This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
First Method. Fig. 184 shows a sectional view of one method of making a break at the end of the day's work, which has been used very extensively and successfully. The stirrups and slab bars form the main bond between the old and the new work, if the break is left more than a few hours. Short bars in the top of the slab will also assist in making a good bond; also, an additional number of stirrups should be used in the beam where the break is to be made. Before the new concrete is placed, the old concrete should be well scraped, thoroughly soaked with clean water, and given a thin coat of neat cement grout. An objection to this method of forming a joint is that the shrinkage in the concrete may cause a separation of the concrete placed at the two different times, so that water will find a passage. The top coat that is generally placed later will greatly assist in overcoming this objection.
Fig. 184. Break in Slab.
Fig. 185. Break in Beam.
Another method of forming stopping-places is by dividing the beam vertically - that is, making two L-beams instead of one T-beam, Fig. 185. Theoretically this is a very good method, but practically it is found difficult to construct the forms dividing the beam, as the steel is greatly in the way.
The method of stopping the work at the center of the span of the beams and parallel to the girders, has been used to some extent. Fig. 186 illustrates this method. Theoretically the slab is not weakened; and as the maximum bending moment occurs at this point, the shear is zero, and therefore the beams are not supposed to be weakened, except for the loss of concrete in tension, and this is not usually considered in the calculation. The bottoms of the beams are tied together by the steel that is placed in the beams to take the tensile stresses; and there should be some short bars placed in the top of these beams, as well as in the top of the slab, to tie them together. The objection made in the description of the first method - in that any shrinkage in the concrete at the joint will permit water to pass through - is greater in the second and third methods than in the first.
Fig. 186. Break in Center of Span.