How-Ji Chen

Use of building rubbles as recycled aggregates

The application of building rubble collected from damaged and demolished structures is an important issue in every country. After crushing and screening, this material could serve as recycled aggregate in concrete. A series of experiments using recycled aggregate of various compositions from building rubble was conducted. The test results show that the building rubble could be transformed into useful recycled aggregate through proper processing. Using unwashed recycled aggregate in concrete will affect its strength. The effect will be more obvious at lower water/cement ratios. When the recycled aggregate was washed, these negative effects were greatly improved. This is especially true for the flexural strength of the recycled concrete. The recycled coarse aggregate is the weakest phase at a low water/cement ratio. This effect will dominate the strength of recycled concrete. This mechanism does not occur in recycled mortar. The quantity of recycled fine aggregate will govern the mortar strength. D 2003 Elsevier Science Ltd. All rights reserved.

DETERMINATION OF THE DIVIDING STRENGTH AND ITS RELATION TO THE CONCRETE STRENGTH IN LIGHTWEIGHT AGGREGATE CONCRETE

Abstract As concrete is considered to be a composite material consisting of mortar and coarse aggregate, its strength depends on the mortar strength and coarse aggregate strength. During the strength development stage of a lightweight aggregate concrete, a critical condition, under which the type of stress distribution changes, occurs as the values of the modulus of elasticity of the lightweight aggregate and mortar become the same. The concrete strength corresponding to this instant is named ‘Dividing Strength (FG)’ by Weigler & Karl (Stahlleichtbeton, 1972, pp. 38–43) [1]. A series of specimens consisting of 252 concrete cylinders (100φ × 200 mm) and 252 mortar cubes (50 × 50 × 50 mm) were made and subjected to compression tests. From the obtained mortar-concrete strength relationship, we were able to identify a dividing strength for a lightweight aggregate concrete made with a 0.4 water cement ratio. The test data also show that both particle density and particle sizes of lightweight aggregate are governing factors for the dividing strength. The concept of a dividing strength can be utilized to optimize a mix design for lightweight aggregate concrete. For instance, concrete that has a designed compressive strength much greater than its dividing strength may consume too much cement and resulting to wasteful cement consumption.

Dynamic Properties of Lightweight Concrete Beams Made by Sedimentary Lightweight Aggregate

This research aimed to investigate the engineering and dynamic properties, such as unit weight, stiffness, natural frequency and damping ratio, of lightweight aggregate concrete (LWAC) beams made with sedimentary lightweight aggregate (LWA) and to compare with companion normal weight concrete (NWC) beams. The primary design variables included compressive strengths of 20, 40, and 60 MPa and reinforcement ratios of 0%, 1.03%, and 2.32%, respectively. A total of 62 beams were made and tested. Test results showed that the unit weight of LWAC beams was about 16–23% lower than that of NWC beams for the same strength level. In addition, the reduced modulus of elasticity of LWAC resulted in a stiffness decrease of reinforced lightweight concrete (RLC) beams of 5–15% related to the reinforced NWC (RC) beams. Nevertheless, the natural frequency of RLC was still higher by about 1–10% than that of RC. In contrast, it was also found that the porous LWA with high damping capacity enhanced the damping ratio of RLC beams by 13–30% for concrete strength in the range from 20 to 60 MPa. As a whole, the lower the concrete strength is (e.g., 20 MPa), the more effective will be for the lightness of LWAC beam and the damping ratio, which in turn is more favorable to the seismic resistant efficiency of LWAC beam.

A monitoring method for scaffold-frame shoring systems for elevated concrete formwork

Abstract This article proposes a monitoring method to prevent scaffold-frame shoring systems, used as a falsework for an elevated concrete formwork, from collapse. Basically, if a scaffold-frame shoring fails, it fails in buckling failure mode. To avoid the buckling failure, it is recommended that two parameters, axial forces and lateral displacements, be monitored in the field. The allowable axial forces and displacements for these two parameters as well as the locations of the monitoring instruments to be installed are proposed based on analysis and full-scale laboratory experiments. A data gathering real-time analysis program should be combined with the monitoring process so that warning signals can be issued in advance of the collapse. The whole monitoring process including the alert giving can be managed using a personal computer. A flowchart to show this management is also proposed herein. Finally, a site test has been performed, and the test result indicates that the suggested monitoring method is basically applicable.

Paper Sludge Reuse in Lightweight Aggregates Manufacturing

The lightweight aggregates used by the civil engineering market are sintered at a high temperature, about 1200 °C. In times of high energy prices and regulation of carbon dioxide emissions, lightweight aggregate products of the high-temperature process in sales marketing are not readily accepted. This study developed a sintered-type paper sludge lightweight aggregate. In order to reduce energy consumption, substitution of some reservoir sediment clay in paper sludge substitutes is to be expected. The study used two types of paper sludge (green clay paper sludge and paper pulp sludge). The sintering temperature was reduced effectively as the green clay paper sludge was substituted for some of the reservoir sediment clay, and the optimum substitute ranges of green clay paper sludge were 10%–50%. The optimum substitute ranges of the paper pulp sludge were 10%–40%. Test results show that the properties of aggregates have a particle density of 0.66–1.69 g/cm3, a water absorption of 5%–30%, and a loss on ignition of 10%–43%. The loss on ignition of aggregate became greater with the increase in paper sludge content. This means that the calorific value provided by the paper sludge will increase as paper sludge content increases. Paper sludge can therefore be considered a good material to provide heat energy for sintering lightweight aggregate.

