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Volume 16, Number 3, September 2023

In this study, the steel fiber and the polypropylene fiber were used to enhance the mechanical properties of fully recycled coarse aggregate concrete. Natural crushed stone was replaced with recycled coarse aggregate at 100% by volume. The steel fiber and polypropylene fiber were used as additive material by incorporating into the mixture. In this test two parameters were considered: (a) steel fiber volume ratio (i.e., 0%, 1%, 1.5%, 2%), (b) polypropylene fiber volume ratio (i.e., 0%, 0.1%, 0.15%, 0.2%). The results showed that compared with no fiber, the integrity of cubes or cylinders mixed with fibers after failure was better. When the volume ratio of steel fiber was 1~2%, the width of mid-span crack after flexural failure was 5~8 mm. In addition, when the volume ratio of polypropylene fiber was 0.15%, with the increase of steel fiber content, the static elastic modulus and toughness of axial compression first increased and then decreased, and the flexural strength increased, with a range of 6.5%~20.3%. Besides, when the volume ratio of steel fiber was 1.5%, with the increase of polypropylene fiber content, the static elastic modulus decreased, with a range of 7.0%~10.5%. The ratio of axial compression toughness first increased and then decreased, with a range of 2.2%~8.7%. The flexural strength decreased, with a range of 2.7%~12.6%. On the other hand, the calculation formula of static elastic modulus and cube compressive strength of fully recycled coarse aggregate with steelpolypropylene fiber was fitted, and the optimal fiber content within the scope of the test were put forward.

Key Words
axial compression; bending; fully recycled coarse aggregate concrete; polypropylene fiber; steel fiber

(1) Weiwei Su, Zongping Chen, Haoyu Liao, Xingyu Zhou:
College of Civil Engineering and Architecture, Guangxi University, Nanning 530004, China;
(2) Zongping Chen:
College of Architecture and Civil Engineering, Nanning University, Nanning 530200, China;
(3) Zongping Chen:
Key Laboratory of Disaster Prevention and Structural Safety of Ministry of Education, Guangxi University, Nanning 530004, China;
(4) Dingyuan Liu:
Civil Engineering Department, Guangxi Technological College of Machinery and Electricity, Nanning 530007, China.

In order to develop and take full advantage of pasture fiber and waste concrete, this article studied how different amounts of pasture fiber influenced the toughness and pore structure of concrete with different replacement rates of recycled fine aggregate. Pasture fiber recycled concrete constitutive equations were established under idealized stiffness and toughness damage rate, based on fracture energy and damage mechanics theories. The relationship between pore structure and toughness was studied utilizing nuclear magnetic resonance and fractal theory. The toughness of text groups (0% (JZ), 10% (ZS10), 20% (ZS20)) first increased and then decreased with increasing amounts of pasture fiber, based on the damage rate of toughness. The toughness of concrete samples with recycled fine aggregate and pasture fiber is negatively correlated to the fractal dimension of small and medium-sized pores with a pore size of 0-500 nm. At a replacement rate of 10% of the recycled fine aggregate, the fractal dimension of the air voids (r: 500-9000 nm, i.e., Lg(r) ∈ [2.7, 3.9]) shows a gradual decrease with the increase of grass fiber dosage, indicating that with such a replacement rate of the recycled fine aggregate, the increase of pasture fiber can reduce the complexity of the pore structure of the air voids (500-9000 nm).

Key Words
damage constitutive equation; fractal dimension; nuclear magnetic resonance; pasture fiber; recycled fine aggregate; toughness

College of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, 306 Zhaowuda Road, Hohhot, Inner Mongolia, 010018, China.

