Last reviewed 16 Sept 2026 · 7 min read
Quality
Quality is conformance to requirements (Crosby) or fitness for use (Juran) — meeting the specified and implied needs of the owner and users.
In construction, quality includes conformance to drawings and specifications, durability, safety, functionality, aesthetics and maintainability.
QC, QA and quality management
| Term | Meaning |
|---|---|
| Quality control (QC) | Operational techniques — inspection, testing, measurement — to verify that products meet requirements; product-oriented, detects defects |
| Quality assurance (QA) | Planned and systematic activities (procedures, audits, documentation) to provide confidence that quality requirements will be met; process-oriented, prevents defects |
| Quality management | Overall management function — quality policy, planning, QC, QA and improvement |
| Total quality management (TQM) | Organisation-wide philosophy of continuous improvement with customer focus and involvement of all employees |
Quality gurus
| Contributor | Key ideas |
|---|---|
| W. Edwards Deming | PDCA (Plan–Do–Check–Act) cycle (Deming/Shewhart cycle); 14 points for management; reduce variation |
| Joseph M. Juran | Quality trilogy — quality planning, quality control, quality improvement; "fitness for use"; Pareto principle applied to quality (vital few, trivial many) |
| Philip B. Crosby | Zero defects; "quality is free"; conformance to requirements; cost of quality |
| Kaoru Ishikawa | Cause-and-effect (fishbone) diagram; quality circles |
| Walter A. Shewhart | Control charts; statistical process control |
| Armand Feigenbaum | Total quality control; cost of quality |
| Genichi Taguchi | Loss function; robust design |
ISO 9001
- ISO 9001 specifies requirements for a quality management system (QMS); the current edition is ISO 9001:2015 (as of the update date of these notes).
- Principles include customer focus, leadership, engagement of people, process approach, improvement, evidence-based decision making, relationship management; the 2015 edition emphasises risk-based thinking.
- Certification is by accredited third-party bodies.
Quality planning in construction projects
- Quality assurance plan (QAP) — scope, organisation and responsibilities, procedures, standards, testing frequencies, documentation.
- Inspection and test plan (ITP) — for each activity: what to inspect/test, method, frequency, acceptance criteria, hold points (work cannot proceed until inspected) and witness points.
- Method statements, checklists, material approvals (samples, mill certificates), field laboratory, third-party quality audits, non-conformance reports (NCR) and corrective/preventive actions.
Seven basic quality control tools
- Check sheet — structured data collection.
- Histogram — frequency distribution of data.
- Pareto chart — defects ranked by frequency — about 80% of problems from 20% of causes.
- Cause-and-effect (Ishikawa/fishbone) diagram — causes grouped (e.g. man, machine, material, method, measurement, environment).
- Scatter diagram — relationship between two variables.
- Control chart — process variation over time.
- Stratification (or flow chart in some lists) — separating data by source.
Statistical quality control
Control charts
A control chart plots a quality characteristic over time with a centre line (CL), upper control limit (UCL) and lower control limit (LCL), commonly at ±3σ of the plotted statistic.
| Chart | For | Plots |
|---|---|---|
| x̄ chart | Variables | Sample means — process average |
| R chart / s chart | Variables | Sample ranges / standard deviations — dispersion |
| p chart | Attributes | Fraction defective |
| np chart | Attributes | Number defective (constant sample size) |
| c chart | Attributes | Number of defects per unit |
FormulaControl limits
x̄ chart: , R chart: , p chart: c chart: (Negative LCL is taken as zero.)
- Points outside limits or non-random patterns (runs, trends) indicate assignable (special) causes; variation within limits is due to chance (common) causes.
- Process capability — is commonly regarded as capable.
Acceptance sampling
- Deciding to accept or reject a lot based on a sample.
- Producer's risk (α) — rejecting a good lot; consumer's risk (β) — accepting a bad lot.
- Operating characteristic (OC) curve — probability of acceptance vs lot quality; AQL (acceptable quality level) and LTPD (lot tolerance percent defective).
Cost of quality
| Category | Examples |
|---|---|
| Prevention costs | Training, quality planning, process design |
| Appraisal costs | Inspection, testing, audits |
| Internal failure costs | Rework, scrap, repairs before handover |
| External failure costs | Defects after handover — repairs, claims, reputation loss |
Investing in prevention reduces failure costs — the total cost of quality falls.
