Quality Practices in Construction

Standards and standardisation

A standard — a document established by consensus and approved by a recognised body that provides rules, guidelines or characteristics for products, processes or services to achieve optimum order.

Standardisation — the process of developing and implementing standards.

Levels of standards

Benefits of standardisation

• Interchangeability and compatibility of parts; variety reduction.
• Quality, safety and reliability assurance.
• Consumer protection and confidence.
• Economies of scale, reduced costs.
• Facilitates trade (removes technical barriers).
• Common language for design, procurement and contracts.

Bureau of Indian Standards (BIS)

• National standards body of India, under the Ministry of Consumer Affairs, Food and Public Distribution; established under the BIS Act, 1986, now governed by the BIS Act, 2016.
• Functions: formulation of Indian Standards (IS), product certification, hallmarking, laboratory services, management system certification, consumer awareness.

• Other standard-setting bodies relevant to civil engineering: IRC (Indian Roads Congress), RDSO (railways), CPWD specifications and schedules, MoRTH specifications for road and bridge works, CEA (electricity).

Quality concepts

Quality — "degree to which a set of inherent characteristics fulfils requirements" (ISO 9000), fitness for use (Juran), conformance to requirements (Crosby).

Dimensions of product quality (Garvin)

Performance, features, reliability, conformance, durability, serviceability, aesthetics, perceived quality.

Quality control, assurance and management

• Quality control (QC) — inspection and testing to detect defects (product-oriented).
• Quality assurance (QA) — planned systematic activities to prevent defects and provide confidence (process-oriented).
• Quality management (QM) — overall coordinated activities including planning, QC, QA and improvement.
• Total Quality Management (TQM) — organisation-wide, customer-focused, continuous improvement involving everyone.

Quality gurus (summary)

Continuous improvement tools and philosophies

Cost of quality

Investing in prevention reduces total cost of quality.

Statistical quality control

• Statistical process control (SPC) — monitoring processes using control charts (x̄–R charts for variables; p, np, c charts for attributes) with limits typically at ±3σ; distinguish common (chance) and special (assignable) causes.
• Process capability $C_p=\frac{USL-LSL}{6\sigma }$; $C_{pk}$ accounts for centring.
• Acceptance sampling — accept/reject lots based on samples; producer's risk (α) and consumer's risk (β); OC curves; AQL.

Management system standards

Common structure (High Level Structure/Annex SL) enables integrated management systems; certification involves documentation, implementation, internal audits, management review and third-party certification audits.

Quality in production (manufacturing)

• Incoming material inspection, in-process inspection, final inspection.
• SPC and control charts; process capability studies.
• Standard operating procedures (SOPs), work instructions.
• Poka-yoke, automation, calibration of gauges.
• Traceability (batch numbers — e.g. cement bags, steel heat numbers and mill test certificates).
• Supplier quality management and vendor rating.

Quality in construction

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

Quality gurus

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

1. Check sheet — structured data collection.
2. Histogram — frequency distribution of data.
3. Pareto chart — defects ranked by frequency — about 80% of problems from 20% of causes.
4. Cause-and-effect (Ishikawa/fishbone) diagram — causes grouped (e.g. man, machine, material, method, measurement, environment).
5. Scatter diagram — relationship between two variables.
6. Control chart — process variation over time.
7. 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.

• Points outside limits or non-random patterns (runs, trends) indicate assignable (special) causes; variation within limits is due to chance (common) causes.
• Process capability $C_p=\frac{USL-LSL}{6\sigma }$ — $C_p\ge 1.33$ 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

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

Sampling frequency (IS 456)

• 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 $f_{ck}$, 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

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 $f_{ck}+1.65s$; 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.