Assessment of material characteristics of ancient concretes, Grande Aula, Markets of Trajan, Rome

Abstract

The Grande Aula, or Great Hall, of the Markets of Trajan is an intact example of the sophisticated, domed, concrete architecture of imperial Rome, which records the expertise of Roman builders in formulating extraordinarily durable pozzolanic concretes between about AD 96 and 115. Petrographic, chemical, X-ray diffraction analyses and SEM images demonstrate that the pozzolanic mortars of the conglomeratic wall concretes contain strätlingite, a complex calcium aluminate cement hydrate (C2 ASH8) that gives modern cements high durability and compressive strength. It has not been previously recognized in ancient pozzolanic mortars. New methods of assessing bulk specific gravity of the porous concretes suggest unit weights of 1495 kg/m³ for the conglomeratic wall cores and 1200-1300 kg/m³ for the pumice-bearing vaulted ceiling mortar.Innovative point load tensile strength testing methods provide a means of evaluating the strengths of small samples of ancient, conglomeratic cementitious materials. Tests of coarse aggregate clasts, pozzolanic mortar, and the adhesive interfacial surfaces of coarse aggregate clasts (caementa) in contact with the mortar, suggest tensile strengths (ft) of 2.7 MPa for brick clasts, 1.2 MPa for Tufo Lionato tuff clasts, and 0.9 MPa for Tufo Giallo della Via Tiberina tuff clasts based on a tentative, approximate correlation with splitting (Brazilian) tests on these materials. The pozzolanic mortar and interfacial zones appear to have lower ft, in the range of 0.8 MPa to 0.5 MPa. The relatively low tensile strengths of the mortar and its somewhat tenuous adhesion to coarse aggregate clasts suggests that the caementa may have had a role in arresting the propagation of tensile microcracks that formed in the mortar, thereby increasing the composite tensile strength of the concrete. Roman builders evidently selected the complex aggregate mixes to optimize the performance and durability of the wall and vault masonry.