How a Screed Works
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2-13
The primary factor affecting this force is changing the width of
the screed (Figure 2-14). Extendible screeds weigh the same
whether they are fully retracted or fully extended. The
difference is the wider the extension of the screed the greater
the surface area of paving material to support. An extended
screed has fewer psi, which means less compaction, causing the
screed to raise.
Figure 2-14. Width of Screed Affects Weight of Screed
Quality of Base Being Paved
There are factors other than the four main forces that can have
an effect on the quality of the mat. An important one is the
quality of the base to be paved. It makes more sense to build
smoothness from the base up, than to try to smooth a road in the
last lift of paving. However, on overlay paving jobs we have to
work with what we have.
Remember that a mat which appears smooth immediately
behind the paver, may actually contain areas of considerably
varying thickness of material as a result of undulations in the
base being paved. Due to the principle of “Differential
Compaction” high spots will not compact as much as the low,
therefore allowing some of the irregularities to be rolled back
onto the mat (Figure 2-15 on page 2-14). To minimize this
problem, lay a leveling course in the low spots or pave multiple
lifts to average out the irregularities.
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