In this article we will discuss about:- 1. Introduction to Practical Grading of Aggregates 2. Grading of Fine and Coarse Aggregates 3. Grading.
Introduction to Practical Grading of Aggregates:
The grading should be such that it could give a reasonable workability with minimum segregation. Segregation is very important in the production of good concrete. A workable mixture which could produce a strong and economical concrete will result in honey combed, weak, undurable and variable end product if segregation takes place.
The process of calculation of the proportions of aggregates of different sizes to achieve the desired grading comes within the scope of mix design. Here properties of some good grading curves have been discussed. Usually aggregate available locally or within an economic distance has to be used.
It can produce satisfactory concrete, if sufficient care and intelligent approach is adopted. In India no ideal grading curves have been prepared. However Road Research Institute U.K. has prepared some curves using 19.0 mm and 38 mm maximum size aggregates. These curves are shown in Fig. 4.14 and 4.15 respectively.
Four curves are shown for each maximum size of aggregate, but due to the presence of over and under size aggregate and also due to variation within any fraction size, practical gradings may be in the vicinity of these curves than to follow them exactly. Therefore it is convenient to specify grading zones rather than a single curve. These zones are marked on the curves.
Curve No. 1 represents the coarsest grading in each set of curves. These gradings are comparatively workable and can be used for mixes with low water/cement ratio or for rich mixes. However it should be ensured that no segregation takes place. At the other extreme, curve No. 4 represents a fine grading. It will be cohesive, but not very workable. In particular, an excess of material between 1.20 and 4.75 mm test sieves will produce a harsh concrete, which may be compacted by vibration, but difficult to place by hand.
If the same workability is to be obtained using aggregates with grading curve No. 4 and 1, the latter would require considerably higher water content. This means a lower strength if both concretes are to have the same aggregate/cement ratio or if the same strength is required the concrete made with fine aggregate would have to be considerably richer. The change between the extreme gradings should be progressive.
In case of gradings lying partly in one zone and partly in another, there is likely hood of segregation if too many intermediate sizes are missing. If on the other hand there is an excess of middle sized aggregate, the mix will be harsh and difficult to compact with hand and possibly even by vibration. Therefore it is preferable to use aggregate with grading similar to type rather than totally dissimilar ones.
Grading of Fine and Coarse Aggregates:
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For all important works coarse and fine aggregates are batched separately, hence their grading should be known and controlled. Grading curves of the four zones of fine sand of table 4.3 as per I.S. 386 Part-I 1963 are shown in Fig. 4.16 to 4.19. The grading curve of any fine aggregate falling wholly within the limits of any one zone is considered suitable.
A tolerance of total amount of 5% on certain sieves is permitted, but the aggregate should not be finer than the exact limits of the finest grading or coarser than the coarset grading. In case of crushed stone 20% is allowed to pass the 150 micron test sieve in all zones.
Sand (fine aggregate) falling into any zone can be generally used in concrete, although under some circumstances the suitability of a given sand may depend on the grading and shape of the coarse aggregate.
The suitability of the fine sand of zone 4 for use in reinforced concrete should be tested. For sands, the sand content of the mix should be low whose greater part is smaller than 60C micron. In general, finer the grading of the fine aggregate, higher should be the coarse/fine aggregate ratio. Typical values are shown in Table 4.23.
On the other hand the coarse sand of zone I produces a harsh mix and a high content of sand may be necessary for higher workability. This sand is more suitable for rich mixes or for use in concrete of low workability.
Zone II represents medium sand, generally suitable for the standard 1:2 fine to coarse mix, when the maximum size of coarse aggregate is 20 mm. The choice of correct proportion is particularly important as the grading of the sand approaches the fine outer limit of zone 4 or the coarse outer limit of zone I. It is worth noting that if proportioned correctly, fine sand can be utilized with success and its use is economical as it is available locally in desired quantity.
Grading of Coarse Aggregate:
The requirements of I.S. 383-1970 for the grading of coarse aggregate are reproduced in Table 4.2 and for all in aggregate in Table 4.24.
The actual grading requirements depend to some extent on the shape and surface characteristics of the particles. For example, sharp, angular particles with rough surfaces should have a slightly finer grading in order to reduce the possibility of interlocking and to compensate for the high friction between the particles. Some of the gradings for single size and for all in aggregate are shown in Fig. 4.20 to 4.21. Curve 4.22 shows the grading limits for all in aggregates.
Oversize and Undersize:
Strict adherence to size limits of aggregate is not possible. Breakage during handling will produce some undersize material and wear of screen in the quarry or at the crusher will result in oversize particles being present. As per I.S. 383-1970, the limit for oversize is permitted between 5% to 15%. However no aggregate must be retained on a sieve one size larger than the nominal maximum size. The allowable limit of undersize material is 5 to 10%. For fine aggregate a total departure of 5% from zone limits is allowed, but not beyond the coarser limit of zone I or finer limit of zone IV.