Define the release decision before choosing equipment
A curing package should not be selected from chamber dimensions, heat-source capacity or a promised number of hours alone. The buyer first needs to define the event that ends curing: form removal, prestress transfer, lifting, movement to storage or another project-specific milestone. Each event may require different evidence.
That decision affects the whole precast concrete production line. Curing occupancy influences how many moulds or chambers are required, when transfer equipment becomes available, how reinforcement and casting crews are scheduled, and whether the planned daily output remains repeatable in cold, hot or changing weather.
The RFQ should therefore connect four items: the approved concrete mixture, the specified strength or other release criterion, the curing profile and the production schedule. A supplier cannot responsibly fix the curing cycle from component length and daily output alone.
Separate release strength from later-age acceptance
For a prestressed beam, the concrete strength required before prestress transfer is not automatically the same as the strength used for later-age structural acceptance. The project designer and governing specification must define the required release strength, the test method, who authorises release and what happens after a failed or delayed result.
The buyer should state whether release is demonstrated by companion specimens, an approved maturity method or another contract method. The US Federal Highway Administration’s plant-inspection guidance, for example, asks whether cylinders used for release strength are cured with the girder while cylinders used for 28-day strength receive standard moist curing. It also checks for continuous temperature records and the required strength before detensioning. This is useful inspection logic, but its numerical limits and referenced specifications are not universal. See the FHWA precast prestressed plant review.
If the project permits maturity, the RFQ must include the work needed to establish and maintain it. ASTM C1074 requires a strength–maturity relationship for the actual concrete mixture and a recorded temperature history for the concrete being estimated. ASTM also notes limitations: hydration conditions must be maintained, early-age temperature can affect later strength, and maturity needs other evidence of potential strength. A generic temperature-time total from another mix is therefore not a release certificate.
Specify the full curing profile
“Steam cure to the required strength” is not a complete control description. The RFQ should define each phase and identify which value comes from the design specification, concrete technologist or equipment supplier.
| Curing phase | Buyer-defined basis | Equipment and record to request |
|---|---|---|
| Initial period | Time of set, moisture protection and conditions before added heat | Covers or enclosure status, concrete and ambient temperature records |
| Controlled heating | Permitted start condition and maximum rate of temperature rise | Modulating heat input, sensors, rate alarm and trend log |
| Hold period | Target range, permitted variation and minimum/maximum duration | Uniform heat and moisture distribution, multi-point records |
| Controlled cooling | Maximum cooling rate and temperature difference before opening | Controlled ventilation or heat reduction, continued logging |
| Release | Required strength or approved maturity result and authorisation | Test result linkage, batch identity, operator approval and timestamp |
Project rules vary. One FHWA sample construction specification requires initial set before accelerated curing, limits heating and cooling rates, controls maximum concrete temperature and calls for recording thermometers until stripping or transfer strength is achieved. Those values belong to that sample specification, not to every bridge or country. The procurement lesson is to place the project’s own limits in the control philosophy and acceptance plan rather than copy a supplier default. See the FHWA sample accelerated-curing requirements.
Sensor locations also matter. A chamber-air reading does not necessarily represent the temperature history of a long or deep member. The buyer’s quality plan should identify representative and potentially critical locations, how sensors are fixed and protected, the calibration requirement and how failed readings are handled.
Convert the production target into curing occupancy
Daily output is a result of the complete process, not a curing-system nameplate. Build the planning model from the required casting starts and the time each mould, bed or chamber remains occupied from entry to authorised release.
A useful first-pass relationship is:
curing positions required ≈ planned casting starts during the curing occupancy window, plus an agreed allowance for changeover, variability and maintenance.
This is a scheduling check, not a final design formula. The actual model should include product families, mould sharing, concrete placing windows, prestress operations, shift calendars, chamber loading rules and transfer routes. It should also test at least three operating cases:
- normal production with the approved reference mix and expected ambient conditions;
- a credible slow-strength-gain case, such as cold-weather production or an approved mix change;
- one curing position or heat source unavailable for planned maintenance or a recoverable fault.
If the line meets target output only when every batch reaches release strength at the earliest assumed time, it has no visible recovery margin. The buyer should decide whether that risk is acceptable before fixing mould quantity and foundations.
Define utilities and system boundaries in the RFQ
The curing proposal needs site data as well as a temperature profile. Give bidders the available electrical supply, water quality and pressure, fuel options where applicable, drainage constraints, ambient design conditions, working calendar and local requirements for pressure equipment, emissions, electrical systems and worker safety.
Then ask each bidder to mark its boundary of supply. The schedule should identify responsibility for:
- heat generation, circulation, enclosure insulation and door or cover operation;
- water treatment, make-up water, condensate collection and drainage;
- distribution pipework, valves, pumps, instruments and equipment supports;
- control panels, field wiring, network interfaces and production-data exchange;
- ventilation, access, guarding, hot-surface protection and safe maintenance isolation;
- civil openings, embedded items, foundations and connections to site utilities.
Compare energy proposals on a common production basis: the same component, concrete mixture, ambient case, curing profile, number of cycles and utility-price assumptions. A percentage saving without those inputs is not an auditable procurement comparison.
Request control records that support production decisions
The control system should help the authorised production and quality staff answer three questions: What happened to this component? Is release permitted? What changed when a cycle deviated?
At minimum, define the required sampling interval, sensor tags, batch or component identifier, recipe version, operator actions, alarm history, manual overrides and data-retention period. Decide which actions require role-based approval and whether the line must block an automatic release command when mandatory evidence is missing.
The RFQ should also state the expected failure behaviour. Loss of one sensor, communication, power, water flow or heat source should produce a defined safe response and a traceable alarm. The recovery procedure must preserve the concrete record; simply restarting the timer can hide the actual temperature history.
Put curing evidence into FAT, SAT and trial production
Acceptance should prove the agreed functions at the stage where they can be observed. A practical division is:
- Factory acceptance test: verify control sequences, simulated sensor failures, alarms, permissions, data export and equipment documentation.
- Site acceptance test: verify installation, utilities, sensor calibration, circulation, interlocks, drainage and safe access under site conditions.
- Trial production: record real component and enclosure temperatures, demonstrate the approved curing profile, link strength evidence to the component and confirm repeatable release decisions across an agreed sample of cycles.
Before testing, define allowable temperature variation, rate limits, missing-data rules, strength evidence, retest procedure and responsibility for corrective work. Do not leave “uniform curing” or “stable operation” as undefined acceptance phrases.
Send bidders one comparable curing brief
A useful RFQ package contains the component drawings and schedule, approved or target concrete-mixture information, release and later-age strength requirements, applicable standards, ambient design cases, desired curing profile, mould and chamber concept, utility data, control architecture, record-retention needs and staged acceptance criteria.
Realjet supplies project-specific production-line solutions, with the final engineering and delivery boundaries defined by the agreed project requirements. Sharing this curing brief together with the target output, shift plan and site conditions allows the discussion about a production line for precast concrete components to focus on a verifiable process: not merely how heat is supplied, but how each component reaches an authorised release point without obscuring quality or production risk.