Investigation on correlation between pulse velocity and compressive strength of concrete using ANNs

Chao-Wei Tang,Yiching Lin,Shih-Fang Kuo 한국계산역학회 2007 Computers and Concrete, An International Journal Vol.4 No.6

The ultrasonic pulse velocity method has been widely used to evaluate the quality of concrete and assess the structural integrity of concrete structures. But its use for predicting strength is still limited since there are many variables affecting the relationship between strength and pulse velocity of concrete. This study is focused on establishing a complicated correlation between known input data, such as pulse velocity and mixture proportions of concrete, and a certain output (compressive strength of concrete) using artificial neural networks (ANN). In addition, the results predicted by the developed multilayer perceptrons (MLP) networks are compared with those by conventional regression analysis. The result shows that the correlation between pulse velocity and compressive strength of concrete at various ages can be well established by using ANN and the accuracy of the estimates depends on the quality of the information used to train the network. Moreover, compared with the conventional approach, the proposed method gives a better prediction, both in terms of coefficients of determination and root-mean-square error.

Effect of presoaking degree of lightweight aggregate on the properties of lightweight aggregate concrete

Chao-Wei Tang 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.1

This study aimed at exploring the effect of presoaking degree of lightweight aggregate (LWA) on the fresh and hardened properties of concrete. Two series (i.e., Series A and Series B) of concrete mixes that were made of LWA with different moisture states were prepared. The presoaking degree of LWA was divided into three types: oven dry state, 1 hour prewetted and 24 hours prewetted. For the Series A, the water content of the lightweight aggregate concrete (LWAC) mixes was adjusted in accordance with the moisture condition of the LWA. Whereas the amount of water added in the Series B mixes was deliberately not adjusted for the moisture condition of the LWA. Slump test, mechanical tests, interfacial transition zone microscopical tests and thermal conductivity test were carried out on the specimens of different concretes and compared with control normal-weight aggregate concretes. The test results showed that the effect of mixing water absorption by LWA with different moisture states was reflected in the fresh concrete as the loss of mixture workability, while in the hardened concrete as the increase of its strength. With the use of oven-dried LWA, the effect of reduction of water-cement ratio was more significant, and thus the microstructure of the ITZ was more compact.

Local bond–slip behavior of fiber reinforced LWAC after exposure to elevated temperatures

Chao-Wei Tang 국제구조공학회 2020 Structural Engineering and Mechanics, An Int'l Jou Vol.73 No.4

The microstructure and mechanical properties of concrete will degrade significantly at high temperatures, thus affecting the bond strength between reinforcing steel and surrounding concrete in reinforced concrete members. In this study, the effect of individual and hybrid fiber on the local bond–slip behavior of lightweight aggregate concrete (LWAC) after exposure to elevated temperatures was experimentally investigated. Tests were conducted on local pullout specimens (150 mm cubes) with a reinforcing bar embedded in the center section. The embedment lengths of the pullout specimens were 4.2 times the bar diameter. The parameters investigated included concrete type (control group: ordinary LWAC; experimental group: fiber reinforced LWAC), concrete strength, fiber type, and targeted temperature. The test results showed that for medium-strength LWACs exposed to high temperatures, the use of only steel fibers did not significantly increase the residual bond strength. Moreover, the addition of individual and hybrid fiber had little effect on the residual bond strength of the high-strength LWAC after exposure to a temperature of 800°C.

Fire resistance of high strength concrete filled steel tubular columns under combined temperature and loading

Chao-Wei Tang 국제구조공학회 2018 Steel and Composite Structures, An International J Vol.27 No.2

In recent years, concrete-filled box or tubular columns have been commonly used in high-rise buildings. However, a number of fire test results show that there are significant differences between high strength concrete (HSC) and normal strength concrete (NSC) after being subjected to high temperatures. Therefore, this paper presents an investigation on the fire resistance of HSC filled steel tubular columns (CFTCs) under combined temperature and loading. Two groups of full-size specimens were fabricated to consider the effect of type of concrete infilling (plain and reinforced) and the load level on the fire resistance of CFTCs. Prior to fire test, a constant compressive load (i.e., load level for fire design) was applied to the column specimens. Thermal load was then applied on the column specimens in form of ISO 834 standard fire curve in a large-scale laboratory furnace until the set experiment termination condition was reached. The results demonstrate that the higher the axial load level, the worse the fire resistance. Moreover, in the bar-reinforced concrete-filled steel tubular columns, the presence of rebars not only decreased the spread of cracks and the sudden loss of strength, but also contributed to the load-carrying capacity of the concrete core.

Uniaxial bond stress-slip behavior of reinforcing bars embedded in lightweight aggregate concrete

Chao-Wei Tang 국제구조공학회 2017 Structural Engineering and Mechanics, An Int'l Jou Vol.62 No.5

This paper presents an experimental study of bond-slip behavior of reinforced lightweight aggregate concrete (LC) and normal weight concrete (NC) with embedded steel bar. Tests were conducted on tension-pull specimens that had cross-sectional dimension with a reinforcing bar embedded in the center section. The experimental variables include concrete strength (20, 40, and 60 MPa) and coarse aggregate type (normal-weight aggregate and reservoir sludge lightweight aggregate). The test results show that as concrete compressive strength increased, the magnitudes of the slip of the LC specimens were greater than those of the NC specimens. Moreover, the bond strength and stiffness approaches zero at the loaded end, or close to the central anchored point of the specimen. In addition, the proposed bond stress-slip equation can effectively estimate the behavior of bond stress and steel bar slipping.

