Compliance (3.1.1)

Compliance in concrete and reinforcement work is fundamental to ensuring the structural integrity and durability of new build homes. The NHBC Standards require that all concrete and its reinforcement must comply with specific Technical Requirements, covering both plain and reinforced concrete, whether it’s precast or constructed in-situ (on site).

The key principle underlying these requirements is that mix design must carefully consider both strength and durability, adhering to the relevant British Standards. While builders can follow recognised standards and practices for concrete design and specification, they also have the option to use mixes that conform to the specific guidance provided in the NHBC Standards chapter. This flexibility ensures that builders can meet the necessary quality requirements while allowing for practical construction methods suited to different project needs.

Information Requirements (3.1.2)

When it comes to concrete work in new builds, proper documentation and communication are essential. The NHBC Standards require that designs and specifications must be produced in a clear, understandable format and distributed to all relevant personnel. This documentation needs to cover crucial details such as ground aggressivity (including design sulfate class and aggressive chemical environment classifications), strength and durability specifications (including strength requirements, water-cement ratios, consistency class, and aggregate specifications), and detailed reinforcement plans. The information must also include specific requirements for formwork, finishing treatments, testing protocols, and curing procedures. This comprehensive approach ensures that everyone involved in the construction process has access to the necessary technical information to complete the work correctly.

Storage of Materials (3.1.3)

The proper storage of concrete materials is crucial for maintaining the quality of the finished product. The NHBC Standards emphasise that all materials must be stored appropriately to prevent any impairment of the concrete’s final performance. Materials should be stored according to manufacturer’s recommendations regarding maximum storage time, with different types of cement kept separate in dry conditions. Aggregates of different sizes must be stored in separate bays to prevent mixing, and both sand and aggregate should be kept clean and dry. The standards specifically note that moisture content in sand and aggregate should be accounted for during concrete batching, highlighting the importance of proper material conditioning before use.

Site-Mixed Concrete (3.1.4)

Site-mixed concrete plays a crucial role in construction projects, and the NHBC Standards outline specific requirements to ensure it meets necessary strength and durability specifications. Understanding these requirements is essential for achieving high-quality construction outcomes.

Materials and Standards All materials used in site-mixed concrete must meet strict quality standards. Cement or cementitious materials must comply with BS 8500-2 2015 including Annex A. Water used should be from mains supply or meet BS EN 1008 requirements. Aggregates must comply with BS EN 12620 and be properly proportioned to ensure reasonable consistency. Special attention must be paid when using shrinkable aggregates or those susceptible to alkali attack or excessive moisture movement.

Mixing Methods and Requirements Except for very small quantities, mechanical mixers should always be used for mixing concrete on site. When hand mixing is unavoidable, the Standards require adding an extra 10% of cement to the quantities shown in the prescribed mix tables. This additional cement helps compensate for the potentially reduced mixing efficiency of manual methods.

Mix Proportions and Specifications The Standards provide detailed tables for mix proportions, considering both weight and volume measurements. These specifications vary based on:

• Cement strength class (32.5 or 42.5 and higher)
• Maximum aggregate size (typically 20mm)
• Required consistency class (measured by slump)
• Intended use of the concrete

Strength Classifications Different applications require different concrete strengths, designated as standardised prescribed mixes (ST1 through ST5). Each mix is designed for specific uses:

• ST1: Suitable for basic applications with minimal structural requirements
• ST2: Common for general construction use
• ST3: Used where higher strength is needed
• ST4: For applications requiring enhanced strength
• ST5: Highest strength classification for demanding structural applications

Quality Control Considerations Several key factors must be monitored during the mixing process:

• Accurate measurement of all materials
• Proper moisture content assessment in aggregates
• Consistent mixing time to ensure uniform distribution of materials
• Regular checking of consistency through slump testing
• Temperature monitoring during mixing and placement

Storage and Handling Proper storage of materials is crucial for maintaining mix quality:

• Cement must be stored in dry conditions
• Different types of cement should be kept separate
• Aggregates must be stored in separate bays by size
• Materials should be protected from contamination
• Storage areas should allow for proper drainage

Consistency and Workability The Standards specify different consistency classes (S1 through S4) measured by slump:

• S1 (10-40mm): Stiff consistency
• S2 (50-90mm): Standard plasticity
• S3 (100-150mm): Increased workability
• S4 (160-210mm): High workability

Time Constraints Site-mixed concrete must be placed within 30 minutes of water being added to the cement. This time limit helps ensure the concrete’s workability and final strength are not compromised by premature setting.

