Polymers, Fibre-Reinforced Composites and Adhesives

Introduction and Context

Polymers and polymer-based composites have changed the design and construction of civil infrastructure. Their low density, corrosion resistance, high specific strength, chemical resistance and ability to form continuous membranes make them useful in repair, strengthening, waterproofing, façades, bridge decks, industrial floors and lightweight enclosures. However, polymers are not simply lightweight substitutes for steel, concrete or masonry. Their behaviour is strongly affected by temperature, time, moisture, ultraviolet radiation, chemicals, fire and manufacturing quality.

The engineer must therefore evaluate both short-term mechanical performance and long-term durability. A polymeric material that performs well in a laboratory tensile test may experience creep, stress relaxation, debonding or softening in service. Fibre-reinforced polymer (FRP) systems can provide exceptional tensile capacity, but their effectiveness depends on fibre orientation, resin properties, bond quality, anchorage and substrate condition. Adhesives and sealants can improve structural continuity and weather resistance, but their performance is sensitive to surface preparation, joint geometry and environmental exposure.

This chapter presents the materials science, mechanics, design applications and limitations of polymers, FRP composites and adhesives used in advanced construction.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain polymer structure, glass transition, viscoelasticity, creep and stress relaxation.

2. Distinguish thermoplastics, thermosets, elastomers, fibres, matrices, adhesives, sealants and coatings.

3. Assess ageing mechanisms and fire-related risks in polymeric construction materials.

4. Explain the mechanics and practical design of FRP reinforcement and strengthening systems.

5. Select suitable materials for sandwich panels, membranes, coatings, sealants and structural adhesives.

6. Identify failure modes, quality-control requirements, sustainability concerns and regulatory issues.

7. Evaluate an applied strengthening problem using appropriate assumptions and design checks.

1. Polymer Fundamentals for Construction

1.1 Molecular structure and classification

A polymer is a material formed from long molecular chains composed of repeating units. The chains may be linear, branched or cross-linked. Molecular architecture controls stiffness, ductility, melting behaviour, chemical resistance and long-term deformation.

Thermoplastics soften when heated and harden on cooling. The process is generally reversible, although repeated heating can cause degradation. Examples include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polycarbonate and polyamide. Thermoplastics are used in pipes, geomembranes, drainage products, insulation and protective sheets.

Thermosets form permanent three-dimensional networks during curing. Once cured, they do not melt and flow in the manner of thermoplastics. Epoxy, polyester, vinyl ester and phenolic resins are important construction thermosets. Epoxies are widely used in structural adhesives and FRP matrices because of their adhesion and relatively low shrinkage.

Elastomers have lightly cross-linked chains and can undergo large reversible strains. Silicone, polyurethane, polysulphide and modified-silicone products are used in sealants, expansion joints and flexible waterproofing systems.

A polymer product normally contains more than the base resin. Fillers, pigments, plasticisers, catalysts, stabilisers, flame retardants, fibres and solvents modify processing and performance. Consequently, a generic material name such as “epoxy” does not define a complete engineering specification.

1.2 Thermal transitions

The glass-transition temperature, denoted by \(T_g\), is the approximate temperature range at which amorphous or amorphous regions of a polymer change from a relatively rigid glassy state to a softer rubbery state. It is not the same as a sharp melting point. Above \(T_g\), modulus and strength may reduce substantially, and creep generally increases.

The service temperature should be assessed relative to \(T_g\), not merely against the material’s room-temperature strength. For a structural adhesive or FRP matrix, the design temperature should provide an adequate margin below the relevant softened-state transition. Moisture can lower the effective \(T_g\) of some resins by plasticisation.

1.3 Mechanical behaviour

Polymer response depends on strain rate, temperature and loading duration. A rapid test may indicate high strength, whereas a slowly loaded or sustained member may exhibit much lower effective stiffness. Important properties include tensile and compressive strength, elastic or secant modulus, fracture strain, shear strength, fatigue resistance, interlaminar strength and fracture toughness.

