The reason for having quality control (QC) is to assure that the requirements and intent of the project, plans, specifications are completely accomplished.
National development and progress depend to a large extent on essential infrastructures, such as housing, power, industry, roads, communication system, bridges, airports, seaports, etc. these infrastructural developments have to be reliable and trouble-free. To have reliable and trouble-free usage it is necessary to exercise quality control (QC) of their constriction works.
Civil construction work involves a large amount of quality control (QC) effort at a site especially because the construction work to a great extent involves the production of concrete at the site. Unlike the production of goods in a controlled factory environment, concrete production at the site is done in a different environment. The site production is quite often done in an uncontrolled manner and without proper inspection.
The term “inspection” as used in concrete construction includes not only visual observation and field measurement, but also laboratory testing and the assembly and evaluation of test data.
Golden Rules of Quality Control (QC)
Before we deal with any aspects of quality control (QC) there are eleven golden rules one must remember for successful quality control (QC) system implementation.
- The TOPMOST MAN in the organization must be convinced that quality must be had.
- The reporting system of the quality control (QC) department must be direct to the TOPMAN and not through the person or persons in any manner connected with the Project Management.
- Every worker or supervisor in the project team plays a vital role in quality control (QC) and hence everyone must be aware of the quality control (QC) system and inspection procedures.
- Quality must be always given preference over speed & economy. Good quality construction will never be deterrent to the project’s speed nor will it be uneconomical. Time-consuming repairs & modifications due to poor quality work generally delay the job and cause an additional financial impact on the project.
- Timely inspection and timely corrective action thereafter, can be the key to successful quality control (QC) system. Delays in inspection or delays in corrective action are the main causes of the failure of the quality control (QC) system.
- Good quality construction can’t be carried out by using poor quality materials but it is often possible that poor construction is done using good quality materials.
- We should learn from mistakes we make during construction but the mistakes should not be allowed to be repeated under any circumstances.
- Quality control (QC) inspector must not only be skillful enough to point out non-conformities but he must also be capable enough to give suggestions to resolve them as well. He should also be able to give suggestions or ideas to prevent their future occurrence.
- Ignorance, negligence, carelessness & greed lead to construction failures. Hence quality control (QC) inspector must have good technical knowledge, must be very watchful, must be very alert, and above all must be honest and a person of high integrity.
- reject the first concrete member which is found to be unacceptable. Any compromise by way of repairs will encourage the construction agency to continue to do poor-quality work. If at first instance instructions are given to demolish the defective concrete member then all precautions will be taken to prevent future occurrence of unacceptable concrete quality.
- Essential material testing & concrete testing must be carried out at the construction site. It’s not possible to control the quality of concrete work if the testing laboratory is located far away from the site.
Quality Control System
A separate team of Engineers and trained personnel under a Manager (QC) is required to be set up for a construction project. This team must be independent of the construction group whose head is generally the Project Manager.
The quality control (QC) group must directly report to a Senior Executive or the topmost man in the organization to whom the project belongs.
Function of Quality Control System
The functions of quality control (QC) system are broadly classified as follows:
- Define or Identify activities affecting quality through instructions and procedures.
- Ensure that equipment & testing instruments are calibrated and properly adjusted to maintain their accuracy within acceptable limits.
- Carry out unannounced spot checks on materials and finished products.
- Identify problems & initiate action which results in solutions.
- Verify implementation of solutions/corrections.
- Have access to responsible management level where appropriate action can be taken or initiated.
- Report regularly on the effectiveness of the system.
The Workers, Foremen, Supervision, Engineers, and managers of the Client (Owner), the Consultant, and the Contractors each form a link of the quality chain. The strength of this chain is dependent on each & every link. The quality control (QC) department has to keep a continuous watch on this chain and spot the weak links before they snap.
The overall responsibility for assuring the execution of work as per required standards, specifications, and drawings rests on the contracting firms. The responsivity for overseeing and checking the quality of work, rests with the project manager, the senior engineer and their group. The organizational approach recommended for any project for non-conformance control is given in the below table.
|Execution of work||Construction firm|
|Overseeing/Checking of work||Project Manager|
|Verification of quality of work||Manager QC|
|Dispositioning site level||Manager QC and Project Manager|
|Management level||Consultant, owner’s senior representative|
|Senior management level||Owner or managing director|
|Dispositioning action||Construction firm/Owner’s workforce on instruction from the project manager.|
Non-compliance observed must be quickly attended to by the concerned authority in consultation with the consultants. This contributes significantly to the quantity and speed of the project.
The inspector of the quality control (QC) department must point out the non-conformity to the supervising engineer in writing and a copy of the same must be sent to the project manager when any work does not conform to the required specified standards of quality, specifications, or drawing.