The penetrated chloride in concrete has different behavior with mix proportions and local exposure conditions, even in the same environments, so that it is very important to quantify surface chloride contents for durability design. As well known, the surface chloride content which is a key parameter like external loading in structural safety design increases with exposure period. In this study, concrete samples containing OPC (Ordinary Portland Cement), GGBFS (Ground Granulated Blast Furnace Slag), and FA (Fly Ash) had been exposed to submerged, tidal, and splash area for 5 years, then the surface chloride contents changing with exposure period were evaluated. The surface chloride contents were obtained from the chloride profile based on the Fick's 2nd Law, and the regression analysis for them was performed with exponential and square root function. After exposure period of 5 years in submerged and tidal area conditions, the surface chloride content of OPC concrete increased to 6.4 kg/m3 ~ 7.3 kg/m3, and the surface chloride content of GGBFS concrete was evaluated as 7.3 kg/m3 ~ 11.5 kg/m3. In the higher replacement ratio of GGBFS, the higher surface chloride contents were evaluated. The surface chloride content in FA concrete showed a range of 6.7 kg/m3 to 9.9 kg/m3, which was the intermediate level of OPC and GGBFS concrete. In the case of splash area, the surface chloride contents in all specimens were from 0.59 kg/m3 to 0.75 kg/m3, which was the lowest of all exposure conditions. Experimental constants available for durability design of chloride ingress were derived through regression analysis over exposure period. In the concrete with GGBFS replacement ratio of 50%, the increase rate of surface chloride contents decreased rapidly as the water to binder ratio increased.

Key Words
durability design; exposure period; GGBFS; surface chloride content

(1) Jong-Suk Lee:
Department of Structural Engineering Research, Korea Institute of Civil Engineering and Building Technology, 283 Goyang-daero, Insanseo-gu, Goyang, 10233, South Korea;
(2) Keun-Hyeok Yang:
Department of Plant and Architectural Engineering, Kyonggi University, 154-42 Gwanggyosan-ro, Yeongtong-gu, Suwon, 16227, South Korea;
(3) Yong-Sik Yoon:
Korean Peninsula Infrastructure Special Committee, Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Insanseo-gu, Goyang, 10233, South Korea;
(4) Jin-Won Nam:
Department of Civil and Environmental Engineering, Inha Technical College, 100 Inha-ro, Michuhol-gu, Incheon, 22212, South Korea;
(5) Seug-Jun Kwon:
Department of Civil and Environmental Engineering, Hannam University, 70 Hannam-ro, Daedeok-gu, Daejeon, 34430, South Korea.

The aim of the study was to establish the influence of particle size, chemical and phase composition of fine microspherical high-calcium fly ash (HCFA), as well as superplasticizer content on the strength of cementless composite materials based on 100% HCFA and mixtures of HCFA with Portland cement (PC). For the initial HCFA fractions, the particle size distribution, chemical and quantitative phase composition were determined. The compressive strength of cured composite materials obtained at W/B 0.4 and 0.25 was determined at a curing time of 3-300 days. For cementless materials, it was found that a change in the particle size d90 from 30 μm (fraction 3) to 10 μm (fraction 4) leads to an increase in compressive strength by more than 2 times. Compressive strength increases by at least another 2.2 times with the addition of Melflux 5581F superplasticizer (0.12%) and at W/B 0.25. The HCFA-PC blends were investigated in the range of 60-90% HCFA and the maximum compressive strength was found at 80% HCFA. On the basis of 80% HCFA-20% PC blend, the samples of ultra-high strength (108 and 150 MPa at 28 and 100 days of hardening) were obtained with the addition of 0.3% Melflux 5581F and 5% silica fume. The quantitative phase composition was determined for composite materials with a curing age of 28 days. It has been established that in a sample with ultra-high strength, a more omplete transformation of the initial phases of both HCFA and PC occurs as compared to their transformation separately.

Key Words
binder; compressive strength; high-calcium fly ash; superplasticizer; X-ray diffraction

(1) Olga M. Sharonova, Leonide A. Solovyov, Alexander G. Anshits:
Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center "Krasnoyarsk Science Center SB RAS", Akademgorodok 50/24, Krasnoyarsk, 660036, Russia;
(2) Alexander G. Anshits:
Siberian Federal University, 79 Svobodny pr., Krasnoyarsk, 660041 Russia.

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