2.9.5 Limitations:
The determination of kT and p should not be carried out on wet surfaces (the moisture entering the unit could damage the membrane in the pressure regulator).
The most accurate values are obtained for dry concrete ( p measurement is superfluous).
In order to obtained an exact idea of the quality of the cover concrete of a structure or of a finished component, several measurements must always be carried out .
The quality classification of cover concrete from table and the monogram from figure related to young concrete i.e. concrete age about 1-3 months. Some measurements on concrete a few years old have shown that the classification in Table and the monogram cannot be directly applied.
The moisture content of the concrete has a major effect on the gas permeability. The correction of this effect by the measurement of the electrical resistance generally leads to satisfactory results in the case of young concrete. For old concrete, further investigations must be carried out.
The investigations were performed using a vacuum pump with a suction capacity of 1.5 m3/h and a motor power of 0.13 kW, this pump makes it possible to achieve a vacuum of a few mbar. Pumps of lower power do not reach the same vacuum and it is therefore advisable to use only pumps of similar power.
There may be three further reasons why the desired vacuum (10-50 mbar ) is not reached.
The concrete cover is too permeable (normal function of the unit).
The concrete surface is too uneven: the rubber seals can compensate only a certain degree of unevenness (abnormal function).
The unit has a leak (abnormal function).
2.10 Bond Test:
The bond testing equipment measures in place bonding or direct tensile strength between two layers e.g. a repair overlay and the parent concrete material or adhesion of shot Crete and membrane. The test location is so selected with the help of a metal detector that reinforcement disturbance, if any, is controlled and minimised during cutting operation.
The test consists of drilling a 100 mm nominal diameter core through the overlay into the parent concrete material. The drilled core is left without breaking. The top surface of the core is cleaned and dried and a cylindrical steel disc 85 mm in diameter is fastened to it with epoxy resin and adhesive. After hardening of the epoxy, the counter pressure ring with an inside diameter of 105 mm is placed concentric with the core on the overlay surface and connected to the pull bolts with countering plate and coupling. Load is applied by turning the instrument handle to a required pull force up to failure of the core in tension. Here also the disadvantage is that a small damage to the concrete is required to be repaired.
2.11 Maturity Method:
ASTM C 1074 has adopted a standard practice on the use of the maturity method to estimate concrete strength. The maturity method is also used in ASTM C 918 for estimating later-age potential cylinder strength based on measured early-age strength.
Compressive strength of well cured concrete increases with time. But this increase is dependent on the temperature of curing also. The combined influence of time and temperature is considered as the maturity. It is thus defined as the integral of time multiplied by temperature with a datum temperature of – 10 ° C, since below this temperature cement in concrete ceases to hydrate. The maturity of in-place can be monitored by thermocouples or by instruments called “maturity maters” The strength of in-place concrete is then estimate using the established correlation graph between maturity and compressive strength of concrete. The advantage of maturity concept is that by prior placing of maturity meters in the formwork at the time of the construction, the strength of early age concrete can be monitored and accordingly formwork can be removed confidently.
2.12 Complete Structural Testing:
2.12.1 Structural Testing System- This integrated technique provides an approach that can be used to detect damage, general evaluation and development of load ratings for all type of steel, concrete and timber structures.
The technique is based on the principle of “semi- static “live load test. It consists of instrumentation removal. In this technique upto 65 recessable strain transducers are indented to the structural members of the bridge and strains recorded vehicle with known weight crosses at crawling speed. High speed passes are also needed to conduct in order to determine actual impact.
2.12.1.1 Advantage:
In this technique no holes or welds are required.
Each strain sensors needs very short period of time (about 5 minutes) to install.
In need, only one lane to be closed at a time.
The subsequent analytical modeling techniques range from a simple planner grid model to a three dimensional finite element representation. The response of the model in systematically compared with the field test results using multiple gauge location and load configurations. Structural parameters such as lateral deck stiffness, rotational restraints are then modified through an interactive process until the analytical responses closely match the field measurement. This calibrated model can be used to predict stress levels at critical locations due to rating and overloaded vehicle. Rating factors can then be developed using either allowable stress method (ASD) or the load factor method (LFD).
This approach is suitable to use on highway bridges, rail road bridges and other structures. Where the live load can be easily applied and load stresses significant, by approaching to this method steel, pre-stressed concrete, reinforce concrete and timber structures can be tested successfully.
2.12.2 Indian Railways has planned to procure the complete structural testing equipment for performing live load tests on short to medium span Railway Bridge
for static load, low speed, full sectional speed i.e. upto 200 kmph, braking and acceleration of trains so as to complete the full test for load rating of bridge.