KR102472007B1 - Crack-proofing method of mass concrete by using hydrated heat and mass concrete thereof - Google Patents

Abstract

The present invention discloses a crack prevention method of mass concrete using heat of hydration and a mass concrete by the method.
According to the present invention, a method for preventing cracks in mass concrete using calorific value of hydration and mass concrete thereby reduces the difference in calorific value of hydration by placing cement with low calorific value in the lower layer and cement with high calorific value in the upper layer. However, this reduces the calorific value difference between the upper and lower parts of the concrete to prevent temperature cracks, and the rapid development of strength in the upper layer concrete speeds up the follow-up process. Depending on the use, material costs are saved and cracks are prevented to reduce repair costs and secure durability. can help

Description

Crack-proofing method of mass concrete by using hydrated heat and mass concrete thereof}

The present invention relates to a method for preventing cracks in mass concrete using hydration calorific value and mass concrete thereby, and more particularly, to prevent temperature cracking by reducing the calorific value difference between the upper and lower parts of concrete, and to rapidly develop the strength of the upper layer concrete so that subsequent processes can be performed. It relates to a method for preventing cracks in mass concrete using heat of hydration, which is helpful in saving material costs, preventing cracks, and securing durability, depending on use, and mass concrete by the same.

In general, as urban buildings become larger, taller, and more in-depth due to the rise in land prices and the development of construction technology, in the case of building foundation mat concrete, construction with mass concrete is inevitable.

However, mass concrete with a thickness of 800 mm or more causes deterioration in the quality of concrete such as cracks if it does not properly cope with the temperature stress generated by the heat of hydration.

Currently, mass concrete construction for building foundations in Korea is divided into two or more floors for reasons such as prevention of settlement cracks.

Therefore, since the upper concrete is poured at the time when the hydration heat of the lower concrete is activated, the temperature difference between the upper and lower concrete becomes very large from this point on, and tensile stress due to the heat of hydration is generated on the upper surface from about 10 hours after the initial casting, and eventually Problems appearing as tensile cracks occur.

Existing methods for solving this problem have been proposed as shown in Table 1 below.

Measures specific measures combination Reduction of calorific value Use of low-heat cement Reduction of cement amount Use of good quality blending materials make the slump small increase the aggregate size Use of good quality aggregate Extension of strength judgment period city ball Minimize temperature change Curing temperature control Insulation (sheet, insulator) heating and curing Adjustment of formwork retention period Adjustment of concrete casting time interval Condensation time control for each lift by using super retardant Reduce temperature rise during construction cooling of materials Strictly manage the planned temperature design design considerations Installation of crack inducing joints Distributing cracks with rebar separate waterproof reinforcement

That is, in order to prevent cracks caused by the temperature difference between the upper and lower concrete placed at different times during the conventional mass concrete pouring, the curing temperature is controlled, the heat-retaining heat curing is performed such as sheet insulation, the form holding period is controlled, and the concrete is poured. A method of giving physical change by adjusting the time interval was generally used (Refer to Korean Patent Publication No. 2011-0058935, etc.), but the problem of worsening workability still remained.

Therefore, the technical problem to be solved by the present invention is to prevent temperature cracking by reducing the calorific value difference between the upper and lower parts of the mass concrete, to speed up the follow-up process by rapidly developing the strength of the upper layer concrete, and to save material costs and prevent cracks depending on use. It is to provide a method for preventing cracks in mass concrete using the calorific value of hydration, which is helpful in reducing repair costs and securing durability, and mass concrete thereby.

In order to solve the above-described technical problem, the present invention reduces the difference in hydration calorific value by placing cement with low calorific value on the lower layer and placing cement with high calorific value in the upper layer. Cracking of mass concrete using calorific value of hydration Provides a way to prevent it.

According to another embodiment of the present invention, the low calorific value cement may be medium heat cement, low heat cement, fly ash cement, blast furnace slag cement or low heat mixed cement.

According to another embodiment of the present invention, the high calorific value cement may be Portland cement, semi-crude cement, or early-crude cement.

