Natural Curing Conditions Research Articles

Effect of nanocomposite cementitious capillary crystalline waterproofing material on self-healing properties of cementitious composites

There are limited studies about the effect of nanocomposite cementitious capillary crystalline waterproofing material (CCCW) on the self-healing properties of cementitious materials in different environments. Compressive strength recovery ratio(CSRR) test, electrochemical impedance spectroscopy (EIS) test, and macroscopic cracks healing observation test were adopted to study the self-healing properties of pure mortar specimens, mortar specimens mixed with CCCW, and nanocomposite CCCW respectively under three kinds of curing conditions. The self-healing mechanism of nanocomposite CCCW was analyzed using thermogravimetric analysis and differential scanning calorimetry (TG-DSC), mercury intrusion porosimetry (MIP), and scanning electronic microscope and energy dispersive spectroscopy (SEM-EDS) techniques. It was found that the curing condition significantly influenced self-healing performance, with touching water curing condition having the highest impact, followed by soaking water curing condition and natural curing condition. Nanocomposite CCCW promoted the healing performance of damaged specimens and significantly increased the CSRR. Under touching water curing condition, the CSRRs of the mortar groups with 3% nano-SiO2, 3% nano-CaCO3, and 2% nano-MgO were 26.1%, 11.5%, and 8.6%, respectively, higher than the CSRR of the single-doped CCCW group. After 28 days of touching water curing, change in charge transfer resistance before and after specimen healing (∆Rct) of the groups with 3% dosage of nano-SiO2, 3% dosage of nano-CaCO3, and 2% dosage of nano-MgO were 5.2, 1.8, and 2.7 times, respectively, higher than that of the single-doped CCCW mortar group. Throughout the healing process, the addition of the three nanocomposite CCCWs accelerated the hydration reaction to generate C-S-H gel, CaCO3, Aft, etc., improved the internal pore structure, reduced the size of the most probable pore diameter (MPPD) of the mortar specimens, and ultimately promoted the self-healing properties of the cementitious materials.

Experimental Study of Mechanical Properties and Theoretical Models for Recycled Fine and Coarse Aggregate Concrete with Steel Fibers.

With diminishing natural aggregate resources and increasing environmental protection efforts, the use of recycled fine aggregate is a more sustainable approach, although challenges persist in achieving comparable mechanical properties. Exploration into the incorporation of steel fibers with recycled aggregate has led to the development of steel-fiber-reinforced recycled aggregate concrete. This study investigates the shrinkage performance and compressive constitutive relationship of steel fiber recycled concrete with different steel fibers and recycled aggregate dosages. Initially, based on different replacement rates of recycled coarse aggregate and different volume contents of steel fiber, experimental results demonstrate that as the replacement rate of recycled coarse aggregate increases, shrinkage also increases, while the addition of steel fiber can mitigate this effect. An empirical shrinkage model for steel fiber recycled concrete under natural curing conditions is also proposed. Subsequently, based on the uniaxial compression test, findings indicate that with an increasing replacement rate of recycled fine aggregate, the peak stress and elastic modulus of concrete decrease, accompanied by an increase in peak strain, and the addition of steel fiber limits concrete crack development and enhances its brittleness while the peak stress and strain of recycled fine aggregate concrete are enhanced. However, the steel fiber volume percentage has a negligible effect on the elastic modulus. A constitutive relationship for concrete considering the effects of recycled fine aggregate and steel fiber is also proposed. This finding provides foundational support for the influence patterns of steel fiber dosage and recycled aggregate ratio on the mechanical properties of steel fiber recycled concrete.

Analysis and Prediction Model Reinforced UHPC Shrinkage Property

This paper explores the shrinkage of reinforced UHPC under high-temperature steam curing and natural curing conditions. The results are compared with the existing shrinkage prediction models. The results show that the maximum shrinkage strain of reinforced UHPC after steam curing is 164 με and gradually becomes zero. As for natural curing, the maximum shrinkage strain is 173 με and the value stabilizes on the 10th day after pouring. This indicated that steam curing can significantly reduce shrinkage time. Compared with the plain UHPC tested in the previous literature, the structural reinforcement can significantly inhibit the UHPC shrinkage and greatly reduce the risk of cracking due to shrinkage. By comparing the results in this paper with the existing models for predicting the shrinkage strain development, it is found that the formula recommended in the French UHPC structural and technical specification is suitable for the shrinkage curve in the present paper.

