Precast or cast in place: how shelter modules are built in the factory

Reinforcement and formwork

The process begins with the fabrication of the reinforcement cage. Steel grades, bar diameters, bending radii, and lap lengths are calculated in accordance with DIN EN 1992-1-1 to resist dead loads, earth pressures, live surcharges, and accidental dynamic loads. Certified steel fixers assemble the cage on precision jigs. This ensures that spacing remains consistent across the entire component, avoiding localized stress concentrations or vulnerable zones that could compromise structural capacity under sudden shock waves or soil displacement.

Dimensionally rigid steel formwork is essential for modular civil-protection structures. Unlike timber formwork used on job sites, precision steel shutters resist hydrostatic concrete pressure without deflection. This ensures strict dimensional accuracy, squareness, and flat sealing faces along all modular connection flanges. Maintaining tight geometric tolerances is vital because adjacent modules must align perfectly on site to establish reliable watertight and gas-tight elastomeric joint seals.

All penetrations and cast-in components are secured directly to the reinforcement cage and formwork prior to pouring. These include gas-tight door frames, blast valve sleeves, fresh air intake conduits, waste pipes, cable transits, and earthing terminals. Fixing these elements before casting eliminates the need for subsequent core drilling or chipping. Post-drilling often cuts through structural rebar and creates micro-cracks in the concrete matrix, introducing long-term leakage pathways.

• —Reinforcement cages assembled on precision steel jigs
• —Rigid steel formwork ensuring sub-millimetre dimensional tolerances
• —Cast-in sleeves and blast door frames positioned prior to pouring
• —Avoidance of subsequent core drilling through structural rebar
• —Documented cover spacers to maintain minimum concrete depth

Concrete mix and curing

Precast shelter modules use high-performance concrete formulated for severe environmental exposures. Concrete mixes correspond to exposure classes XC4, XD3, and XA2 in accordance with DIN EN 206, providing robust resistance to moisture, de-icing salts, and aggressive soil chemistry. A low water-cement ratio, achieved through optimized plasticizers, produces a dense cement matrix with minimal capillary porosity. This low permeability forms the primary defence against groundwater ingress and prevents chloride-induced reinforcement corrosion over decades of subterranean service.

The hall environment allows complete control over the hydration process. While site concrete is vulnerable to rapid drying, temperature swings, and rain, factory curing takes place under monitored thermal and moisture conditions. Controlled hydration moderates peak hydration temperatures and prevents thermal differential cracking between the inner core and the outer surface of thick wall elements. Formwork insulation and controlled ambient conditions ensure that the concrete achieves uniform early and long-term strength.

Concrete quality is verified systematically throughout the batching and casting sequence. Fresh concrete is tested for slump flow, air content, density, and temperature before placement. Test cubes and cylinders are cast from the same batches and cured alongside the main modules for compressive strength testing at 7 and 28 days. Every production lot is documented in a factory production control log, creating full traceability for the client and approving authorities.

• —Controlled factory climate preventing thermal and shrinkage cracking
• —Low water-cement ratio for minimum capillary porosity
• —Documented testing of fresh concrete and companion test cubes
• —Uniform concrete cover ensuring passive corrosion protection
• —Continuous compliance with DIN EN 206 and DIN EN 1992 standards

Joint technology and water ingress protection

Underground structures face continuous hydrostatic groundwater pressure and soil moisture. For modular systems, joint design between individual precast elements is critical. Factory production allows joints to be formed with cast-in tongue-and-groove profiles or stepped rebate geometries that cannot be reliably cast on site. These precision profiles ensure that sealing gaskets are compressed uniformly along the entire perimeter when modules are drawn together during assembly.

Sealing strategies typically employ multiple independent barriers. Primary sealing is achieved using elastomeric compression profiles or hydrophilic swellable rubber strips cast directly into the joint recesses. When exposed to moisture, these hydrophilic materials expand into any micro-voids, creating an active barrier against water ingress. Secondary external seal coats and bituminous membranes are applied across outer joint faces before backfilling, providing layered protection compliant with DIN 18533.

In contrast to cast-in-place construction, where cold joints between floor slabs and rising walls are vulnerable to compaction defects and leakage, precast monolithic box sections eliminate horizontal base-to-wall joints entirely. By casting the floor, walls, and ceiling as a single integrated unit or utilizing specialized vertical joint systems, the number of potential leakage paths exposed to groundwater is minimized.

Factory integration of technical systems

A functional shelter requires complex life-support infrastructure, including chemical, biological, radiological, and nuclear filtration systems, blast protection valves, overpressure control, and emergency power. Installing these mechanical and electrical systems in a factory environment provides significant quality and speed advantages over site installation. Technicians work in clean, illuminated conditions with direct access to calibrated tooling and test benches.

