Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate

Laboratory tests are performed to investigate the effects of a new method of mixture proportioning on the creep and shrinkage characteristics of concrete made with recycled concrete aggregate (RCA). In this method, RCA is treated as a two component composite material consisting of residual mortar and natural aggregate; accordingly, when proportioning the concrete mixture, the relative amount and properties of each component are individually considered. The test variables include the mixture proportioning method, and the aggregate type. The results show that the amounts of creep and shrinkage in concretes made with coarse RCA, and proportioned by the new method, are comparable to, or even lower than, those in similar concretes made entirely with natural aggregates. Furthermore, it is demonstrated that by applying the proposed “residual mortar factor” to the existing ACI and CEB methods for calculating creep or shrinkage of conventional concrete, these methods could be also applied to predict the creep and shrinkage of RCA-concrete.     

颗粒整形对再生粗骨料性能的影响

为了有效提高再生粗骨料的性能,必须对再生骨料进行机械强化处理.首次提出了利用高速(线速度≥80m/s)运动的再生骨料之间的反复相互冲击与摩擦作用,有效地打掉较为突出的棱角和除去颗粒表面附着的砂浆和水泥石的一种新技术.研究表明:颗粒整形使再生骨料的颗粒堆积密度平均提高了9.3%、表观密度从2.56g/cm3提高到2.59g/cm3、空隙率从53.3%降至48.5%、吸水率从4.7%降至2.9%、压碎指标值从15.8%降至9.4%,而且堆积密度、密实密度和针片状骨料含量等指标均优于天然碎石骨料,完全能够满足配制普通混凝土的要求.     

再生混凝土力学性能的研究进展

再生混凝土的应用,不仅能够解决废弃混凝土处理问题;又能降低因资源过度开采所引起的生态环境破坏,因而具有广阔的发展前景。相比于普通混凝土,再生混凝土的抗压强度、弹性模量以及抗疲劳性能较低,主要与再生骨料多方面因素的影响有关。对近年来再生混凝土力学性能相关研究进展进行了综述,再生骨料总吸水率是降低抗压强度的主要原因,疲劳性能则主要与再生骨料取代率和附着砂浆含量有关。在再生混凝土中掺加矿物掺合料能够改善新、旧双界面从而提高抗压强度和劈裂抗拉强度,掌握多个因素的影响和作用对再生骨料和再生混凝土进一步研究和应用具有重要意义。     

Short and long-term behavior of structural concrete with recycled concrete aggregate

Recycling concrete construction waste is a promising way towards sustainable construction. Coarse recycled concrete aggregates have been widely studied in recent years, however only few data have been reported on the use of fine recycled aggregates. Moreover, a lack of reliable data on long-term properties of recycled aggregate concrete has to be pointed out.In this paper the effects of both fine and coarse recycled concrete aggregates on short and long-term mechanical and physical properties of new structural concrete are investigated. The studied concrete mixes have been designed by adjusting and selecting the content and grain size distribution of concrete waste with the goal to obtain medium–high compressive strength with high content of recycled aggregates (ranging from 27% to 63.5% of total amount of aggregates).Time-dependent properties, such as shrinkage and creep, combined with porosity measurements and mechanical investigations are reported as fundamental features to assess structural concrete behavior.     

The use of building rubbles in concrete and mortar

Abstract

The main components of building rubble collected from demolished structures are waste concrete, brick and tile. A series of experiments using recycled aggregates of various compositions from building rubble were conducted. The test results show that building rubble can be transformed into useful recycled aggregate through proper processing. When the recycled aggregate was washed, the negative effects on the recycled concrete were greatly reduced. This is especially meaningful for flexural strength. Recycled coarse aggregate is the weakest phase given a low water/cement ratio. This effect will dominate the mechanical properties of recycled concrete. On the contrary, using recycled aggregate in concrete has little effect on its mechanical properties if the water/cement ratio is high. This mechanism does not occur in recycled mortar. The quantity of recycled fine aggregate will govern the mortar strength reduction percentage. Although using brick and tile in concrete will affect its mechanical properties, the effect is limited.     