DESIGN OF SCAFFOLD SHORES FOR CONCRETE BUILDINGS DURING CONSTRUCTION

ABSTRACT The aim of this paper is to offer some practical information for consideration in designing scaffold systems commonly used in concrete building construction. To this end, the content of this work is divided into two parts: the load part and the resistance part. In the load part, the loads during the process of concrete placing are discussed. Some field data have been collected. The intensities and distributions of the gravity loads are based on the investigations and calculations of 20 working sites. An amplification factor (A.F.) is also suggested to cover the effects caused by some seriously unsymmetrical concrete-placing paths. In the resistance part, the load-carrying capacities and the variations of the scaffolds are discussed. Some laboratory and field tests along with analytical work have been made. The unit structure which serves as a basic unit to develop the entire scaffold system is also suggested Based on the load and resistance information, a step-by-step design procedure for scaffol...

The accelerated method for estimating corrosion of reinforced concrete structure in seawater

The corrosion of concrete for offshore structures is generally caused by the penetration of chloride ion deriving from the salts in seawater. Up to now, lots of researches have been done concerning the corrosion of concrete by seawater and durability of concrete. However, the investigation for corrosion of reinforced concrete structure by seawater are relatively rare, because of the needs for long term tests (at least 3 to 5 years usually). The structures sited in the intertidal zone always accompany severer deterioration by seawater. Under such environmental conditions, frequent wetting and drying will aggravate the effect of sulfate attack, while the crystallization of sea salts in the concrete on evaporation may also contribute to expansive forces. For these reasons above, the purpose of this paper is to study the corrosion of reinforced concrete structure in the intertidal zone by a accelerated method presented. The critical variables considered in this research include the temperature and concentration of seawater. During the experimental period (1 year), the specimens were cured in wetting and drying situation (for each circle, submerged by seawater for 24 hours and then drying at 70°C for the following 24 hours), and this repeatedly cyclic procedures is to accelerate the corrosion of reinforced concrete structure by seawater. According to the experimental results, the increased concentration and temperature of seawater would accelerate deterioration of concrete obviously. The duration of testing could be shortened about 75% by using the sweater with 10 times the concentration. As the curing temperature increased, the corresponding strength concrete at earlier age would increase, and which could reduce the corrosion of reinforced concrete.

Fire Performance and Thermal Insulation of Reinforced Lightweight Aggregate Concrete

Structural lightweight aggregate concrete (LWAC) members have demonstrated greater fire endurance periods than equivalent thickness members made with normal-weight aggregates. Superior performance is due to a combination of lower thermal conductivity, lower coefficient of thermal expansion, and the inherent thermal stability developed by aggregates that have been exposed temperature greater than 1050°C during preprocessing. Furthermore, LWAC exhibits relatively high thermal insulating value, of which the thermal conductivity can be half as much as that of ordinary normal-weight concrete (NWC). Therefore, the main objective of this paper is to implement fire resistance testing for structural and non-structural elements made of LWAC and NWC to assess and compare their fire behavior.

Properties of Lightweight Concrete Masonry Units Made from Lightweight Aggregates

In the paper the properties of concrete masonry unit (CMU) made from sedimentary lightweight aggregate (LWA) were investigated. The main variables include water to cementitious material ratio (W/CM), filling ratio of paste or mortar in voids between coarse aggregate particles (Fv), filling ratio of sand in mortar (Fm), and cement replacement level by slag (Sc). Test results of representative CMU specimens show that unit weight ranged from 1585 to 1743 kg/m3, which was 30-25% lower than that for a normal weight CMU (2300 kg/m3); compressive strengths ranged from 8.4 to 18.7 MPa; water absorption was found to vary between 0.05 to 0.13 g/cm3; and thermal conductivity ranged from 0.27 to 0.41 W/mK. The research findings demonstrate that the use of sedimentary LWA as coarse aggregate in various concrete mixtures could produce high performance lightweight CMU, which comply with the requirements of Chinese National Standards (CNS) standards.

Pre-Failure Deflection and Residual Load Capacity of Reinforced Lightweight Aggregate Concrete Beams under High-Cycle Fatigue

The present study experimentally investigated the pre-failure and post-fatigue behavior of reinforced concrete (RC) beams constructed with lightweight aggregate concrete (LWAC) in comparison with that constructed of normal weight concrete (NWC) of the same compressive strength (40 MPa). A total of twelve RC beams were tested under different fatigue loadings. Based on the experimental observations, the midspan total deflection measured in the fatigue testing consisted of the elastic and plastic components. The mechanismof the two deflection components developed with load cycles was different. The experimental results showed that the fatigue resistance of LWAC beams was better than that of NWC beams for the same fatigue loading levels. It was reflected in both the lower evolution of fatigue damage and the smaller growth of midspan residual deflection. After 2 million cycles, an average increase in residual load capacity of about 8% was found in the NWC beams, while that in the LWA beams remained virtually unchanged.