Work Sampling in Precast Concrete Plants
Design sampling in precast concrete production with attention to phases, batches, safety and process variability.
Updated:
Mould preparation, reinforcement, casting, curing, demoulding and dispatch use different resources. Sampling may estimate person or equipment states, but a product's curing time is not its operator's waiting time.
Choose the unit before classifying
A product can cure while its crew prepares another mould. For product-instant, curing is a process state; for person-instant, record actual activity. Define mould or crane occupancy as separate units when relevant.
Batch flow does not itself invalidate sampling. Proper temporal sampling does not automatically underrepresent long phases; observing a product over time makes longer states more likely. Stratifying by phase, family or shift may help depending on the objective, but requires compatible populations and weights.
Practical categories
For people, separate mould preparation, reinforcement, casting/vibration, inspection/documentation, transport, confirmed waiting, maintenance, breaks and other. Defect repair is not planned finishing. Conversation may be necessary coordination; a snapshot does not establish an unjustified break.
Category labels do not authorise removing controls, shortening curing or early demoulding. Those decisions depend on product, required strength, procedures and competent technical staff.
Plan precision and effort
The simple reference n=ceiling[z²p₀(1−p₀)/e²] gives 547 records with z=1.96, p₀=0.35 and e=0.04. Using z=2 gives 569 at approximately 95.45% confidence. State the choice rather than present both as exact 95% calculations.
Five people per round and sixteen daily rounds produce up to 80 participations daily. Reaching 569 takes at least eight days, not seven. This calculates volume only: check shift coverage, losses and correlation. Half-hour rounds do not guarantee independence.
Teaching case: 38 workstations and two shifts
Assume 569 valid records:
| Defined group | Count | Proportion |
|---|---|---|
| Direct work | 267 | 46.92% |
| Support | 182 | 31.99% |
| Waiting/rework for investigation | 120 | 21.09% |
| Total | 569 | 100% |
The third group is not entirely removable. Direct work's approximate normal 95% margin is 4.1 points under independence; observed precision need not equal the margin planned at p₀=0.35.
Investigate reinforcement waits, crane availability and production decisions through records. Coordination meetings or relocating stock are hypotheses: assess time, safe travel, lifting, ergonomics and crane constraints before implementation.
Quality, traceability and regulation
Spain's Structural Code and the European construction-products framework require product- and regime-specific assessment. Documentation, assessment and marking duties are neither replaced by an activity percentage nor certified by WorkSamp.
Regulation (EU) 2024/3110 introduces a new framework and transitional provisions. Do not assume all product standards, editions and assessment systems change simultaneously. Quality staff must verify applicable specifications and current requirements.
Evaluate results
If a later scenario assumes OEE rises from 62% to 76%, this is fourteen points and a 22.6% relative increase. It needs times, ideal cycle and total/good units; it is not derived from the person-state table.
Nor does €12 million revenue establish €600,000 contribution or payback: saleable volume, incremental margin and full costs are missing. An assumed study budget of 97 hours at €36/h is €3,492, not a market rate; add implementation and follow-up.
WorkSamp supports planning and recording. Report coverage, intervals and limitations alongside verifiable proposals.
Sources
NIST: proportions. Structural Code, RD 470/2021. EU Regulation 2024/3110. OEE.