Effects of Curing Methods on the Permeability and Mechanism of Cover Concrete

Curing methods are one of the most important factors in determining the quality and compactness of cover concrete. The effect of curing methods on the water absorption and sorptivity coefficient of cover concrete with the substitution ratio of fly ash (FA) and ground granulated blast slag (GGBS) for cement between 30 and 40 wt % was studied by capillary water absorption test. The vacuum saturation test and mercury intrusion test were employed to characterize these differences in the pore structure of cover concrete under different curing methods. With further analysis of the compactness of microstructure by SEM, the mechanism of the impact of curing methods on the permeability of cover concrete was revealed. The results obtained indicate that the effect of curing methods on the water absorption, sorptivity coefficient and porosity of cover concrete shows the trend of natural curing > cover curing > water curing > standard curing. It is also shown that reasonable curing is advantageous to reduce the porosity and permeability of cover concrete. In natural curing conditions, the appearance of porosity increasing and pore structure coarsening is more critical for cover concrete with mineral admixtures than for pure cement concrete. Therefore, the permeability of cover concrete with mineral admixtures is more sensitive to the early-age curing methods.

Solidification and stabilization of harmful elements in antimony tailings and synergistic utilization of multiple solid wastes

The massive generation and accumulation of tailings potentially lead to environmental pollution. Antimony tailings (AT) have higher environmental risks than other types of tailings and thus require special attention. Hence, this article used a variety of industrial solid wastes to solidify and stabilize the harmful components in AT. The solidified bodies prepared by adding different ratios of AT were cured under standard and natural curing conditions, and a series of performance tests and leaching toxicity analysis were performed. The results show that the solidified bodies containing 1000 g AT demonstrates excellent mechanical properties after standard curing for 28 d, with a compressive strength of 15 MPa and flexural strength of about 2.5 MPa. The solidified bodies have a water absorption rate of around 27%, softening coefficient of about 0.7, dry density of 900 kg/m3 and wet density of 1100 kg/m3, which meet waterproof and lightweight requirements. In addition, according to leaching toxicity analysis, the solidified bodies meet environmental standards. The bonding effect of needle-shaped ettringite and the encapsulation and filling effect of gelling substance are beneficial to the solidification and stabilization of AT. This article provides theoretical basis and research ideas for AT treatment and disposal.

Effect of Biopolymeric Stabilization on the Strength and Compressibility Characteristics of Cohesive Soil

Sustainable engineering discourages the use of conventional soil stabilizers such as lime and cement owing to their higher carbon emission rates. To address these challenges, many researchers considered biopolymer stabilization, a promising green technique in geotechnical engineering practice for treating expansive soils. This study evaluated the performance of two biopolymers, xanthan gum (XG) and guar gum (GG), in improving the unconfined compressive strength (UCS) and consolidation characteristics of an expansive soil subjected to different testing conditions. The dosage of the biopolymers (XG and GG) was maintained at 0.5%, 1%, 2%, and 4% by weight of dry mass of soil mass. Biopolymer treated samples were subjected to desiccator and natural curing conditions (i.e., specimens were exposed to different temperature and relative humidity conditions) for 7, 14, 28, 45, and 60 days. Results showed that the UCS of biopolymer-treated soil increased with increased dosage up to 2% and 1% for XG and GG, respectively; strength significantly improved during the first 28 days of curing under controlled conditions. Improvement of strength was due to increased adhesive strength between gum strands and soil particles. One-dimensional fixed-ring oedometer consolidation test results indicated that the compressibility and swelling characteristics of biopolymer-treated soil increased due to the hydrophilic nature of gum and an increase in the repulsive forces between negatively charged clay particles and the respective functional groups of XG and GG. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were performed on both untreated and biopolymer-treated soil to evaluate the interaction mechanism between soil and biopolymer strands. The microlevel studies confirmed the crosslinking of soil particles by gum strands, which aids in the strength improvement.

