Q.: This detail shows a construction joint we are using on several underground structures.

In one structure that's already been built we're experiencing seepage in several areas where the wall meets the floor. It was hard to keep the keyway and the polyvinylchloride (PVC) waterstop properly aligned while consolidating the concrete curb.

We were not allowed to omit the curb and put the keyway and waterstop into the floor slab because the engineer wanted to ensure that the waterstop did not touch the rebar in the slab. Consequently we suggested using a metal waterstop welded in place to permit the vibrators better access to the curb, but the engineer did not like metal waterstops. Can you provide any details showing how to hold the waterstop and keyway in place?Is it necessary to keep the waterstop from touching the rebar?

Do you have any data com paring PVC and metal waterstops?

A.: We see no harm in the waterstop touching the reinforcing bars. Perhaps the engineer meant that if the keyway were not elevated by the curb the waterstop would extend below the tops of the reinforcing bars. Thus it would have to be intersected by the bars or be bent flat on top of them. Either case would reduce the value of the waterstop.

Various articles discuss the merits of metal and PVC types of waterstop in general terms but we don't know of any direct comparison of the relative performance of the two kinds.

Both types are discussed in "Waterstops: Choose Them Wisely and Install Them With Care," published in our June 1984 issue, page 569.

Based on that article a metal waterstop could presumably be used in the construction joints that you discuss.The configuration of the joint may help explain why you are having difficulty in building it. Because your keyway is depressed, it can catch sawdust and other construction debris. Some builders prefer an upturned keyway as shown in the drawing on page 577 of the June 1984 article.

Other methods of holding waterstops are shown on page 573. Since your wall is wide, why couldn't you place the waterstop at the side of the keyway, lashed to the vertical bars, using a waterstop 8 inches high?

This would decrease congestion and the difficulty of construction.The leakage you are experiencing might be repaired by a method shown in an article "An Effective Repair for Leaking Waterstops," also published in the June 1984 issue, page 594.