Table 3.12: General grading requirements for aggregates according to BS EN 12620: 2002
Aggregate Size* Percentage passing by mass
2D 1.4D D d d/2 Coarse Did < 2 and D < 11.2 mm 100
100 98-100 98-100 85¬
80--99 -99 0
0 -20 -20 0-5 Did > 2 and D > 11.2 mm 100 100 98-100 0-5
95-100 90¬
85--99
-99 0 -5 0-5 Fine D < 4 mm and d = 0 100 95-100 85--99 -Natural
graded D = 8 mm and d = 0 100 98-100 90--99 -All-in Z) < 45 mm and d = 0 100
100 98-100 98-100 90
85--99 -99
-* D = upper sieve size, d = lower sieve size and Did > 1.4
Gap-graded aggregate
As mentioned earlier, aggregate particles of a given size pack so as to form voids that can be penetrated only if the next smaller size of particles is sufficiently small. This means that there must be a minimum difference between the sizes of any two adjacent particle fractions. In other words, sizes differing only slightly cannot be used side by side, and this has led to advocacy of gap-graded aggregate, as distinct from continuously graded conventional aggregate. On the grading curve, gap-grading is represented by a horizontal line over the range of sizes omitted (see Fig. 3.5).
ASTM sieve number or size
200 100 50 30 16 8 4 3/8 %
75 150 300 600 1.20 2.36 5.00 10.0 20.0 Sieve size - mm
Fig. 3.5: Typical gap gradings
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Gap-graded aggregate can be used in any concrete, but there are particular uses: preplaced aggregate concrete (see page 141) and exposed aggregate concrete where a pleasing finish is obtained, since a large quan-tity of only one size of coarse aggregate becomes exposed after treatment.
However, to avoid segregation, gap-grading is recommended mainly for mixes of relatively low workability that are to be compacted by vibration;
good control and care in handling are essential.
Bibliography
3.1 ACI COMMITTEE 221 R-89 (Reapproved 2001). Guide for use of normal weight and heavyweight aggregates in concrete, Part 1, ACI Manual of Concrete Practice (2007).
Problems
3.1 What is meant by surface texture of aggregate?
3.2 What is meant by sphericity of aggregate?
3.3 Can an aggregate particle be both flaky and elongated?
3.4 Why do we determine the elongation index?
3.5 Why do we determine the flakiness index?
3.6 What may be the consequences of impurities in aggregate?
3.7 What is meant by soundness of aggregate?
3.8 What is the property of sea-dredged aggregates which requires special attention?
3.9 How does the shape of aggregate particles influence the properties of fresh concrete?
3.10 What is bulking of sand?
3.11 How would you determine whether aggregate contains organic material?
3.12 What are the consequences of organic material in concrete?
3.13 Define the fineness modulus of aggregate.
3.14 What is angularity number?
3.15 What is a gap-graded mix?
3.16 What are the advantages of a gap-graded mix?
3.17 How is gap-grading noticed on a grading curve?
3.18 How is the quality of bond assessed?
3.19 Discuss the influence of aggregate grading on density (unit weight in air) of concrete.
3.20 What is the maximum size of fine aggregate?
3.21 What is oversize?
3.22 What is undersize?
3.23 What are the common deleterious materials which may be found in aggregate?
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PROBLEMS
3.24 Why is grading of aggregate important with regard to the properties of hardened concrete?
3.25 Why is the grading of aggregate important with regard to the pro-perties of fresh concrete?
3.26 How would you assess the shape of aggregate particles?
3.27 How does the shape of aggregate particles affect the properties of fresh concrete?
3.28 How can the shape of aggregate particles be relevant to the proper-ties of hardened concrete?
3.29 What is the influence of the fineness modulus on the properties of concrete mixes?
3.30 What is meant by the saturated and surface-dry and bone-dry con-ditions of aggregate? Define absorption and moisture content.
3.31 What do you understand by the term aggregate grading?
3.32 How does the grading of aggregate affect the water requirement of the mix?
3.33 Explain the difference between apparent specific gravity and bulk specific gravity of aggregate.
3.34 What are some of the common deleterious materials in natural aggregates?
3.35 How would you assess the strength of aggregate?
3.36 Explain the 10 per cent fines value.
3.37 Define toughness of aggregate.
3.38 How would you assess resistance of aggregate to wear?
3.39 How would you measure the apparent specific gravity of coarse aggregate? State a typical value for natural aggregate.
3.40 What are bulk density and voids ratio?
3.41 What is buoyancy meter test?
3.42 State a typical value of coefficient of thermal expansion of common aggregate.
3.43 What are the effects of clay and very fine material on the properties of concrete?
3.44 How can aggregate cause efflorescence in concrete?
3.45 How does the maximum size of aggregate affect the workability of concrete with a given water content?
3.46 How does the variation in moisture content of the aggregate affect the workability of fresh concrete and the strength of hardened concrete?
3.47 Is there an ideal grading for aggregate? Discuss this with reference to workability of fresh concrete.
3.48 Calculate: (i) the apparent specific gravity, (ii) the bulk specific gravity, (iii) the apparent particle density, and (iv) the bulk particle density of sand, given the following data:
mass of sand (oven-dry) = 480 g mass of sand (SSD) = 490 g mass of pycnometer full of water = 1400 g mass of pycnometer plus sand and topped up with water = 1695 g
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Answer: (i) 2.59 (ii) 2.51 (iii) 2594 kg/m3 (iv) 2513 kg/m3
3.49 If the mass of a vessel full of water is 15 kg (33 lb), the mass of the empty vessel is 5 kg (11 lb) and the mass of the vessel with compacted coarse aggregate is 21 kg (46 lb), calculate the bulk density and voids ratio of the coarse aggregate.
Answer: 1600 kg/m3 (99.8 lb/ft3); 0.38
3.50 Calculate the absorption of the sand used in Question 3.48. If the sand in the stockpile has a total water content of 3.5 per cent, what is the moisture content?
Answer: 2.1 per cent; 1.4 per cent
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4
Q u a l i t y o f w a t e r
When we consider the strength of concrete in Chapter 6, the vital influence of the quantity of water in the mix on the strength of the resulting concrete will become clear. At this stage, we are concerned only with the individual ingredients of the concrete mix: cement, aggregate, and water, and it is the quality of the latter that is the subject matter of this chapter.
The quality of the water is important because impurities in it may interfere with the setting of the cement, may adversely affect the strength of the concrete or cause staining of its surface, and may also lead to corrosion of the reinforcement. For these reasons, the suitability of water for mixing and curing purposes should be considered. Clear distinction must be made between the effects of mixing water and the attack on hard-ened concrete by aggressive waters because some of the latter type may be harmless or even beneficial when used in mixing.