According to another embodiment of the present invention, the difference in setting time may be reduced when placing the cement with low calorific value and the cement with high calorific value.

According to another embodiment of the present invention, an upper layer of cement having a high calorific value may be placed on the surface of the lower layer following the lower layer of cement having a low calorific value.

According to another embodiment of the present invention, cracking of the mass concrete may be prevented by reducing the temperature difference between the interface between the surface of the lower layer and the upper layer and the surface of the upper layer.

In addition, the present invention provides mass concrete in which cracks are prevented by the above-described crack prevention method in order to solve the above-mentioned other technical problems.

According to the crack prevention method of mass concrete using the heat of hydration according to the present invention and the mass concrete thereby, temperature cracking is prevented by reducing the calorific value difference between the upper and lower parts of the concrete, and the follow-up process is faster due to the rapid development of strength of the upper concrete layer. Depending on the method, it can help save material cost, prevent cracks, reduce maintenance costs, and secure durability.

1 is a graph showing the temperature change over time when the mass concrete pouring method of the present invention and a general, low-heat mixing method are applied,
2 is a perspective view of a formwork for pouring concrete according to an embodiment according to the present invention;
Figure 3 is a picture showing the location of the temperature sensor and the concrete hardened by the formwork of Figure 2,
4 is a graph showing the temperature difference between the lower layer and the upper layer of concrete according to a comparative example;
5 is a graph showing a temperature difference between a lower layer and an upper layer of concrete according to an embodiment according to the present invention.

Advantages and characteristics of the present invention, and methods for achieving them will become clear with reference to the embodiments described later in detail in conjunction with the accompanying drawings, but the present invention is not limited to the embodiments disclosed below, but different It will be implemented in various forms, but the present embodiments are provided to complete the disclosure of the present invention and to completely inform those skilled in the art of the scope of the invention to which the present invention belongs, is defined by the recitation of the claims.

Meanwhile, terms used in this specification are for describing the embodiments and are not intended to limit the present invention. In this specification, singular forms also include plural forms unless specifically stated otherwise in a phrase. As used herein, "comprises" or "comprising" means the presence or absence of one or more other elements, steps, operations and/or elements other than the recited elements, steps, operations and/or elements; Additions are not excluded.

1 is a graph showing the temperature change over time when the mass concrete pouring method of the present invention and a general, low-heat mixing method are applied, FIG. 2 is a perspective view of a formwork for placing concrete according to an embodiment according to the present invention, FIG. is a figure showing the location of the temperature sensor and the concrete hardened by the formwork of FIG. 2, FIG. 4 is a graph showing the temperature difference between the lower layer and the upper layer of concrete according to the comparative example, and FIG. 5 is an embodiment according to the present invention It is a graph showing the temperature difference between the lower layer and the upper layer of concrete, refer to this.

The crack prevention method of mass concrete using the calorific value of hydration according to the present invention is characterized in that cement having a low calorific value is placed on the lower layer and cement having a high calorific value is placed on the upper layer to reduce the difference in calorific value of hydration.

The mass concrete is an integral structure that is usually cast with a thickness of 800 mm or more, and usually concrete (2 in FIG. 1. Low heat-generating mixing method) is mainly used by substituting a mineral admixture for one type of ordinary cement, but the second in FIG. In the low-heat mixing method, the temperature difference due to the difference in casting time between the upper and lower parts and the temperature difference between the upper part and the upper part still occur when the lower temperature is the highest, causing cracks in the mass concrete.

In the present invention, this mass concrete is divided into two layers, a lower layer and an upper layer.

First, the concrete poured into the lower layer is cement with a low calorific value, and the concrete poured into the upper layer is cement with a high calorific value, through which the difference in heat of hydration between the upper and lower layers is reduced and reduced, eventually preventing cracking of the mass concrete structure. You can do it.

The low calorific value cement may be used individually or in combination with medium heat cement, low heat cement, fly ash cement, blast furnace slag cement or low calorific value mixed cement, and these cements with low calorific value are shown in Tables 2 and 3 below It can be specified as a cement having the following properties.

