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It can also be filled with concrete , in which case the filling itself become able to bear load in addition to the load carried by the ring of voussoir s. If the voussoir stones are thin they can not take a lot of weight so instead it is the concrete filling that becomes the structural part of the arch .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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