Unlike an ideal elastic solid, a polymer may store and dissipate energy simultaneously. This behaviour is called viscoelasticity. A useful conceptual representation is the standard linear solid, consisting of an elastic spring in parallel with a Maxwell element (a spring and dashpot in series). It captures an instantaneous elastic response followed by time-dependent adjustment, although real polymers require more advanced constitutive models for accurate prediction.

For a constant applied stress \(\sigma_0\), the total strain can be represented conceptually as

\[

\varepsilon(t)=\varepsilon_e+\varepsilon_v(t)

\]

where \(\varepsilon_e\) is the recoverable instantaneous strain and \(\varepsilon_v(t)\) is the time-dependent component. The expression is a model description rather than a universal material law; the parameters must be obtained from appropriate tests.

Two practical phenomena are especially important:

• Creep: strain increases with time under sustained stress.
• Stress relaxation: stress decreases with time when strain is held approximately constant.

For an ideal linear viscoelastic material, the strain response to a constant stress is described by a creep compliance \(J(t)=\varepsilon(t)/\sigma_0\), while the stress response to a prescribed strain is described by a relaxation modulus \(E(t)=\sigma(t)/\varepsilon_0\). These functions are time- and temperature-dependent and should not be replaced by a single short-term modulus in long-term design.

The time–temperature superposition principle is often used to estimate long-term behaviour from tests at different temperatures. It assumes that increasing temperature accelerates molecular relaxation without changing the fundamental mechanism. This assumption must be validated; it is unreliable near phase transitions, crystallisation changes, chemical degradation or moisture-induced damage.

2. Fibre-Reinforced Polymer Composites

2.1 Components and anisotropy

An FRP composite contains high-strength fibres embedded in a polymer matrix. Fibres carry most of the tensile load; the matrix transfers stress between fibres, maintains geometry, protects fibres and contributes to transverse and shear properties.

Common reinforcement fibres are:

• Glass fibre: economical and widely used; E-glass is common, while higher-strength glass types provide improved performance.
• Carbon fibre: high modulus, high strength-to-weight ratio and good fatigue resistance; electrically conductive and comparatively expensive.
• Aramid fibre: low density and high tensile strength; sensitive to ultraviolet exposure and some chemical environments.
• Basalt fibre: derived from volcanic rock and increasingly used where moderate performance and thermal resistance are required.

FRP is anisotropic. A unidirectional laminate has very high stiffness and strength along the fibre direction but much lower transverse and through-thickness capacity. Biaxial, triaxial and woven fabrics provide multidirectional reinforcement but normally have lower efficiency in the principal direction than perfectly aligned fibres.

For a simple aligned, continuous-fibre composite loaded longitudinally, the rule of mixtures gives the approximate longitudinal modulus:

\[

E_L \approx V_f E_f+V_m E_m

\]

where \(E_L\) is the longitudinal composite modulus, \(E_f\) and \(E_m\) are fibre and matrix moduli, and \(V_f\) and \(V_m\) are their volume fractions with \(V_f+V_m=1\). This relation assumes uniform strain, continuous aligned fibres, adequate bond and elastic behaviour. It is not appropriate for transverse modulus, woven systems or damaged laminates without modification.

2.2 Manufacture and construction forms

FRP is supplied as pultruded plates, bars, tendons, grids, prefabricated laminates, dry fabrics and pre-cured strips. Wet lay-up systems are impregnated and cured on site; pre-cured systems are manufactured in controlled conditions and bonded to the structure. Pultruded sections provide consistent fibre alignment and are used as bars, plates, channels and profiles.

The matrix may be epoxy, vinyl ester or polyester. Epoxy generally provides strong adhesion and low shrinkage, while polyester is economical and vinyl ester offers a compromise between chemical resistance and cost. Resin selection must match the exposure, cure temperature, working time, substrate and fire requirements.

2.3 Failure mechanisms

Typical FRP and bonded-strengthening failures include:

1. fibre rupture in tension;

2. matrix cracking and transverse splitting;

3. interlaminar delamination;

4. plate or laminate buckling;

5. adhesive cohesive failure;

6. adhesive–substrate interfacial failure;

7. concrete cover separation or substrate fracture;

8. debonding initiated at cracks, ends or defects; and

9. premature failure from impact, moisture, heat or poor impregnation.

