Main technical difficulties of crack control measures

1. Exterior wall and slab structures do not have expansion joints and have a thick protective layer, so concrete must meet early and late cracking requirements.
2. The base slab and pile abutment are large in thickness and pouring volume, which is a large volume concrete, so it is necessary to solve the problems of high heat of hydration, water secretion and easy construction cold joints of large volume concrete.
3. The construction is carried out in May and June, the concrete is poured under higher ambient temperature, which makes the structure have higher starting point of cooling deformation, and often accompanied by strong wind, the surface air drying significant.
4. During the construction period, the material properties of the concrete parameters in the development and change, the ambient temperature changes a certain degree of randomness, the construction and curing process inevitably occur irregularities, the effect of these factors superimposed on the short-term difficult to quantitative analysis, so that the value and distribution of the tensile stress in the structure is uncertain, increasing the difficulty of design and construction to take joint countermeasures.
5. At present, it is impossible to determine the specific part, time and maximum width of structural cracks by theoretical calculation, but only by semi-theoretical, semi-empirical method or more relying on the empirical method, combining with the measured results in the field, under the ideal assumptions, to obtain the roughly accurate presumed results.
6. The spacing of reserved expansion joints and after-pouring strips of concrete structures is significantly larger than the current design specifications, which greatly reduces the actual effect of these structural measures to control concrete forming and cooling shrinkage cracks.
7. Due to the adjustment of the structural scheme leading to the modification of the construction drawings, it is difficult to obtain the detailed calculation and analysis data from the designer in time during the preliminary construction technical preparation work.

Main technical measures for crack control

Design measures

1. Reduction or relaxation of the confinement of concrete shrinkage deformation by the use of post-cast zones and control joints.
2. To channel or disperse shrinkage deformation of the concrete by means of post-casting strips and control joints.
3. Combine with calculation and analysis, increase the anti-cracking reinforcement appropriately; change the guide angle of the bearing where stress is concentrated from right angle to slope shape.

Technical measures for the design of raw materials and compound ratio

 

1. Raw materials

Cement The lower alkali content and early heat of hydration are used.

Additive High efficient water reducing agent for slow setting, with high water reduction, micro air attraction, plastic preservation, etc., has obvious effect on controlling the time loss of concrete slump, pumping construction, delaying the appearance of peak hydration heat and strength development. In order to compensate for the shrinkage of concrete, it is mixed with the right amount of expansion agent.

Aggregate Coarse aggregate is Sanhe 5-25mm mountain crushed stone, with mud content less than 1.0%, needle content less than 10%, water absorption less than 2.0%, and no chlorine ion content detected in the submitted crushed stone. The fine aggregate is Chengde sand with fineness modulus of 2.6-2.8 and mud content less than 2.0%.

Mixing material is a finely ground composite powder (78% slag powder + 22% fly ash), which is a new industrial product with stable and reliable quality and can play the effect of 1+1>2, and can significantly improve the construction performance of concrete.

2. Concrete proportioning and related properties

According to the above construction requirements and design principles, a lot of trial mixing research work has been carried out to determine the proportion from the following angles, with a view to being applied in actual projects and finalizing the proportion.

(1). New industrial products with large admixtures were used instead of cement to improve concrete durability, later strength and reduce the heat of hydration.

(2). Adopt 60-day age strength as the strength assessment index to better reduce the amount of cement and reduce the heat of hydration and shrinkage in the early stage.

(3). Expansion agents are added to compensate for the shrinkage of concrete.

(4). Extend the setting time of the concrete by admixture adjustment to delay the peak of heat of hydration.

In the actual application, the slump of the concrete was controlled at 180±15mm and the extension was 400-460mm, the concrete did not secrete water and segregation, and the cohesion and fluidity were good, which met the requirements of pumping construction.

Technical measures in construction

1. Crack control measures during construction

(1). Choose a more appropriate temperature for pouring bulk concrete, and avoid construction during hot noon and windy weather as much as possible.

(2). Reasonable arrangement of the construction process, control the uniform rise of concrete in the pouring process, to avoid the accumulation of concrete mixes with excessive heights.

(3). Adopt layered pouring of mass concrete to relax the degree of restraint, reduce the heat storage of each pouring length, prevent the accumulation of heat of hydration and reduce temperature stress.

(4). Strengthen concrete tamping to improve the compactness and tensile strength of concrete, reduce expansion and deformation, and ensure construction quality.

2. Crack control measures during maintenance

(1). To avoid early shrinkage and cracking of concrete, when constructing under high temperature, dry and strong wind conditions, the concrete moisture conservation should be started immediately after the first troweling, and the second troweling.

(2). According to practical experience, the first three days to adopt water storage curing concrete, and cover the surface of 1 ~ 2 layers of straw bags (depending on the thickness of the straw bag and watering interval may be), keeping the straw bag wet maintenance methods.

(3). Take 14 days long time maintenance, delay the cooling time and speed, give full play to the concrete stress relaxation effect.

4. Concrete temperature change law

The actual temperature of the concrete is measured by the pre-buried thermometer tube, and the time of temperature measurement is: every 2 hours from day 1 to day 4 after pouring, every 4 hours from day 5 to day 9, every 6 hours from day 10 to day 14, and every 12 hours from day 15 to day 28, see Figure 1 for concrete internal temperature rise and fall (the average ambient temperature is 33℃).

The temperature values displayed by the concrete pyrometer and the temperature map show that.

(1). The temperature of concrete entering into the mold is 22～25℃, the temperature rise in the center of concrete is 50～55℃, and the maximum temperature rise time of concrete is 48～96 hours.

(2). The maximum temperature difference between the center of the concrete and the surface is 24℃, which effectively controls the rate of heat dissipation and avoids temperature cracks due to the water storage curing and other measures. There are no harmful cracks later on.

Although the initial effect is not very satisfactory, but later by adjusting the concrete setting time, the peak of the heat of hydration of concrete is delayed and reduced, and temperature cracks are inhibited, and the expected effect is obtained. It has been proved that the 60-day strength was used as the evaluation criterion in the large volume concrete project, which is beneficial to the regulation of the heat of hydration and plays a leading role in the crack control of concrete.

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