钢纤维橡胶再生混凝土的抗冻性试验

为使废弃混凝土和再生橡胶在北方地区混凝土工程中得以应用,采用正交试验法研究再生粗骨料掺量、再生粗骨料强化方式、钢纤维掺量与橡胶掺量对钢纤维橡胶再生混凝土(C45)立方体抗压强度和抗冻性的影响规律。利用扫描电镜和螺旋CT扫描技术研究了钢纤维橡胶再生混凝土的宏观和细观结构及其对抗冻性能的影响机理。结果表明:橡胶颗粒掺量是影响再生混凝土含气量、抗压强度和相对动弹模量的重要因素,再生粗骨料掺量是影响相对动弹模量和强度损失率的次要因素,钢纤维掺量对混凝土抗压强度增强作用较小,粗骨料强化方式对混凝土性能影响不大;橡胶颗粒与砂浆界面的裂缝宽度在5~55μm之间,二者之间的相容性较差;当橡胶颗粒掺量(与砂的体积比)大于20%后,随橡胶颗粒掺量增大,混凝土内部孔洞数目增多,钢纤维橡胶再生混凝土抗压强度降低、抗冻性减弱。     

Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR consideration

There is a growing interest of using recycled crushed glass (RCG) as an aggregate in construction materials especially for non-structural applications. Although the recycled crushed glass is able to reduce the water absorption and drying shrinkage in concrete products due to its near to zero water absorption characteristics, the potential detrimental effect of using glass due to alkali–silica reaction (ASR) in cementitious materials is a real concern. The extent of ASR and its effect on concrete paving blocks produced with partial replacement of natural aggregates by crushed glass cullet are investigated in this study. This study is comprised of two parts. The first part quantified the extent of the ASR expansion and determined the adequate amount of mineral admixtures that was needed to reduce the ASR expansion for concrete paving blocks prepared with different recycled crushed glass contents using an accelerated mortar bar test in accordance with ASTM C 1260 (80 °C, 1 N NaOH solution). In the second part, concrete paving blocks were produced using the optimal mix proportion derived in the first part of this study and the corresponding mechanical properties were determined.It was found from the mortar bar test that the incorporation of 25% or less RCG induced negligible ASR expansion after a testing period of 28 days. For mixes with a glass content of higher than 25%, the incorporation of mineral admixtures such as pulverized fuel ash and metakaolin was able to suppress the ASR expansion within the stipulated limit but the results need to be confirmed by other test methods such as the concrete prism test.The study concluded that the optimal mix formulation for utilizing crushed waste glass in concrete paving blocks should contain at least 10% PFA by weight of the total aggregates used.     

Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: durability performance

The growing difficulty in obtaining natural coarse aggregates (NCA) for the production of concrete, associated to the environmental issues and social costs that the uncontrolled extraction of natural aggregates creates, led to a search for feasible alternatives. One of the possible paths is to reuse construction and demolition waste (CDW) as aggregates to incorporate into the production of new concrete. Therefore, a vast and detailed experimental campaign was implemented at Instituto Superior Técnico (IST), which aimed at determining the viability of incorporating coarse aggregates from concrete and ceramic brick wall debris, in the production of a new concrete, with properties acceptable for its use in new reinforced and pre-stressed structures. In the experimental campaign different compositions were studied by incorporating pre-determined percentages of recycled coarse concrete aggregates and recycled coarse ceramic plus mortar particles, and the main mechanical, deformability and durability properties were quantified, by comparison with a conventional reference concrete (RC). In this article, these results are presented in terms of the durability performance of concrete, namely water absorption, carbonation and chlorides penetration resistance.     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Expansion behaviour of glass aggregates in different testing for alkali-silica reactivity

The potential for reaction between amorphous silica in recycled glass used as aggregate in concrete and alkalis in cement is the subject of debate in current concrete literature. Whilst the ASTM C1260 accelerated mortar bar method is conventionally used for rapid ASR assessment, there is doubt about its suitability for glass aggregates. This paper reports upon a comparison of the relative ASR reactivity of various colours of recycled glass aggregates using the ASTM C1260 and C227 test methodologies. The results show that with limited exception the ASTM C1260 method does not cause glass aggregates to react by the end of the prescribed test period. In contrast, the ASTM C227 method causes all glass aggregates to react within 2 weeks, despite the test being designed for 12 months or even longer if necessary. This paper compares and contrasts the results of the two methods over a wide range of glass aggregate and cementitious systems made with two sizes of mortar bar, draws conclusions about the reasons for the differences observed and makes remarks on the expansion behaviour of glass aggregates in cementitious systems.     

Micromechanical investigation on size effect of tensile strength for recycled aggregate concrete using BFEM

In this paper, the validity and performance of base force element method (BFEM) based on potential energy principle was studied by some numerical examples. And the BFEM on damage mechanics is used to analyze the size effect on tensile strength for recycled aggregate concrete (RAC) at meso-level. The recycled aggregate concrete is taken as five-phase composites consisting of natural coarse aggregate, new mortar, new interfacial transition zone (ITZ), old mortar and old ITZ on meso-level. The random aggregate model is used to simulate the meso-structure of recycled aggregate concrete. The size effects of mechanical properties of RAC under uniaxial tensile loading are simulated using the BFEM on damage mechanics. The simulation results agree with the test results. This analysis method is the new way for investigating fracture mechanism and numerical simulation of mechanical properties for RAC.     

