fread - read all columns as character

Your colClasses argument is in the wrong place. It needs to be inside fread(), not inside data.frame(). Try this:

prop1 <- data.frame(fread("C:\\myFile.csv", 
                          colClasses = c(rep("character", 58))),
                    stringsAsFactors = FALSE)

More canonical use of data.table to accomplish this would be:

prop1 <- fread("C:\\myfile.csv", colClasses = 'character', data.table = FALSE)

simply put:

colClasses=c("character")