Modern high-rise concrete buildings are models of design skill in the use of materials. It is now a commonplace to specify high strength concrete, usually 5,000 psi, for compression members in the lower stories of multi-story buildings. Concentration of reinforcing bars in these members is very high. The trend toward buildings other than rectilinear in shape is producing some unusual structural members. All of these factors spell placing problems unless a mix design is formulated that produces concrete of considerable flowability and high quality.
The problem of obtaining uniform, high-quality concrete in this work hinges mainly on two basic considerations: (1) mix design; and (2) placing and curing procedures. The narrow, high forms often used for columns and shear walls aggravate the tendency of mixes to bleed and produce laitance. These problems are quite common in tall building construction field, but they can be prevented by close attention to two aspects of the mix design: (1) cement paste consistency and physical composition; and (2) characteristics of the aggregate. Air entrainment and minimum water content can also play and important role in halting bleeding, formation of laitance and segregation.
Without proper placing procedures, however, even the best designed concrete will segregate and bleed, resulting in honeycombing, poor bond to steel and other woes. Because column and wall forms are usually rather high and reinforcing bars are often spaced quite closely, workmen must be careful in depositing and vibrating concrete in this work. Concrete should be dropped in a true vertical plane and have a free fall of no more than four feet. Vibration can be a great help in concreting high-rise buildings because it permits use of lower water contents. However, vibration can be misused if it is employed with wet mixes or continued for important bearing on the quality of concrete flatwork
These desirable proportioning and placing practices will also largely hold true for lightweight concrete. However, uniform workability of the mix is more difficult to maintain with lightweight aggregates. This can be improved with air entraining and water reducing agents.