It turns out that that unusual-looking intersection element is a 'truck corner.' It's a sort of compromise between street and pedestrian space, for the benefit of large trucks.
The concrete beyond the curb functions as a sidewalk to pedestrians and cars, but can be used as part of the street by large trucks making the tight turn.
It's explained pretty well in this document from the Portland Bureau of Transportation:
Feb 5, '08 Open House presentation (pdf) (link goes directly to relevant page)
This is all part of the St Johns Truck Strategy (pdf). From this page at the Portland Bureau of Transportation:
Adopted by City Council in 2001, the goal of the St Johns Truck Strategy is to reduce or eliminate conflicts between freight movement and neighborhood livability within theSt JohnsTownCenter.
...
The basic challenge is to provide a transportation system inSt Johnsthat supports efficient freight access connections between theSt JohnsBridge, Rivergate and other nearby industrial districts, while at the same time supporting the revitalization of theSt JohnsTownCenter.