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Why does pumping sometimes occur at joints in concrete carriageway?

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Pumping at joints in concrete carriageway occurs in the presence of the following factors:

(i) Fine-grained subgrade;
(ii) Seepage of water into subgrade due to improper or inadequate drainage design;
(iii) The presence of heavy vehicular loads.

It involves the pumping out of water-borne particles of the subgrade owing to the deflections at the end of concrete slab. The first mechanism of pumping involves the softening of subgrade by water and the reduction in bearing capacity. It causes a larger instantaneous deflection at the slab ends under heavy traffic loads. During deflection, water containing fine soil particles is pumped out at the joints. Consequently, voids are formed in subgrade region and the void size grows by repeating the above sequence.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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