The Vermont Transportation Agency is using what it calls a ''slide-in construction technique'' to minimize disruption for travelers when it replaces two Interstate 91 bridges in White River Junction. The highway will remain open during construction except for two weekends, probably in late August. On those two weekends the road will be closed in one direction and traffic detoured while an existing bridge is rapidly demolished and a new span is slid into place. Deputy Transportation Secretary Chris Cole said the bridges are near the interchange with Interstate 89 so it is essential to minimize the time the spans are closed. The bridges will be built on temporary supports next to those they are replacing this spring and summer. The foundations for the new bridges will be built under the existing bridges. The new bridges will be slid into place when they are done.