ROMAN URBAN TOPOGRAPHY in Britain and the western Empire

Fig 30 ‘Temple of Janus’ at Autun, showing putlog holes
in regular courses (Photo: author)
in the form of slaves and prisoners of war. It was also a
relatively speedy method of building; the main delaying
factor would have been the time required for the concrete
to set.
Work in concrete, known to Vitruvius as structura
caementicia or simply caementicium (ii.4.1; 7.5; 8.16, etc),
evolved in central Italy during the 2nd century BC and
achieved its greatest successes there. Its invention was
probably a spontaneous outcome of experiments with a
pisé technique of the type familiar in Punic Africa; it is
perhaps no accident that the first tentative examples
appeared in the late 3rd century BC, as at Terracina, in the
wake of Hannibal’s invasion of Italy. Progress was rapid,
especially after the discovery that lime-mortar made with
pulvis puteolanus (pozzolana) and other volcanic earths
readily available in the Rome and Naples areas possessed
extraordinary cohesive strength and hydraulic properties
(ie the ability to set under water and in damp conditions).
The result was a material whose quality has rarely been
equalled before modern times. The standard of the
concrete in the Roman provinces was certainly inferior to
that of the best work in Italy; in fact, when used to
describe the mortared rubblework of Asia Minor (Fig 28)
and certain other areas, the term ‘concrete’ is something of
a misnomer. That is partly why provincial architects
resorted to frequent bonding-courses of brickwork in
23
Ling: The mechanics of the building trade
their concrete walls, and why, in Asia Minor and the east,
they even constructed brick vaults or all-brick, as opposed
to brick-faced, walls. In Britain the vault of the Great Bath
at Bath (Fig 29), composed entirely of bricks and
box-tiles, may be a witness to the same lack of confidence
in local mortar.
Walls of caementicium were built up in stages roughly
equivalent to the height of one stage of scaffolding: that is
the height that a man could conveniently handle from a
given position. Examination of surviving walls in Pompeii
and elsewhere, for instance in the so-called Temple of
Janus at Autun, will show either horizontal breaks in the
fabric (building-lines) or series of putlog holes at regular
intervals of about 1 to 1.50 m (Fig 30). In brick-faced walls
there is often a levelling- and capping-course of extra-large
bricks, generally ‘two-footers’ (bipedales), above each
stage of work (every three or four Roman feet in
Domitianic buildings). The purpose here was not so much
to provide a bond through the thickness of the wall, for
builders in Italy were by now more confident of the
strength of their material, as to compartmentalize the
areas of concrete and thus prevent settling within the wall.
The actual method of construction (Fig 31) was to
shovel the concrete mixture, which was rather stiffer than
its modern counterpart, into the cavity between two
purpose-built faces. In the foundations this was formed by
timber shuttering which was removed as soon as the
concrete had set, but above ground it was formed by the
actual surface dressing of the wall, carried out in stone or
brick. This would be built up first and could be sustained
by a minimum of planking while the concrete was inserted
and left to harden. It is this facing which is the most
characteristic feature of the wall and which, though
structurally otiose once the concrete had set, still tends to
dominate the historical study of Roman wall construction
— not least because of its value as a dating tool. Three
principal techniques were used (Fig 32). The earliest,
incertum (‘irregular work’), was a carefully constructed
patchwork of pieces of rubble roughly as big as a human
Fig 31 The method of constructing a concrete wall (Drawn
by R Lea, after J P Adam)