Reinforced concrete columns under compression loads and under little or no moment may exhibit cracking. Some cracks develop at early ages and others years later under sustained axial loads or no significant loads at all. Flexural cracking may be expected from externally applied loads on columns within the tension-controlled zone in the axial load-moment diagram. However, for columns within the compression-controlled zone of the diagram, cracking is not normally expected to occur under allowable service loads. Concrete shrinkage and creep, temperature variations and loading history cause all these cracks. In this paper, the causes of these cracks are described, analyzed and illustrated with photos of cracked columns. Design and construction recommendations to prevent or reduce these cracks are provided.

Creep and shrinkage of concrete were studied under constant load and restrained conditions during the first week after casting. Concrete behavior was characterized by a uniaxial test that measures shrinkage deformation and restrained shrinkage stress. The extent of stress relaxation by tensile creep was determined using superposition analysis. The experimental measurements were compared with current creep and shrinkage models to assess their validity for early age prediction. The ACI 209 equation for creep is currently not applicable to early age, but modifications are proposed that fit a database of early age behavior. The B3 model has been previously modified to accommodate early age creep, and this modification was employed in the current study. Test results for normal concrete with different w/c ratios are discussed.

SP227 Structural engineers are concerned with the consequences of shrinkage, creep and cracking on the serviceability and durability of their structures. Creep increases deflections, reduces prestress in prestressed concrete elements, and causes redistribution of internal force resultants in redundant structures. Shrinkage can cause warping of slabs on grade due to differential drying and increased deflections of non-symmetrically reinforced concrete elements. Materials scientists are concerned with understanding the basic phenomena and assessing new materials and the effects of admixtures on the mechanical behavior of concrete. Concrete is an age stiffening material that has little tensile strength, shrinks, and exhibits creep in sealed conditions and additional creep in drying environments. Predicting the amount of shrinkage and deflection that may occur is not easy and is especially complicated in concrete that contains supplementary materials, chemical admixtures, and lightweight aggregates. Supplementary cementing materials and waste products are being used in increasing volumes in response to environmental concerns. Admixtures have been developed to modify the behavior of fresh and hardened concrete. Self consolidating concrete is being used in more applications. A recent development is the marketing of shrinkage reducing admixtures. This volume contains papers presented during four sessions sponsored by ACI Committee 209, Creep and Shrinkage in Concrete, and ACI Committee 231, Properties of Concrete at Early Ages, held at the Spring 2005 Convention. The subjects addressed by the authors are diverse and cover many aspects of shrinkage and creep. Some papers pay special attention to the development, use, and evaluation of models to predict shrinkage, creep, and deflection, while other papers consider the behavior of early age concretes that are restrained from shrinking, resulting in the development of residual stress and cracking.

This paper presents the results of a 10-year instrumentation and monitoring program on the North Halawa Valley Viaduct, a major prestressed box girder viaduct on the Island of Oahu, Hawaii. The long-term monitoring program was initiated in 1994 during construction of the long-span post-tensioned box-girder viaduct. Over 200 electrical strain, displacement, temperature and load sensors were installed in one unit of the structure and have been monitored continuously since. These instruments monitor vertical deflections, span shortening, prestress loss, longitudinal strains and temperature in the box-girder concrete. The long-term response of this structure is presented and compared with the initial predictions made during the design process. Modified material properties based on short-term shrinkage and creep tests were incorporated into the long-term prediction model to produce significantly improved comparisons. A procedure is proposed for prediction of upper and lower bounds for the long-term response of long-span prestressed concrete bridges. This improved prediction model is applied to the other five units making up the NHVV to verify its performance as a design tool. The results of this study were then incorporated into the development of an instrumentation system for the planned Kealakaha Bridge on the Island of Hawaii. Application of the prediction model is demonstrated using shrinkage and creep data determined from short-term tests performed on the concrete mixture proposed for this new long-span box-girder bridge structure.

Temperature effects are the predominant cause for volume change in concrete pavements. This paper describes an experimental investigation of thermal volume change conducted to improve the understanding of joint movement in concrete pavement. Four slab strips containing embedded strain gauges and thermocouples were monitored in a controlled environment under four heating rates. Each strip was monitored for translation, rotation, and warping height. Key findings of the experiment include the internal strain distribution and non-linear thermal gradients produced by asymmetrical heating. The laboratory data are compared with long-term data from an instrumented parking lot pavement. Analysis of the data provides insight into the prediction of thermal movements and determination of thermal stress development in pavements.