By Serena Gilles, PE
Pouring concrete in hot and cold weather conditions requires special attention in order to achieve desirable strength and quality.
Reinforced concrete slabs have been used in buildings from the middle of the 19th century. Post-tensioned concrete slabs have been used since the 1930’s, and have become commonplace in the last 30 years.
Architects like concrete slabs because the designs can allow for longer spans and thinner slabs. Longer spans give more column-free space and more available square footage. Thinner slabs allow for higher floor-to-floor heights.
Post tensioned concrete is a method of casting sheathed steel cables inside of the concrete. The concrete is allowed to cure for a period usually between two and seven days, and afterward the cables are tensioned using hydraulic jacks. The application of tension provides internal stresses in the concrete to counteract forces that the concrete member is subjected to during the life of the structure. A post-tensioned concrete member can be a reduced member size compared to a conventionally reinforced concrete member subject to the same forces thus making post tensioning an attractive option for owners, architects and engineers. Post tensioning can also reduce the potential for cracking and the amount of conventional steel reinforcing. Using post tensioned concrete construction can improve the quality and durability of the structure often also saving on the cost of the construction.
Finding the information necessary to understand the current coding requirements for tilt-up wall panels within the ACI Standard and Report can be frustrating and confusing for new engineers as the information seems to be scattered among multiple sections. The current ACI318 forces designers to seek out information contained in different sections and have a deep understanding of the current code to meet necessary requirements. The new code organization and simplified design process eases the process and makes design procedures more accessible.
ACI318, Building Code Requirements for Concrete and Commentary, is updated every three years. These code updates and changes stem from comments submitted to the American Concrete Institute (ACI) from professors, practicing professionals, and industry users about the modernization, common practices, industry consistency, and engineering accuracy. ACI 318-14* will completely change the format of the code from previous versions to a more user friendly and logical focus for designers.
These are simple questions that require a complex answer. Reshoring is the process of utilizing multiple levels of shores below the story being cast to distribute the applied construction loads to multiple stories. Concrete is heavy and without a sufficient number of levels to support the weight the slabs can become overloaded.
Concrete formwork is the temporary structure built to support and confine concrete until it hardens and it is commonly broken into two categories: formwork and shoring. Formwork refers to vertical forms used to form walls and columns whereas shoring refers to horizontal formwork to support slabs and beams.
Forms must be designed to resist all vertical and lateral loads exposed onto the formwork during transport and in-use. Forms can be either pre-engineered panels or custom-built for the job. The advantage of pre-engineered panels is the speed of assembly and the ease of reconfiguring the forms to cycle to multiple pour locations. The disadvantages are fixed panel and tie dimensions that limit their architectural applications and allowable design loads that may limit their use for certain applications. Custom-built forms are designed to maximize the efficiency for each application but they are not as easy to reconfigure for other pour locations. Custom forms can be built to accommodate any architectural consideration or loading condition.