APPLICATION, MAINTENANCE and PRECAUTIONS

ISSUES TO BE TAKEN INTO CONSIDERATION IN VIBRATOR APPLICATIONS :

• The depth of concrete should not exceed the length of the plunger end. Each concrete layer should be subjected to vibration.   
• Vibrator must be plunged into the fresh concrete at a right angle (in vertical axis it may bend maximum 10 degrees),  
• The plunger end of the vibrator should be plunged in the fresh concrete at complete depth, even plunged so that it reaches and plunges a few cm into the former layer.  
• Generally, the distance between the plunges of the vibrator end is approximately 40 - 50 cm and should not exceed 75 cm.
• The plunger end should not be held near the side surfaces of the formwork.  
• The plunger end should be plunged into the concrete quickly and should not be held in concrete neither too short nor too long, (normal duration is 5 - 15 seconds), and the plunger end should be taken out of concrete slowly (5 - 10 cm / second).  
• The plunger end should not touch the outfits in concrete.
• Vibration process should not be applied to highly fluid concretes.

STRAIGHTENING THE SURFACE OF FRESH CONCRETE

To attain the required evenness on the hardened concrete surface, the surface straightening process should be materialised right after the consolidation of the fresh concrete, in other words it has to be applied just before concrete hardens to attain the sufficient resistance and expected performance.  

1. First straightening: The first straightening process is carried our so as to get rid of the bulges and cavities on the surface of the concrete and reach to an evenness that had been placed in the formwork and consolidated.  For this purpose templates with smooth surfaces are used.  

2. Final straightening: This is done to bring the surface of concrete that has been brought to an even level at the end of the first straightening, to a complete evenness. During the final straightening floating method process is carried out by using a wooden tool with a very smooth base, with 10-12 cm width and 30 cm length, by lightly applying pressure to the surface of concrete and floating over the surface the grainy parts and thin cracks on the surface of the concrete are cured.  

In the straightening process by the floating method, machines that touch the concrete for evenness with their rotating disks are also used. 

But, for a smoother and even surface straightening with a steel trowel is required. This process is applied for a smoother and crackles surface short after the surface has been straightened by the floating process. To maintain a very smooth and even concrete surface the final straightening processes, such as floating and trowelling should start right after the first straightening process.  

To start the final straightening process the perspiration of the concrete has to come almost to an end, no free water should be left on the surface and the concrete must be hardened at a certain extent.  

Determination of the time for the final straightening is carried out as:

• There should be no water at the surface of concrete; the concrete should not have lost the surface glaze.  
• The surface of the concrete should have hardened enough to carry a man's weight (a step should leave a footprint with maximum 3-5 mm depth).

CONCRETE CURE - MAINTENANCE

Especially in the first days, to keep enough water and heat in the concrete to make hydration and preservation of this environment is referred to as, "Cure of Concrete or Maintenance of Concrete".  

Evaporation is the main reason for the reduction of the water in concrete that has been dumped in its place. Speed of evaporation is related to the relative humidity in the air, outside temperature, heat of the concrete and the speed of the wind.   

After the dumping and straightening processes the surfaces of the concrete that are in direct touch with air should be preserved and kept damp for the possible longest time right after they reach the state to weigh a man load. Curing blankets and concrete covers are suitable for the purpose. Besides, preservation can be attained by spraying the liquid curing material all over the surface short after trowelling.  
In case the preservation processes are not applied properly contraction stretching constitute on the surface of concrete and since the concrete has not yet reached the strength to resist these tensions and the concrete cracks. The cracks that are called plastic contraction cracks are in a map appearance, scattered all around but nor in continuous forms.