JPH0220773B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for constructing underwater concrete.

When constructing a concrete structure underwater or underground, if it is not possible to drain the water in the excavation hole, well or temporary frame, concrete should be poured directly into the water using a tremie pipe or special concrete bucket. This is often done by pouring concrete and is called underwater concrete. In particular, when constructing underground continuous walls and cast-in-place piles, which are increasingly being constructed in urban areas, concrete is poured into the bottom of the hole using a tremie pipe, etc., and bentonite slurry is used as a stabilizing liquid for the hole wall during excavation. It is not possible to construct homogeneous concrete underground walls and concrete piles unless the muddy water is completely replaced with concrete.

For this purpose, the conventional practice has been to pour concrete with a rich mix and good fluidity into the bottom of the hole and push the muddy water and slime upwards during construction. However, in order to improve the fluidity of concrete by mixing it, the water/cement ratio, that is, the ratio of the amount of mixing water to the amount of cement, must be increased, and each of the water, cement, and aggregate that make up concrete must be increased. It has a strong tendency to separate, especially if the concrete is moved in water, resulting in complete separation and a significant reduction in strength after hardening.

The reason why concrete separates in this way is thought to be because excess water causes a breathing phenomenon when the water/cement ratio increases during mixing, and fluidity is particularly important for underwater concrete construction. Frequently, there is a need for concrete that does not cause bleeding.

The present invention has been made to solve the above-mentioned problems, and provides a construction method using underwater concrete that has very good fluidity and does not cause separation.

This will be explained below using examples.

A portion of the water in the designed amount of mortar or concrete is added to the cement in advance and stirred to form the cement into a ball, and then the ball-shaped cement is subjected to forced vibration using a high-frequency vibrator. The cement paste obtained by adding the remaining amount of water in the designed mixture and stirring again has extremely fluidity compared to cement paste made by conventional conventional methods. It has a high viscosity and produces almost no separated water. By mixing this cement paste with sand or sand and gravel, high-strength mortar and concrete can be manufactured. It is related to the method of pouring in water using.

In general, if you add water equivalent to 55% of the weight of Portland cement, stir well, and leave it in a glass cylinder, you will soon observe that the cement paste settles somewhat and that separated water collects at the top. I can do it. Leave the cylinder as it is for 5-6
Since the settled cement paste will solidify if left to stand for a period of time, the separated water can be easily taken out by tilting the cylinder. Such separated water is
The same problem occurs in mortar and concrete made by mixing cement paste with sand or sand and gravel, and it has a great effect on the quality of the mortar and concrete.

When we prepare a cement paste with a water/cement ratio of 55% without adding an admixture to ordinary Portland cement and measure the rate of formation of separated water, that is, the breathing rate, we find that breathing reaches 7 to 8% of the paste volume. I did.

However, according to the method of the present invention, water equivalent to 25% of the weight of the cement is first added and stirred, then high-frequency vibration is applied using a vibrator or the like to cause liquefaction, and the remaining water is then stirred. When a cement paste with a water/cement ratio of 55% was prepared by adding water and stirring again, and the breathing was measured in the same manner, the breathing rate decreased to 1 to 2%. Furthermore, when the cement paste prepared by the method of the present invention is mixed with a suitable amount of sand and gravel, it is possible to produce concrete with much less breathing and better fluidity than concrete produced by conventional methods. was completed.

In the case of underwater concrete, the common method of constructing concrete is to install tremie pipes inside an excavated hole or a temporary frame underwater, and the number of tremie pipes to be installed is limited for technical reasons. Unless the concrete has good fluidity, it will not be possible to insert reinforcing bars or the like, or replace it with muddy water such as bentonite, to reach every corner of the hole or frame, and to form dense concrete.

In addition, when constructing underwater concrete, the first concrete poured underwater will trap slime and muddy water, so new concrete that is poured successively later is used to remove slime and muddy water from the first concrete poured into the water. The work is carried out on the principle that high-quality concrete is formed by pushing up old concrete that has deteriorated due to muddy water.

However, in reality, due to the fluidity of concrete, the construction method, or obstacles such as inserted reinforcing bar cages, the deteriorated concrete that was poured first is wrapped up in the subsequent concrete, causing localized or Overall, this will reduce the quality of the constructed concrete structure.

For this reason, underwater concrete is required to have good fluidity, a viscosity that does not involve slime or muddy water when moving underwater, and physical properties that do not cause breathing or separation.

According to the examples, it was found that according to the method of the present invention, the breathing rate, which is an indicator of concrete separation, was reduced to 1/5 to 1/6 compared to the case of ordinary concrete. The fluidity has also improved significantly, making it possible to construct underwater concrete of much higher quality than before.

Figure 1 shows the results of a mortar experiment conducted to investigate the situation in which the first concrete placed underwater is pushed up by the subsequently placed concrete. , it can be seen that it is being pushed up very well due to its good liquidity.

Figure 2 shows the water/cement ratio 55 obtained by the method of the present invention.
% concrete, the slump value of the mixed concrete changes greatly depending on the water/cement ratio of the cement paste during vibration, and the water/cement ratio is 0.25. The paste subjected to vibration had the best fluidity and the lowest slump. As can be seen from the breathing diagram in Figure 3, this
This means that by shaking a cement paste with a low cement ratio, a highly viscous concrete that is difficult to separate even in water is formed, and the underwater concrete constructed by the method of the present invention does not mix well with slime or muddy water. By replacing the concrete, we were able to obtain homogeneous concrete with high strength.

Figure 4 shows that a cylinder with a diameter of 60 cm is filled with bentonite mud water, and ordinary underwater concrete and underwater concrete of the present invention are poured to a depth of 70 cm each through a 10 cm diameter pipe using the tremie pipe method. This is a comparison of the physical properties of cores taken in the horizontal direction. The specimen number is φ10×10 core.
These numbers are assigned in order of the depth of the sample taken when formatted to cm, and these data are the average values of the specimen data taken from two cores. As is clear from this figure, the underwater concrete cast by the method of the present invention has a higher density than the underwater concrete cast by the conventional method, and has a compressive strength on the 10th day after casting. It can be seen that the degree of deterioration in the bottom and top concrete, which tends to deteriorate during ordinary underwater concrete construction, is extremely small.

[Brief explanation of drawings]

Fig. 1 is a mortar experimental diagram, Fig. 2 is a graph showing the slump of concrete according to the present invention, and Fig. 3 is a graph showing the slump of concrete according to the present invention.
The figure is a breathing diagram, and FIG. 4 is a comparison diagram between the conventional method and the method of the present invention.

Claims (1)

[Claims] 1 Within the water/cement ratio range of the specified design mix for underwater concrete, Portland cement is stirred with 25% or more of the weight of the cement and subjected to high-frequency vibrations to cause liquefaction. The remaining amount of water was added to this, stirred again, mixed with the designed amount of sand and gravel, and other admixtures, and then submerged in water in the borehole or underwater using a tremie pipe, etc. An underwater concrete construction method characterized by pouring concrete into a well or temporary frame.