Dowel bars

In Europe comparatively little FBECR is used, except in dowel bars for concrete pavements, where they have performed well for several years. They are also used for electrical isolation in tram and light railway system and for power transmission systems. 

It is common to place dowel bars at the position of the contraction joint. The dowel bars assist the transfer of loads over the contraction joint and are placed approximately in the middle of the slab. Dowel bars, since no bonding with concrete is required, are smooth and covered by corrosion-resistant coating. Their diameter is typically 20 or 25 mm and they are usually spaced every 300 to 400 mm. 

Dowel bars may not be used when the joint spacing is smaller than 4 to 4.5 m or when traffic volume is low. 

The use of dowel bars is necessary in the expansion joints, because of the gap created between slabs, ensuring smooth load transfer from one slab to the other and increasing concrete s resistance to bending and shear. 

Because of the expected expansion, one end of the dowel bar is free to move owing to the provision of a waterproofing cap filled with compressible material. A typical cross section of an expansion joint is shown in Figure 14.8. 

The dowel bars for expansion joints are slightly longer than those used in the contraction joints, typically 500-600 mm, and are usually placed every 300 mm. The bar diameter is always greater than that of the dowel bars placed in the contraction joints, usually 25 or 32 mm. As in contraction joints, the dowel bars are made of smooth steel and coated by a corrosion-resistant material. 

For the exact determination of the diameter, length, spacing length, steel quality of the dowel bars or any other related construction detail, the designer should follow precisely the instructions of the methodology used. 

Dowel bar covered by polymeric corrosion-resistant coating... 

The proper anchorage of the tie bars is achieved either by welding the tie bars on longitudinal supportive steel bars to be placed at a certain depth (or height) or by bending both ends. Suffice it to say that the tie bars are not covered with a corrosion-resistant coating at their full length, in contrast to the dowel bars in the expansion or contraction joints. 

As the tie bars are not designed to act as a load transfer mechanism, they have a smaller diameter and usually are longer than the dowel bars. 

The determination of the spacing and diameter of dowel bars is left to the designer s experience. However, the following general guide is given the bar diameter should be 1/8 of the slab thickness and the spacings should be 12 to 18 inches. 

The dowel bars are recommended to be plain round steel bars of grade 250 N and 450 mm long, placed at 300 mm centres. More than half of the length of the dowel bar should be coated with a debonding agent to ensure effective debonding from the concrete on that side of the joint. The dowel diameters vary from 24 to 36 mm, depending on the slab thickness. 

Corner cracking is caused by load repetitions combined with a loss of corner support (subbase or subgrade), poor load transfer across the joint, dowel bar restrain near the edge of the slab and ingress of solids into the joint. The loss of support may be created by pumping or warping stresses. 

In old buildings timber often fails where it enters a masonry wall or in roofs at the junction of rafter and tie beam where the truss is supported on the wall plate. In both cases leaking water over a long period has resulted in wet rot. The repair process consists of replacing the decayed timber with reinforced epoxy mortar. The reinforcement may consist of steel plate or bars which are drilled or set deep into the sound portion of the timber to ensure a good bond and load transfer. Fig. 6.23 shows bonded dowel bars used to repair the end section of a column or beam while Fig. 6.24 illustrates a typical repair to a roof truss. 

Sandstone indent fixed with stainless steel dowel bars... 

Option (b) is an indirect connection the smaU-diameter starter bars are omitted the dowels play the role of starter bars by extending into the web by at least the lap length of the web bar they intend to anchor into the frame member. The dowel bar is dimensioned in this case per fib 2012) for simultaneous tension (as an anchor) and shear (as a dowel) it is epoxy-grouted into the frame member for 8-times its diameter, or for the length needed to anchor the full yield force of the web bar(s)... 

Similarly, other FRP products (rebar and dowel bars for concrete as shown in Fig. 31.7) have been field implemented in Ueu of steel reinforcement because they offer unique performance advantages including better thermal and stiffness matching with... 

Figure 31.7 (Top) FRP rebar for continuously reinforced concrete pavements. (Bottom) FRP dowel bars for conventional concrete pavement. (Photos courtesy of CFC-WVU).

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