Properties of architectural mortar prepared with recycled glass with different particle sizes

The recycling of glass waste as a source of aggregate for the production of concrete products has attracted increasing interest from the construction industry. However, the use of recycled glass in architectural mortar is still limited. This study attempts to develop a self-compacting based architectural mortar using white cement and 100% recycled blue glass as key ingredients. To improve the aesthetic qualities, a certain minimum quantity of glass cullets of larger particle size must be present. The influence of particle size of the recycled glass on the engineering properties of fresh and hardened architectural mortar is investigated. The experimental results demonstrate that it is feasible to utilize 100% recycled glass as the aggregate for the production of self-compacting based architectural mortar. These products have an average compressive strength of 40 MPa and flexural strength of 6 MPa at 28 days which are appropriate for some architectural and building applications. Also, the overall performances of all the architectural mortars prepared with different particle sizes of glass aggregates are comparable to those of control mortar mix prepared with river sand.     

Lightweight mortar made with recycled polyurethane foam

This paper presents results of an experimental study on the use of rigid polyurethane foam wastes with cement-based mixtures to produce lightweight mortar. Several mortar grades were obtained by mixing cement with different amounts of polyurethane, aggregate and water. Dosages were varied to replace aggregates with recycled polyurethane, while the amount of water was optimized to obtain good workability. Rigid polyurethane was ground to particle sizes of less than 4 mm prior to use as an aggregate substitute. The characteristics of the test specimens were defined and they were tested in both a fresh and a hardened state. Results show that an increase in the amount of polyurethane affects the mortar, decreasing its density and mechanical properties while increasing its workability, permeability, and occluded air content. These results confirm that mortar produced with recycled polyurethane is comparable to lightweight mortar made with traditional materials.     

自保温再生混凝土空心砌块的热工性能研究

在再生粗骨料取代率为100%的情况下,以再生细骨料取代率和玻化微珠体积掺量为主要变量,进行自保温再生混凝土导热系数的试验研究,再生细骨料取代率为0%、50%、100%,玻化微珠掺量占混凝土总体积比为60%~120%,揭示了自保温再生混凝土多因素的影响规律并得出综合性能优化的配合比。在此基础上,进行了自保温再生混凝土空心砌块墙体的热工性能研究,结合理论计算和数值模拟分析,对自保温再生混凝土空心砌块和普通混凝土空心砌块的保温性能进行了对比分析,结果表明:自保温再生混凝土空心砌块的热阻值比同强度普通混凝土的对应值增大41.9%,在室内外温差23℃的条件下,1h后普通混凝土空心砌块墙体的室内温升较自保温再生混凝土空心砌块墙体高44.9%,当墙体内外各采用一层20mm厚保温砂浆粉刷时,自保温再生混凝土空心砌块墙体可达到节能65%的要求。     

Macroscopic and microstructural properties of engineered cementitious composites incorporating recycled concrete fines

Recycled concrete fines (RCF) are fine aggregates and particles from the demolition waste of old concrete. Unlike recycled coarse aggregates, RCF is seldom used to replace sands in concrete due to its high surface area and attached old mortar on the surface of RCF. This study investigated potential use of RCF as microsilica sand substitute in the production of engineered cementitious composites (ECC), a unique high performance fiber-reinforced cementitious composites featuring extreme tensile strain capacity of several percent. The results showed that it is viable to use RCF as microsilica sand substitute in the production of ECC and the resulting RCF-ECCs possess decent compressive strength and strain capacity. Microstructure investigation on the component level revealed that RCF size and content modify matrix toughness and fiber/matrix interface properties. The influence of RCF size and content on ECC properties was clearly revealed and explained by the resulting fiber bridging σ(δ) curves of RCF-ECCs calculated from the micromechanical model. Micromechanics-based design principle can therefore be used for ingredients selection and component tailoring of RCF-ECCs.     

考虑孔隙及微裂纹影响的混凝土宏观力学特性研究

混凝土是一种典型的多孔介质材料,孔隙分布错综复杂,孔径尺寸跨越微观尺度和宏观尺度,对混凝土弹性模量及强度等力学参数产生巨大影响.认为混凝土是由骨料、孔隙及砂浆基质组成的三相复合材料,采用Monte Carlo 法将孔隙、微裂纹及微缺陷与骨料颗粒随机投放在砂浆基质中.根据三相球模型及中空圆柱形杆件模型得到含孔材料的有效力学性质,并推导得到含孔材料的等效本构模型.建立含孔隙混凝土试件的细观单元等效化力学模型,对二级配含孔隙混凝土试件在单轴拉伸及压缩条件下的反应进行了非线性分析.结果表明:孔隙、微裂纹的存在对混凝土宏观弹性模量、强度及残余强度等力学性质都有很大影响,在对混凝土宏观力学特性分析及研究混凝土损伤断裂时不应忽略其影响.     