Brittleness of Concrete under Different Curing Conditions.

In order to shorten construction periods, concrete is often cured using steam and is loaded at an early age. This changes the performance and even the durability of the concrete compared to concrete that has been cured under normal conditions. Thus, the pattern and the mechanism of concrete performance change under different curing conditions, and loading ages are of great significance. The development of brittleness under different curing conditions and loading ages was studied. The evaluation methods that were used to determine concrete brittleness were expounded. Steam, standard, and natural curing conditions were carried out on single-side notched concrete beams as well as on a concrete prism and cubic blocks. The compressive strength and splitting tensile strength of the concrete blocks along with the fracture performance of the concrete beams were tested after 3, 7, 28, and 90 days. The steam curing condition significantly improved the strength of concrete before 28 days had passed, and the standard curing condition improved the strength of concrete after 28 days. Based on the experimental fracture parameters, a two-parameter fracture model was applied to study the development of fracture toughness , critical crack tip opening displacement , and critical strain energy release rate with hydration age under different curing conditions. With respect to long-term performance, the standard curing condition was better at resisting concrete crack propagations than the steam curing condition was. The characteristic length and the material length under the three curing conditions and the long-term development of brittleness in the concrete indicated that steam curing increased the concrete brittleness. Considering the effects of the curing condition and the loading age, a time-dependent concrete fracture toughness model was established, and the predicted value of the model was verified against the measured value. The results indicated that the model was able to accurately predict the fracture toughness with an error rate of less than 16%.

Study on manufacturing method of improving the strength of UHPC under natural curing conditions

Standard or steam curing is generally used in the production of ultra-high performance concrete (UHPC), which puts forward higher hardware conditions for the production of this material. In order to promote the popularization and application of UHPC, it is necessary to explore its fabrication method under natural curing, that is, without accurate control of temperature and humidity. The research method used in this paper is experimental analysis. By adjusting the components of UHPC materials, adjusting the proportion of each component, humidifying with watering and other simple measures, the methods to improve the strength of UHPC under natural curing conditions are explored. The experimental results show that the addition of high-efficiency solid water reducer can make the UHPC blocks have higher compressive strength than liquid water reducer. When the ratio of silica fume to cement is 30%, the compressive strength of UHPC reaches the maximum. Reducing or increasing the silica fume will reduce the strength of UHPC. Humidification by simple watering can be used to improve the compressive strength of UHPC. UHPC with strength over 100MPa can still be obtained under natural curing condition.

The influence of containing supplementary cementitious materials on preparation and properties for UHPC

The preparation of ultra-high performance concrete usually required expensive materials and harsh curing conditions, which limited its application in engineering widely. It may be solved by adding supplementary cementitious materials (fly ash, ground granulated blast slag, silica fume and so on). This paper is dedicated to determining the optimal mix ratio of UHPC under common preparation methods and raw materials and under common maintenance conditions to expand the applications of UHPC. The influence of the ratio of water to binder, the ratio of sand to binder, the content of supplementary cementitious materials on UHPC slump spread, compressive strength of 7 days and 28 days, apparent density and tensile strength of 28 days were researched by orthogonal test. The results revealed that the method of semi-dry mixing could enhance the homogeneity of steel fiber and improve the rate of its utilization. When the replacement of cement achieved 50% by the supplementary cementitious materials, the performances of UHPC were still guaranteed and confirmed. When the ratio of sand to binder reached 1.1, the ratio of water to binder reached 0.17, ground granulated blast slag was 25%, fly ash was 10% and silica fume was 15%, its slump spread reached 580 mm, compressive strength and tensile strength reached 152.9 MPa and 14.2 MPa respectively in 28 days under natural curing conditions. Therefore, the UHPC with supplementary cementitious materials should be considered as one of the sustainable development and environmental protection materials.