Statistical data must be complied with by the quality control (QC) department, on non-conformities observed on the project. This can give excellent feedback for the subsequent projects. Specification and contract conditions can be reviewed with a view to eliminating such non-conformities in future projects or to at least reduce them considerably.
Mechanized concreting and construction practices can go a very long way in achieving excellent quality. Manual methods are most often prone to human error. Even mechanized equipment operating on manual control often causes serious quality problems. Automatic control is most reliable, superior, and faster and must be followed and insisted upon.
Inspection and testing of concrete works
All materials must be inspected and tested, on arrival at the site. All batches of cement must be tested for physical properties and stored separately. Samples at random can be sent to outside laboratories for the determination of chemical properties and properties like the heat of hydration for special applications. Cement test certificates and the monthly standard deviations of cement production must be insisted upon by the cement manufacturing company. Sand, natural and crushed aggregates must be inspected for deleterious materials and grading. If concrete is manufactured at the site or at a central batching and mixing plant, frequent changes in aggregate proportion may be required to be made to suit the grading, shape, and size of aggregates. Frequent variations in cement strengths and aggregate quality and grading require close monitoring and continuous inspection. Cement and aggregates should be preferably obtained from a single source to achieve consistency of quality.
Time-consuming quality control (QC) procedures for testing must be made simple and yet comprehensive enough to ensure good quality of works as stipulated in the specifications. For example, the determination of silt content in the sand by weight is not possible for each and every truckload. Therefore, silt content must be checked by volumetric measure in the fixed concentration of a salt solution or alum solution, if on visual inspection the sand is found to be silty in nature.
A correlation between time and silt content by volume in the sand can be arrived at. Silt content at 10 minutes can be made acceptable criteria instead of waiting for 2 to 3 hours. For example, if silt by volume is less than 20% in 10 minutes then trucks can be permitted to unload the sand at the site if it is observed that similar samples were well within the acceptable limit of silt content of 8% in 3 hours. Sand that does not clear this criterion must not be unloaded at site proper testing is done.
Cube failures are often observed due to the coating of silt on the rounded gravel aggregates, even though the silt content in aggregate is within the acceptable limits, as per standards and specifications. This is especially observed when aggregates are scaped from the dead storage areas of the batching plant bin in the event of a short supply. Especially at the end of a large concrete pour. A study was made in a power plant project and it was found necessary to reduce the aggregate to cement ratio (A/C) by 0.20 in case silt coating was between 0.5% to 0.8% by weight of gravel and by 0.5 in case the coating was between 0.8% to 1.5% by weight of gravel. This not only prevented low concrete strengths but also helped in completing the concrete pour whenever completing the concrete pour whenever such shortages arose.
Concreting operations must be commenced only after inspection of formwork, reinforcement, inserts, equipment, and concrete materials. The adequacy of skilled and unskilled personnel must be also checked. Schemes for large concrete pours and their method statement must be prepared and reviewed well in advance by the QC department. Close monitoring while placing is required to avoid cold joints, segregation, and poor compaction. The layout of reinforcement, formwork design, and method of placing and compacting concrete must be reviewed critically for every concrete pour to minimize defects which might lead to time-consuming and costly repairs.
Placing and compacting concrete in small pours such as columns is most often neglected as the quantity is small. Columns are compression members and therefore the quality of concrete in a column must be given utmost importance.
Concrete must come right at first instance. Repaired or patched-up concrete will have lower performance than concrete made correctly in one go. Test results of concrete cubes do not give an indication of the strength of the structure. The strength of concrete cubes only gives indications of whether or not the concrete mix was satisfactory.
A statistical data of likely reasons for cube failure, if prepared can give useful information and feedback data to improve the quality of concrete mix and reduce the strength variations. Statistical data of cube strength variations give useful feedback on the extent of QC being exercised on the quality of concrete materials and their batching and mixing process. The lower the standard deviation, the more superior is the quality control. Consistently very high concrete strength results than those specified do not mean good quality control, it only means that concrete mix is uneconomical.
Quality of work should never be sacrificed at the cost of speed and economy. Unsophisticated pursuit of fast production and the false economy does not pay in the long run. Costly and time-consuming repairs often slow down the progress of work. If quality is given preference, then speed and economy in work can be easily achieved. Understand the eleven golden rules of quality control.
Negligence, ignorance due to lack of technical training and knowledge, inadequate designs, bad quality of materials, and poor workmanship are some of the main causes of non-compliance in concrete works and often result in failures of structures, it is always better to “Catch” an error before it becomes a mistake. The true success of the QC system is in the timely detection of such errors. This success cannot be measured or quantified. Efficient, reliable, and maintenance-free service given by a structure during its life span is by itself a testimony of strong and durable concrete work.