Item fly ash cement blast furnace slag cement low heat mixed cement characteristic characteristic characteristic Specific surface area (cm2/g) Over 2,500 Over 3,300 Over 3,300 stability
(autoclave expansion degree) ±0.5 or less 0.2 or less 0.8 or less congelation
hour Initial (minute) over 60 60 over 60 closure (hours) below 10 10 below 10 compression
burglar
(MPa) 3 days 7.5 or higher 7.5 or higher 10.0+ 7 days 15.0+ 15.0+ 20.0+ 28 days 32.5+ 40.0 or higher 40.0 or higher chemistry
ingredient
(%) SO ₃ 3.0 or lower 4.5 or lower 3.5 or less MgO 5.0 or lower 6.0 or lower 5.0 or lower Ignition loss (%) 5.0 or lower 3.0 or lower 3.0 or lower

Item medium heat cement low heat cement characteristic characteristic density 3.00 or higher 3.00 or higher Specific surface area (cm2/g) Over 2,800 Over 2,800 stability
(autoclave expansion degree) less than 0.8 less than 0.8 congelation
hour Initial (minute) 60 awards over 60 closure (hours) below 10 below 10 compression
burglar
(MPa) 3 days 7.5 or higher - 7 days 15.0+ 7.5 or higher 28 days 32.5+ 22.5+ 91 days - 42.5 or higher chemistry
ingredient
(%) SO3 3.0 or lower 3.5 or less MgO 5.0 or lower 5.0 or lower Ignition loss (%) 5.0 or lower 5.0 or lower

In addition, after the low calorific value cement is poured, the high calorific value cement placed on top may be Portland cement, semi-crude strong cement or early strong cement, and these cements with high calorific value are characterized as shown in Table 4 below, despite their names. It can be specified as a cement having

Item Ordinary Portland
cement early steel cement semi-crude cement characteristic characteristic characteristic density 3.00 or higher 3.00 or higher 3.00 or higher Specific surface area (cm2/g) Over 2,800 Over 3,300 Over 4,500 stability
(autoclave expansion degree) less than 0.8 less than 0.8 less than 0.8 congelation
hour Initial (minute) 60 awards over 45 over 60 closure (hours) below 10 below 10 below 10 compression
burglar
(MPa) 1 day - 10.0 or higher - 3 days 12.5 or higher 20.0 or higher 12.5+ 7 days 22.5 or higher 32.5 or higher 22.5+ 28 days 42.5 or higher 47.5 or higher 42.5 or higher chemistry
ingredient
(%) SO3 3.5 or less 4.5 or lower 3.5 or less MgO 5.0 or lower 5.0 or lower 5.0 or lower Ignition loss (%) 5.0 or lower 5.0 or lower 5.0 or lower

On the other hand, the fine aggregate and coarse aggregate in which the cement with low calorific value and the cement with high calorific value are mixed may use 100 to 300 parts by weight of fine aggregate and 200 to 400 parts by weight of coarse aggregate based on 100 parts by weight of cement. If it is less than 300 parts by weight, there may be a problem of material separation of concrete due to insufficient fine aggregate, and if it exceeds 300 parts by weight, there may be a problem of poor workability due to reduced fluidity due to increased viscosity. Also, if coarse aggregate is 200 parts by weight If it is less than part, there may be a problem of poor durability due to increased drying shrinkage, and conversely, if it exceeds 400 parts by weight, watertightness and filling may be deteriorated, resulting in poor material separation and finish.

In addition, S / a (%) is 40 to 60, and the high-performance AE water reducing agent / C may be 0.2 to 3%. If the S / a (%) is less than 40, the amount of fine aggregate may be insufficient and material separation may occur, and S If /a(%) exceeds 60, there are many fine aggregates, and fluidity may decrease due to viscosity increase.
Here, S of S/a (%) represents sand (indicated by the initial 'S' in Sand) and a represents coarse aggregate (indicated by the initial 'a' in aggregate).

