Tsong Yen

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.

OPTIMIZING MIXTURE PROPORTIONS FOR FLOWABLE HIGH-PERFORMANCE CONCRETE VIA RHEOLOGY TESTS

This paper describes several series of tests involving binder combinations, water-binder ratios, and high-range water-reducing admixture-binder ratios conducted to optimize mixture proportions for flowable high-performance concrete (HPC). Test methods used include the standard slump, slump-flow spread, and rheology test procedures. Test samples were made with freshly mixed paste, mortar, and concrete. Measured data was used to calculate volumetric fractions of coarse aggregate and filling ratios of sand and to correlate the flow characteristics of tested paste, mortar, and concrete. Analytical results show that a slump value of at least 235 mm and slump-flow spread of 500 mm or more under conditions where there is no aggregate segregation are considered viable indicators for producing flowable HPC. A slump range of 235-270 mm and a slump-flow spread range of 520-750 mm verified by rheology test data are recommended for reference in optimizing mixture proportions for flowable HPC.

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.

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...

Research on the Shrinkage Deformation and Crack Development of High-Strength High-Performance Concrete Containing Silica Fume

High-strength high-performance concrete containing silica fume may easily cause cracking at early age. In this research, self designed edge restrained plate specimens of 600 mm × 600 mm × 100 mm in dimension were cast with silica fume concrete and embedded optic fiber sensor (SOFO) for tests, which include image analysis, crack development observation, and continuously detection of the shrinkage deformation of concrete at early age. Test results indicated that the shrinkage deformation of concrete at the age of 1-3 hours is 0.01 mm, equal to a strain of 20×10 -6 (mm/mm) or a tensile stress of about 2.4 MPa, so the plastic cracking may occur within one hour. The shrinkage deformation of concrete at the age of 3-12 hours increased to a strain of 278×10 -6 (mm/mm), indicating a relatively great growing rate of crack development. After the age of 12 hours, the volume change tends to a stable situation that the shrinkage crack will occur only when the shrinkage stress becomes greater than the corresponding tensile strength of concrete.

Earthquake damage to the strength of newly placed concrete

Abstract The purpose of this paper is to study the 28‐day strength of concrete which suffers shaking, such as an earthquake, during its strength developing period. Focusing on moderate strength concrete, 34.31 MPa (5000 psi), groups of concrete cylinders were made and tested on a shaking table. The PGA level of the earthquake and the cylinder age are the two main variables considered in this study. Some testing results can be given as follows: If the concretes age is older than three days, the earthquake shaking will not reduce the 28‐day strength significantly. For an age less than two days, shaking will reduce its strength. However, the tendency is not monotonic. The 28‐day strength can increase for concrete aged between initial setting and 12 hours old, and concrete exposed to intense earthquake (say 0.33g).

Optimizing mix‐proportions for flowable high performance concrete via mortar rheology

Abstract Rheology of flowable high performance concrete(HPC) is pretty much governed by the viscous property of its cement paste. In other words, the flowability of mortar determines the movability of coarse aggregates in a fresh concrete, less friction along interfaces of coarse aggregate enhances smoother flow of fresh concrete. Our experimental study concluded that a proper rheological range and suitable mortar thickness coating on coarse aggregate collectively control the flow of high performance concrete. Based on Newtons viscous flow law and the mortar overfill/coating thickness, a series of test programs using mix proportions that yielded 60 cm slump flow were carried out to determine the relationship between the flowability of mortar and the volumetric fraction of coarse aggregate used. In addition, mortars made with 12 mix proportions similar to those used in the concrete mixes having 600 mm slump flow were carried out to verify the validity of the test results obtained in the concrete test series. This report describes the test program along with test data and findings on optimizing mix proportions for production of flowable high performance concrete.