NZS 3109:1987 Specification for Concrete Construction

The NZS 3109:1987 is a vital standard within the New Zealand construction industry, serving as the definitive specification for concrete construction. While superseded in part by newer standards for specific design aspects, it remains a referenced document for general concrete construction practice not specifically covered by the design code, NZS 3101. Its primary purpose is to provide the minimum requirements for the materials, methods, and quality of workmanship necessary to produce durable and structurally sound concrete.

Scope and General Requirements

This standard sets out the specifications for the production, transportation, placing, compaction, finishing, and curing of concrete. It applies to structural and architectural concrete elements. The standard emphasizes that the construction must result in concrete that meets the required strength, durability, and surface finish as specified in the design documents. It defines the responsibilities of the contractor, the engineer, and the testing authorities to ensure compliance throughout the construction process.

A key aspect of NZS 3109 is its focus on the "on-site" reality of concrete work. It acknowledges that theoretical mix designs must be translated into physical structures with precision. Therefore, it places heavy emphasis on tolerances, formwork quality, and the environmental conditions during placement.

Formwork and Falsework

Formwork is critical in defining the shape, dimensions, and surface texture of the concrete. NZS 3109 specifies that formwork must be sufficiently rigid, tight, and strong to support the weight of wet concrete and construction loads without deformation or leakage of mortar.

• Tolerances: The standard sets strict tolerance limits for the size, position, and alignment of formwork. This ensures that the final structural element matches the design dimensions accurately.
• Surface Finish: Requirements for the formwork lining material are detailed to achieve the specified concrete surface finish, whether it is rough formwork or an architectural "as struck" finish.
• Removal: The standard provides guidance on the stripping times, ensuring that concrete has achieved sufficient strength to support its own weight and construction loads before the formwork is removed.

Reinforcement

For reinforced concrete, the placement and fixing of reinforcement bars (rebar) are governed by strict rules to ensure the structural integrity of the final element.

• Positioning: Bars must be placed and maintained in the correct position within the formwork to ensure the required concrete cover (the distance between the bar and the concrete surface). Correct cover is essential for both bond strength and durability (protection against corrosion).
• Support: Chairs, spacers, and bolsters must be used to support the reinforcement and prevent it from being displaced by the weight of wet concrete or during the compaction process.
• Splicing and Fixing: The standard covers the methods of lapping bars and fixing them together with ties or welds, ensuring that the reinforcement acts as a continuous structural unit.

Concrete Materials and Production

While the specific design of the concrete mix is often determined by NZS 3104 or specific project specifications, NZS 3109 references the requirements for materials to ensure quality.

• Cement: Must comply with relevant New Zealand standards for general purpose or blended cements.
• Aggregates: Aggregates must be clean, hard, durable, and free from harmful substances such as organic impurities or excessive clay that could affect the concrete's strength or setting time.
• Water: Water used for mixing and curing must be potable or free from impurities that could adversely affect the concrete.
• Admixtures: If chemical admixtures are used to retard or accelerate setting, improve workability, or enhance durability, they must comply with relevant standards and be used in strict accordance with manufacturer recommendations.

Transportation and Placing

The standard highlights that concrete should be transported from the batching plant to the site as quickly as possible to minimize segregation and loss of slump. Methods of transportation, such as agitating trucks or conveyor belts, must prevent contamination and evaporation.

Placing involves depositing the concrete into the forms. NZS 3109 requires that concrete be placed as close as possible to its final position to avoid unnecessary movement (segregation) of the aggregate. It stipulates rules on the rate of placing and the height from which concrete can be dropped. Generally, concrete should not be dropped from a height greater than 1.5 to 2 meters to prevent separation of the coarse aggregate from the mortar.

Compaction

Once placed, concrete must be compacted thoroughly to remove entrapped air voids and ensure it completely surrounds the reinforcement and fills the formwork.

• Vibration: Mechanical vibration is the standard method. The operator must insert the vibrator at regular intervals vertically into the fresh concrete.
• Duration: Vibration must continue only long enough to achieve compaction; over-vibration can cause segregation, bringing too much mortar to the surface and leaving the aggregate at the bottom, while under-vibration leaves honeycombing (voids).

Finishing and Curing

Finishing refers to the process of leveling and texturing the concrete surface after compaction. For slabs, this may involve screeding, floating, and troweling. The standard requires that finishes be applied at the appropriate time in the setting process to achieve a durable, even surface.

Curing is arguably the most critical factor for achieving durable concrete. NZS 3109 mandates that concrete be kept moist and at a suitable temperature for a specific period (usually a minimum of 7 days for general construction) to ensure proper hydration of the cement. Without adequate curing, concrete will fail to reach its potential strength and will be prone to surface cracking and dusting. Methods include covering with plastic sheets, applying curing compounds, or continuous water spraying.

Hot and Cold Weather Concreting

Recognizing New Zealand's varied climate, NZS 3109 provides specific guidelines for concreting in extreme conditions.

• Hot Weather: In high temperatures, measures must be taken to reduce the rate of evaporation and setting time. This includes cooling aggregates, adding ice to mixing water, using retarding admixtures, and protecting the concrete with sunshades or windbreaks.
• Cold Weather: In low temperatures, precautions must be taken to prevent the concrete from freezing before it gains strength. This involves heating mixing water and aggregates, insulating formwork, and using accelerating admixtures.

Conclusion

NZS 3109:1987 serves as a foundational document in the New Zealand building code framework regarding concrete work. By standardizing the expectations for materials, workmanship, and site practices, it ensures that structural concrete performs as intended over its lifespan. Adherence to this specification minimizes the risk of structural defects, ensures safety, and guarantees the durability of the built environment.