diff --git a/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb b/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
index 21e4fd672f..9fb7dfb779 100644
--- a/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
+++ b/activesupport/lib/active_support/number_helper/number_to_rounded_converter.rb
@@ -29,9 +29,11 @@ module ActiveSupport
 
           formatted_string =
             if BigDecimal === rounded_number && rounded_number.finite?
-              s = rounded_number.to_s('F') + '0'*precision
+              s = rounded_number.to_s('F')
+              s << '0'.freeze * precision
               a, b = s.split('.'.freeze, 2)
-              a + '.' + b[0, precision]
+              a << '.'.freeze
+              a << b[0, precision]
             else
               "%00.#{precision}f" % rounded_number
             end