Why is the slump specified in concrete carriageway comparatively low (30mm) when compared with normal concrete (75mm)?

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The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance.

In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom. Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.


Moreover, a low-slump concrete facilitates the use of slipforms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the
paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.

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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  • Manilal Nakum August 9, 2011 at 1:34 am

    What is the basic difference between 53 Grade and 43 Grade of cemet? Why 53 Grade cement is used in concrete work now a days ?

  • nagraj June 7, 2012 at 1:44 pm

    compressive strength of cement.. for 53n/mm2 n for 43 n/mm2

  • Harsh chaudhari August 6, 2013 at 7:50 am

    Max value of slump without using of admixture ?

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