• Computers and Concrete
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• v.29 no.6
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• pp.407-418
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• 2022

This paper numerically investigates the effect of changes in the mechanical properties (displacement, strain, and stress) of the ultra-high-performance concrete (UHPC) without rebar and the reinforced concrete (RC) using steel re-bars. This reinforced concrete is mostly used in the concrete bridge decks. A mixture of sand, gravel, cement, water, steel fiber, superplasticizer, and micro silica was used to fabricate UHPC specimens. The extended finite element method as used in the ABAQUS software is applied for considering the mechanical properties of UHPC, RC, and ordinary concrete specimens. To calibrate the ABAQUS, some experimental tests have been carried out in the laboratory to measure the direct tensile strength of UHPC by the compressive-to-tensile load converting (CTLC) device. This device contains a concrete specimen and is mounted on a universal tensile testing apparatus. In the experiments, three types of mixed concrete were used for UHPC specimens. The tensile strength of these specimens ranges from 9.24 to 11.4 MPa, which is relatively high compared with ordinary concrete specimens, which have a tensile strength ranging from 2 to 5 MPa. In the experimental tests, the UHPC specimen of size 150×60×190 mm with a central hole of 75 mm (in diameter)×60 mm (in thickness) was specially made in the laboratory, and its direct tensile strength was measured by the CTLC device. However, the numerical simulation results for the tensile strength and failure mechanism of the UHPC were very close to those measured experimentally. From comparing the numerical and experimental results obtained in this study, it has been concluded that UHPC can be effectively used for bridge decks.

• Computers and Concrete
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• v.32 no.3
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• pp.327-337
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• 2023

Concrete is the most widely used building material, with various types including high- and ultra-high-strength, reinforced, normal, and lightweight concretes. However, accurately predicting concrete properties is challenging due to the geotechnical design code's requirement for specific characteristics. To overcome this issue, researchers have turned to new technologies like machine learning to develop proper methodologies for concrete specification. In this study, we propose a highly accurate deep learning-based predictive model to investigate the compressive strength (UCS) of lightweight concrete with natural aggregates (pumice). Our model was implemented on a database containing 249 experimental records and revealed that water, cement, water-cement ratio, fine-coarse aggregate, aggregate substitution rate, fine aggregate replacement, and superplasticizer are the most influential covariates on UCS. To validate our model, we trained and tested it on random subsets of the database, and its performance was evaluated using a confusion matrix and receiver operating characteristic (ROC) overall accuracy. The proposed model was compared with widely known machine learning methods such as MLP, SVM, and DT classifiers to assess its capability. In addition, the model was tested on 25 laboratory UCS tests to evaluate its predictability. Our findings showed that the proposed model achieved the highest accuracy (accuracy=0.97, precision=0.97) and the lowest error rate with a high learning rate (R2=0.914), as confirmed by ROC (AUC=0.971), which is higher than other classifiers. Therefore, the proposed method demonstrates a high level of performance and capability for UCS predictions.

• Journal of the Korean GEO-environmental Society
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• v.24 no.6
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• pp.13-20
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• 2023

This paper is aimed to evaluate the mechanical properties and durability of high-flowable retaining wall material (RWM) with different levels of steel fiber (SF) content. To produce the specimens of RWM, some chemical agents such as superplasticizer (SP), air-entrained agent (AEA) and viscosity modifying agent (VMA) were added in the fresh RWM. The compressive and split tensile strength measurements were performed on the hardened RWM specimens at the predetermined periods. Additionally, surface electric resistivity and absorption tests according to ASTM standards were carried out to examine mechanical properties of RWM mixes. The durable performances such as chloride ions penetrability and freezing-thawing resistance of RWM mixes were experimentally investigated. As resutls, it was found that the performance of RWM mix with SF were much better than that without SF, especially at the 2% addition of SF. Thus, it is noted that the proper addition of SF in the RWM mix may have a beneficial effect to improve mechanical properties and durability of RWM mixes.

