The grading curve graphically represents the proportion of different grain sizes which the aggregate is composed of and which form part of the shortcrete mix. It provides useful information to find out:

• Whether the distribution of the different aggregate sizes is suitable for pumping
• The fines content of the aggregates (particles with a diameter $\lt$ 0,125 mm), which affects the properties of workability, internal adhesion and early strength development of the shortcrete mix.

Screening test to separate the aggregate particles

The process to obtain the grading curve consists in passing the aggregate sample through a series of standardized sieves of different diameters mounted on a column. The aggregates are exposed to vibration and rotation movements in order to obtain the classification by size. Afterwards, the sieves are removed and the retained material in each one is weighed.

Continuous and discontinuous grading curves

A continuous curve, in contrast to a discontinuous one, represents an aggregate composition with particles of all sizes.

• In the graphic, the black curve represents a continuous curve and within the grading envelope recommended by the norm, therefore the aggregate composition is adequate for the shortcrete mixture.
• The light gray curve represents the most unfavorable scenario: discontinuous and beyond what is recommended by the norm.

Passing and retaining grading curves

The difference consists in measuring the percentage of the aggregate which passes through the sieve, like in the latest examples (passing curves), or the percentage which is retained (retaining curve); in this case the curve is declining:

Pay special attention to the fines content

The fines content of the mixture is important to achieve a good a compression of the shortcrete mixture and to improve its workability.

Besides the fines in the aggregate, the total fine material of the shortcrete mix includes binder content, consisting of:

Cement

Additives (fly ash, silica fume, etc)

The recommended fines content of the mixture lies between 450 and 500 kg/m3, thus if there is a lack of fines in the aggregate, it should be compensated with binder content.