For this reason the maximum aggregate size chosen should be the largest sized aggregates available at a reasonable cost.There are however a number of factors which need to be considered when specifying particularly large aggregates for concrete works. Answer plzzwhat is common maximum size of coarse aggregate available in the market? Gravel, cobble and boulders come under this category. 7 FACTORS INFLUENCING REBOUND HAMMER TEST OF CONCRETEDesign procedures for a Building Foundation (Step by Step)10 PRACTICAL PROBLEMS TO THINK BEFORE DESIGNING A FOUNDATION For the same strength or workability, concrete with large size aggregate will require lesser quantity of cement than concrete with a smaller size aggregate. BS EN 13877-1 limits the max aggregate size to one third of the depth of the slab, and this is consistent with US standards and general practice around the world, though some agencies may have slightly different restrictions.Within the above physical limits there may be a great deal of scope for including large or very large sized aggregates, particularly in thick unreinforced concrete slabs. The commentary R22.214.171.124(a)(5) of ACI 318-19 states that the size limitations on aggregates are provided to facilitate placement of concrete around the reinforcement without honeycombing … Using the maximum nominal aggregate size avoids any issues with very small quantities of slightly oversized aggregates bumping the whole coarse aggregate grading into a higher band which is not really appropriate.The optimal maximum aggregate size needs to be determined both by the desired properties of the fresh and hardened concrete and by physical limitations. 6 CHARACTERISTIC PROPERTIES OF BITUMEN In a mass concrete work the use of larger size aggregate will be useful due to the lesser consumption of cement. In principle larger sized aggregates have a number of benefits to the mechanical performance of the concrete.
BS EN 13877-1 limits the maximum aggregate size in concrete to one third of the spacing between reinforcing bars. Less cement also reduces the amount of shrinkage in the cement paste which reduces stress on the slab. The maximum size of coarse aggregate that can be used in a mix depends on the size, shape, and reinforcing of a member, based on these "not-to-exceed" guidelines (see Photos 5 and 6): 3/4 of the clear spacing between rebar or between rebar and forms (cover distance) 1/3 the depth of the slab 1/5 the narrowest dimension of a member The larger sizes will require less mixing water and hence, for a given cement factor, will produce concrete of lower water-ratio than the smaller sizes. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete. The physical limitations are typically the gaps between reinforcing steel bars or simply the depth of the slab for a concrete pavement or floor.The most common physical limitation on the maximum nominal aggregate size in concrete is the gaps between steel reinforcing bars. The maximum size aggregate used may be dependent upon some conditions.
This is a more onerous restriction than in US standards which specify a maximum aggregate size of ¾ of the minimum spacing. For heavily reinforced concrete members the nominal maximum size of the aggregate should be usually restricted to 6 mm less than the minimum clear distance between the main bar or 5 mm less than the minimum cover to the reinforcement, whichever is less. The designer can make a judgement if the maximum aggregate size is likely to cause issues with closely spaced reinforcement in reinforced concrete.For concrete pavements the maximum size of coarse aggregate must be small enough to sit suspended within the fresh concrete mix without breaking the surface of the concrete or impeding the compaction of the concrete. The size range of various coarse aggregates given below: Fine gravel: 4mm – 8mm. For this reason most national standards limit the maximum size of the aggregate to a proportion of the depth of the slab.