Is Hempcrete stronger than concrete?

This is not as straight forward as you might think. However, the short answer is no. Hempcrete is typically not used to form structural elements in a building. For example, in the UK, a hempcrete block has been developed with a void in each block that allows reinfrocement to be run through and concrete poured, so that a composite structural element can be made. In designing these elements, the strength of the hempcrete is ignored as it’s less that 0.2MPa in compression and concrete design for structural elements typically starts at 30MPa in compression. In effect, the hempcrete provides a “permantent shuttering” to the reinforced concrete element.

Hempcrete does not exhibit the compressive strength assoicated with concrete- one reason being its not a dense material. It contains lots of small voids, with the result being that there are fewer points of contact between all the individual particles. The other reason is that it is made with Lime, as opposed to portland cement, so takes a very long time to reach its final strength. Lime mortars and limecrete inherently have a lower final compressive strength than concrete. However, concrete is a very brittle material because it has a very limited youngs modulus- ie it fractures very suddenly- or to put it another way, it isn’t ductile and lacks plasticity. Lime based mortars and limecrete similarly lack ductility or plasticiity and a re brittle. However, if the lime is “Fat lime” as opposed to hydraulic lime, it will gain its strength from exposure to air as opposed to being mixed with water. This is important as it means the hempcrete, assuming Fat lime or slaked lime is used, will accomodate “small” long term movement, without permanent cracking and significant loss of strength. The lime will simply heal itself as newly exposed surfaces react with the CO2 in the atmosphere.

However, hempcrete has many properties that concrete lacks and consequently, hempcrete cannot be compared directly to concrete. For example, the use of hemp in of itself results in a carbon neutral product. The hemp absorbs CO2 as it grows and this offsets the CO2 released during the manufacturing of the lime binder.

Hempcrete is a better insulator. In construction, we refer to cold bridging, where a material such as concrete acts as a bridge between the exterior and interior of a building and causes condensation to form on the interior where is condenses out of the atmosphere on a colder surface. Hempcrete differs from concrete due to the presence of copious voids, trapping air and slowing the transfer of heat from the interior to exterior of the building.