A design based only on nominal fibre tensile strength is unsafe. The governing limit state may be bond or substrate capacity rather than fibre capacity.

3. Design Applications of FRP Reinforcement and Strengthening

3.1 Flexural strengthening

Externally bonded FRP plates or fabrics can increase the tensile capacity of reinforced-concrete beams and slabs. The strengthened section must be checked for compatibility of strain, equilibrium, concrete compression, steel yielding, FRP rupture, debonding and serviceability.

For a simplified rectangular reinforced-concrete section, the internal tensile force may be approximated as

\[

T=A_s f_s+A_f f_f

\]

where \(A_s\) and \(A_f\) are the steel and FRP areas, and \(f_s\) and \(f_f\) are their design stresses. The compression force in concrete, often represented using an equivalent stress block, must satisfy equilibrium with \(T\). The nominal flexural resistance is then obtained from the internal force couple, approximately \(M_n=Tz\), where \(z\) is the lever arm. This simplified expression applies only after a compatible strain distribution and appropriate code stress limits have been established.

FRP strain is limited by the lesser of material rupture strain, bond-related strain, environmental reduction and strain compatible with concrete compression. The FRP should not automatically be allowed to reach its coupon tensile strain in a bonded beam.

3.2 Shear strengthening

U-wraps, side-bonded strips and closed FRP jackets can increase shear resistance. Fibre direction is selected to intercept diagonal cracks, commonly at approximately 45° to the member axis or vertically, depending on the strengthening objective. The effective strain is limited by debonding, anchorage, curvature, corners and existing cracks. Rounded corners are essential for wrapping because sharp corners concentrate stress and can cut or kink the fibre.

3.3 Columns and confinement

FRP jackets provide passive confinement to concrete columns. As axial load causes lateral dilation, hoop-oriented fibres develop tensile stress and confine the core. Circular sections are generally more efficiently confined than rectangular sections because the pressure distribution is more uniform. Rectangular columns require corner rounding and may experience reduced effectiveness at large side dimensions. The engineer must also check eccentric loading, lap zones, fire protection, impact and termination details.

3.4 Near-surface-mounted reinforcement

In near-surface-mounted (NSM) strengthening, FRP bars or strips are inserted into grooves and bonded with resin or cementitious grout. NSM systems have greater protection from impact and can develop efficient bond, but groove geometry, cover, anchorage, resin compatibility and moisture condition are critical.

3.5 Durability and environmental reduction

Design strength should account for environmental exposure, sustained stress, fatigue and construction variability. Carbon FRP is generally resistant to corrosion, but the polymer matrix and bond system can degrade. Glass and aramid fibres may be more sensitive to alkaline, moisture or ultraviolet environments depending on sizing and protective treatment. Reduction factors and detailing requirements must be taken from a recognized design standard or qualified manufacturer data; they should not be improvised.

4. Sandwich Panels and Lightweight Composite Enclosures

A sandwich panel consists of two relatively stiff faces separated by a lightweight core. Faces resist membrane tension and compression and contribute to bending stiffness; the core carries transverse shear and maintains separation of the faces.

Typical cores include expanded or extruded polystyrene, polyurethane, polyisocyanurate, phenolic foam, mineral wool, balsa and honeycomb. Face sheets may be steel, aluminium, fibre-cement, glass FRP or carbon FRP.

For an ideal symmetric sandwich beam, the bending stiffness is dominated by the faces. If each face has thickness \(t_f\), separation between face centrelines is \(d\), and face modulus is \(E_f\), the face contribution to bending rigidity per unit width is approximately

\[

D_f \approx 2E_f t_f\left(\frac{d}{2}\right)^2

\]

This neglects the individual face self-bending and assumes effective composite action. Core shear deformation may dominate deflection, especially for short spans or low-shear-modulus cores. A practical design must check face wrinkling, core shear failure, core crushing, local indentation, debonding, fastener pull-out, moisture ingress, thermal movement and fire spread.

