【正文】 Electronic Journal of Structural Engineering, 1 ( 2021) 15 Shrinkage, Cracking and Deflection the Serviceability of Concrete Structures . Gilbert Professor and Head, School of Civil and Environmental Engineering The University of New South Wales, Sydney, NSW, 2052 Email: ABSTRACT This paper addresses the effects of shrinkage on the serviceability of concrete structures. It outlines why shrinkage is important, its major influence on the final extent of cracking and th e magnitude of deflection in structures, and what to do about it in design. A model is presented for predicting the shrinkage strain in normal and high strength concrete and the timedependent behaviour of plain concrete and reinforced concrete, with and without external restraints, is explained. Analytical procedures are described for estimating the final width and spacing of both flexural cracks and direct tension cracks and a simplified procedure is presented for including the effects of shrinkage when calculating longterm deflection. The paper also contains an overview of the considerations currently being made by the working group established by Standards Australia to revise the serviceability provisions of AS3600 1994, particularly those clauses related to shrinkage. KEYWORDS Creep。 Cracking。 Deflection。 Reinforced concrete。 Serviceability。 Shrinkage. 1. Introduction For a concrete structure to be serviceable, cracking must be controlled and deflections must not be excessive. It must also not vibrate excessively. Concrete shrinkage plays a major role in each of these aspects of the service load behaviour of concrete structures. The design for serviceability is possibility the most difficult and least well understood aspect of the design of concrete structures. Service load behaviour depends primarily on the properties of the concrete and these are often not known reliably at the design stage. Moreover, concrete behaves in a nonlinear and inelastic manner at service loads. The nonlinear behaviour that plicates serviceability calculations is due to cracking, tension stiffening, creep, and shrinkage. Of these, shrinkage is the most problematic. Restraint to shrinkage causes timedependent cracking and gradually reduces the beneficial effects of tension stiffening. It results in a gradual widening of existing cracks and, in flexural members, a significant increase in deflections with time. The control of cracking in a reinforced or prestressed concrete structure is usually achieved by limiting the stress increment in the bonded reinforcement to some appropriately low value and ensuring that the bonded reinforcement is suitably distributed. Many codes of practice specify maximum steel stress increments after cracking and maximum spacing requirements for the bonded reinforcement. However, few existing code procedures, if any, account adequately for the gradual increase in existing crack widths with time, due primarily to shrinkage, or the timedependent development of new cracks resulting from tensile stresses caused by restraint to shrinkage. For deflection control, the structural designer should select maximum deflection limits that are appropriate to the structure and its intended use. The calculated deflection (or camber) must not exceed these limits. Codes of practice give general guidance for both the selection of the maximum deflection limits and the calculation of deflection. However, the simplified procedures for calculating deflection in eJSE International Electronic Journal of Structural Engineering, 1 ( 2021) 16 most codes were developed from tests on simplysupported reinforced concrete beams and often produce grossly inaccurate predictions when applied to more plex structures. Again, the existing code procedures do not provide real guidance on how to adequately model the timedependent effects of creep and shrinkage in deflection calculations. Serviceability failures of concrete structures involving excessive cracking and/or excessive deflection are relatively mon. Numerous cases have been reported, in Australia and elsewhere, of structures that plied with code requirements but still deflected or cracked excessively. In a large majority of these failures, shrinkage of concrete is primarily responsible. Clearly, the serviceability provisions embodied in our codes do not adequately model the inservice behaviour of structures and, in particular, fail to account adequately for shrinkage. The quest for serviceable concrete structures must involve the development of more reliable design procedures. It must also involve designers giving more attention to the specification of an appropriate concrete mix, particularly with regard to the creep and shrinkage characteristics of the mix, and sound engineering input is required in the construction procedures. High performance concrete structures require the specification of high performance concrete (not necessarily high strength concrete, but concrete with relatively low shrinkage, not prone to plastic shrinkage cracking) and a high standard of construction, involving suitably long stripping times, adequate propping, effective curing procedures and rigorous onsite supervision. This paper addresses some of these problems, particularly those related to designing for the effects of shrinkage. It outlines how shrinkage affects the inservice behaviour of structures and what to do about it in design. It also provides an overview of the considerations currently being made by the working group established by Standards Australia to revise the serviceability provisions of AS36001994 [1], particularly those clauses related to shrinkage. 2. Designing for Serviceability When designing for serviceability, the designer must ensure that the structure can per