International Concrete Abstracts Portal

This paper presents the state-of-the art in the evaluation of the flexural crack width development and crack control of flexural cracks in reinforced and prestressed concrete structures It is based on extensive research over the past five decades in the United States and overseas in the area of macro-cracking in reinforced and prestressed concrete elements. Mitigation and control of cracking has become essential in order to maintain the integrity and aesthetics of concrete structures and their long-term durability performance. The trend is stronger than ever towards better utilization of concrete strength, use of higher strength concretes in the range of 12,000-20,000 psi and higher compressive strength, more prestressed concretes and increased uses of limit failure theories - all these trends require closer control of serviceability requirements of cracking and deflection behavior. The paper discusses and presents common expressions for the mitigation and control of cracking in reinforced concrete beams and thick one-way slabs, prestressed, pretensioned and post-tensioned flanged beams, reinforced concrete two-way action structural floor slabs and plates, and large diameter circular tanks In addition, recommendations are given for the maximum tolerable flexural crack widths in concrete elements based on the cumulative experience of many investigators over the past five decades. The expressions include the ACI 3 18-99 crack control provisions in reinforced concrete beams and one-way slabs, and the Concrete Euro Code 1999 for the design of concrete buildings.

A case history is presented in which a laser-screeded slab showed more cracks than would be expected in conventional strip-cast slabs. It was determined that laser-screeded slabs are more sensitive to early-age, thermal cracking than strip-cast slabs because of extra restraint provided by fixed-edge boundary conditions. Among the possible solutions is closer spacing of contraction joints and fog curing during the first day.

This paper provides an outline of the provisions for design for cracking given in the current version of Eurocode 2; the Eurocode for the design of concrete structures. The basic theory underlying the clauses is derived, the content of the clauses themselves are outlined and the development of simplified detailing rules for the control of cracking is considered.

A number of fundamental concepts relevant to all types of cracking are examined. A tension stiffening relationship derived from first principles indicates that traditional empirical relationships include significant residual tension stresses from untracked concrete. Service load crack strains should not be estimated using an empirical tension stiffening expression. While primary cracks continue to form up to strains of 0.00 10, due to deformation of concrete between visible cracks, the minimum strain that should be used with the stable crack spacing is 0.0005. A magnification factor must be applied to crack spacings at smaller strains, or a minimum strain of 0.0005 used to estimate crack width. Test results indicate that the 9Sth percentile crack width is 2.0 times the average crack width. Procedures for diagonal crack inclination, spacing and width are reviewed, and a simplified expression for estimating diagonal crack widths is presented. Diagonal crack widths are generally larger than flexural crack widths in members with orthogonal reinforcement due to diagonal strains being larger than reinforcing bar strains. Current code requirements for side-face reinforcement were developed to control flexural cracking, and may not be adequate to control diagonal cracking in certain exposure conditions. The simplified expression for diagonal cracking was used to develop an expression for the maximum spacing of side face reinforcing bars to control flexural and diagonal cracking in large members. A design example illustrates the proposal. Finally, it is shown how the proposed methodology can be used to extend the current AC1 expression for spacing of reinforcement near a surface in tension to treat the case of diagonal cracking.

Precast prestressed girder bridges can be made continuous for live load if the deck and diaphragm are cast with sufficient positive and negative moment reinforcements. The continuity eliminates costly joints and enhances seismic performance, structural integrity and overall durability of the structure. If diaphragm is poured with sufficient negative moment reinforcement before the deck is cast, continuity may also apply to the dead load of the slab. Although, connection of the girders at the diaphragm varies from state to state, it generally consists of bent bars or bent strands. Also, a short length of the girder may be embedded into the diaphragm. The continuity connection is a doubly reinforced section, which requires a time-dependent analysis including differential shrinkage, creep due to prestressing and dead loads, and temperature effects. These time-dependent effects can result in considerable positive restraining moments at the supports, which can in turn crack the diaphragm or pull the girder out of the diaphragm. These positive moment cracks are not only unsightly, but may also result in durability issues for the bridge. Furthermore, it questions the integrity of the continuity connection. The paper examines the extent of positive moment cracking based on field observations, time-dependent analysis, and previous studies.