Hydration properties of cement pastes containing high-volume mineral admixtures

Chao-Wei Tang 사단법인 한국계산역학회 2010 Computers and Concrete, An International Journal Vol.7 No.1

This research aimed to investigate the influence of high-volume mineral admixtures (MAs), i.e., fly ash and slag, on the hydration characteristics and microstructures of cement pastes. Degree of cement hydration was quantified by the loss-on-ignition technique and degree of pozzolanic reaction was determined by a selective dissolution method. The influence of MAs on the pore structure of paste was measured by mercury intrusion porosimetry. The results showed that the hydration properties of the blended pastes were a function of water to binder ratio, cement replacement level by MAs, and curing age. Pastes containing fly ash exhibited strongly reduced early strength, especially for mix with 45% fly ash. Moreover, at a similar cement replacement level, slag incorporated cement paste showed higher degrees of cement hydration and pozzolanic reaction than that of fly ash incorporated cement paste. Thus, the present study demonstrates that high substitution rates of slag for cement result in better effects on the short- and long-term hydration properties of cement pastes.

Shear behavior of reinforced HPC beams made of a low cement content without shear reinforcements

Chao-Wei Tang,Yu-Ping Chen,How-Ji Chen,Chung-Ho Huang,Tsang-Hao Liu 사단법인 한국계산역학회 2013 Computers and Concrete, An International Journal Vol.11 No.1

High-performance concrete (HPC) usually has higher paste and lower coarse aggregate volumes than normal concrete. The lower aggregate content of HPC can affect the shear capacity of concrete members due to the formation of smooth fractured surfaces and the subsequent development of weak interface shear transfer. Therefore, an experimental investigation was conducted to study the shear strength and cracking behavior of full-scale reinforced beams made with low-cement-content high-performance concrete (LcHPC) as well as conventional HPC. A total of fourteen flexural reinforced concrete (RC) beams without shear reinforcements were tested under a two-point load until shear failure occurred. The primary design variables included the cement content, the shear span to effective depth ratio (a/d), and the tensile steel ratio (ρw). The results indicate that LcHPC beams show comparable behaviors in crack and ultimate shear strength as compared with conventional HPC beams. Overall, the shear strength of LcHPC beams was found to be larger than that of corresponding HPC beams, particularly for an a/d value of 1.5. In addition, the crack and ultimate shear strength increased as a/d decreased or ρw increased for both LcHPC beams and HPC beams. This investigation established that LcHPC is recommendable for structural concrete applications.

Local bond stress-slip behavior of reinforcing bars embedded in lightweight aggregate concrete

Chao-Wei Tang 사단법인 한국계산역학회 2015 Computers and Concrete, An International Journal Vol.16 No.3

This paper aims to study the local bond stress-slip behavior of reinforcing bars embedded in lightweight aggregate concrete (LWAC). The experimental variables of the local bond stress-slip tests include concrete strength (20, 40 and 60 MPa), deformed steel bar size (#4, #6 and #8) and coarse aggregate (normal weight aggregate, reservoir sludge lightweight aggregate and waterworks sludge lightweight aggregate). The test results show that the ultimate bond strength increased with the increase of concrete compressive strength. Moreover, the larger the rib height to the diameter ratio (h/db) of the deformed steel bars is, the greater the ultimate bond stress is. In addition, the suggestion value of the CEB-FIP Model Code to the LWAC specimen’s ultimate bond stress is more conservative than that of the normal weight concrete.

Local bond-slip behavior of medium and high strength fiber reinforced concrete after exposure to high temperatures

Chao-Wei Tang 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.4

This study aims to investigate the influence of individual and hybrid fiber on the local bond–slip behavior of medium and high strength concrete after exposure to different high temperatures. Tests were conducted on local pullout specimens (150 mm cubes) with a reinforcing bar embedded in the center section. The embedment lengths in the pullout specimens were three times the bar diameter. The parameters investigated include concrete type (control group: ordinary concrete; experimental group: fiber concrete), concrete strength, fiber type and targeted temperature. The test results showed that the ultimate bond stress in the local bond stress versus slip curve of the high strength fiber reinforced concrete was higher than that of the medium strength fiber reinforced concrete. In addition, the use of hybrid combinations of steel fiber and polypropylene fiber can enhance the residual bond strength ratio of high strength concrete.

Residual properties of high-strength fiber reinforced concrete after exposure to high temperatures

Chao-Wei Tang 사단법인 한국계산역학회 2019 Computers and Concrete, An International Journal Vol.24 No.1

Thermal energy from high temperatures can cause concrete damage, including mechanical and chemical degradation. In view of this, the residual mechanical properties of high-strength fiber reinforced concrete with a design strength of 75 MPa exposed to 400-800°C were investigated in this study. The test results show that the average residual compressive strength of high-strength fiber reinforced concrete after being exposed to 400-800°C was 88%, 69%, and 23% of roomtemperature strength, respectively. In addition, the benefit of steel fibers on the residual compressive strength of concrete was limited, but polypropylene fibers can help to maintain the residual compressive strength and flexural strength of concrete after exposure to 400-600°C. Further, the load-deflection curve of specimen containing steel fibers exposed to 400-800°C had a better fracture toughness.