what is Destructive testing system in construction

DESTRUCTIVE TESTING SYSTEMS IN CONSTRUCTION

The most common destructive test is load test and is used to assess the strength of concrete structural elements. Load testing is sometimes used as an alternative method of assessing structural capacity. Load tests are usually carried out for one of the following reasons: 

• There are still doubts about the satisfactory performance of the structure under load after a survey and local testing. 
• It is difficult or impossible to determine adequate information about the structure and ft materials.
• Verification of structural analysis in cases where the complexity of the structural form does not lend itself to rigorous analysis 

Deficiencies in detail, material or construction are suspected and such deficiencies would mean that the normal procedures or assumptions on which structural analysis is based were not appropriate. After confirming that the reinforcement did not go in to plastic, it was decided to conduct full scale load tests on the beams, so as to determine the load carrying capacity of the cracked beams and compare it with original design strength before any remedial measures are suggested. Accordingly load tests were conducted on most distressed beams. The following aspects were set for during the testing programme.

1. To conduct the load test on beams up to 1.25 times the designed live load. 
2. To monitor the deflections and recovery of the beams during incremental loading and unloading 
3. To compare the actual deflections with that of theoretical deflections

what is Destructive testing system in construction

Assessment of existing concrete structures 

It has been learnt that concrete structures require a closer inspection, not only immediately after construction but also periodically at a regular interval. The quality control measures during construction, generally, consist of workability tests on fresh concrete and cube compressive strength of concrete samples, after some specified days of curing. It is a well known fact that the results of the above tests do not reflect the true quality of the concrete, existing in the concrete structure because the quality of a concrete structure depends on many factors such as method of mixing, transporting, placing, compacting and curing of concrete. While concrete members with certain amount of imperfections can satisfy the requirements relating to strength and serviceability, such concrete may not satisfy durability requirements.

Assessment of quality of concrete is necessary to ensure that the quality of execution. satisfactory and to identify any deficiencies so that they can be rectified. This can be achieved only by conducting some in-situ tests on the structures besides visual inspection. The in-situ tests are non-destructive tests and partially destructive tests. These tests measure indirectly the strength of concrete except in the case of core test, where direct evidence on the condition of concrete and a measurement on compressive strength are possible. These testing methods are in use for evaluating existing concrete structures with regard to their strength apart from assessment and quality control of hardened concrete.

Direct load test 

This is a method of assessing the strength of the in-situ concrete member. In most cases this test is performed for the proof of structure capacity, not for suspect or critical location. The principal aim in this testing is to demonstrate satisfactory performance under an over load, above the design working value. This is usually judged by measurement of deflections under this load which may be sustained for a specific period. The selection of specific members or portion of a structure to be tested will depend upon the general features of convenience as well as the relative importance of strength and expected load effect at various locations. Selection of member may often be assisted by non-destructive methods coupled with visual inspection to locate the weakest zones or elements. 

There are certain inherent problems in this test. This method is very expensive and time consuming. In in-situ test whether the member under test is actually subjected to the assumed test load during fire is difficult to predict to predict due to load sharing effect. All the direct methods of assessing residual strength of concrete provide regarding average potential cube compressive strength and are not indicative of strength profile across the section. Direct load testing again may not be possible where the loading may damage the structure permanently or where the structural system in too stiff to show any deflection.

Load test on structural element 

A live load of 1t/sqm was taken for imposed load calculation. Floor finishing load and partition wall load was also considered. Thus imposed load was taken as 1.25 times live load as per IS 456: 2000 Code recommendations. The load was imposed on the beam as udl spreads over an area of 55.5 sqm. Thus a total load of 84 t was arrived at and imposed in 12 stages of 7t each at an interval of 15 minutes. The load of 7t was imposed with the help of 200 sand bags weighing 35 kg each as first step and increased in steps up to 84t. Dial gauges and electrical strain gauges were fixed at different locations. Separate platform was provided to read and record the deflections from the dial gauges, strain from strain indicator during loading and unloading. 

Deflections for the above mentioned loading steps were recorded. Total time for reaching maximum load was 26 hrs and the maximum load was kept for more than 43 hrs during Which creep effect was monitored and the time taken for unloading was 32 hrs during which deflection recovery was monitored. The recorded deflections and theoretical deflections were compared.