Sandwich panels are used in cold stores, industrial buildings, modular construction, cleanrooms, façades and acoustic enclosures. Joints, penetrations and edge details often govern performance more than the panel field. A panel with a non-combustible or fire-resistant core may still have combustible facings, adhesives or seals; the complete assembly must be tested.

5. Sealants, Coatings and Membranes

5.1 Sealants

Sealants close joints against water, air, dust and chemicals while accommodating movement. They are generally classified as elastomeric or plasto-elastic. Silicone sealants offer excellent weathering and ultraviolet resistance; polyurethane sealants provide strong adhesion and abrasion resistance; polysulphide products have useful fuel and chemical resistance; acrylic products are economical but commonly less suitable for large movement or severe exposure.

Joint design must consider expected movement, sealant movement capability, bond width, depth, backing material and three-sided adhesion. A compressible backer rod is used to control depth and prevent adhesion to the back of the joint. The sealant should normally bond to the two joint faces but not the back, allowing strain to develop in a controlled manner. Primer selection and compatibility testing are essential.

5.2 Protective coatings

Polymer coatings protect concrete, steel and masonry against water, carbonation, chloride ingress, abrasion, chemicals and biological attack. Typical systems include epoxy, polyurethane, acrylic, methacrylate, polyurea and cement-polymer coatings.

A coating system includes substrate preparation, primer, body coat and topcoat where required. Performance depends on adhesion, surface cleanliness, moisture content, roughness, film thickness, curing conditions and compatibility between layers. Coatings may conceal cracks rather than repair them; active cracks and moving joints require separate treatment.

5.3 Membranes and geosynthetics

Polymeric membranes are used for roofs, basements, tunnels, reservoirs, landfills and bridge decks. PVC, thermoplastic polyolefin, HDPE, EPDM and bitumen-polymer sheets are common. Membrane design requires assessment of tensile and tear strength, puncture resistance, seam integrity, permeability, chemical compatibility, thermal movement and repairability.

A membrane is only as reliable as its laps, penetrations and terminations. Welded seams require controlled temperature, pressure and speed; adhesive seams require suitable surfaces and curing conditions. Water testing and electronic leak detection can be used where appropriate.

6. Structural Adhesives

Structural adhesives transfer load continuously over an area and can join materials without drilling or welding. Epoxy, acrylic, polyurethane and methacrylate adhesives are used in steel, concrete, timber, FRP and hybrid joints.

Adhesive performance depends on:

• cohesive strength within the adhesive;
• adhesion at each interface;
• shear, tensile and peel stresses;
• bond-line thickness and uniformity;
• surface preparation;
• cure temperature and humidity;
• sustained-load creep;
• differential thermal and moisture movement; and
• fatigue and fracture toughness.

Adhesive joints should be designed primarily for shear and compression. Peel and cleavage stresses are highly damaging because they concentrate at edges. Tapered ends, spew fillets, flexible adherends, scarf joints or mechanical load-sharing details can reduce stress concentrations.

A simplified average shear stress is \(\tau_{avg}=P/(bL)\), where \(P\) is transferred load, \(b\) is effective bond width and \(L\) is bond length. This is only a preliminary estimate. Real stresses are non-uniform, especially near the ends, and final design requires a recognized joint model, test evidence or validated numerical analysis.

Adhesive choice should consider the weakest adherend, not just the advertised adhesive strength. Concrete may fail by cohesive substrate fracture, timber may split, and FRP may delaminate. The surface must be sound, clean and appropriately roughened. Dust, laitance, oil, release agents and uncontrolled moisture are common causes of failure.

7. Ageing, Fire and Sustainability

7.1 Ageing mechanisms

Polymeric materials age through physical and chemical processes. Important mechanisms include:

• ultraviolet degradation and chalking;
• oxidation and chain scission;
• hydrolysis in susceptible polymers;
• plasticiser migration;
• moisture absorption and swelling;
• thermal cycling and differential expansion;
• freeze–thaw effects;
• chemical attack by acids, alkalis, solvents or fuels;
• biological attack in selected materials; and
• fatigue and creep under repeated or sustained loading.

