Types of Cracks in Prestressed Concrete Beams with Openings and its Control

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There are five types of cracks that may develop in prestressed concrete beams with openings. These crack types and method of controlling those cracks will be explained in this article. Various kinds of cracks in prestressed concrete beams with openings are illustrated in Figure-1.

Fig.1: Types of Cracks Around Openings in Prestressed Concrete Beams

Types of cracks in prestressed concrete beams with openings

Following are the types of cracks in prestressed concrete beams around openings: a) Cracks at mid depth of opening due to the prestressing force b) Cracks at corner of opening resulted from the framing action at the opening region c) Shear cracks in chord d) Flexural cracking in chord caused by flexural stresses which arise from secondary moment e) Cracking of tension chord due to normal tensile stresses It should be said that, the first type of cracks (a) are developed at prestress transfer stage in which the beam is subjected to prestress force, and the remaining (b, c, d, e) are occurred during service load stage. In the latter stage, prestressing force and vertically applied loads are exerted on the beam.

Cracks at Mid Depth of Openings

Vertical splitting tensile stresses at the edge of holes in concrete beams are rose because of prestressing forces and create horizontal cracks. The highest values of stresses are occurred near to the middle of opening depth regardless the location of prestressing force or position of the holes. The stress magnitude is increased when opening depth is increased or when the opening is moved up vertically in the direction of horizontal line action of prestressing force. The splitting tensile force is withstood by both concrete and steel reinforcement before the element is cracked. Adequate stirrups should be provided around the openings to control this type of cracks and they should be placed as close as possible to the edge of the openings.

Cracks at Corners of the Opening

There are two different stresses which lead to generate cracks at corners of the prestressed beam openings. Firstly, vertical tensile stress which resulted from prestressed force, reach ultimate value, as denoted by (f_t) in figure 2, at the middle of the opening and still considerably large at the corner, denoted by (f_tf). Secondly, vertical applied load develops stresses that spread along horizontal plane and goes through the top corner of the hole at the high moment end. This stress modifies from tension to compression at both top and bottom corner and the same stress distribution occur at other corners at low moment end as shown in Figure-3. The combined effect of both aforementioned stresses cause the peak splitting tensile stress to move downward at low moment end of the opening and move upward at high moment end of the hole. That is why cracks are initiated at bottom corner of low moment end of the opening and top corner of high end moment of the opening as shown in Figure-1. Since, the combination effect of both splitting stress resulted by prestress force and applied load is nearly vertical, therefore installing vertical stirrups is substantially influential to control cracks at the corners.

Fig.2: Vertical Splitting Stress at Opening of Prestressed Beam

Fig.3: Vertical Stresses Developed by Vertical Loads

Shear Cracks in Chords

Shear stresses close to the opening corners and in the opening chords are produced by prestressing forces and vertical loads respectively as shown in Figure-4. The magnitude of these stresses is influenced by the depth of the opening and its vertical position. Finally, vertical stirrups which the amount and arrangement is computed in usual way, are employed to control shear cracks.

Fig.4: Shear Stress Around the Opening of Prestressed Beam

Flexural Cracking in Chords

Primary moment in the beam and secondary moment in the chord induce flexural stresses in top and bottom corners of the opening at high end moments and low end moments respectively and consequently flexural cracks are initiated. Even though flexural cracks develop in the tension region of the beam but flexural cracking in the compression region of the chord can be observed that caused by the presence of secondary moment. The secondary moments are produced by shear stresses in the chord elements. Lastly, Flexural reinforcement is used to prevent or limit flexural cracking.

Cracking of Tension Chord

In the case where chord members are shallow and net axial stresses at the middle of the chord is greater than the allowable tensile stress of concrete, cracks might develop over the entire depth of the chord. It can be controlled by providing flexural reinforcement and positioned equally at the upper and lower face of the chord element. Read More: Prestressed Concrete Mix Design Proportions Maintenance of Prestressed Concrete Structures from Deteriorations