Fineness modulus and size of particle inversely proportional
Answers
) Grain Gradation and Coarseness of Sand
The grain gradation and coarseness of sand are determined by screen residue analysis. Grading region and fineness modulus can be used to express the gradation and the coarseness of sand particles respectively. A set of square-hole sieves whose diameters are 4.75 mm, 2.36 mm, 1.18 mm, 0.60 m, 0.30 mm and 0.150 mm are adopted specifically. Screen 500 g dry sand obtained by sample splitting device from coarseness to fineness, then weigh their screen residue, and calculate their percentage a1, a2, a3, a4, a5, a6 (which means the mass of screen residue to the mass of the total sample sand) and the percentage of cumulative screen residue A1, A2, A3, A4, A5, A6 (which refers to the screen residue of one sieve to the sum of all the unit screen residue percentages whose sieves are thicker than it). The relationship between the cumulative screen residue and the unit screen residue is shown is Table 5.4. The cumulative screen residue in each group represents a gradation.
According to GB/T 14684-2001, there are three grading regions of sand when it is calculated by the percentage of the cumulative screen residue of 0.60 mm square-hole sieve, shown in Table 5.5. Compared with the standard indexes in Table 5.5, the real gradations of sand particles are allowed to exceed the limits slightly (except 4.75 mm and 0.60 sieves), but the total percentage should not be more than 5%. Taken the cumulative screen residue percentage and the size of sieve holes respectively as the ordinate and the abscissa, the numbers regulated in Table 5.5 can be drown into sieve-analysis curves of the upper and the lower limits of three grading regions, shown in Figure 5.2. The sand of zone II is proper for the preparation of concrete; if the sand of zone I is selected, the sand ratio should be improved and the amount of cement should be enough to satisfy the workability of concrete; if the sand of zone III is used, the sand ratio should be lowered properly to guarantee the strength of concrete.