Table of Contents for Top 5 Concrete Quality Control Methods at Construction Site-
- Introduction to Concrete Quality Control
- Need of Concrete Quality Control
- Tip 1: QC for Material Storage
- Tip 2: QC for Preparing Concrete
- Tip 3: QC for Handling
- Tip 4: QC for Pouring Concrete
- Tip 5: QC for Post-Pouring Concrete
- Final Words
Introduction to Concrete Quality Control
Concrete is a mixture of cement, sand, aggregate and water. Concrete is an essential building construction product. The cement binds the concrete elements together. The sand fills the space between the aggregate and provide a larger contact area for cement binding. The aggregate provides the bulk and strength to the concrete mixture. The lime can also be used in place of cement but it has lesser strength as compared to the cement, keeping all the other parameters the same. The concrete with lime is called lime concrete and the concrete with cement is called cement concrete.
The concrete properties can be altered by adding additives to it. The concrete can be further strengthened by reinforcing it with steel bars. Such concrete is called the reinforced cement concrete or RCC, while the concrete without steel bars is called plain concrete or PCC.
The popularity of concrete is due to the various properties of concrete. The ability to set on mixing with water, suitability to be cast into any shape, ease in handling, strength, lower heat transmission, etc. and being economical than steel.
The primary requirement of the concrete is to provide strength. The strength of the concrete is controlled by changing the ratio of its’ constituent. The concrete mix can be prepared by any of the two methods- nominal mix and design mix. The nominal mix is used for concrete grades up to M20. The nominal mix has a fixed ratio of the constituents for each grade. The design mix is used for the high-performance concrete and the ratio of the constituent is calculated for each grade.
Need of Concrete Quality Control
Every project has its’ own quality control program. The QC procedures, checks, control methods, remedial measures are mentioned in the agreement. Moreover, one of the main objectives of project control is to ensure quality.
Concrete quality control is necessary to ensure the strength of the structure. Quality control has various aspects from selecting the right material to the curing of the concrete. Every step involved in the process should be monitored for its’ quality compliance.
The quality maintained throughout the construction project not only helps in maintaining the desired lifetime of the structure but also keeps aesthetics. Moreover, it creates a reputation for the contractor, provides better value for money, lower maintenance cost etc.
As per the research, the cost incurred due to the poor quality is 6-15% of the construction cost for defective components and 5% for rework of defect. The reasons for the defects can be either human error like lack of supervision, low-quality material, poor workmanship.
Tip 1: QC for Material Storage
The quality control starts with the material brought to the site. The materials have properties that can be utilised only if it has been preserved. The material quality deteriorates if its storage is improper.
The storage of material is to be done to protect it from pollutants and undesirable weather condition. For example, the cement should be kept away from dampness, steel reinforcement bars should be kept away from water source etc.
Following are the methods to store construction materials for concrete-
The cement should be kept away from moisture. The cement sets in the presence of moisture and forms lumps. These lumps deteriorate the cement quality. The storage should protect the cement from all weather conditions and have proper ventilation.
The storage area floor should ideally be of concrete to avoid moisture from the ground. If storage is to be done on the ground, the storage area should be made 300 mm higher than the ground surface. The cement bags should be covered with waterproof plastic or tarpaulin. The storage area should also have a stable shed over it.
The storage area should be decided as per the required stock built. Ideally, 10 cement bags are piled up to facilitate the manual loading and unloading. The storage is to be kept in such a way that older stock can be used first. Generally, the storage is kept for 3 times the daily consumption. The criteria may change as per the delivery time, storage conditions, cash flow etc.
Recordkeeping- To ensure the quality following records should be maintained-
- Delivery date
- Manufacturing date
- Storage location
- The brand with cement type and cement grade
The reinforcement steel bars are prone to corrosion. To protect the steel bars from corrosion, they should be kept away from the potential waterlogging area and covered with waterproof plastic or tarpaulin. The steel bars should also be protected from foreign materials that can reduce its’ bonding with concrete.
The correct method to store the bars is by providing the wooden sleepers beneath the bars. The ground is made level and wooden sleepers are provided that maintain the 100 mm distance between the bars and the ground. The steel bundles should also be separated into layers by the wooden sleepers.
Recordkeeping- The manufacturer’s test certificate for steel bars is to be kept in the records. Samples taken randomly should also be tested as per the quality plan.
Sand and Aggregate-
Sand and aggregate accumulate the water and becomes bulkier. This can lead to a higher water-cement ratio of the concrete. Thus, producing poor-quality concrete. To avoid this, the water content in sand and aggregate is determined prior to mix and water content is reduced suitably. The sand and aggregate are such stored that the water is naturally drained out.
The storage of sand and aggregate is made on the slightly inclined ground. The shed above the storage would prevent higher water content during the rainy season.
The other major concern is the mixing of pollutants. When piling up and reclaiming the material stock, soil, twigs, dust and debris etc. gets mixed with it. Thus, the area should be cleaned prior to the storage of the material and attention is to be paid when stacking.
The steel tank for storing the water should be treated with an anti-corrosion coating. The water should be free from oil, grease or any other foreign material and should be potable.
Recordkeeping- The water is to be tested for suspended solids, chloride contents and pH value prior to mixing.
Tip 2: QC for Preparing Concrete
IS 456:2000 recommends to collect the following minimum data before preparing the concrete-
- Type of mix, either nominal mix or design mix
- Grade Designation
- Type of Cement
- Maximum nominal size of aggregate
- Maximum cement content (for design mix)
- Maximum water-cement ratio
- Mix Proportion (for nominal mix)
- Maximum temperature of concrete at the time of placing
- Degree of supervision
To ensure the quality a design mix should be prepared and tested for the target mean strength.
