Complex sentence: In 1991 Dr. Ronin joined Luleå University of Technology in Sweden, first as a Guest Researcher, then as a senior Researcher, and in 1999 became an Professor at LTU's Division of Structural Engineering, Cement Chemistry and Technology.
Split into simpler sentences: In 1991 Dr. Ronin joined Luleå University of Technology (LTU) in Sweden, first as a guest researcher. In 1999 Dr. Ronin became a Professor at LTU's Division of Structural Engineering, Cement Chemistry and Technology.