Non-Structural Crack

Structural cracks and non-structural cracks differ primarily in their impact on the integrity and safety of a building or structure.

1. Structural Cracks:
   • Definition: These are cracks that affect the load-bearing capacity or stability of a structure. They can occur in various elements like shear walls, beams, columns, slabs, or foundations.
   • Causes: Structural cracks can result from factors such as foundation settlement, soil movements, overloading, differential settlements, seismic activity, design flaws, improper construction practices, inadequate maintenance etc.
   • Impact: They are a significant concern as they compromise the structural integrity of the building, potentially leading to structural failure or collapse if not addressed promptly and appropriately.
   • Repair: Repairing structural cracks often involves comprehensive assessments by structural engineers, followed by specialized repair techniques such as underpinning, strengthening, or reinforcement of affected elements.
2. Non-Structural Cracks:
   • Definition: These are cracks that do not compromise the load-bearing capacity or stability of a structure. They may appear in both load-bearing and non-load-bearing elements.
   • Causes: Non-structural cracks can result from factors like temperature and moisture variations, settling of materials, corrosion, minor movements in the building due to environmental changes, or wear and tear of the material/element/component/system.
   • Impact: While non-structural cracks are generally less critical than structural ones, if not managed timely, deterioration may escalate and even turn into a structural one. An example is reinforcement corrosion due to chloride ion attack, concrete suffer from alkali-silica reaction etc.
   • Repair: Repairing non-structural cracks typically involves cosmetic fixes such as filling with sealants, chemical injection, impregnation, patching materials, or repainting. However, it’s essential to investigate and address any underlying causes to prevent recurring cracks.

In summary, structural cracks are a serious concern requiring thorough assessment and specialized repairs to maintain the safety and stability of a building, while non-structural cracks are typically less critical and can often be addressed through cosmetic repairs.

Case Study

Type of Building: Commercial

In this case, cracks has been developed randomly on the basement floor. The crack ,in this case, is known as shrinkage crack, with drying induced shrinkage of concrete yield the formation of random crack lines . Drying shrinkage is defined as the contraction of a hardened concrete mixture due to the loss of capillary water. As a result, unevenness of shrinkage, which in turn produces stress, may occur. They could display patterns as shown in I, J or K in the figure below.

Cause of Defects

Shrinkage cracks typically result from the failure to cure the concrete properly. However, it may also be the result of poor site preparation, improper mixing, placing and finishing of concrete.

Inappropriate spacing of control joints and the prevailing weather conditions may also contribute to cracking due to thermal movements.

Good Practices

Concrete

Material

Ensure that the concrete mix is appropriate for construction. If the concrete is ready mix, it shall comply with SS 289.

Potential excessive shrinkage may be reduced by using cement with the following properties such as:

1. as low a water content as possible. Add only enough water to obtain an even and workable mix.
2. maximising coarse aggregates content
3. use of aggregates and cement with low shrinkage characteristics
4. use of a suitable admixture, so that shrinkage can be controlled

Construction

To control the occurrence of concrete cracks caused by drying shrinkage, the following steps are recommended:

Concreting should be started early in the day or night to minimize excessive drying due to hot weather. In addition, ensure that there are sufficient able-bodied helpers to assist in placing, compacting, and finishing the concrete until entire job is complete.

Ensure the provision of correct placement of steel reinforcement to control cracks resulting from shrinkage and thermal movement. Spacers can be used to place the re-bars in correct positions.

Place the concrete carefully. If concrete is placed directly from a truck or concrete pump, place concrete vertically into position. Do not allow the concrete to fall from a height of more than 1 to 1.5 metres.
Ensure thorough compaction of the concrete during placement. The concrete should be properly cured.

Quality Control

(a) Check for quality of concrete before placing e.g. water cement ratio, slump test, etc.

(b) Avoid the followings during concreting in order to minimise cracks:

• Avoid excessive manipulation of the surface, which can depress the coarse aggregate, increase the cement paste at the surface, or increase the water-cement ratio at the surface.
• Do not finish the concrete before it has completed bleeding.
• Do not dust any cement onto the surface to absorb bleed water.
• Do not sprinkle water on the surface while finishing.

Construction Joint

Design

Sufficient movement joints should be provided at a suitable distance to accommodate movements.