In addition, if the high-performance AE water reducing agent / C is less than 0.2%, workability and durability may be deteriorated due to insufficient fluidity and AE air amount, and if it exceeds 3%, due to excessive addition, material separation and a large amount of voids, strength and durability this can be degraded
Here, C in the high-performance AE water reducing agent / C means cement (indicated by the initial 'C' in cement).

In addition, there is a feature that the difference in setting time is reduced when the cement with a low calorific value and the cement with a high calorific value are placed. Following the lower layer of the cement with a low calorific value, the upper layer is poured with cement with a high calorific value on the surface of the lower layer, Mass concrete has a thickness of 800 mm or more, and at most sites, concrete of 800 mm or more cannot be poured at once to prevent subsidence cracking. ) height, and the remaining 15 to 50% of the upper concrete is poured. At this time, as can be seen in Figure 1, a difference in casting time occurs between the upper and lower concrete depending on the applied method (1. General method) and (2. In the case of the low heat mixing method), since the upper and lower concrete are the same mixture, the lower concrete is poured first and condensation starts first, and the upper concrete is poured after 2 to 10 hours (it varies from site to site, but it is a normal value), so it takes 2 to 10 hours As condensation starts later, a difference in condensation time due to a difference in casting time between the upper and lower concrete is inevitably generated, causing a temperature difference at the beginning of casting, which causes cracks. cracks may become more severe.

However, in the present invention, concrete is poured using cement with low calorific value and delayed setting time for the lower concrete to be cast first, and cement with high calorific value and fast setting time is used for concrete to be cast later, By reducing the difference in setting time caused by the difference in pouring time, the upper and lower concrete can generate hydration heat at the same time without a temperature difference, and even when the temperature of the lower concrete is the highest, the temperature difference between the upper concrete can also be reduced. By using it, the overall heat of hydration can be lowered.

Therefore, the temperature difference between the interface between the surface of the lower layer and the upper layer and the surface of the upper layer is reduced, and cracking of mass concrete can be prevented by reducing tensile stress due to the decrease in temperature difference.

Examples and Comparative Examples

A mass concrete sample was prepared as an example by pouring the comparative examples according to Tables 5 and 6 below and the concrete mixture according to the present invention into a die.

combination
matters W/B(%) One 45 Slump (mm) One 150±25 Air volume (%) One 4.5±1.5 sample Size (mm) 2 300×300×400 (two-stage casting) pouring method 10 2nd stage
pouring upper floor
(200mm) lower floor
(200mm) comparative example plain portland cement Example plain portland cement fly ash cement Experiment
matters unhardened
concrete 5 - Slump, slump flow
- Air volume, unit volume mass
- condensation time hardened concrete One - Heat of hydration temperature history of simulated member
(upper center, lower center, outside temperature)
- Measurement of compressive strength (standard curing and core specimen) Lap time difference: 4 hours

Combination combination W/B
(%) unit
Quantity
(kg/m ³ ) S/a
(%)
high performance
AE water reducing agent
/C
(%) Mass mix (kg/m ³ ) cement fine aggregate thick
aggregate plain cement 45 175 43 0.40 389 708 984 fly ash cement 0.55 389 692 961

Experimental example

The characteristics of unhardened concrete, compressive strength, and temperature history were evaluated according to Examples and Comparative Examples.

- Characteristics of unhardened concrete

Experimental results including setting time for the Examples are shown in Table 7 below.

Referring to Table 7, the fluidity of the concrete according to the examples was found to be in a range satisfying both the target slump of 150 ± 25 mm, and the slump flow was around 250 mm in proportion to the slump. In addition, the air amount all satisfied the range of 4.5±1.5%, which is the target air amount.

division slump
(mm) slump
flow
(mm) air volume
(%) unit volume
mass
(kg/m ³ ) Condensation time (h) first resolution closing plain cement 170 250 5.0 2301 11.4 13.5 fly ash cement 160 275 5.8 2248 16.0 21.4

In addition, as for the setting characteristics, the completion time of the mixture using ordinary Portland cement was 13.5 hours, and the mixture using fly ash cement was 21.4 hours. Therefore, the difference in setting time between the two mixtures was about 7.9 hours. .