These requirements for site-mixed concrete ensure that the final product meets structural requirements while maintaining durability and longevity. Proper adherence to these standards is essential for achieving high-quality construction outcomes that meet NHBC requirements.

Ready-Mixed Concrete (3.1.5)

Ready-mixed concrete must come from suppliers operating under recognised quality control systems that meet NHBC approval. The Standards specifically recognise suppliers operating under the Quality Scheme for Ready-Mixed Concrete (QSRMC) or the BSI Kitemark scheme, though other suppliers may be acceptable if they maintain equivalent quality standards. When ordering ready-mixed concrete, it must conform to BS 8500 and BS EN 206 specifications. For designated mixes, suppliers only need the mix designation and consistency class to fulfil the order correctly. It’s crucial that delivery information is checked upon arrival to verify that the concrete meets all design requirements. The inclusion of quality control requirements ensures that ready-mixed concrete maintains consistent standards across all deliveries.

Concrete Specification (3.1.6)

Concrete specification must ensure adequate strength and durability for its intended use. The Standards outline several critical factors that must be considered, including exposure to climatic and atmospheric conditions, aggressive ground conditions, sulfates and acids in groundwater, effects of chlorides, potential alkali-silica reactions, and aggregate characteristics. Concrete mixes should be specified according to BS 8500-1 as either designated mixes for ready-mixed concrete or standardised prescribed mixes for site mixing. The specification must also account for the geographical location of the site and the specific location of the concrete element within the structure. Higher grade concrete, which offers greater resistance to chemical and mechanical damage, should be specified where appropriate.

Fundamental Specification Requirements Concrete mixes must be suitable for their intended use and specified according to BS 8500-1 as either:

• Designated mixes for ready-mixed concrete
• Standardised prescribed mixes for site mixing Specifications must consider geographical location, element position within the structure, and potential exposure to damaging elements. Higher grade concrete, offering enhanced resistance to chemical and mechanical damage, should be specified where appropriate.

Environmental and Exposure Considerations

Climatic and Atmospheric Conditions The Standards identify several exposure classes that affect concrete specification:

• XC1: Dry or permanently wet environments (e.g., concrete inside buildings with low humidity)
• XC2: Wet, rarely dry conditions (e.g., foundations with long-term water contact)
• XC3: Moderate humidity environments (e.g., external concrete sheltered from rain)
• XC4: Cyclic wet and dry conditions (e.g., exposed concrete surfaces)
• XF1: Moderate water saturation without de-icing agents

Aggressive Ground Conditions Where concrete will be exposed to aggressive ground conditions, specifications must account for:

• Design sulfate class (DS class)
• Aggressive Chemical Environment for Concrete class (ACEC class)
• pH levels of groundwater
• Presence of sulfates and other harmful chemicals For lower ACEC classes (AC-1 to AC-2z), standard guidance tables can be used; higher classes require specialist advice.

Protection Against Chemical Attack

Sulfates and Acids The Standards require careful consideration of:

• Ground water mobility
• Chemical attack risk
• ACEC classification
• Additional protective measures where required For higher ACEC classes, specialist advice should determine:
• Design chemical class (DC class)
• Appropriate additional protective measures (APM)

Chloride Considerations Chloride content must be limited according to BS EN 206 Table 15 because:

• Chlorides increase metal corrosion risk
• They can reduce concrete’s chemical resistance
• Exposure can come from ground conditions, sea spray, or de-icing products

Alkali-Silica Reaction Prevention To prevent damaging alkali-silica reactions, specifications must consider:

• Alkali sources
• Moisture content
• Aggregate reactivity Alkali content should not exceed 3kg/m³ when calculated according to BRE Digest 330 or Concrete Society Technical Report 30.