• Journal of the Korea Concrete Institute
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• v.26 no.1
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• pp.23-34
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• 2014

This study investigates the relationship between the performance of fresh and hardened Ultra-High Performance Concrete (UHPC) without heat treatment. The performance of fresh UHPC is determined by the slump flow test related to the fluidity of concrete mixtures, and the air content test. The variables of these tests are the water to binder ratio, superplasticizer dosages and volume fractions of steel fiber. Generally, insufficient fluidity and excessive air contents in concrete mixtures lead to the insufficient packing density related to the performance of harden concrete. The performance of hardened UHPC is determined by the compressive and flexural tensile tests. The results of the fresh UHPC tests show that there is the linear correlation between each variable and the slump flow diameter, and that the slump flow diameter is linearly decreased as the air content ratio increase. Using these results, the formula is developed to predict the fresh performance before mixing UHPC. The results of the hardened UHPC tests show that the hardened performance is not influenced by the air content ratio in the range of 3.2 to 4.2 per cent. However, the flexural tensile strength dominantly influenced by the volume fractions of steel fiber.

• Journal of the Korea Concrete Institute
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• v.26 no.1
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• pp.63-70
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• 2014

This experimental study was intended to improve the compressive strength of multi-walled CNT reinforced cementitious composites with efficiency. The variables considered are the degree of sonication, the amount of surfactant, the replacement ratio of silica fume, etc. Optical microscope informed that fiber dispersion of CNT was improved with the increase of sonication time, and the compressive strength was proved to be enhanced as the degree of sonication increased. When superplasticizer as a surfactant had SP/CNT ratio of 4~6, the best improvement in strength was obtained. Silica fume was shown to produce the highest compressive strength at 10% replacement. Microstructure of CNT composites was also analyzed; XRD and SEM results indicated that CNT addition hardly changed hydration products and microstructure, and MIP analysis found the reduction of total porosity as well as the increase of nano-pores with the size of tens of nm instead of the decrease of pore distribution in the region of around 10 ${\mu}m$ and 100 nm. The results of microstructure analysis explains that the strength improvement is closely related to physical contribution rather than chemical influence by adding CNT.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.5
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• pp.12-20
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• 2017

This report presents the results of an investigation on the fundamental properties of mortars high fluidity high volume slag cement(HVSC) activated with sodium silicate($Na_2SiO_3$). The ordinary Portland cement(OPC) was replaced by ground granulated blast furnace slag(GGBFS) from 40% to 80% and calcium sulfoaluminate(CSA) was 2.5% or 5.0% mass. The $Na_2SiO_3$ was added at 2% and 4% by total binder(OPC+GGBFS+CSA) weight. A constant water-to-binder ratio(w/b)=0.35 was used for all mixtures. The research carried out the mini slump, V-funnel, setting time, compressive strength and drying shrinkage. The experimental results showed that the contents of superplasticizer, V-funnel, setting time and drying shrinkage increased as the contents of CSA and $Na_2SiO_3$ increase. The compressive strength increases with and an increase in CSA and $Na_2SiO_3$. One of the major reason for these results is the accelerated reactivity of GGBFS with CSA and $Na_2SiO_3$. The maximum performance was CSA 5.0% + $Na_2SiO_3$ 4% specimens.

• Applied Chemistry for Engineering
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• v.9 no.3
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• pp.336-341
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• 1998

