2. Mixing Method
Beyond composition, the concrete mixing or production process significantly influences compressive strength. Steps such as proportioning, mixing, placing, and compaction must be executed properly to ensure homogeneous, dense, and defect-free concrete.
2.1. Mix Proportioning
Mix design is a critical initial step. The ratio of cement, water, fine and coarse aggregates, and admixtures must align with:
- Target compressive strength.
- Required workability for placement methods.
- Environmental and structural conditions.
Poor mix design can lead to overly dry (hard to compact) or overly wet (prone to segregation) concrete, both of which compromise strength. Proportioning generally refers to standards like SNI 7656:2012 for optimal mix design.
2.2. Mixing Process
Mixing aims to evenly combine all components into a uniform concrete. Inadequate mixing can cause segregation, weak zones, or composition variability. Key considerations:
- Mixing sequence: Aggregates and cement are typically mixed dry before water and admixtures are added to avoid clumping.
- Equipment type: Examples include portable mixers, truck mixers, and batching plants.
- Mixing time: Short mixing may lead to inconsistency; prolonged mixing can cause water loss and reduced workability.
2.3. Placing
Concrete should be placed immediately after mixing. Delays can lead to initial setting before placement, weakening layer bonding and compressive strength. Key factors include:
- Drop height: Free-fall from over 1.5 meters without control can cause segregation and microstructural damage.
- Transport time and distance: Concrete from batching plants must arrive in time to avoid slump loss or early setting.
- Handling method: Consider segregation risk (e.g., when using buckets, pumps, or manual placement).
2.4. Compaction
Compaction removes air voids and ensures cement paste fully envelops aggregates. Poor compaction leads to porous, low-strength concrete.
Common compaction methods:
- Manual (rodding or tamping): Suitable for small volumes.
- Internal vibrator (poker): Standard on construction sites for effective compaction.
- External vibrator or vibrating table: Used in precast concrete production.
Improper compaction causes honeycombing, poor reinforcement bonding, and drastically reduced strength.
Here we continue with the discussion on the Curing Factor on the next page.