- Compressive strength characteristics

The following <Table 8> shows the compressive strength test results of standard cured specimens, and <Table 9> shows the compressive strength test results of core specimens. Referring to this, in the case of standard cured specimens, when using ordinary Portland cement, It was found that the overall strength was higher than that of fly ash cement, and there was a lot of difference in the initial strength in particular, but it was confirmed that there was almost no difference in the long-term strength of 91 days.

In addition, in the core specimen, the age of 14 days was as low as 3.3 MPa, but the age of 91 days was 1 MPa, and the difference was insignificant. Therefore, when using low-heat cement to reduce cracks in mass concrete, if the management age is maintained at 91 days, quality control is considered to be good for

(Unit: MPa) elapsed time
division 1 day 3 days 7 days 14 days 28 days 91 days plain cement 5.8 20.4 26.6 33.0 35.0 39.0 fly ash cement 0.6 11.2 17.7 23.1 27.7 38.5

(Unit: MPa) elapsed time
division 14 days 28 days 91 days plain cement 35.7 40.0 47.0 fly ash cement 32.4 37.5 46.0

- Temperature history characteristics

As can be seen in FIGS. 4 and 5, in the case of the comparative example, the lower concrete generated hydration heat first, but in the case of the example, hydration heat was gradually generated from the upper concrete to the lower concrete, thereby reducing the probability of crack occurrence.

In addition, the overall heat of hydration could be lowered by 8.3℃, and the rate of heat of hydration was also slowed down, indicating that the overall durability was improved.

In addition, Table 10 below shows the maximum heat of hydration temperature and the time to reach the maximum temperature in Comparative Examples and Examples. In the case of the Comparative Example, the heat of hydration was as high as 58.6 ° C and the time to reach the maximum temperature was very fast at 41 hours, but in the case of Examples has a maximum temperature of 50.3 ° C, which is 8.3 ° C lower than the comparative example, and the arrival time is 64 hours, which is 23 hours longer than the comparative example.

Maximum hydration temperature and time to reach Absence type
division comparative example Example highest temperature
(℃) 58.6 50.3 arrival time
(h) 41 64

Claims (7)

Cement with low calorific value is placed on the lower layer, and cement with high calorific value is placed on the upper layer to reduce the difference in heat of hydration,
Following the placement of the lower layer of cement with a low calorific value, the upper layer is cast with cement with a high calorific value on the surface of the lower layer,
Prevent cracking of mass concrete by reducing the temperature difference between the interface between the surface of the lower layer and the upper layer and the surface of the upper layer,
The low calorific value cement, the fine aggregate mixed with the high calorific value cement, and the coarse aggregate include 100 to 300 parts by weight of fine aggregate and 200 to 400 parts by weight of coarse aggregate based on 100 parts by weight of cement,
S / a (%) is 40 to 60, high-performance AE water reducing agent / C is 0.2 to 3% (where S / a (%) of S / a (%) represents sand, a represents coarse aggregate, and the high-performance AE water reducing agent C in /C stands for cement),
The decrease in the difference in setting time when placing cement with low calorific value and cement with high calorific value is achieved by placing concrete using cement with low calorific value and delayed setting time in the lower concrete that is poured first, and the calorific value in the concrete that is placed later Crack prevention method of mass concrete using heat of hydration, characterized in that it is performed by placing concrete using cement having a high and fast setting time.
According to claim 1,
The cement with low calorific value is medium heat cement, low heat cement, fly ash cement, blast furnace slag cement or low heat mixed cement.
According to claim 1,
Crack prevention method of mass concrete using calorific value of hydration, characterized in that the high calorific value cement is usually Portland cement, semi-crude cement or early-crude cement.
delete delete delete Mass concrete whose cracks are prevented by the crack prevention method according to any one of claims 1 to 3.

Images