A comparative study on the effects of mean particle diameter and substitutional ratio of II-anhydrite on the fluidity of cement paste has been conducted. Three different mean particle diameters(4, 14 and $35{\mu}m$) and four different substitution ratios(3, 5, 10, 15wt%) have been tested while the dosage of two superplasticizers - naphtalenic(NSF) and polycarboxylic(NT-2) - has been varied from 0 to 2.0wt%. To investigate the effects of those parameters, the variation in fluidity and apparent viscosity of the cement paste has been observed as a function of the elapsed time. The initial fluidity of the cement paste is more affected by the increase in substitution ratio than by the mean particle size of the II-anhydrite. When the substitution ratio of the II-anhydrite is fixed at 10wt%, addition of 1.0wt% NT-2 improves the fluidity of paste cement more than that for NSF. This case has showed the best improvement of the fluidity in the range of parameters investigated. At the addition of 1.0wt% NT-2, apparent viscosity of the cement paste has been noticeably decreased as substitutional ratio of II-anhydrite increases. It was found that it would be more effective to substitute II-anhydrite at a certain ratio to improve the fluidity of the cement paste, in addition to adding NT-2.

• International Journal of Highway Engineering
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• v.11 no.2
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• pp.185-194
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• 2009

The autogenous shrinkage of high-performance concrete, including very-early strength latex-modified concrete(VES-LMC), is generally bigger than that of normal strength concrete because of the low water/cement ratio, high binder contents, and usage of superplasticizer. Mix. proportion of VES-LMC has low water/cement ratio(0.38), high cement content(390kg/m$^3$), and aid of latex(15% of cement weight). Thus, these factors of VES-LMC, rapid water self-dissipation and evaporation within 3 hours of concrete placement would increase the autogenous shrinkage. The purpose of this study was to evaluate the early-age shrinkage, thermal deformation and autogenous shrinkages of VES-LMC with retarder contents(retarder solids-cement ratio, by weight) using to secure working time in field. The experimental results showed that retarder contents do not affect of the maximum hydration temperature. Early-age expansion of VES-LMC was mostly caused by thermal expansion and partly by autogenous expansion. The autogenous shrinkage is decreased by increasing the retarder contents within this study. On the other hand, the usage of retarder should be decided carefully considering the field conditions because an excessive usage of retarder can cause handful early-age expansion.

• Journal of the Korea Concrete Institute
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• v.16 no.5 s.83
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• pp.605-612
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• 2004

This study investigated the fundamental properties and shrinkage properties of high performance concrete with water/binder ratio of 0, 30 and with combination of expansive additive and shrinkage reducing agent. According to the results, the fluidity of high performance concrete showed lower the using method in combination with expansive additive and shrinkage reducing agent than the separately using method of that, so the amount of superplasticizer increased when the adding ratio of expansive additive and shrinkage reducing agent increased. However the air content of concrete increased when used in combination with expansive additive and shrinkage reducing agent, so the amount of AR agent decreased. The compressive strength showed the highest at $5\%$ of expansive additive, and decreased with an increase of the amount of shrinkage reducing agent. Furthermore, in order to reduce the shrinkage of high performance concrete, it was found that the using method in combination with expansive additive and shrinkage reducing agent was more effective than separately using method of that. Therefore, it analyzed that the combination of expansive additive of $5\%$ and shrinkage reduction agent of $1\%$ was the most suitable mixture, considering to the fluidity, strength and shrinkage under the condition of this experiment.

• Journal of the Korean Recycled Construction Resources Institute
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• v.5 no.4
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• pp.442-449
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• 2017

In this study, a method to inject small amount of activation agent from the outside of the pipeline to the inside wall of the pipe was newly proposed to enhance pumping efficiency of cementitious materials. The activation agent is injected into the slip-layer, which is generally formed in the vicinity of the inside wall of the pipe during pumping cementitous materials. Through the injections, it is expected to decrease viscosity of slip-layer, namely, the friction between the mateirals and the pipe. The proposed method was verified by lab-scale pumping tests with mortars having water to cement ratio of 47%. The tests were performed with two different type of activation agents(superplasticizer and anionic surfactant) and three different amount of the agents(0.14, 0.28, 0.42% of the mortar volume). The compressive strength were measured with and without injecting the activation agent, and the internal pressures of pipeline were measured. When the anionic surfactant was used, there was no change in the compressive strength. As the amount of anionic surfactant increased, the pumping pressure decreased up to 71.4% at the maximum.