the water absorption decreased by over a half. For mechanical properties, the compressive and flexural strength were increased by 22.8% and 42.4%, respectively. Compared to the untreated cement mortar samples, the total porosity of cement mortar was reduced by approximately 33% and the densified microstructure therefore resulted in a higher microhardness.