Question 2 of 5: Aggregate Quality Tests and Sieve Analysis

Question 2: Aggregate Quality Tests and Sieve Analysis (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — significance and application of the three tests (6 marks). All three are acceptance tests run on the aggregate before it is allowed into a concrete or asphalt mix, and each answers a different question about the stockpile.

The durability test, in North American practice the Los Angeles abrasion and impact test of ASTM C131/C535 (CSA A23.2-16A in the Canadian series), tumbles a graded sample with a charge of steel spheres for a set number of revolutions and reports the percentage of material broken down finer than the 1.70 mm sieve. Its significance is that it measures resistance to abrasion, impact and degradation — the mechanical toughness of the particle itself. Its application is to screen aggregates for the loads and handling they will actually see: a maximum loss of about 40 % is typical for concrete aggregate and 30 to 35 % for a surface-course asphalt aggregate, because a soft particle will be crushed during compaction, will degrade under traffic and will generate fines that change the mix gradation after the design was fixed. The same test is used to compare quarry sources and to detect a stockpile that has drifted onto a weaker seam.

The soundness test (ASTM C88, CSA A23.2-9A) subjects the aggregate to five cycles of immersion in saturated sodium or magnesium sulphate solution followed by oven drying; the salt crystallising in the pores generates internal pressures that mimic the disruptive effect of ice, and the reported result is the weighted percentage loss. Its significance is that it measures resistance to weathering — specifically to freeze–thaw and wetting-and-drying disintegration — which is a property of the pore structure rather than of the particle strength. Its application is decisive in Canada: aggregate for exposed concrete, bridge decks and pavement surfaces is limited to roughly 12 % loss with sodium sulphate (18 % with magnesium sulphate), and an unsound aggregate produces popouts, D-cracking of pavement joints and scaling long before the paste itself fails.

The sieve analysis (ASTM C136, CSA A23.2-2A) shakes a dried, weighed sample through a nest of sieves and reports the mass retained on each. Its significance is that it establishes the particle-size distribution, from which the maximum size, the nominal maximum size, the fineness modulus and the position of the curve relative to the specification band all follow. Its application runs through every mix: gradation controls the void content the paste or binder has to fill and therefore the cement or asphalt demand, it controls workability and the tendency to segregate and bleed, it controls the aggregate interlock that carries shear in an asphalt mix, and it is the routine production-control test by which a supplier demonstrates that a stockpile still matches the approved design. The three tests are complementary: sieve analysis says what sizes are present, LA abrasion says whether the particles are strong enough to stay that size, and soundness says whether they will survive the climate.

Part (b) — sieve analysis, fineness modulus and gradation (14 marks).

Given. The masses retained on a six-sieve nest plus the pan, as printed:

Find. The percent retained, cumulative percent retained and cumulative percent passing on every sieve; the fineness modulus of the sample; and a comment on the shape of the semi-logarithmic gradation curve.

Approach. Total the masses, convert each retained mass to a percentage of that total, accumulate downwards to get cumulative percent retained, subtract from 100 to get percent passing, sum the cumulative retained values to obtain the fineness modulus, and then compare the plotted curve with the maximum-density (Fuller) line for the same nominal maximum size.

1. Total the sample and check for loss. The sieve analysis is a mass balance, so the first step is the total dry mass: $$M_{tot} = 0 + 45.2 + 289.6 + 145.7 + 128.8 + 64.4 + 4.3 = \boxed{678.0\ \text{g}}.$$ Every later percentage is referred to this figure. In the laboratory this total would be compared with the mass weighed out before sieving; ASTM C136 rejects the test if the two differ by more than 0.3 %.
2. Convert each retained mass to a percent retained. Individual percent retained is $R_i = 100\,m_i/M_{tot}$. Working down the nest, $100(45.2)/678.0 = 6.67\ \%$ on the 9.5 mm sieve, $100(289.6)/678.0 = 42.71\ \%$ on the 4.75 mm sieve, and so on; the 25 mm sieve retains nothing, so the sample passes 25 mm entirely.
3. Accumulate downwards and subtract from 100. Cumulative percent retained is $C_i = \sum_{j \le i} R_j$ and cumulative percent passing is $P_i = 100 - C_i$. Carrying the running sum down the nest gives the full analysis:

   Sieve (mm)	Retained (g)	Percent retained	Cumulative percent retained	Cumulative percent passing
   25	0.0	0.00	0.00	100.00
   9.5	45.2	6.67	6.67	93.33
   4.75	289.6	42.71	49.38	50.62
   2	145.7	21.49	70.87	29.13
   0.425	128.8	19.00	89.87	10.13
   0.075	64.4	9.50	99.37	0.63
   Pan	4.3	0.63	100.00	0.00

   The pan fraction is only 0.63 % of the sample, so this aggregate carries almost no material finer than 0.075 mm.
4. Sum the cumulative percent retained to obtain the fineness modulus. The fineness modulus is defined as $$FM = \frac{\sum C_i}{100}$$ taken over the specified sieve series. Summing the cumulative retained column, $0.00 + 6.67 + 49.38 + 70.87 + 89.87 + 99.37 = 316.15$, so $$FM = \frac{316.15}{100} = \boxed{3.16}.$$ A fineness modulus of 3.16 corresponds to a coarse sand: it sits just above the 2.3 to 3.1 band that ASTM C33 and CSA A23.1 allow for fine aggregate, and each unit of FM represents one sieve-size step in average particle size.
5. Plot the curve and read its shape. Plotting cumulative percent passing against sieve size on a logarithmic size axis gives Figure 2.1. Three features govern the comment. First, the nominal maximum size is 9.5 mm, because 9.5 mm is the largest sieve retaining less than 10 % of the sample (6.67 %) while the next sieve down retains far more. Second, the curve is dominated by a single step: 42.7 % of the whole sample lies in the one band between 9.5 mm and 4.75 mm, which appears as a near-vertical drop from 93.3 % passing to 50.6 % passing. Third, the curve then flattens abruptly at the fine end, running from 10.1 % passing at 0.425 mm to only 0.63 % passing 0.075 mm.
6. Compare with the maximum-density line and conclude. The Fuller maximum-density curve for a 9.5 mm nominal maximum size, $P = 100(d/D)^{0.45}$, would pass 73.2 % at 4.75 mm, 49.6 % at 2.00 mm, 24.7 % at 0.425 mm and 11.3 % at 0.075 mm, against measured values of 50.6, 29.1, 10.1 and 0.6 %. The sample lies below the maximum-density line on every sieve finer than 9.5 mm, by 22.6 points at 4.75 mm, 20.5 points at 2.00 mm, 14.6 points at 0.425 mm and 10.7 points at 0.075 mm. Combined with the single 42.7 % step across the 9.5 to 4.75 mm band, the shape is a steep coarse end followed by a flat fine tail rather than the smooth S of a densely graded blend, so the aggregate is $\boxed{\text{gap- (uniformly) graded and deficient in fines}}$. Practically, such a blend has a high void content, will need extra cement paste or asphalt binder to fill those voids, and is prone to segregation during handling and to a harsh, unworkable mix; it should be blended with a natural sand before use.

Final results.