By Javed B. Malik
First published in Concrete International Magazine, May 2006
Constructible solutions to a common problem
Waterstops are commonly used at cold joints in concrete structures, such as water tanks, water treatment plants, and below-grade structures, to prevent the seepage of fluids through the joint. Although they come in several forms and shapes, the two most commonly used types are adhesive and mechanical waterstops as shown in Fig. 1 and 2, respectively. Adhesive waterstops can be hydrophilic or hydrophobic. Hydrophilic waterstops prevent the seepage of fluids by swelling when they come in contact with moisture, hydrophobic waterstops act as internal joint sealants, and mechanical waterstops rely on embedment into the concrete on both sides of a joint to form a diaphragm that seals off liquids.
Because they are typically smaller than mechanical waterstops and don’t have to be embedded on both sides of a joint, adhesive waterstops can generally be installed without conflicting with the reinforcing bars. Mechanical waterstops, however, can often conflict with reinforcement when their size and location are not properly taken into account during design and detailing.
The most common conflict between mechanical waterstops and reinforcement occurs at the joint between a slab or mat and a wall, as shown in Fig. 3. The waterstop is generally embedded into the slab 3 in. (75 mm) or more, producing the potential for interference with the top layer of reinforcing bars in the slab. If this conflict isn’t addressed on the contract drawings, the steel detailer will specify the height of the reinforcing bar supports based on the cover requirements shown on the drawings, and the iron worker will place the bars accordingly. Because the waterstop is typically the last item installed before the concrete is placed, the workers will either curl the waterstop so it lies above the steel, or cut notches in the waterstop so it clears the bars. Neither of these remedies is acceptable practice.
Potential conflicts become even more pronounced when the contract documents call for shear keys at wall-to-slab joints (Fig. 2). Not only does the shear key effectively increase the embedment of the waterstop in the slab, the concrete contractor must split the form for the shear key and install the shear key between the resulting form components. After the concrete hardens, it’s difficult to remove the form pieces without damaging the waterstop. It’s therefore a good idea to consider the use of alternate means for shear transfer at the cold joint, such as roughening the surface of the slab.
The three most common solutions to the interference are:
- Form a “starter” wall that raises the waterstop clear of the top slab bars;
- Deflect the top slab bars so they pass below the waterstop; and
- Lower the top slab bars to clear the waterstop.
The first option, shown in Fig. 4, is to form and place a small portion of the wall (called a starter wall) monolithically with the bottom slab, thus raising the waterstop sufficiently clear of the top steel in the slab. This may be the ideal solution from a designer’s viewpoint. Many builders consider it problematic, but others like having the starter wall to tighten the wall forms against.
The second option, shown in Fig. 5, is to deflect the top slab bars below the waterstop. For smaller-diameter bars, this can be accomplished by pushing the bars down at the waterstop location, but larger bars need to be bent by the fabricator in the shop. Because the moment capacity of the slab is reduced due to a smaller lever arm, this option works best if the negative moment in the slab is small. The end of the deflected top bar will be very low in the slab if it’s deflected at a steep angle or over a long distance. Therefore, this solution becomes less practical the farther the wall is from the slab edge, and it’s not practical at interior walls.
The third option, shown in Fig. 6, is to lower the top mat of steel to clear the waterstop. If the depth of the slab is not increased, this solution may require additional top bars in the slab because of the reduced effective steel depth. However, if the reinforcing quantity is controlled by creep and shrinkage or temperature requirements, the reduced moment capacity may not be a concern. For either the second or third option, a shear key will increase the required correction.
WHICH OPTION WORKS BEST
From a constructibility point of view, forming a starter wall is the best option when the slab has a large amount of top reinforcing bars, or when the wall is an interior wall. Forming the starter wall incurs a certain amount of cost, but can be offset by the savings due to better constructibility. If creep and shrinkage criteria control the steel quantity, or the bars are of a relatively large diameter and closely spaced, lowering the top layer of reinforcement in the slab to clear the waterstop may be a good option. For slabs with smaller-diameter bars, deflecting the top bars to clear the waterstop should only be considered if the reduced moment capacity of the slab is not a concern. No matter which option is selected, it’s best if the design engineer addresses the condition before the construction phase and indicates the preferred method on the contract documents.
Thanks to ACI member Dick Birley for his contributions to this article.