Learning from Roman Concrete

By Luke Carothers

For years, engineers and laypeople alike have pondered over the great concrete structures of Ancient Rome, admiring their enduring beauty and awing at the strength and resilience the structures still possess.  Buildings like the Parthenon, the Colosseum, and Caecillia Metella’s Tomb have endured conditions, such as earthquakes and fires, that would have caused the failure of modern concrete structures that use Portland cement.  Moreover, Roman concrete, unlike  its modern counterpart, seems to get stronger as time goes on.

One of the researchers trying to uncover the secrets of Roman concrete is Dr. Marie Jackson at the University of Utah.  Dr. Jackson and her colleagues are studying the tomb of a first century noblewoman named Caecillia Metella which is located along the Appian Way.  While not much is known about Caecilia Metella’s life, her tomb has been a source of both inspiration and awe.  On top of being one of the most visited antiquity sites in Rome, Caecilia’s tomb is one of the most well preserved.  To study the substructure of the concrete in the tomb, Dr. Jackson traveled to Rome in 2006 to collect samples.  Dr. Jackson also collected samples from and studied other Roman concrete structures such as Trajan’s Markets.

There are several key differences between Portland and Roman concrete that impact the trajectory of their structures.  Portland cement is made by heating limestone and clay to form clinker, then grinding that clinker and adding a mixture of gypsum.  When mixed with water and aggregate, it forms concrete.  According to Dr. Jackson, Roman concrete was made using the “antithesis” of this process, meaning there is no cement in the material.  Roman concrete was almost entirely composed of reactive rock (usually volcanic) and lime.

The resulting reactions between the rock and lime, known as pozzolanic reactions, give the Roman concrete its “unique framework”, according to Dr. Jackson.  When Roman concrete was mixed and poured, these reactions resulted in a phase known as C-A-S-H binding (calcium-aluminum-silicate-hydrate).  This means that Roman concrete was intentionally designed to change and grow stronger over time.  Concrete made from Portland cement is also capable of forming these C-A-S-H bonds if there are supplementary materials added, but Dr. Jackson points out that “most times when Portland concrete changes, that’s a bad sign.”

When Dr. Jackson and her team analyzed the samples from Caecillia Metella’s tomb, they were initially focused on how the boundaries of the reactive aggregate changed over time and formed new minerals, but they also soon realized that the C-A-S-H bonds were undergoing significant changes.  In their samples from Roman concrete structures, Dr. Jackson and her colleagues found that, contrary to previous research, these C-A-S-H materials “dissolved, split, grew nanocrystals, and elongated.”

This process was well understood by the Romans.  Part of the work Dr. Jackson and her colleagues performed around the time of their first trip to Rome was a retranslation of de Architectura  from the perspective of a geotechnical professional.  In his famous tome de Architectura, Roman architect and intellectual Vitruvius spoke about refined concretes made from hydrated lime and volcanic ash mortar that bind over time around a rock framework, an insight revealed after this new translation was completed.  This historical approach also reveals that much of the aggregate material used in these concrete structures comes from the same pyroclastic flow.  While some classic Roman structures, such as the Theatre of Pompeii, were built using materials from the top part of this pyroclastic flow, the Tomb of Caecilia Metlla was built using materials from the lower portion of the pyroclastic flow.  This means there is a higher natural presence of the mineral leucite in the mortar used to build Caecillia Metella’s Tomb.  Over the centuries, groundwater and rainwater have seeped into the substance, dissolving the leucite and releasing potassium into the structure.

In a modern concrete building, such a flood of potassium would be bad news.  In such a structure, a flood of potassium creates expansive gels, eventually leading to microcracks, spalling, and deterioration of the structure.  However, in the case of Caecillia Metella’s Tomb, this potassium dissolved and reconfigured the C-A-S-H bonds, preventing cracks and microfractures.

Dr. Jackson and her colleagues are hoping to apply their research to modern forms of concrete.  They are currently working with the U.S. Department of Energy ARPA-e to develop modern concrete using the same principles as Roman concrete, replacing tephra with engineered cellular magmatics.  This could prove a huge paradigm shift in terms of sustainability with a stated goal of reducing the energy emissions of concrete production and installation by 80 percent and improving the lifespan of marine concrete by four times.

Luke Carothers is the Editor for Civil + Structural Engineer Media. If you want us to cover your project or want to feature your own article, he can be reached at lcarothers@zweiggroup.com.