Question

Why is slump specified in concrete carriageway comparatively low like 30mmwhen compared with normal concrete like 75mm?

Answer

Slump of concrete carriageway is purposely specified to be a relatively low value such as 30mm. For concrete carriageway, traffic loads directly do something on concrete pavement surface and therefore surface strength is detrimental to its future performance. In newly placed concrete, segregation may be in the form of bleeding occurs within mixture of cement paste and aggregates. The Degree of resistance to segregation is related to workability of concrete. If substantial segregation is authorized to take place, then relatively porous and weak laitance layer will be formed on carriageway surface and aggregates will concentrate in bottom. Therefore, concrete which has insignificant bleed possesses a stronger surface layer and is additional abrasion resistant. Thus, a small slump value is specified to increase wearing resistance of concrete and to reach an apposite surface texture of concrete pavements.

Besides, a low-slump concrete facilitates use of slip forms when constructing concrete pavement. With concrete of a low slump value, it motionless remains its compacted shape and is not liable to deform when paving machines go away. Though, if a high slump concrete is used instead, pavement surface would drop and the edges might deform readily.