Question

1. How do different long term climatic conditions (including air temperature, humidity,
solar radiation, precipitation) affect the performance of concrete, steel and timber?
2. Discuss the formation of different types of cracking in concrete. What will be the potential problems if these cracks are not repaired? Discuss the methods to prevent and fix the cracks.
3. There are two parts below:
a. What is difference between Minor Destructive Test (MDT) and Non Destructive Test (NDT) of concrete? In which situation, NDT will be preferred for assessing the concrete?
b. In order to assess the conditions of slabs and columns of this 30 years old reinforced concrete structure, give your suggestions for (i) assessment methods (ii) testing parameters

Answer 1

Hope it helps.

1- Air temperature, humidity, solar radiation and precipitation affects the construction units including steel, concrete and timber.

--Concrete expands and contracts due to the air temperature change. Change in humidity will change the pore size and radiation will affect the coloration which is change in tone or color of concrete. Swelling or bleeding of concrete will caused by the water cement ratio change (happens due to precipitation).

-- Steel get the problem of corrosion due to precipitation and humidity. Temperarure rise and radiation will cause expansion.

-- Timber get the problem of fungi attack caused due to increase in moisture content i.e humidity or precipitation. And solar radiation will create cracks in timber due to decrease in sap water level.

2-Types of cracks in concrete are -

- Due to increase in temperature water level in concrete get reduced which creates a crack termed as plastic shrinkage crack.

- Excess of heat expands concrete which may oppose by the obstacles like brick which doesn't absorb the energy so the expansion energy strick back the concrete and creates crack known as expansion concrete crack.

-In the cold weather, freezing effect takes place in the ground concrete. The cycle of freezing and thawing creates a crack known as heavy concrete crack.

- Sometimes compaction on a ground before concreting not done accordingly, in this case, after concreting , settling concrete crack develops

- Cracks also developed in concrete due to heavy load. In the rain or snowfall, concrete become soft and can't bear the weight of heavy loads and get cracks.

-Top layer of concrete loss water and get minor cracks known as crazing cracks.

The strength and durability of concrete affected by the presence of cracks. So it become very important to prevent cracks on concrete. The aesthetic of a structure also ruin due to the presence of crack.

To prevent cracks we should -

Make proper mix design and follow it accordingly.

Should provide proper curing for atleast 7 days. The method of compaction, use of steel and minimum cover should be used according to coubtry codes.

Stitching, dry packing, vacuum impregnation, bandage etc are some methods to repair a cracks.

3-A-The difference between destructive and non destructive test are-

Destructive test are the direct while non destructive test are indirect significance in service.

Destructive test are quantitative measure of loads and NDT do not measure failure's load.

Destructive test works as a separate unit i.e. work in fraction while NDT take work as a single unit.

Minor and non destructive testing are same... The question should be.. Difference between destructive or partially destructive testing and NDT. NDT are generally preferred because in this method of test, we obtain compressive strength and concrete's property. It gives the actual value of strength and properties of concrete.

B- To assess the condition of slab and column of 30 year old RC structure. Visual observation and documentation should be use for assessment which study the condition of slab and column and test for the cracks present in it. The presence of voids and honeycomb causes damge in slab and column. The test parameter for such structure can be done by ultasonic pulse velocity test which test wave of 54 kHz frequency, core sampling and testing and carbonation and chloride content test which checks for the corrosion level in reinforcement.