Construction of tall buildings becomes an unstoppable trend all over the world in recent several decades. In Hong Kong, hundreds of high-rise buildings have been built during the period, and the city expands vertically due to the limit of available land. It is well known that concrete is the main construction material for these skyscrapers, playing an important role in the structural behavior. When designing tall buildings, conventionally, structural engineers have to use large cross-section area for beams and columns to control the structural deformation, as the stiffness of the structure is much more critical than strength for high-rise buildings. However, there is no doubt that enhancing the modulus of concrete itself is much more efficient than increasing the cross-section area of s...[ Read more ]

Construction of tall buildings becomes an unstoppable trend all over the world in recent several decades. In Hong Kong, hundreds of high-rise buildings have been built during the period, and the city expands vertically due to the limit of available land. It is well known that concrete is the main construction material for these skyscrapers, playing an important role in the structural behavior. When designing tall buildings, conventionally, structural engineers have to use large cross-section area for beams and columns to control the structural deformation, as the stiffness of the structure is much more critical than strength for high-rise buildings. However, there is no doubt that enhancing the modulus of concrete itself is much more efficient than increasing the cross-section area of structural members when the stiffness of a structural member needs to be improved. The objectives of this thesis include: (1) to optimize concrete mix proportions for high modulus concrete, (2) to characterize dimensional stability properties, (3) to apply the high modulus concrete in producing structural members, (4) to verify high modulus concrete does not have alkali aggregate reaction risk, and (5) to promote the high modulus concrete developed in this study to local construction industry.

To implement the objectives of this study, concrete formulations with different types of coarse aggregate, fine aggregate, different proportions of sand/coarse aggregate ratio, have been prepared and their strength and modulus of elasticity were measured. An innovative ultrasonic technique was utilized during mixing process. The microstructure of interfacial transition zone between cement paste and aggregate was investigated by scanning electron microscope. The majority of the factors influencing the modulus of elasticity of concrete have been studied experimentally and systematically. After that, dimensional stability properties including shrinkage and creep were studied. Meantime, accelerated mortar bar test was conducted to analysis whether alkali silica reaction would happen in high modulus concrete mixture with volcanic aggregate. High modulus concrete developed in this study was then applied in producing structural members, and the different performances of high modulus concrete and conventional concrete were compared.

A new type of concrete, which contains volcanic aggregate and well-graded river sand, having both high strength and high modulus of elasticity has been developed for tall buildings, and its properties were characterized. The experimental results showed that concrete with volcanic rock as coarse aggregate and river sand as fine aggregate had higher stiffness than that with granite rock and crushed stone fines. Types of coarse and fine aggregate had significant effects on modulus of elasticity but had limited effects on compressive strength. Sand ratio around 43% and silica fume content about 10% were found suitable for making high modulus concrete. Water to cement ratio was the key issue in determining the concrete stiffness, and it should be minimized so that the lowest porosity and highest stiffness of cement paste can be achieved. Meanwhile, well-graded size distribution of river sand was better than uniform grading when developing high modulus concrete, and the aggregate to cement ratio cannot be too large because there must be adequate cement matrix surrounding aggregates. Mineral admixtures such as metakaolin and nano-silica led to higher modulus of elasticity of concrete, but the water/cement ratio had to sacrifice. At last, ultrasonic mixing technique was used for making high modulus concrete, and it was verified to be helpful for concrete stiffness. So far, a particular concrete with the cubic compressive strength of 146 MPa and modulus of elasticity of 53.5 GPa was developed for tall buildings, which is 20% stiffer than normal concrete in the code of practice. For the dimensional stability properties, test results indicated that high modulus concrete had much smaller drying shrinkage value although its autogenous shrinkage was a little larger than normal concrete. Therefore, the total shrinkage of high modulus concrete was still much smaller than that of normal concrete. Meanwhile, high modulus concrete also showed smaller creep deformation than normal concrete. Alkali aggregate reaction test results indicated that the potential alkali reactivity of volcanic rock was innocuous and 30% of fly ash can help to further reduce this risk. The load capacity of reinforced ultra-high modulus concrete beam was approximately 35% higher than that of C45 concrete beam with the same reinforcement ratio. At actual service stress level, the mid-span deflection of ultra-high modulus concrete beam was only roughly 46% of normal concrete, demonstrating its advantages in structural deformation control.

High modulus concrete has shown a very good workability, excellent mechanical property, outstanding dimensional stability and durability. By using this concrete in tall buildings, 15-20% materials can be saved and up to 2-5% more useable space can be achieved. It provides a new option for construction of tall buildings and also gives a practical manual to produce such high modulus concrete. It is no doubt that high modulus concrete will lead to an unprecedented revolution in construction industry.