Standards

Standards / Codes of Practice

• ASTM C33/C33M-16e1 Standard specification for concrete aggregates
• ASTM C1193-16 Standard guide for use of joint sealants
• BS 1881-210 Testing hardened concrete. Determination of the potential carbonation resistance of concrete. Accelerated carbonation method.
• BS 6093 Guide. Design of joints and jointing in building construction
• BS 6150 Code of practice for painting of buildings
• BS 6213 Guide for selection of construction sealants
• BS 6398 Specification for bitumen damp-proof courses for masonry
• BS 8000-0 Workmanship on construction sites — Introduction and general principles
• BS 8004 Code of practice for foundations
• BS 8102 Code of practice for protection of below ground structures against water ingress
• BS 8204-1 Code of practice for screeds, bases and in situ floorings — Concrete bases and cementitious levelling screeds to receive floorings
• BS 8210 Code of practice for facilities maintenance management
• BS 8215 Code of practice for design and installation of dampproof courses in masonry construction
• BS 8221-1 Code of practice for cleaning and surface repair of buildings — Cleaning of natural stone, brick, terracotta and concrete
• BS EN 1504-2 Products and systems for the protection and repair of concrete structures. Definitions, requirements, quality control and evaluation of conformity. Surface protection systems for concrete.
• BS EN 1504-3 Products and systems for the protection and repair of concrete structures. Definitions, requirements, quality control and evaluation of conformity. Structural and non-structural repair.
• BS EN 1504-9 Products and systems for the protection and repair of concrete structures. Definitions, requirements, quality control and evaluation of conformity. General principles for use of products and systems.
• BS EN 1992 Design of concrete structures
• BS EN 1992-3 Design of concrete structures — Liquid retaining and containment structures
• BS EN 8204-2 Code of practice for screeds, bases and in situ floorings: Concrete wearing surfaces
• BS EN 14630 Products and systems for the protection and repair of concrete structures. Test methods. Determination of carbonation depth in hardened concrete by the phenolphthalein method.
• BS EN 15651-3 Sealants for non-structural use in joints in buildings and pedestrian walkways. Sealants for sanitary joints.
• BS EN ISO 11600 Building construction. Jointing products. Classification and requirements for sealants
• CP 52 Code of practice for automatic fire sprinkler system
• CP 65-1 Code of practice for structural use of concrete — Design and construction
• SS 150 Specification for emulsion paint for decorative purposes
• SS 509-1 Code of practice for cleaning and surface repair of buildings — Part 1: Cleaning of natural stone, brick, terracotta, concrete and rendered finishes
• SS 542 Code of practice for the painting of buildings
• SS 554 Code of practice for indoor air quality for airconditioned buildings
• SS 637 (formerly CP 82) Code of practice for waterproofing of reinforced concrete buildings
• SS EN 1992-1-1 Eurocode 2: Design of concrete structures, Part 1-1: General rules and rules for buildings
• SS EN 1992-1-2 Eurocode 2: Design of concrete structures, Part 1-2: General rules — Structural fire design

Maintenance and Diagnostics

• Regular inspections should be carried out methodically at regular intervals to detect the defects.
• Checklists could be used so that a thorough checking could be made. Defects could be recorded, monitored and not be left unattended.
• Cleaning of car parks should be carried out at appropriate intervals of time to ensure the removal of dirt before they stain the
  1. Daily spot cleaning to clear up oil spots on the drive way to prevent oil stains from becoming stubborn and difficult to clean off.
  2. The floor of the car park is scrubbed clean once a week
  3. The surface drains have to be flushed to clear silt and dirt twice a year to prevent blockages.

Diagnostics of Defect (see also NDT)

Cracks can be visually detected. However, following equipment may be used to analyze a crack.

Ultrasonic Pulse Velocity (UPV)

UPV can identify non-homogeneous conditions such as voids, cracks and honeycombs using the optional hand-held terminal. This method can also be used to estimate the depth of cracks.

Remedial (see also Remedial)

The repair method for a shrinkage crack shall be cosmetic and is different from that of a structural repair. The main purpose of repairing a shrinkage crack is to seal the crack, not to re-introduce stresses into an already “relieved” previously stress concentrated area. Hence, materials used for the sealing are usually elastomeric types which allow movements but provide adequate sealing functions.

The following methods can be used to repair the crack.

1. Repair of crack lines (Procedure of Surface Repair)

Surface repairs can be done on the floor slab if there are no signs of water seepage from underneath or widening of gap with time.

2. Polymer Modified Cementitious Grouting

This can be applied on the crack so that the final appearance of the concrete structure closely resembles the concrete that is being repaired, in respect of strength and long term movement. Its characteristics are also improved, with regards to bonding, flexibility and time within which strength is gained.