Effect of expansive agent and curing condition on the properties of low-cost polyvinyl alcohol engineered cementitious composites

At present, the traditional ECC material has the disadvantages of high cement content and high cost of PVA fiber, which seriously limits its application in engineering. In order to develop a sustainable building material for engineering, a low-cost and high toughness engineered cementitious composites (ECC) was prepared by using domestic polyvinyl alcohol (PVA) fiber. The effects of expansive agent (EA) and curing condition on the mechanical properties, shrinkage, early hydration degree and volume water loss rate of PVA-ECC were studied. The microstructure of PVA-ECC was analyzed by scanning electron microscopy (SEM). The test results show that EA can effectively improve the flexural and uniaxial tensile properties of PVA-ECC, as well as the shrinkage resistance, early hydration degree and volume water loss rate of PVA-ECC. Unexpectedly, the maximum uniaxial tensile strain of PVA-ECC material can reach 4.68%. In addition, it was found that PVA-ECC exhibited ultra-high tensile toughness when EA content was 10% under natural curing conditions. The above results verify the feasibility of domestic PVA fiber in ECC. This study can open up a new development direction for PVA-ECC in the future.

Effect of Silicone Waterproofing Agent on the Properties of Ceramic Recycled Aggregate Cement Mortar

The ceramic waste were crushed and sieved to replace partially the sand to prepare cement mortar as aggregate, and different quality of silicone waterproofing agents were added to study the effects on the mechanical properties and water absorption under natural curing conditions. The experimental results indicated that through 28 days of natural curing, the flexural strength of ceramic recycled aggregate cement mortar was 1.18% higher than that of ordinary cement mortar, the compressive strength is 2.94% lower, the water absorption is 0.5% lower, which means the performance difference is not obvious. With the increasing amount of the silicone waterproofing agent, the compressive and flexural strength of the samples gradually decreased, and the water absorption decreased first and then increased. When the amount of waterproofing agent is 2%, the comprehensive performances of the sample were the best: the28-day flexural strength was 6.84 MPa, the compressive strength was 42.8MPa, and the water absorption was 4.66%.

Influence of humidity on the mechanical properties of polymer-modified cement-based repair materials

Portland cement–aluminate cement–hemihydrate gypsum ternary binder repair mortars, as a type of road repair material, have the advantages of fast hardening and early strength, micro-expansion, stable strength, and good mechanical performance. However, problems on high brittleness, fast loss of fluidity, and poor toughness are also encountered. The effects of styrene butadiene copolymer and styrene acrylate on the construction performance of a ternary binder repair mortar composed of Portland cement, aluminate cement, and hemihydrate gypsum and the effects of different curing humidity conditions on the mechanical properties of the polymer-modified ternary repair mortar were investigated. Both polymers enhanced the fluidity and decreased the fluidity loss rate of the ternary binder cement paste and prolonged its setting time. Moreover, both polymers improved the toughness and reduced the water absorption of the ternary repair mortar in concordance with their filling effect and film-forming roles under different humidity curing conditions. Different curing humidity conditions affected the ettringite crystal morphology and polymer film formation of the polymer-modified ternary binder repair mortar. The polymer under natural curing conditions presented the best modification effect in terms of mechanical properties, and the inner structure was the densest of the repair mortar for the ternary binder. The toughness and flexural strength of the specimens were markedly improved.

Preparation of Photocatalytic Decorative Concrete Based on Nano-Titanium Dioxide Adsorbed by Clay Brick from Building Waste and Its Application in Pavement Structure of Railway Freight Yard

In this paper, based on the clay brick of building waste, nano-titanium dioxide photocatalytic decorative concrete was prepared, and its application in the pavement structure of a railway freight yard was studied. The results show that the loading capacity of nano-titanium dioxide on clay sand carrier is better than that on zeolite sand. With the increase of powder content, the photocatalytic efficiency of decorative concrete surface decreases, but the degree of reduction is small. Compared with laboratory conditions, the reduction rate of NOX in surface layer of decorative concrete under natural curing condition is lower. The photocatalytic efficiency did not decrease significantly at 90 days. Cement cementing could greatly improve the bonding effect of nano-titanium dioxide, which made the photocatalytic activity of decorative concrete surface layer have good durability. In this paper, decorative concrete is applied to the pavement engineering of a railway container yard in Liaoning Province. The pavement structure combination design, cement concrete pavement thickness design, pavement panel design and joint design are analyzed.

