Shown here are six concrete floors that were cast over time in one of the buildings by a “Pre-Pottery Neolithic Part B” culture 9800 years ago. It's hoped that an examination of the concrete in each floor will reveal how their concrete technology developed over time.
Credit: Howard Kanare
There is much talk about green and sustainability these days. For concrete, the key word is sustainability, which has to do with constructing slabs and structures with low carbon footprints. There are many ways to do this, but the best way is to use the inherent qualities of the material to provide longer life cycles—replacement being the most energy intensive. So how long can the service life of concrete be? How about 9800 years.
The concrete industry is more fascinated with concrete than other trades are about the materials with which they work. Those in the industry push the limits of what it can do, learn about the technical developments that can make it even better, and keep up with the ever-changing upgrading of tools to form, place, and finish it. Concrete professionals point with pride to projects that use concrete to push the limits of what's possible.
Recent discoveries add another dimension to humans' experience with concrete. As a building material, it actually predates the invention of pottery made from clay and the discovery of metal for making tools and implements. Along with wood and rock, it's the oldest building material.
The beginnings for concrete
In the summer of 2008, Howard Kanare was invited by Hamoudi Khalaily, Ph.D., Israel's Antiquities Authority (IAA), to participate in an archaeological excavation in northern Israel known as Yiftahel. The site was discovered in 1982 when a bulldozer struck unearthed ancient buildings while excavating for a new highway. IAA archaeologists walked several acres of the highway construction path and found artifacts, including flint tools, on the ground's surface, leading them to believe there was an ancient buried settlement. What they found is historically significant, especially for the concrete industry.
This cross section of concrete from the site clearly shows the typical ingredients of concrete: large and small aggregate and cement paste. The small black marks indicate air voids.
Credit: Sang Lee, CTLGroup
Yiftahel is located in Northern Israel about 15 miles east of Haifa and 5 miles west of Nazareth. The archaeological remains are embedded within the eastern bank of Yiftahel Valley and are estimated to cover at least 7.4 acres. This particular site is likely the oldest permanent village ever found, and it has a number of concrete floors.
The future of Yiftahel
After years of debates about the site and the road system in the area, the Israel government decided that Yiftahel will be covered by the new construction of a high-speed interchange to alleviate congestion at the intersection of Highways 77 and 79. From September 2007 until August 2008, excavations were conducted for the IAA by Khalaily and his colleagues Ianir Milevski and Nimrod Getzov to document the remains of the several occupation layers. The goal is to record and learn as much about the site's remains before the highway is constructed. The discovery of extensive concrete floors gives an unprecedented opportunity to explore the earliest known concrete.
The first concrete
This aerial photo shows the archaeological site at Yifthel. The square excavations are each 16x16 ft., and the 18x40-ft. building footprint with parts of its floor shown is oriented in the direction of the North Star. Mixing and placing concrete to cover the 720 sq. ft. of floor is a major undertaking.
Credit: Ianir Milevski
The Pre-Pottery Neolithic Part B (PPNB) culture used rock and mud to construct walls and concrete for the floors. The buildings were rectangular, and aligned to true north, indicating that the inhabitants developed some knowledge of how to do layout work. Archeologists found as many as six layers of floors, one on top of the next, providing the opportunity to find out how their technology developed. The floor finishes vary, indicating differences in ability. They may even have used large stones to polish floor surfaces. However, no evidence of the tools they used exists. Careful laboratory examination of samples may explain how the floors were placed and finished.
Investigations by several scholars for the past 25 years revealed some information about the concretes, but there is still much to learn. Archaeologists usually refer to these materials as plasters, but they are more correctly designated lime-concrete and lime-mortars. Wood-fired kiln temperatures achieved at this point in human history were between 850º to 900° C, sufficient to calcine limestone to make lime, but not enough to fire clay pottery or to produce hydraulic cements.