Aggregate Specifications

Quality Requirements Aggregates must:

• Be of appropriate grade for durability
• Meet BS EN 12620 standards
• Be properly assessed if using proprietary or recovered materials Special considerations apply for:
• Shrinkable aggregates
• Materials susceptible to alkali attack
• Aggregates prone to excessive moisture movement

Mix Design Considerations The specification must also address:

• Strength requirements
• Maximum free water/cement ratio
• Minimum cement content
• Consistency class
• Air content (where required)
• Aggregate size
• Colour requirements

Quality Control and Documentation All specifications must include:

• Testing requirements
• Sampling frequency
• Documentation requirements
• Quality control measures

These specification requirements ensure that concrete used in construction meets both structural and durability requirements while considering site-specific conditions and potential exposure risks. Proper adherence to these specifications is crucial for achieving long-lasting, high-quality construction outcomes.

Admixtures (3.1.7)

The use of admixtures in concrete must be carefully controlled and should only be employed to enhance concrete performance and durability. The NHBC Standards specify that admixtures must comply with BS EN 934-2 and should only be used with full knowledge of their limitations and effects. Key considerations include improved workability, waterproofing capabilities, and the effects of accelerators or retarders. Importantly, air-entraining agents should not be used as an anti-freeze for fresh concrete, though they can improve frost resistance in cured concrete for elements like paths and drives. The Standards specifically warn against using admixtures containing chlorides in reinforced concrete due to the risk of metal corrosion, and all admixtures must be added to the mix water to ensure complete dispersal.

Special Types of Concrete (3.1.8)

The NHBC Standards recognise that certain construction situations may require special types of concrete, such as proprietary concrete, no-fines concrete, or lightweight concrete. These special mixes must be appropriate for their intended use, with quality and density suitable for the specific conditions and application. When no-fines concrete is used, the Standards require that a render, cover coat, or cladding must be applied to the finished structure for protection. Any proprietary reinforcement methods, such as glass fibre, must undergo proper assessment according to Technical Requirement R3. The structural design must also comply with Technical Requirement R5, ensuring that all special concrete types are properly detailed and specified.

Design of Reinforced Concrete (3.1.9)

Reinforced concrete design must carefully consider several crucial factors to ensure structural integrity and longevity. The Standards require compliance with appropriate British Standards, proper consideration of end restraint in slabs, adequate concrete cover for reinforcement, appropriate fire resistance, and protection against carbonation. For concrete not designed by an engineer, specific minimum cover requirements apply: 75mm for concrete in contact with the ground, 50mm for external conditions, 40mm when cast against a DPM on sand blinding, and 25mm for protected or internal conditions. The Standards emphasise that carbonation can reduce corrosion protection of reinforcement by increasing porosity and decreasing alkalinity, making proper concrete cover and compaction essential.

Compliance with Standards

British Standards Requirements All reinforced concrete design must comply with key standards:

• BS EN 1992-1: Design of concrete structures
• BS 4449: Steel for reinforcement specification
• BS 4482: Steel wire reinforcement specification
• BS 4483: Steel fabric reinforcement specification
• BS 6744: Stainless steel reinforcement bars
• BS 8103-1: Structural design for low-rise buildings

Documentation Requirements Design documentation must include:

• Steel type, grade, and size specifications
• Detailed drawings and bending schedules to BS 8666
• Complete dimensions for site work
• Reinforcement details at supporting edges
• Requirements for openings and movement joints

Structural Considerations

End Restraint Special attention must be paid to:

• Surface cracking risk over supports where slabs are cast monolithically
• Appropriate reinforcement provision according to BS EN 1992-1-1
• Prevention of structural issues at slab ends

Concrete Cover Requirements

Minimum Cover Specifications The Standards specify minimum cover requirements for different situations:

• 75mm for concrete in contact with ground
• 50mm for external conditions
• 40mm when cast against DPM on sand blinding
• 40mm against adequate blinding concrete
• 25mm for protected or internal conditions

Cover Implementation Cover requirements apply to:

• Main bars and stirrups
• All reinforcement types
• No protrusion of ties or clips into cover zone

Fire Resistance

Fire Protection Requirements

• Cover specifications must meet fire resistance requirements
• BS EN 1992-1-2 provides fire resistance guidance
• Standard cover from BS EN 1992-1-1 typically provides one hour fire resistance for:
  • Columns
  • Simply supported beams
  • Floors

Carbonation Protection

Prevention Measures To protect against carbonation-induced corrosion:

• Maximise concrete cover where possible
• Ensure good quality wet concrete
• Achieve proper compaction to reduce carbonation rate
• Consider environmental exposure conditions

Effects of Carbonation The Standards recognise that carbonation:

• Reduces reinforcement corrosion protection
• Increases concrete porosity
• Decreases alkalinity
• Can be mitigated through proper design and execution

Design Considerations for Durability

Environmental Factors Designs must account for:

• Local climate conditions
• Exposure to aggressive substances
• Ground conditions
• Expected service life

Quality Control Measures The design should specify:

• Required testing procedures
• Inspection points during construction
• Documentation requirements
• Quality assurance measures

These design requirements ensure that reinforced concrete structures meet both structural and durability requirements while providing adequate protection against various degradation mechanisms. Proper adherence to these design specifications is crucial for achieving long-lasting, high-quality construction outcomes.

Installation of Reinforcement (3.1.10)

The proper installation of reinforcement is critical for structural integrity. The Standards specify that main reinforcing bars must be parallel to the span, and in slabs, the main reinforcement should be located near the bottom with secondary bars placed on top. For beams, main reinforcing bars must be placed inside the links. All reinforcement must be clean and free from loose rust and contaminants, particularly shutter-releasing agents and oil. When it comes to support, spacers should be either concrete blocks (no larger than 50x50mm) or ready-made steel or plastic components. These supports must be placed no more than one metre apart, or closer if necessary. For parallel bars, spacers should be staggered to avoid creating planes of weakness in the concrete. The Standards also emphasise the importance of proper lapping of bars and mesh according to design specifications, with any additional laps requiring designer approval.

Placement and Positioning Requirements

Main Reinforcing Bars

• Must be installed parallel to the span unless otherwise detailed
• Position must precisely match design specifications
• Installation must ensure stability during concrete placement
• Proper alignment is crucial for structural performance

Slab Reinforcement

• Main reinforcing bars must be located near the bottom of the slab
• Installation sequence: main reinforcing bars placed first
• Secondary bars positioned on top of main bars
• Positioning must maintain specified concrete cover
• Proper support required to prevent displacement during concreting

Beam Reinforcement

• Main reinforcing bars must be placed inside the links
• Links must be properly spaced according to design
• Adequate support required to maintain position
• Cover requirements must be maintained on all sides

Material Condition and Preparation

Reinforcement Quality

• Must be clean and free from loose rust
• No contamination from shutter-releasing agents
• Oil and other contaminants must be removed
• Surface condition must not compromise bond strength

Bending Requirements

• All bending must use appropriate equipment
• Bending radii must comply with design specifications
• Cold bending of bars must follow proper procedures
• No re-bending of bars unless specifically approved

Support Systems

Spacer Requirements

• Concrete blocks limited to 50x50mm maximum size
• Ready-made steel or plastic spacers acceptable
• Placement at maximum one-metre intervals
• Closer spacing where required by design
• Material must be appropriate for exposure conditions

Spacer Layout

• Must be staggered for parallel bars
• Arranged to avoid creating planes of weakness
• Positioned to maintain specified cover
• Additional support for top steel using chairs

Chair Support

• Must be properly sized for required height
• Stable installation essential
• Appropriate material for exposure conditions
• Sufficient number to prevent sagging

Lapping and Continuity

Bar Lapping

• Must follow designer’s specifications for type and size
• Loads must transfer fully across lap joints
• Additional laps require designer approval
• Proper alignment essential for load transfer

Mesh Lapping

• Specific overlap requirements based on mesh type
• Proper securing of overlaps
• Maintenance of specified cover at laps
• Additional support may be required at laps

