|
|
| Maintaining concrete in summer |
|
Quality of buildings are always the most concern of householders. In there, quality of concrete is a key factor bulding a “strong” house. Another factor which is very important is the concrete maintenance. After casting hardly, workmen are usually permitted to take a rest. Therefore, concete maintenance is often passed, which is really dangerous, especially under strong sun. Concrete is sucked out of water before natural solidification, which generates cracks and decreases quality of concrete.
|
|
1. Important of producing mortar, concrete
– Mixing mortar and concrete has to be carried out based on the mixing proportion of producer or the mixing proportion calculated and selected by construction professionals.
– The rate of water has to used enough to ensure the workability of mortar
-
If the water to cement ratio is high, the setting time and intensity of mortar and concrete will reducce, which makes shrinkage phenomenon on surface. As a result, cracks will appear.
-
If the water to cement ratio is low, the quality of mortar and concrete will be affected, which makes difficulties during construction.
-
In the process of construction, it is necessary to control the water/cement ratio carefully in order not to excess 0.55 ( 1 bag 50 kgms x 0.55 = 27.7 ltrs water)
– Mixing mortar, concrete by mixer is the best way to be sure of homogene, mixing time and quality of mortar and concrete.
– In the mortar mixing process, water has to be poured into mixture after mixing dry sand and cement in such a way that mixing color is sample. Besides, mixing has to be sure of mortar’s necessary workability.
– Time from mixing with water to finishing construction mustn’t to be over 60 minutes. After mixing with water, it mustn’t to spread use of mortar and concrete. It had best mix dry mortar and mix with water when starting building.
– At the same mixing proportion, using PCB40 will create stronger concrete than using PCB30.
– According to TCVN 6260:2009, compression strength after three –day time and and twenty-eight-day time respectively as below
Cement PCB30:
After 3 days ≥ 14 N/mm2
After 28 days ≥ 30 N/mm2
Cement PCB40:
After 3 days ≥ 18 N/mm2
After 28 days ≥ 40 N/mm2
Therefore, it should use PCB40 in the important items, which is better than use PCB30. Now, some areas still use PCB30 to change roof, beam …. It is alright. However, as a cement producer, we recommend that customer should use PCB40.
|
|
2. Important of maintaining concrete
– Concrete only reach the best quality when it solidifies in a wet and non impact environment. So after being poured, concrete has to be keep wet as long as possible. No matter how a little dry, even look like solidified, concrete surface is, solidification continue to occur inside to achieve max concrete strength.
– With hot weather at temperature of above 40 degree celsius, water in concrete quickly evaporate, the remaining amount of water is not enough for solidification process. Concrete strength can stop expanding, leading to crackings.
– We can keep wet for concrete by many ways as below:
|
|
2.1 Covering with a thin layer of nylon
– This maintenance is very important for the construction of concrete in hot summer conditions. After the concrete was done the final work to finish the surface of the concrete (rubbing face), it is immeditely covered with a thin layer of nylon on the concrete surface in order to keep the water in the first stage of the curing process.
|
|
|
|
2.2. Keep the formwork in place
– A simple maintenance is to keep the formwork constant, not dismantled. Formwork has the effect of maintaining moisture well. At the same time, it can directly spray water into formwork to enhance moisture content. Concrete surface exposed from formwork need to be protected against moisture loss with coverings. Formwork must be watered fully. If the weather is hot, concrete needs to be maintained continuously within the first week.
|
|
2.3 Spraying water and soaking in the water to keep moisture
– Spraying water into wood formwork is the most effective way of moisturizing. It is noted to spray evenly, not to leave any dry area because if wood formwork is not sprayed evenly, surface will appear cracks and spray small jet of water continuously.
– When concreting the floor, the roof has a smooth surface, it is possible to build brick walls for water immersion.
Note: + After 2 hours from pouring concrete, we carry out maintenance by directly watering on the plastic film covered on concrete surface. It is noted to watering once with low pressure, not watering under plastic film.
+ After 24 hours, pumping water to submerge the surface of the concrete or watering every 3 hours, at least once at night for the concrete area without edge. Maintaining the amount of water at least for the first 7 days after construction.
– We can go on concrete floor after 1.5 days in summer and is 3 days in winter.
|
|
|
|
2.4 When can formwork be removed
– Formwork can only be removed when the concrete structure reaches the material strengtht stabilizing the structure. In common, people remove formwork after pouring from three to four weeks in normal conditions (20 degrees Celsius – 30 degrees Celsius). However, it should be kept as long as possible.
– In many cases, because of removing the formwork before the prescribed time, the structure is collapsed, which causes serious accidents. After removing the formwork, it should be noted that the concrete only reaches its own weight bearing capacity. At least after 28 days to withstand the weight of other fixtures.
|
|
|