Interfacial transition zone of cement composites with steel furnace slag aggregates

As previous studies of mortar and concrete with steel furnace slag (SFS) aggregates have shown increases or decreases in the bulk mechanical properties, this study investigated the microstructural cause of these opposing trends through characterization of the interfacial transition zone (ITZ) with quantitative image analysis of backscatter electron micrographs. Three SFS types – basic oxygen furnace (BOF), electric arc furnace (EAF), EAF/ladle metallurgy furnace (EAF/LMF) – were examined as aggregates in a portland cement mortar. The ITZ size for all SFS mortar mixtures was similar, with the ITZ of BOF and EAF/LMF being slightly more porous than mortar mixtures with EAF or dolomite. Microstructural examinations of the SFS particle revealed that BOF and EAF/LMF aggregates have different outer and interior compositions, with the outer composition consisting of a porous layer, which likely contributes to the reduced strength relative to EAF. The imaging results demonstrated that the type of SFS and its spatial composition greatly influences the bulk properties of mortar and concrete, mainly as a function of porosity content in the ITZ and the outer layer and interior porosity of the SFS aggregate.     

Mechanical and durability properties of concrete using contaminated recycled aggregates

The degradation of concrete structures due to chlorides and sulphates penetration is of obvious importance in civil engineering as having major impact on structural durability. In this paper, the results of an investigation on the effect of contaminated crushed concrete aggregates on mechanical properties and durability of recycled concrete are presented. Natural aggregates concrete (NC) slabs were cured in water, sea water, chloride solutions or sulphate solutions and then crushed to obtain virgin and contaminated (polluted) recycled aggregates. The properties of natural (NA) and recycled aggregates (RA) and the mechanical properties and durability performances of a new concrete made from 100% of RA are analysed. The results show that contaminated RA are much sensitive to chlorides than sulphates and are rapidly leached when soaked into water. Significant differences were observed between the properties of original and new concrete and the results clearly show the necessity of taking these contaminations into account.     

Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates

In this study, the fresh and hardened properties of self-compacting concrete (SCC) using recycled concrete aggregate as both coarse and fine aggregates were evaluated. Three series of SCC mixtures were prepared with 100% coarse recycled aggregates, and different levels of fine recycled aggregates were used to replace river sand. The cement content was kept constant for all concrete mixtures. The SCC mixtures were prepared with 0, 25, 50, 75 and 100% fine recycled aggregates, the corresponding water-to-binder ratios (W/B) were 0.53 and 0.44 for the SCC mixtures in Series I and II, respectively. The SCC mixtures in Series III were prepared with 100% recycled concrete aggregates (both coarse and fine) but three different W/B ratios of 0.44, 0.40 and 0.35 were used. Different tests covering fresh, hardened and durability properties of these SCC mixtures were executed. The results indicate that the properties of the SCCs made from river sand and crushed fine recycled aggregates showed only slight differences. The feasibility of utilizing fine and coarse recycled aggregates with rejected fly ash and Class F fly ash for self-compacting concrete has been demonstrated.     

The assessment of ceramic and mixed recycled aggregates for high strength and low shrinkage concretes

Very few studies on recycled aggregate concretes (RC) have been extended to the use of recycled ceramic and mixed aggregates in relation with high strength concretes. In the main they concentrate only on the analysis of the physical and mechanical properties. This study deals with the investigation of the influence that different percentages (up to 30% substitution for natural aggregates) of high porous ceramic and mixed recycled aggregates have over the plastic, autogenous and drying shrinkage of the concretes. The physical and mechanical properties as well as the chloride resistance were also determine in order to assess the viability of the use of ceramic and mixed recycled aggregates in high strength concretes. The results revealed that the employment of highly porous recycled aggregates reduced the plastic and autogenous shrinkage values of the concrete with respect to those obtained by conventional concrete (CC). Although the total drying shrinkage of the recycled concrete proved to be 25% higher than that of the CC concrete, the CC concrete had in fact a higher shrinkage value than that of the RC from 7 to 150 days of drying. It can be concluded that the RC concrete produced employing up to 30% of fine ceramic aggregates (FCA, with 12% of absorption capacity) achieved the lowest shrinkage values and higher mechanical and chloride ion resistance. In addition, the concrete produced with low percentage (10–15%) of recycled mixed aggregates also had similar properties to conventional concrete.