為什麼是放形容詞而不是副詞呢?

as ... as 是在做同等的比較，中間放原級的「形容詞」或「副詞」：
1. 放形容詞，前面的動詞是state verb（表示狀態）：
I am as old as he (is).
I am old.與He is old.做同等的比較，old是做為「主詞補語」的原級形容詞。is/am是state verbs。

2.放副詞，前面的動詞是action verb（表示行為）：
Tom can run as fast as I (can).
Tom can run fast.與I can run fast.做同等的比較，原級副詞fast是做為動詞run的「修飾語」。run是acton verb。

所以究竟as ... as之間放形容詞或副詞，主要是看句義裡做同等比較的是「主詞補語」，還是「動詞的修飾語」，如果前面的動詞是state verb，就是在做主詞補語的同等比較，我們思考的順序應該這樣才對。有些action verb可能接「受詞補語」（即不完全及物動詞）；有些state verb才可能接「主詞補語」（即不完全不及物動詞），而主詞補語才可能用「形容詞」（主詞補語通常是形容詞、名詞，副詞是例外）。

The new material will be twice as durable as ordinary concrete (is). 主詞補語做同等比較，但多了一倍(twice)。

(1) "will be" is the future tense for the linking verb "be". The linking verb is normally followed either by (a) a noun (called predicate nominative) or (b) an adjective (called predicative adjective) to rename or modify the subject (function as complement of the subject).

example:
(a) The new material will be a major breakthrough in construction. (predicate noun)
(b) The new material will be very durable. (predicative adjective).

(2) Only adjectives or adverbs can be used for comparison.

As a conclusion based on (1) and (2), the choice has to be (b) durable, an adjective.

因本文中之”耐用”1字在修飾ordinary concrete，對人、事、物(名詞)的修飾須使用形容詞，副詞則是用於修飾動詞及副詞，本題之ordinary concrete是物，是名詞，故必須使用形容詞durable來修飾。

因為 "twice as durable as ordinary concrete"是 主詞捕語 用來形容 "a new material that".
當然適用 形容詞.

As -----as or so----as=="as"or "so" is an adverb to modify an adjective and then followed by " as" which acts as a conjunctive. For example: It's not so difficult as I expected. In the case of comparisons of equality,in the patterns :as x adj. or adv.x as, not so x adj or adv. x as: For example; I want a box twice as large as this. It isn't so(as) big as you think it is. Yip