Precast elements made of expanded clay (LECA) concrete

Expanded clay concrete is a lightweight concrete in which natural aggregate is replaced with expanded clay (LECA) — clay fired in a rotary kiln at around 1150°C, with a porous structure. The air enclosed in the granules reduces the element's weight and its thermal conductivity, while the sintered shell of each granule retains the strength needed in a load-bearing structure.

In practice, this means a wall that, at the same thickness, weighs noticeably less than its normal-weight concrete counterpart and retains heat better — without giving up load-bearing capacity, non-combustibility or moisture resistance.

The history of expanded clay begins in 1913 at Hayde's brickworks in the United States. During a kiln failure, at too high a temperature, a lightweight ceramic aggregate was created by accident. The owner patented it immediately and production began in 1917 — the material was named haydite after him and was used to make lightweight concrete and small building components.

By 1939 haydite plants were already operating in Canada, Sweden, Norway and Denmark. After the war, demand for lightweight aggregate grew so much that factories sprang up around the world, and the material found uses in construction — from lightweight concretes to thermal insulating mortars — as well as in geotechnics, horticulture and environmental protection. In western countries the name LECA — Lightweight Expanded Clay Aggregate — took hold. In Poland it was first called GLINIEC, after the clay it is made from, and later the name keramzyt was introduced, inspired by the Greek keramos, meaning potter's clay.

Expanded clay concrete itself emerged in response to the need for lighter, more energy-efficient structural solutions. Since the mid-twentieth century the material has been used in residential, industrial and infrastructure construction, combining the durability of concrete with the insulating properties of expanded clay.

Today expanded clay concrete is used in modern precast production, where manufacturing precision and element repeatability allow projects to be delivered faster and more efficiently.

We produce elements in classes LC12/13, LC16/18 and LC20/22, in thicknesses from 10 to 24 cm. Layered walls achieve fire resistance of R60–R240 and EI30–EI60 with insulation up to 300 mm. All elements are covered by certificate 0761-CPR-1152 issued by IBMB MPA TU Braunschweig, in accordance with EN 14992:2007+A1:2012.

The same material comes in three wall build-ups. The choice depends on how many layers are made in the factory and how many on site.

Three density classes are made here from the same base recipe — what differs is the aggregate skeleton. In D1.4 almost all the aggregate is expanded clay, so the element comes out lightest and warmest. In D1.6 and D2.0 the share of natural aggregate grows: the wall gains load capacity and gives up part of its lightness. That is why the class is chosen for the building element, not from a catalogue — LC12/13 where mass and conductivity rule, LC20/22 where loads size the cross-section. Boxes, conduits and openings are cast into the mould, and every element passes Factory Production Control under the IBMB MPA TU Braunschweig certificate.

It is not the concrete that demands the most practice — it is the water. Under the guidelines of the Danish Letbetonelementgruppen, a lightweight concrete element carries 10–20% moisture by weight at delivery, while the residentially dry state is 2–3% — in between, the material gives up water and shrinks, ultimately up to 0.8‰ against 0.3–0.5‰ for normal concrete. The process cannot be skipped, only planned: expansion joints every 8–10 m and division of longer buildings every 15–20 m are drawn into the shop drawings before production starts — on site it is too late.

The second half of the plan belongs to the finishing contractor: before painting and tiling, roughly half the shrinkage should have developed, which at 18–20°C and air humidity up to 30% usually takes 4–6 weeks. Element joints are reinforced with 100 mm glass-fleece strips. In return the material repays with something normal concrete cannot: shrinkage ends with drying, so cracks do not come back years later. These rules were written down by producers who have worked with lightweight concrete for decades — exactly the knowledge that catalogue tables do not show.

Źródło: Letbetonelementgruppen, Hæfte 11 — Forebyggelse af revner (2023)

Expanded clay concrete works wherever a lightweight structure, good insulation and durability matter. See where it is used most often. The boundary is clear: we make walls from lightweight concrete — solid, two-layer and sandwich. Floor slabs, stairs and balconies we produce in normal-weight concrete up to C35/45 — these are slender, heavily loaded elements where the strength of the section matters more than saving weight. What confuses the picture is that expanded clay does appear in rib-and-block floors as the infill block between the ribs; that is a different thing from a load-bearing element cast in expanded clay concrete.

Expanded clay concrete is chosen where warmth and weight matter: a 10 cm wall in class LC12/13 weighs 140 kg/m² compared with 235 kg/m² for C20/25 concrete, and thermal conductivity drops from 2,0 to 0,9 W/(m·K). Lower weight means lighter foundations, fewer transport runs and a smaller crane on site.

Normal-weight concrete remains the better choice for the highest loads and where sound insulation is decisive — the same 10 cm wall achieves Rw 47,9 dB versus 39,3 dB for expanded clay concrete. In practice we match the material to the specific wall, often combining both in a single building. The comparison only holds at equal thickness, though. In the laboratory measurements an 18 cm wall in LC16/18 lightweight concrete reached 55 dB — one decibel more than 16 cm of normal-weight concrete. Moving from LC16/18 to LC20/22 does not change the rating, so the denser lightweight class is chosen for load-bearing capacity, not for acoustics.