Quality Control in Concrete Construction

Quality of concrete construction on site can be accomplished in three distinct stages as follow

1. Quality control before concreting
2. Quality control during concreting
3. Quality control after construction

Stage-1(Quality Control Before Concreting)

This stage of quality control consists of two steps.

1. Checking of specification requirements regarding excavation, forms, reinforcement and embedded fixtures etc.
2. Control test on concrete ingredients (i.e. on cement, aggregate & water)

Cement

Quality of cement is ascertained by making compressive strength tests on cement cubes. However for effective control cement:

• Should be tested initially once for each source and subsequently once for every two months
• Should be protected from moisture
• Should be retested after 3 months of storage, if long storage in unavoidable
• Should be rejected if large lump are found in cement bags.

Aggregate

Concrete aggregates should confirm to specified values as per standard specification.

The quality of concrete is affected by different physical and mechanical properties of aggregate, i.e. shape, grading, durability, specific gravity and water absorption etc. these properties of aggregated should be tested before using it for concrete production.

还应测试有害材料和有机杂质的量。新利客户

Bulking of sand is also an important property in several ways. It gives wrong results when volume batching is done. It increases water cement ratio which in turn reduces strength.

For effective control aggregates:

• Are required to be tested once initially for approval of source
• 随后应每天进行一次或两次进行两次，用于水分含量，应对聚集体的水分含量进行余量。

水

The quality of water should be checked for the requirements as specified in respective standard. Chemical analysis shall be conducted for approval of source. In case of suspended impurities, it is necessary to store water for some time to allow them to settle down. In case of doubt concrete cubes made with this water are tested. Average 28 days compressive strength of at least three cubes or cylinders or specified size, prepared with water proposed to be used shall not be less than 90% of the average strength of three similar concrete cubes prepared with distilled water.

Stage-2(Quality Control During Concreting)

Careful supervision during concrete manufacture is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing. Following precautions should be taken during concreting operation.

1. The concrete mix should be designed in the laboratory with the materials to be used on site.
2. 尽可能具体应该成批的weight. If weight batching is not possible, then volume batching may be permitted through proper supervision in the presence of engineer in charge.
3. 在混合过程中，应将混合器充分收取。材料应以新利客户适当的顺序喂食。混合器的速度应为每分钟15到20转的范围。在任何情况下，混合时间不应小于2分钟。在从混合器卸下混凝土的同时应避免隔离。
4. Workability of concrete is an important property of concrete while concrete is in its fresh state. Therefore slump test or compaction factor test should be performed to check workability of concrete. About three tests should be carried out for every 25 m³of concrete.
5. Care should be taken so that no segregation takes place during transportation of concrete.
6. 混凝土不应从超过1米的高度掉落。如果跌落高度超过1米，则应使用斜槽。
7. To avoid rehandling of concrete it should be placed at its final position as far as possible.
8. Vibrators should be used for compacting concrete. The insertion spacing of internal vibrators should not be more than 0.6 m. It should be drawn out slowly so that no holes remain in the concrete. The frequency of vibrators should not be less than 7000 cycles/minutes.
9. Curing should be done for a specified period so that concrete develops requisite strength. Concrete should be covered with hessian as soon as it becomes hard.
10. 表格工作应对应于结构的最终形式。应该在开始混凝器之前检查它。应清洁和涂上表格的内部。在指定期间后应删除表格。
11. Concrete should be protected from hot and cold weather at early ages. Concreting should not be done at temperature below 4.5⁰C and above 40⁰In very hot weather water and aggregates should be cooled. Retarders of approved quality can be used.
12. In very cold weather water and aggregates should be heated. Accelerators of approved quality can also be used.

Stage-3（建设后质量控制）

Once the concrete is laid and compacted, compression tests are made on the cubes made out of this concrete. For ordinary concrete, cubes are made out of the concrete made at work site.

The hardened concrete has to be checked for trueness in dimensions, shape and sizes as per design specification. General surface appearance of concrete should also be checked. Dimensions are ascertained by different measurements. Reinforcement should have adequate concrete cover and if the reinforcement is visible in part of a structure, the part should be rejected or necessary actions should be taken accordingly.

Concrete strength is normally to be ascertained from cube or cylinder samples tested at 28 days. In case the strength obtained is less than the specific minimum, one or more of following steps may be taken.

• Load test and measurement of deflection and / or strain (the quality of the structure can then be ascertained by calculating back the concrete strength)
• Cutting cores from the structures and testing them for strength
• Non destructive tests like Schmidt rebound hammer or ultrasonic pulse velocity test. These tests give only a very rough idea and are primarily used to ascertain the uniformity of construction.
• Chemical analysis of hardened concrete.