International Concrete Abstracts Portal

An analytical procedure is described for predicting the development of vertical cracks in thin-walled and thick-walled cylindrical structures subjected to membrane forces and thermal loads. Sustained, axisymmetrical (thermal) loads and thermal cyclic loading may jeopardize, due to cracking, the serviceability (in this case the tightness) of thin-walled cylinders. Mathematically obtained crack patterns have been compared with field observations: a good agreement between theory and practice could be established from this comparison. On the basis of a reliable prediction of crack patterns, cost-benefit analyses are feasible to weigh crack control measures against possible repair costs in case these measures were neglected. An example of such an analysis shows an initial increase of reinforcement in view of crack control to be preferable to repair (grouting) of cracks.

A rational method of analysis is developed that can be used on a computer to determine the behavior of reinforced concrete columns under sustained service loads. At specific time intervals, trial-and-error procedures are used to establish strain compatibility and equilibrium conditions at each of several cross sections of a member. Curvatures are integrated to find the deflected shape, and an iterative approach is used to find the stable deflected shape if there are secondary moments. The analysis calculates the effect of creep on the stress redistribution between concrete and steel and on the deflections of members subjected to variable axial loads and variable moments. The effects of shrinkage and cracking are also included. The applicability of the analysis is partially verified by comparison with laboratory and field investigations reported by various researchers. In most cases, a good correlation is obtained between the analytical results and the measured results.

Differential elastic, creep, shrinkage, and thermal deformations of vertical concrete elements, columns, and walls in tall building structures require special attention to insure proper behavior for both strength and serviceability of the structure and the attached nonstructural elements. The long-term serviceability problems include out-of-level floors in both concrete and composite buildings, and cracking and deformations of internal partitions and external cladding elements. A procedure is developed to predict the long-term deformations of reinforced concrete columns, walls, and composite columns. The procedure incorporates the effects of concrete properties, construction sequence, and loading history. For composite columns, the effects of load transfer from the steel erection column to the reinforced concrete column are also included. Methods to minimize differential shortening of columns and walls are discussed. The methods involve corrections during both design and construction phases. Differential shortening effects for three tall buildings, in Chicago, which were designed using the procedure, are discussed. Results from six years of field measurements of column shortening are compared with predicted values.

Long-term behavior of in situ reinforced concrete columns in two structures during construction, occupancy, and service is reported. Extensive strain and moisture movements were made up to about 10 years of service, and long-term movements at 25 to 30 years were then estimated. The stress history experienced by the columns in the second structure was monitored through a stress meter embedded in one of the columns, and the influence of reinforcement and the time-dependent movements on the stress history is described. The gradual but significant stress redistribution with time and the resulting concrete-steel load transfer is also discussed. Results show that the time-dependent deformation in in situ columns occurred over a very long period of time and continue to occur at a very small rate. However, the majority of movements in the columns occurred during the first 5-year period after construction of the columns. It is shown that dead load appears to be the predominant load carried by the columns. The design steel stress has been exceeded in several columns within 10 years of service life; however, none of the steel is expected to yield in 50 years of service life. Based on the in situ performance of columns along with other available data, a design recommendation is made to incorporate the effects of load transfer from concrete to steel at the design stage.

Presents a simple design aid for predicting long-term (up to 50 years) movements in reinforced concrete columns and bridge beams made of normal and lightweight aggregate concrete. The method is based on the principle of superposition using a creep factor chart, which takes into account varying sizes of members, age at loading, exposure conditions, and the percentage of reinforcement, and it requires only a knowledge of the concrete strength and the loading history of the member. The method is developed from the study of in situ movements in two reinforced concrete structures subjected to increment loading. The shrinkage strains in columns are predicted using a shrinkage chart, which requires only a knowledge of elastic modulus of concrete at 28 days. The predicted load-induced and basic strains show excellent agreement with measured strains in the two structures, and the method shows good agreement with literature. The paper demonstrates how the simple method of predicting long-term movements in buildings and bridges can be utilized by the structural engineer as a designer's tool.