How to stop String.Concat(); from removing whitespaces?

Question

This code is supposed to reverse all substrings which are five characters or more, but it keeps removing the whitespaces from the string, is there a reason to this, or am i just dumb?

   public class Kata
{
    public static string ReverseString(string myStr)
    {
        char[] myArr = myStr.ToCharArray();
        Array.Reverse(myArr);
        return new string (myArr);
    }

    public static string SpinWords(string sentence)
    {
        string[] words = sentence.Split(' ');
        string s1;
        string owo = "";
        foreach (var word in words)
        {
            if (word.Length >= 5)
            {
                s1 = ReverseString(word);
                owo = String.Concat(owo, s1 );
       ;
            }
            else
            {
                owo = String.Concat(owo, word );
            }
        }

        return owo;
    }
    }