Ageing may cause embrittlement, softening, loss of adhesion, colour change, cracking or reduced fracture toughness. Accelerated laboratory exposure can rank materials, but it does not reproduce all field mechanisms. Long-term predictions should use service records, validated ageing models and conservative environmental factors.

7.2 Fire risks

Many polymers are combustible and can produce heat, smoke and toxic gases. Fire performance is governed by ignition, flame spread, heat release, smoke production, dripping, structural loss of strength and interaction with the supporting assembly. FRP may retain some fibre strength while the matrix softens or burns; adhesive bonds may fail before the fibres rupture.

Fire design may require non-combustible barriers, sacrificial protection, intumescent coatings, mineral boards, fire-resistant cores, increased cover, mechanical anchorage or separation from ignition sources. The complete assembly—not merely a resin or panel coupon—should be assessed under the applicable building and fire regulations. Fire resistance, reaction to fire and smoke toxicity are distinct properties.

7.3 Sustainability and circularity

Polymers can reduce whole-life impacts through lightweighting, longer service life, corrosion avoidance and reduced installation time. Conversely, thermoset composites are difficult to recycle, and some additives or resins create environmental and occupational hazards. Responsible specification includes service-life assessment, repairability, recycled content where technically suitable, take-back routes, low-emission formulations, controlled waste handling and avoidance of unnecessary overdesign.

8. Quality Assurance, Safety and Governance

A specification should identify resin type, fibre type and orientation, nominal fibre content, mechanical properties, environmental class, cure requirements, bond strength, fire classification, allowable installation temperature and inspection procedures.

Quality control commonly includes:

1. substrate inspection and moisture measurement;

2. verification of batch numbers and shelf life;

3. trial application or mock-up;

4. control of mixing ratio, pot life and temperature;

5. measurement of wet and cured thickness;

6. inspection for voids, wrinkles, dry fibre and delamination;

7. pull-off or adhesion testing where appropriate;

8. monitoring of cure and environmental conditions; and

9. as-built records and photographic documentation.

Workers require ventilation, gloves, eye protection and respiratory controls appropriate to the resin and solvent. Isocyanates, amines, styrene and reactive diluents can present significant health hazards. Waste resin and contaminated materials should be managed under environmental regulations.

Digital inspection, infrared thermography, acoustic methods, laser scanning and machine vision can assist detection of voids, delamination and surface defects. Artificial intelligence may help classify images or predict deterioration from monitoring data, but it cannot replace engineering judgement, calibration, traceable data or code compliance. AI-based decisions should be auditable and accompanied by uncertainty assessment.

9. Illustrative Applied Case: FRP Strengthening of a Corroded Beam

Background

An illustrative reinforced-concrete beam in a parking structure has lost flexural capacity because of reinforcement corrosion and increased imposed loading. The existing beam has a factored design moment of 320 kN·m, while investigation indicates that its reliable existing resistance is approximately 250 kN·m. The strengthening objective is therefore an additional design resistance of about 70 kN·m, subject to complete code verification.

Problem and assumptions

Suppose a carbon-FRP plate is bonded to the soffit. For preliminary sizing, use a design FRP stress of 1,200 N/mm², an effective internal lever arm of 0.45 m, and assume that the FRP force is developed without premature debonding. These are illustrative assumptions, not a final design; environmental, bond and fire factors must be verified from the governing standard.

The required additional tensile force is estimated from

\[

F_f=\frac{\Delta M}{z}

\]

where \(\Delta M=70\) kN·m and \(z=0.45\) m. Thus,

\[

F_f=\frac{70}{0.45}=155.6\text{ kN}

\]

The corresponding preliminary FRP area is

\[

A_f=\frac{F_f}{f_{fd}}=\frac{155.6\times10^3}{1200}=129.7\text{ mm}^2

\]

A commercially available plate system with an area equal to or greater than this value could therefore be considered, subject to width, thickness, anchorage and detailing constraints.

Analysis and solution

Before installation, unsound concrete is removed, corrosion is treated, and the substrate is repaired with a compatible repair mortar. The beam soffit is abrasive-cleaned and checked for tensile surface strength. The plate is cut, the adhesive is mixed under controlled conditions, and a uniform bond line is formed. Plate ends are tapered or otherwise detailed to reduce stress concentration. If required by the design, U-wraps or mechanical anchorage are added to control end debonding.