Quality Control Requirements

Installation Verification

• Regular checks of bar positions
• Verification of cover dimensions
• Inspection of support systems
• Documentation of completed installation

Pre-Concrete Checks

• Final position verification
• Support system stability
• Cleanliness of reinforcement
• Cover measurements
• Documentation compliance

Blinding Concrete (3.1.11)

Blinding concrete serves specific purposes in construction and should only be used in particular circumstances according to the NHBC Standards. The use of blinding concrete is appropriate in several key situations: to protect the bottom of trenches or excavations when there will be a delay before pouring structural concrete, where foundations have been slightly overdug, to provide adequate support for maintaining proper cover to reinforcement, or where localised soft spots have been removed. This targeted approach to using blinding concrete ensures it serves its intended purpose of aiding construction while avoiding unnecessary application.

Formwork (3.1.12)

The NHBC Standards emphasise that formwork must be both structurally adequate and constructed with proper workmanship. Accurate setting out is crucial, with formwork positioned precisely in relation to reference lines and benchmarks. This accuracy is particularly important for maintaining correct cover to reinforcement. The formwork and its supports must be rigid enough to maintain correct positioning and withstand additional loads and accidental impacts during concrete placement and compacting. For concrete that will be left untreated or with minimal finishing, the Standards require tight formwork joints to prevent grout loss and avoid ragged edges. Additionally, formwork must be designed to allow removal without damaging the concrete, with support for load-bearing elements maintained until the concrete reaches sufficient strength.

Before Concreting (3.1.13)

The Standards outline critical preparations that must be completed before any concreting work begins. All services, ducts, and inserts that will be embedded in the concrete must be securely installed in their correct positions and tested where appropriate. Completed reinforcement must undergo inspection and, when required, receive approval from the designer or their representative. The formwork must be thoroughly cleaned and checked for any fallen debris, with particular attention paid to removing nails and wire clippings. These preparatory steps are essential for ensuring the quality and integrity of the finished concrete structure.

Casting (3.1.14)

The NHBC Standards provide specific requirements for concrete casting to achieve proper design strength and durability. The concrete temperature at the point of use must not fall below 5°C, as fresh concrete is vulnerable to frost damage that might not be immediately apparent. The Standards specify strict timing requirements: site-mixed concrete should be placed within 30 minutes of adding water to cement, while ready-mixed concrete has a two-hour window. The concrete should be deposited as close as possible to its final location and transported efficiently on site to prevent segregation. For unreinforced concrete, areas cast in one operation should not exceed 16m², while reinforced concrete can cover up to 60m². Construction joints should only be formed when unavoidable and in consultation with the engineer.

Curing (3.1.15)

Proper curing is essential for concrete to achieve its full design strength, and the NHBC Standards outline specific requirements for this crucial process. Freshly poured concrete must be kept moist by covering it as soon as the surface is hard enough to resist damage, which is particularly important in hot, windy, or cold weather to prevent rapid surface drying or freezing. Acceptable covering materials include damp hessian, damp sharp sand, or impervious sheets such as polyethylene. The Standards specify minimum curing periods: plain unreinforced concrete made with ordinary Portland cement requires at least four days, while reinforced concrete or concrete containing cement replacements needs seven days. During this period, the concrete structure should not be loaded. Any curing agents used must comply with Technical Requirement R3 and should never be applied to floors that will receive topping or screed, as this could compromise future bonding.

Testing (3.1.16)

The NHBC Standards require that concrete testing must be carried out to their full satisfaction. All testing must be conducted according to BS EN 12390 by UKAS approved laboratories. Test cubes should be prepared as requested by the engineer, with proper marking, curing, and safe storage until testing occurs. The Standards emphasise the importance of maintaining comprehensive documentation, including test reports, certificates, and allied documentation, which must be kept for reference and made available to NHBC upon request. For ready-mixed concrete, suppliers should prepare test cubes in accordance with their quality assurance procedures. This rigorous testing regime ensures that concrete used in construction meets the required specifications and maintains consistent quality throughout the project.