Garfinkel (1987) studied an 1 1180-square-foot area referred to as “Structure 700”. The floor was 1 3/16 to 2 3/8 inches thick, and required 7 tons of calcined limestone. This early work revealed a highly organized, technological society that developed an industry to produce the lime cement and to construct the concrete floors. The following steps must have been developed:
- planning for quantities needed
- quarrying limestone from nearby rock outcrops
- transporting to the kiln site
- crushing/sizing raw limestone
- gathering fuel/cutting to size
- constructing the kiln
- loading kiln with limestone and fuel
- burning (calcining) limestone to make lime
- removing and inspecting/sorting of properly burned lime
- laying out the architecture of the building: rectangular and oriented
- transporting lime cement and aggregates to the construction site
Bentur (1991) examined 2-inch-thick, two-layer floor samples from a building at Yiftahel and found compressive strengths of 1-inch cubes to be 4900 psi for the base layer and 6500 psi for the combined base plus the finish coat. Chemically, the samples consisted of nearly pure calcium carbonate (carbonated lime) and a small percentage of silica, attributed to sand. The base layer had porosity and crystal sizes suggesting it rough compaction, while the finish layer was dense with very fine crystals, suggesting it had been wet and well troweled.
Some of the floor samples recovered in 2008 clearly show a rough, porous base layer and a dense, bright white, sanded finish coat. One small area was painted red, so some decoration was used but it's unknown to what extent. The floors with white finish coats were probably decorative wearing surfaces, but other areas are rougher and somewhat easily abraded; it is speculated that some areas might have been covered with boards or textiles.
Many large holes were found in the floors and pits, some with intact skeletons or modeled plastered skulls. It's known that Neolithic cultures often buried their dead within the boundaries of a structure; in this case, the floors were excavated, bones interred, and floor patched over with fresh concrete.
During this past summer, samples were collected from 12 separate floors, along with samples of limestone likely used as the raw material for the cement and local sand that might have been used as fine aggregate. Subbase materials under the floors also were collected as well as calcined limestone from the remains of a kiln (by now, completely carbonated). The quantity and diversity of these samples will help estimate the load-bearing capacity of the floors, and lead to an understanding of the ancient construction means and methods used.
This larger section of floor shows one of the various crack patterns for the concrete floors at Yiftahel. Subgrade under the concrete was compacted with smooth surfaces allowing for movement in the concrete as shrinkage occurred. When all these cracks occurred will never be known. They could have developed at any time for the past 10,000 years.
Credit: Howard Kanare
The site investigations raised many intriguing questions, such as: Were the floors constructed by the local inhabitants or by itinerant skilled crafters (the earliest cement masons)? Were the buildings used as homes or stone-tool workshops, and used for storage or animal care? Why were the buildings sited and oriented as found? How long would they have taken to construct? How long were they occupied? What did the people learn about making concrete over time?
Carbonized wood and seeds at the site provide accurate and precise dates for occupancy, but the precise reason for abandonment of the site is unknown. Some scholars suggest that swift climate changes brought about by northern Atlantic Ocean currents reduced the ability to grow crops and caused game animals to migrate south, with humans following. It's possible this event probably occurred 1000 years after the initial occupation of the area.
The archaeologists working at Yiftahel have extensive experience investigating ancient sites from the viewpoints of archaeology, anthropology, sociology, and history, but not as construction experts. The mission now is to examine the samples using “concrete construction eyes” to better understand concrete materials and construction techniques. It is hoped that funding will help the concrete industry learn all it can from these remarkable ancient people and the world's oldest concrete floors.
Howard Kanare (firstname.lastname@example.org) is senior principal scientist at CTLGroup, Skokie, Ill. He consults on design-phase, construction, and post-construction troubleshooting of concrete floors. He is chair of ASTM F06.40.05 Task Group on Moisture Measurement in Concrete Floors and author of the PCA book, “Concrete Floors and Moisture.” Ianir Milevski (email@example.com), Hamoudi Khalaily (firstname.lastname@example.org), and Nimrod Getzov (email@example.com) are research archaeologists at the Israel Antiquities Authority. They have excavated and researched extensively in Israel, focusing on late prehistoric sites. Joe Nasvik (firstname.lastname@example.org) is senior editor for Concrete Construction magazine.