The final design must check: existing and strengthened section equilibrium; concrete compression strain; FRP strain limit; steel stress; shear capacity; plate debonding; concrete cover separation; serviceability crack width and deflection; fatigue; fire protection; and durability. Installation is accepted only after inspection of bond-line continuity and appropriate adhesion tests.

Outcome and learning points

The preliminary calculation illustrates the relationship between required moment increase, lever arm and FRP area. It does not establish safety by itself. The governing failure mode may be debonding or concrete cover separation, and the strengthened beam may require simultaneous shear strengthening. The case demonstrates that FRP design is a system problem involving material properties, structural mechanics, substrate condition, detailing, construction control and fire protection.

10. Best-Practice Design Workflow

A robust project workflow is:

1. Define structural demand, exposure, temperature, fire and service-life requirements.

2. Investigate the existing substrate, defects, moisture and load path.

3. Select a complete compatible system rather than isolated products.

4. Establish material properties using relevant standards and realistic environmental conditions.

5. Analyse both strength and serviceability, including time-dependent effects.

6. Design bond, anchorage, joints, terminations and transitions.

7. Conduct a mock-up and approve installation procedures.

8. Inspect, test and document the work.

9. Provide fire, impact and maintenance provisions.

10. Monitor performance where consequences of failure are high.

Chapter Summary

Polymers are time-, temperature- and environment-dependent materials. Their viscoelasticity produces creep and stress relaxation, while glass transition governs the loss of stiffness and strength at elevated temperature. Thermoplastics, thermosets and elastomers have different processing and service characteristics.

FRP composites combine fibres that carry load with polymer matrices that transfer stress and protect the fibres. Their anisotropy makes fibre orientation, bond and detailing central to design. FRP can strengthen beams, slabs and columns through externally bonded, near-surface-mounted and confinement systems, but debonding, substrate failure, fire and durability must be checked.

Sandwich panels achieve high stiffness at low mass by separating stiff faces with a lightweight core. Sealants accommodate joint movement, coatings protect surfaces, and membranes provide barriers against water and chemicals. Structural adhesives distribute load over bonded areas but are vulnerable to peel, poor surface preparation, creep and environmental degradation.

Ageing, fire, occupational safety, recyclability and whole-life performance are essential governance issues. Quality assurance must cover substrate preparation, mixing, curing, bond integrity, inspection and documentation. Successful polymer-based construction is therefore achieved not by material selection alone, but by integrated design, controlled installation and informed maintenance.

Key Terms

Polymer: A material composed of long chains of repeating molecular units.

Thermoplastic: A polymer that can soften on heating and harden on cooling.

Thermoset: A permanently cross-linked polymer formed by curing.

Elastomer: A flexible polymer capable of large reversible deformation.

Glass-transition temperature: Temperature range associated with transition from glassy to rubbery polymer behaviour.

Viscoelasticity: Combined elastic and time-dependent viscous response.

Creep: Increase in strain under sustained stress.

Stress relaxation: Decrease in stress under sustained strain.

FRP: Fibre-reinforced polymer composite.

Matrix: Polymer phase that surrounds and transfers load to fibres.

Anisotropy: Dependence of properties on direction.

Delamination: Separation between layers of a composite.

Debonding: Loss of adhesion between strengthening material and substrate.

Sandwich panel: Composite panel with stiff faces separated by a lightweight core.

Sealant: Flexible material used to close and weatherproof joints.

Membrane: Thin polymeric barrier used to resist water, vapour or chemicals.

Structural adhesive: Adhesive intended to transfer sustained or significant structural load.

Pot life: Usable working time after adhesive or resin components are mixed.

Cohesive failure: Failure within the adhesive or substrate rather than at the interface.

Pull-off test: Test used to measure surface or bond tensile adhesion.