Study on the transfer and anchorage length of steel strand in ultra high performance concrete material

Ultra-high-performance concrete (UHPC) material, a new type of cement-based composite material, is usually employed in the bridge engineering. The transfer and anchorage length of steel strand in UHPC material is different from that in ordinary concrete; nevertheless, few design standards are found that how to anchor the transfer and anchoring length of steel strand in UHPC material under normal curing. Through central pull-out test under the different conditions of protective layer thickness and embedded length, the load-slip curves, failure modes, and bond strength of 36 UHPC material specimens under normal curing were studied. The experimental results showed that the ultimate bond stress between UHPC material and steel strand under natural curing conditions is 7.01∼11.68 MPa. When the compressive strength of cube was 157 MPa; the bond strength under natural curing was smaller than that under thermal curing; when the thickness of the protective layer of steel strand with a diameter of 15.2 mm is greater than 30 mm, it had a little influence on bond strength. The regression analysis of the test results based on the experimental results proves that the recommended formulas for the design of transfer length and anchorage length of steel strand in UHPC material were in great agreement with the results of published studies.

Experimental Study on Basic Performances of Reinforced UHPC Bridge Deck with Coarse Aggregates

In order to reduce the self-weight of conventional steel-concrete composite girders and to improve the crack-resisting behavior of normal strength concrete (NC) bridge decks for long-span cable-stayed bridges, an innovative steel–ultrahigh performance concrete (UHPC) composite girder was proposed. Coarse aggregates (CA) were added to UHPC to control shrinkage and cost. On the basis of the Fifth Nanjing Yangtze River Bridge (FNYRB), a 2 × 600 m cable-stayed bridge in China, a series of experimental tests was conducted. The studies aimed to reveal the material properties of UHPC with and without CA and the static and fatigue flexural performances of the reinforced UHPC deck with CA. The test results implied that the UHPC with CA had compressive strength and flexural strength comparable to UHPC without CA, but the former exhibited much lower shrinkage strains under the natural curing condition. The static bending test results showed that the steel fibers and steel reinforcement bars could effectively control the propagation of cracks in UHPC. The average nominal cracking strength of the UHPC bridge deck specimens was 13.8 MPa, about 1.97 times the maximum tensile stress in the design requirement of the FNYRB under serviceable design loads. Further, the fatigue bending test results demonstrated that the cracks in UHPC propagated very slowly under cyclic tensile stresses, and the residual flexural loads were only 7.2% and 10.9% lower than the ultimate loads obtained for specimens free from the fatigue loading processes. Thus, the current study verified the safety of applying UHPC with CA to long-span cable-stayed bridges.

Experimental evaluation of shear connectors using reactive powder concrete under natural curing condition

In hybrid girder bridges, an efficient type of steel-concrete joint, the so called concrete filled steel cells (CFSCs), can be used to transfer forces between the steel and concrete girders. In the CFSCs, where shear connectors embedded in the concrete core, the bearing capacity of shear connectors is significantly affected by the side confinement effects of steel cells. Shrinkage of concrete may change the interfacial restraints from the steel cells. So far, the influence of concrete shrinkage on the behavior of shear connectors in CFSCs has not been deeply addressed. This paper presents an experimental study of the structural response of shear connectors using reactive powder concrete (RPC) encased in CFSCs. To determine the shrinkage effects of RPC, results from two series of experimental programs are discussed: the first series included testing of twelve plug-in type push-out specimens to assess the performance of perfobond strip (PBL) connectors. The second series consists of testing two plug-in type push-out specimens and eight CFSC type push-out specimens to evaluate the behavior of PBL and steel stud connectors. Results indicated that initial cracks due to shrinkage were observed at the steel cell/RPC interface, which was very detrimental to connectors’ stiffness and strengths. Comparing to connectors with exposed RPC, the average ultimate bond strength at the CFSC steel cell/RPC interface dropped by 15%. Results also indicated that failure of steel stud and transverse steel rebar was barely influenced by RPC shrinkage. Furthermore, a comparison between experimental and predicting results was conducted to assess the applicability of the existing analytical models in predicting the behavior of PBL embedded in RPC with shrinkage.

