def assign_national_road_conditions(x):
    """Assign road conditions as paved or unpaved to national roads

    Parameters
        x - Pandas DataFrame of values
            - loai_mat__ - String value of road surface

    Returns
        String value of road as paved or unpaved
    """
    road_cond = x.loai_mat__

    if road_cond is None:
        return 'paved'
    elif 'asphalt' in road_cond.lower().strip():
        # Assume asphalt for all roads with no condition
        return 'paved'
    else:
        # Anything else not included above
        return 'unpaved'