Review Questions

Short Answer Questions

1. Distinguish thermoplastics, thermosets and elastomers.

2. Define creep and stress relaxation.

3. Why is FRP considered anisotropic?

4. State two common causes of adhesive joint failure.

5. What functions do the faces and core perform in a sandwich panel?

Long Answer Questions

1. Explain polymer viscoelasticity and discuss its significance in long-term construction design.

2. Describe the materials, failure modes and design checks for externally bonded FRP flexural strengthening.

3. Discuss ageing mechanisms and fire risks in polymer-based construction systems.

4. Explain the design and construction requirements for sealants, coatings and membranes.

5. Compare structural adhesives and mechanical fasteners for joining construction materials.

Analytical / Application Questions

1. A beam requires an additional moment resistance of 90 kN·m. If the effective lever arm is 0.50 m and the design FRP stress is 1,000 N/mm², calculate the preliminary FRP area, then identify the additional checks required before selection.

2. Recommend a material strategy for a sandwich façade panel exposed to high humidity, ultraviolet radiation and stringent fire requirements. Justify the face, core, joint and protection choices.

3. An FRP strengthening installation shows blistering and low pull-off strength after curing. Analyse possible causes and propose a diagnostic and remedial plan.

Multiple Choice Questions with Answers

Question 1

Which property most directly describes deformation increasing with time under sustained stress?

A. Creep

B. Elastic recovery

C. Thermal conductivity

D. Hardness

Correct Answer: A. Creep

Question 2

In a typical FRP composite, the primary function of the fibres is to

A. Provide the main tensile load-carrying capacity

B. Prevent all moisture absorption

C. Reduce the glass-transition temperature

D. Act as a flexible sealant

Correct Answer: A. Provide the main tensile load-carrying capacity

Question 3

Why must FRP strengthening design consider debonding?

A. Bond or substrate failure may occur before fibre rupture

B. FRP has no tensile strength

C. Debonding increases concrete density

D. Debonding is always beneficial for ductility

Correct Answer: A. Bond or substrate failure may occur before fibre rupture

Question 4

For a sandwich panel, the core primarily

A. Maintains face separation and carries transverse shear

B. Provides all longitudinal tensile resistance

C. Acts only as a decorative finish

D. Eliminates the need for joint design

Correct Answer: A. Maintains face separation and carries transverse shear

Question 5

Which failure mode is especially associated with concentrated tensile stress at an adhesive-joint edge?

A. Peel or cleavage failure

B. Uniform compression only

C. Fibre crystallisation

D. Concrete carbonation only

Correct Answer: A. Peel or cleavage failure

Question 6

For a simple aligned composite, the rule-of-mixtures expression for longitudinal modulus is

A. \(E_L=V_fE_f+V_mE_m\)

B. \(E_L=E_f/E_m\)

C. \(E_L=V_f+V_m\)

D. \(E_L=E_f-E_m\)

Correct Answer: A. \(E_L=V_fE_f+V_mE_m\)

Question 7

A strengthening system requires 60 kN·m additional moment resistance and has an effective lever arm of 0.40 m. The approximate required tensile force is

A. 24 kN

B. 150 kN

C. 240 kN

D. 1.5 kN

Correct Answer: B. 150 kN

Question 8

Which condition generally improves the reliability of a bonded strengthening system?

A. Clean, sound and suitably roughened substrate

B. Uncontrolled moisture and surface dust

C. Three-sided sealant adhesion in a movement joint

D. Unverified resin mixing ratios

Correct Answer: A. Clean, sound and suitably roughened substrate

Question 9

Why is glass-transition temperature important in polymer construction materials?

A. Properties may reduce substantially as the polymer approaches or exceeds it

B. It is the temperature at which concrete hydrates

C. It defines the fibre volume fraction

D. It measures adhesive bond width

Correct Answer: A. Properties may reduce substantially as the polymer approaches or exceeds it

Question 10

Which statement about fire performance is most accurate?

A. The complete assembly should be assessed because matrix, adhesive, core and protection interact

B. Fibre tensile strength alone proves fire resistance

C. A non-combustible facing makes every core safe

D. Fire performance is unrelated to smoke production

Correct Answer: A. The complete assembly should be assessed because matrix, adhesive, core and protection interact