Weighing for Concrete Quality Control
The accuracy in measuring the quantity of material should be ensured. The tolerance limit as per IS 456:2000 for material variation in one batch is as follows-
The material should be proportioned only on the basis of weight. The volume proportioning can be allowed if the actual bulk densities have been established.
Mixing for Concrete Quality Control
The mechanical mixing of the concrete is recommended. The mixing is generally done in batching plants. However, for small scale production, portable concrete mixers can also be used.
The batching plants have automated weighing systems and can provide the batch report. The quality control is better. They operate on electricity. For better quality control, the batching plants need to be calibrated for their weighing accuracy.
The concrete mixers are operated by attached engines operated by fossil fuels. The production capacity is lesser than the batching plant. The concrete mixers are of two types tilting drum type and reversing drum type.
Recordkeeping- Material quantity for each batch with, time, date, slump at the time of dispatch, concrete samples for cubes.
Tip 3: QC for Handling
The chemical reactions in the concrete start as soon as the mixture is mixed with the water. The reactions make the compounds that set the concrete. Thus, it is necessary to timely transport, pour, compact and finish the concrete before it gets hard. The workability of concrete decreases with the increasing time. The fluidity is lost and the finishing becomes difficult.
Transportation for Concrete Quality Control
The concrete supply should be continuous and the gap between the two batches should not exceed 20 minutes. The casting should be monolithic. It is recommended, the duration from mixing to pouring should not exceed. For higher transportation time, suitable admixtures should be used to retard the setting of concrete.
The temperature change during the transportation should be calculated and suitable measures to be taken to ensure the correct pouring temperature.
The concrete if transported for a short distance, dumpers can be used. For longer distances, concrete millers are used.
Recordkeeping- In case of a concrete purchase from a supplier, the time of dispatch and the time of arrival should be noted.
Tip 4: QC for Pouring Concrete
Cleaning of Surface
The pouring surface should be cleaned of dirt, dust, other foreign material by blowing air or water.
Checking of Rebars, formwork and staging/scaffolding
The rebars should be checked for their diameter and arrangement as per the drawing. The rebars should be strongly held into their position when pouring concrete.
The staging and scaffolding to be checked for their connections and load-carrying capacity. The formwork used should be well oiled, straight and free from the defects that may prevent the finishing.
Concrete pouring should be done in layers of 40-50 cm for horizontal members. For vertical members, the pouring should be 2-3 m in height per hour to avoid the lateral pressure on the formwork. The allowable time between the two layers of pouring is 120-150 minutes for a temperature below 25-degree centigrade else 60-120 minutes.
The concrete pumps, bucket or chute can be used for pouring. The capacity and methodology of each piece of equipment are different from each other.
The concrete pumps can pour concrete in any direction and through long distances up to 500m. Their capacity ranges from 20-70 cum/hr. The pumps are of two types- piston type and squeeze type. A slurry mixture of 1 cement and 3 sand is used prior to the pumping of concrete to avoid the choking of the pipeline. The minimum slump for the concrete to be pumped is 80mm.
The bucket is used with a crane. The bucket has an opening and closing arrangement at the bottom and offers lesser productivity and the least segregation.
The chutes work only vertically downward and diagonally downward. The funnel is attached at the bottom to minimise segregation. The tip of the chute should not be more than 1.5m higher than the pouring surface. The use of chutes is not recommended.
The compaction of concrete is the process of removing air voids from the concrete. The concrete becomes denser and achieves more strength than the uncompacted concrete. The compaction of concrete is to be done by means of vibrators. The vibrator can be of needle type or plate type. The diameter of the needle of the vibrator varies from 30mm to 60mm. The higher the needle diameter, the greater is the consolidation area. The vibrator should comply with the IS 2505, IS 2506, IS 2514 and IS 4656.
The concrete is to be compacted at the outer periphery first and then at the centre of the poured concrete. The vibrator should be vertically dipped into the concrete and taken out and not be left in it for a long time. The prolonged exposure of concrete to the vibration causes grout formation near the vibrator which is undesirable. The distance between two vibration points should not exceed the 1.5m distance. The vibration of a very fine mix should be avoided.
For slump lesser than 100 mm, the vibration time is 15-30 seconds, else 7-10 seconds.
The concrete surface should be made smooth with a trowel. No undulation, aggregate, voids should be visible at the surface. If another layer is to pour over the surface, it should be left rough and provide a shear key if possible.
The use of additional water or cement for finishing should be avoided. Further, the concrete should not be disturbed once placed as it affects the strength.
Tip 5: QC for Post-Pouring Concrete
The concrete loses moisture after setting. This lost moisture needs to be made up by providing external water so that the concrete can attain strength. The process is called curing. The curing is the most important method of post pouring quality control of the concrete.
The curing can be done by wet curing, liquid membrane curing, temperature control methods.
The minimum curing period for OPC is 7 days and cement with mineral admixtures is 10 days. For, hot and dry weather conditions, the curing period should not be smaller than the 10 days for OPC and 14 days for cement with mineral admixture.
The best way to ensure the quality of concrete is by preparing the Quality Assurance Plan (QAP) and implementing it strictly. The IS 456:2000 lays the foundation for QAP, further ISO 9001:2015 lays the general guidelines to ensure the quality.
The effective implementation of QAP is to be done through the execution and operation team only. The quality supervisor can only check and guide the execution team to maintain the quality. Thus, the training of execution and operation staff is an important step of QAP implementation.
The 5 tips of concrete quality control plan as mentioned above will help in developing the QAP.