Six Sigma
- Data-driven approach to reduce variation; a Six Sigma process has about 3.4 defects per million opportunities (DPMO) (allowing a 1.5σ shift).
- Methodology DMAIC — Define, Measure, Analyse, Improve, Control; roles such as Champions, Black Belts and Green Belts.
Quality control of concrete
FormulaTarget mean strength (IS 10262)
= standard deviation (assumed values are given in IS 10262 when site data are not available); 1.65 corresponds to not more than 5% of results falling below .
Sampling frequency (IS 456)
| Quantity of concrete in the work (m³) | Number of samples |
|---|---|
| 1–5 | 1 |
| 6–15 | 2 |
| 16–30 | 3 |
| 31–50 | 4 |
| 51 and above | 4 + 1 for each additional 50 m³ or part |
- Each sample consists of three cubes; the sample strength is the average of the three (individual variation limits apply).
- Acceptance is judged using the mean of groups of consecutive non-overlapping results and individual results relative to , as specified in IS 456 (as amended).
Other site QC for concrete
- Tests on materials — cement (setting time, soundness, strength), aggregates (grading, silt, flakiness), water.
- Slump test for workability; temperature; air content where required.
- Non-destructive tests on hardened concrete — rebound hammer (surface hardness), ultrasonic pulse velocity (homogeneity, cracks), cover meter, half-cell potential (corrosion risk), core tests (actual strength).
Worked examples
Worked ExampleExample 1 — target mean strength
Find the target mean strength for M25 concrete with a standard deviation of 4 N/mm².
Solution. 31.6 N/mm²
Worked ExampleExample 2 — number of samples
How many cube samples are required for 140 m³ of concrete in a slab, and how many cubes?
Solution. Up to 50 m³: 4 samples; the remaining 90 m³ is one full 50 m³ plus a part → 2 more samples. Total = 6 samples = 18 cubes
Worked ExampleExample 3 — p chart limits
In 20 samples of 50 bricks each, the average fraction defective is 0.04. Find the control limits.
Solution. UCL 0.123; LCL → 0
Worked ExampleExample 4 — Pareto analysis
Defects in plaster: hollow sound 45, cracks 30, uneven surface 15, stains 6, others 4. Which defects should be tackled first?
Solution. Total = 100. Hollow sound (45%) and cracks (30%) together account for 75% of defects → address these first.
Frequently tested points
- QC = inspection/testing (product, detect); QA = systematic process (prevent, confidence); TQM = organisation-wide continuous improvement.
- Deming — PDCA; Juran — trilogy, fitness for use; Crosby — zero defects, quality is free; Ishikawa — fishbone, quality circles; Shewhart — control charts.
- ISO 9001 — QMS standard (2015 edition).
- Seven QC tools: check sheet, histogram, Pareto, fishbone, scatter, control chart, stratification/flow chart.
- Control charts: x̄ and R (variables); p, np, c (attributes); limits ±3σ.
- Cost of quality: prevention, appraisal, internal failure, external failure.
- Six Sigma: 3.4 DPMO; DMAIC.
- Target mean strength ; IS 456 sampling: 1, 2, 3, 4 samples for up to 5, 15, 30, 50 m³, then +1 per 50 m³; 3 cubes per sample.
- NDT: rebound hammer, UPV, cover meter, half-cell potential, cores.
Common MistakeCommon mistakes
- Using QC and QA interchangeably.
- Using p charts for number of defects per unit (use c charts).
- Counting cubes instead of samples when applying IS 456 sampling frequency.
Revision SummaryChapter summary
- Quality means conformance to requirements and fitness for use; QC inspects products while QA assures processes.
- Deming, Juran, Crosby, Ishikawa and Shewhart shaped TQM, PDCA, zero defects, fishbone diagrams and control charts.
- ISO 9001, quality assurance plans and inspection and test plans structure project quality management.
- Seven QC tools, control charts, acceptance sampling, cost of quality and Six Sigma support improvement.
- Concrete quality is controlled through target mean strength, sampling frequency, acceptance criteria and non-destructive testing.