Blast valves and overpressure valves are mounted directly to pre-installed steel counter-flanges cast into the concrete walls. Seals are inspected for alignment, torque values on fastening bolts are recorded, and valve mechanisms are mechanically tested before the module leaves the plant. This level of pre-commissioning ensures that life-safety components meet specified opening and closing pressure thresholds in accordance with civil protection guidelines.

Electrical wiring, emergency backup batteries, lighting circuits, and air ducting are pre-routed through designated cast-in conduits and fixing channels. Cable entries are sealed using modular elastomer transits that provide certified gas tightness and water resistance. Pre-assembling these systems off site protects sensitive filter media and electronic control panels from construction dust, moisture, and accidental mechanical damage common on active building sites.

• —Mechanical pre-assembly in clean, climate-controlled conditions
• —Calibrated torque verification for blast valve and hatch mountings
• —Gas-tight and water-tight cable transit pre-installation
• —Pre-testing of overpressure and ventilation control assemblies
• —Protection of sensitive filtration equipment from site dust

Testing before delivery

Quality verification concludes with factory acceptance testing before any module is loaded for transport. Inspectors verify overall dimensions, diagonal squareness, wall thicknesses, and the exact positioning of connection flanges against structural drawings. Concrete surfaces are examined for surface voids, honeycombing, or micro-cracks. Any minor surface deviation is addressed in the factory under controlled conditions using approved polymer-modified repair mortars.

Movable components undergo rigorous functional checks. Heavy reinforced-steel blast doors and emergency escape hatches are hung, adjusted, and tested through their full swing range. Seal compression is verified using feeler gauges and contact paste to guarantee continuous airtight closure against the frame. Latching mechanisms, central locking bars, and internal escape levers are lubricated, adjusted, and certified for smooth operation under specified manual operating forces.

Identifying and rectifying an installation issue or seal misalignment inside the factory requires minimal time and standard workshop resources. In contrast, discovering a misaligned door frame, an obstructed conduit, or a dimensional deviation while a mobile crane is idling on site causes severe delays. Factory testing eliminates costly site interruptions, secondary crane deployments, and unexpected remediation work in the excavation pit.

Logistics, heavy transport and lifting

Precast concrete shelter modules represent heavy, monolithic structures that require detailed logistical planning. Production schedules are coordinated with heavy haulage carriers and road transport authorities to secure route permits well in advance. Modules are designed within transportable dimensions and weight limits, typically utilizing low-loader semi-trailers equipped with hydraulic axle compensation to navigate regional road networks safely.

Lifting safety is built directly into each module during casting. Certified threaded lifting anchors or spherical head transport anchors are engineered into the concrete body, calculated to support dynamic lifting factors during demoulding, transport, and site placement. Prior to dispatch, lifting lugs are inspected and marked with their working load limits. Lifting beam configurations and sling angles are documented to ensure safe crane operations on arrival.

Coordinated delivery ensures that modules arrive on site precisely when the mobile crane and excavation are prepared. By utilizing direct offloading from the transport vehicle straight into the excavation, the need for intermediate ground storage on site is eliminated. This approach reduces the required construction footprint, which is particularly beneficial for residential properties or restricted industrial plots.

What this means for the schedule

The primary operational benefit of prefabrication is the decoupling of site preparation from shelter construction. In traditional cast-in-place construction, work proceeds sequentially: excavation, blinding layer, reinforcement fixing, wall formwork, pouring, curing, stripping, ceiling formwork, and backfilling. Each step depends on weather conditions and requires consecutive weeks of site presence, during which the excavation remains vulnerable to collapse, rain accumulation, and ground softening.

With precast technology, module production in the factory runs in parallel with site groundworks. While the excavation is being dug, drainage installed, and the reinforced concrete base slab poured and cured, the shelter module is simultaneously manufactured, fitted out, and tested in the plant. When the site is ready, the finished module is delivered and set directly onto the prepared foundation in a matter of hours.

Total on-site installation time is reduced from several weeks to typically one to two days for placement, connection, and initial sealing. Backfilling can begin almost immediately after joint inspection, allowing the surrounding terrain to be restored quickly. For clients and general contractors, this drastically reduces site management overheads, minimizes disturbance to adjacent properties, and provides fixed milestone dates with high scheduling reliability.

• —Parallel workflows between off-site fabrication and site excavation
• —On-site installation and crane positioning completed in one to two days
• —Reduced exposure of open excavation pits to weather and groundwater
• —Elimination of curing waiting times on the active construction site
• —Predictable project milestones and minimized contractor disruption