求翻译：丁he fine aggregate shall be so graded that when mixed with the coarse aggregate and cement it shall produce a concrete of maximum density. Crushed sand may be added to natural sand in proportions approved by the Engineer in order to achieve the required grading Crushed sand alone may only be used with the approval by t是什么意思？

待解决 悬赏分：1 - 离问题结束还有