Normally formwork ties are designed to remain permanently in place for a length about 3 inches less than the thickness of the structure. Under service conditions, leakage is frequently observed as moisture or dampness around the mortar-filled end of a tie. A research program was carried out to establish the influence of the following factors: the thickness of the concrete; the diameter, surface finish and cleanliness of the tie; and the degree of compaction. Leakage was found to take place along two paths: cavities under the tie, and cracks extending laterally into the concrete on either side of the tie for distances up to 6 inches. The cavities under a tie are similar to those sometimes found under reinforcement. The general pattern of leakage path occurrence, for 6 foot high walls, can be summarized as follows: top 20 percent, cavities and lateral cracking, down to approximately 80 percent, cracking decreases until no longer visible, bottom 20 percent, cavities are infrequent. It was found that leakage paths are normally due to bleeding and settlement of the plastic mix. They can best be avoided by using the leanest possible mix and reducing workability near the top of a lift. Under these conditions, lateral pressures on the form will also be reduced, which means that less ties are needed.