Experimental research on seawater erosion resistance of magnesium potassium phosphate cement pastes

This paper aims to study the resistance of magnesium potassium phosphate cement (MKPC) paste to seawater erosion. MKPC paste specimens were prepared and their compressive strength, mass change, hydration temperature and water absorption were measured. The phase composition and microstructure were also analyzed. The results show that the initial hydration age of the reference sample M1 under natural curing condition has a significant effect on its compressive strength development in seawater. The residual ratios of the compressive strengths of M1(3d) and M1(28d), hydrated for 3 and 28 days, respectively, under natural curing condition, are 76.6% and 91.1% at 360-day seawater immersion age, respectively. This also indicates that early-age M1 has poor resistance to seawater erosion. Adding some limestone (LS) powders and silica fume (SF) can significantly improve the strength of early-age MKPC paste. The 3-day compressive strength of M2 with LS and SF is about 50% higher than that of M1. After immersion for 360 days in seawater, the residual ratios of the compressive strength of M2(3d) and M2(28d) are both close to 93%. This implies that initial hydration age of M2 under natural curing condition has almost no effect on its compressive strength development in seawater. Compared with M1, M2 has higher resistance to seawater erosion, which is attributed to the higher hydration degree and more compact structure of M2. This paper might be useful for preparing MKPC-based materials that can meet the requirements of marine concrete.

Research on dynamic mechanical properties of alkali activated slag concrete under temperature-loads coupling effects

The 28-day dynamic mechanical properties of alkali-activated slag concrete (AASC) under standard and natural curing conditions were investigated using a dynamic mechanical analysis (DMA) device. The DMA results showed that the dynamic mechanical properties of the standard cured AASC were higher than the natural cured AASC when concrete heated from −45°C to 250°C. The dissipated energy reached its maximum value when AASC under stress were heated to approximately 180°C. The microstructural analysis revealed that the pores of standard cured AASC were smaller than those under natural conditions in terms of the number of pores, volume and specific surface area. An economic and ecological analysis showed that compared with ordinary Portland cement concrete (OPC) in the same strength grade, the production of AASC consumes 2.4% less energy and produces 20.2% less CO2 emissions.

Development and recovery of mechanical properties of self-healing cementitious composites with MgO expansive agent

This paper presents the performance of engineered cementitious composites (ECCs) produced with MgO utilized as expansive self-healing agent to heal micro cracks. ECCs capability to keep small crack width of less than 60µm even under ultimate loads can stimulate the autogenous crack healing and thus, improving the mechanical properties of structural elements. Different test schemes were adopted to quantify the long-term self-healing capability of proposed ECC-MgO system (made of 5% MgO expansive agent ‘MEA’ as fly ash replacement in ECC matrix) through studying the development and recovery of strength (compressive and flexural) and Ultrasonic Pulse Velocity (UPV) of pre-loaded multiple damaged (cracked) cubic and prismatic specimens subjected to long term water and natural curing conditions. Test results indicated that preloaded cracked ECC-MgO specimens had high tendency to recover original mechanical properties of virgin (un-cracked) specimens through healing of micro-cracks. This study confirmed the self-healing capability of proposed ECC-MgO system.

The effect of seawater curing on properties of magnesium potassium phosphate cement

This paper investigated the effect of early seawater curing on properties of magnesium potassium phosphate cement (MKPC) paste. First, the MKPC paste was prepared by mixing overburning MgO powders, KH2PO4, the composite retarder (CR) and water in a certain proportion. Then, we measured the compressive strength, drying shrinkage deformation, and mass change of it under different curing conditions. The results were as follows. For MKPC paste specimens under seawater curing condition, the hydration ages before they were soaked in seawater had a significant effect on their compressive strength, drying shrinkage and mass increase. For MKPC paste specimens cured in seawater after 3-day natural curing, their 28-day and 60-day compressive strengths were 112.8% and 105.4% of those of MKPC paste specimens with same hydration ages under natural curing condition, respectively. In addition, the 60-day shrinkage strain of them (0.82×10−4) was significantly less than that of MKPC paste specimens with the same hydration age under natural curing condition (5.14×10−4). Moreover, they had higher mass increase ratio (1.23%). These were due to higher production, higher degree of crystallinity and less defects of MKP crystals, and more perfect pore size distribution in MKPC specimens soaked in seawater after 3-day natural curing.