Abstract: The quality of highway bridges and culverts and other structures is decisively affected by concrete. Therefore, strengthening the control of concrete quality is the most important part of the construction process. The workability of fresh concrete is an important technical property in concrete quality control, which plays a decisive role in concrete quality. The main factors affecting concrete workability are analyzed, and improvement measures are proposed.

 

Keywords: fresh concrete; influencing factors; improvement measures

 

Before cement concrete sets and hardens, it is called cement concrete mixture. The technological property of fresh concrete is called workability (or ease of handling). The workability of fresh concrete, also known as ease of handling, is the performance of concrete mixture that is easy to construct (mixing, transportation, pouring, vibration) and has uniform and dense quality after molding. In fact, the workability of concrete mixture is a comprehensive technical property, including three aspects: fluidity, cohesiveness, and water retention. Fluidity is the property of concrete mixture that can flow under its own weight or mechanical vibration and fill the formwork. Cohesiveness is the property that the constituent materials of concrete mixture have a certain cohesive force during construction, so as not to produce stratification and segregation. Water retention is to ensure that concrete has a certain water retention capacity during construction, so as not to produce serious bleeding. It can be seen that the quality of fresh concrete depends on the fluidity, cohesiveness, and water retention of concrete. Therefore, construction must ensure that these three aspects meet the specifications and design requirements.

 

I. Problems existing in engineering construction

 

In bridge construction, we can often see or participate in some concrete pouring operations, such as concealed works (bored cast-in-place piles), tie beams between piles (pile caps), pier columns, bent caps, and beam body concrete operations. Usually, cast-in-place construction is adopted. During construction, no matter which pouring process is used, the workability of fresh concrete must be ensured. However, we often find some problems, such as when pouring fresh concrete, the concrete has a problem of large or small fluidity. When the fluidity is large, the construction personnel take measures to return to the site or directly pour without the attention of supervisors; however, when the fluidity is small, the construction personnel take measures to add water to the mixing tank of the concrete transport truck, and then complete the mixing operation by rotating the mixing tank. Their purpose of increasing the fluidity of concrete is to facilitate vibration or pouring. We should stop this situation in time, because their practice seriously affects the workability of concrete, often causing concrete stratification, bleeding, and even segregation. This results in engineering economic losses or engineering quality problems. In particular, attention should be paid to the concrete pouring of some concealed works, and the workability of fresh concrete should be strictly controlled.

 

II. Main factors affecting the workability of fresh concrete

 

1. Influence of water-cement ratio. Under the condition of fixed water consumption, when the water-cement ratio is small (more cement content), the cement paste becomes thicker and the fluidity of the mixture is smaller; if the water-cement ratio is increased (cement content reduced), the cement paste becomes thinner and the fluidity increases, but it will cause the mixture to flow and segregate, affecting the strength of concrete. Therefore, the water-cement ratio should be reasonably selected.

 

2. Admixtures. Adding certain admixtures to the concrete mixture can effectively improve the workability of the concrete mixture without increasing water consumption and cement content.

 

3. Unit water consumption. Practice has proved that the factor that has the greatest impact on slump is still unit water consumption. Increasing water consumption increases fluidity, but after hardening, concrete will have larger pores, thereby reducing the strength and durability of concrete. In addition, too much water will cause stratification and bleeding of fresh concrete, but reduce workability. Therefore, under the condition of ensuring the strength and durability of concrete, the unit water consumption should be determined according to the fluidity of concrete.

 

4. Sand ratio. The sand ratio reflects the relative proportion of coarse and fine aggregates, which allows for the voids and total area of concrete aggregates. When the amount of cement paste is constant, if the sand ratio is too large, the total surface area of the aggregate increases, the cement paste layer wrapping the sand becomes thinner, the friction resistance between sand particles increases, and the fluidity of the mixture decreases; if the sand ratio is too small, although the surface area is reduced, due to insufficient mortar amount, the cement paste, in addition to filling the voids of stones, has a thinner cement mortar layer wrapped on the surface of stones, and the fluidity of the mixture decreases. At the same time, due to insufficient sand amount, it is also easy to cause segregation, bleeding, and other phenomena, affecting workability. Therefore, the sand ratio has a reasonable value. When the amount of cement paste is constant, the sand ratio that can make fresh concrete obtain the maximum fluidity without segregation and bleeding is the reasonable sand ratio.

 

5. Cement type and aggregate properties. Different cement types have different water consumption when reaching the standard. Under the same other conditions, the cement with the smallest water consumption for standard consistency has a larger fluidity of its concrete mixture. Generally, the concrete mixture of ordinary cement has better workability than that of slag cement and pozzolanic cement. Although the fluidity of slag cement mixture is large, its cohesiveness is poor, and it is prone to bleeding and segregation. Pozzolanic cement has small fluidity but the best cohesiveness. Under the same water consumption, the mixture made with pebbles with smooth surface, round shape, and few edges and corners has large fluidity, but its strength is lower than that of crushed stone with rough surface and edges and corners.

 

6. Amount of cement paste. The cement paste in the concrete mixture, in addition to filling the voids between aggregates, wraps the surface of aggregates and has a slight surplus, giving the mixture a certain fluidity. Under the condition of a certain water-cement ratio, the more cement paste, the greater the fluidity, but if there is too much cement paste, the aggregate is relatively reduced, and bleeding will occur. The stability of the mixture is poor, which not only wastes cement but also reduces the strength and durability of the mixture; if the amount of cement paste is too small, it cannot well wrap the surface of aggregates and fill their voids, and the mixture is prone to collapse and lose stability. Therefore, the amount of cement paste in the mixture should meet the fluidity requirement.

 

7. Temperature and mixing time. The higher the temperature, the faster the water in the concrete mixture evaporates, and the smaller the fluidity. For every 10°C increase in temperature, the slump decreases by about 20mm--40mm. This must be noted in summer construction. In addition, the length of mixing time also affects the workability of the concrete mixture. If the mixing time is insufficient, the workability of the mixture is worse and the quality is uneven. Therefore, the specification stipulates that the minimum mixing time is 1-3 minutes.

 

III. Main measures to improve the workability of fresh concrete

 

1. Adjust the composition of concrete. Under the premise of ensuring the strength, durability, and economy of concrete, appropriately adjust the concrete mix proportion to improve workability.

 

2. Add various admixtures to make the workability of the concrete mixture meet different use requirements.

 

3. Improve the efficiency of vibrating machinery. Due to the improvement of vibrating efficiency, the requirements of construction conditions on concrete mixture can be reduced.

 

IV. Conclusion

 

The factors affecting the workability of fresh concrete can be summarized into two major aspects: first, human factors, including the selection of materials, the accuracy of mix proportion, and the standardization of construction. Second, natural factors, including temperature, rainfall, etc. In response to the above two factors, we can take the following measures: for natural factors, we should try to avoid rainfall, and we can also improve the workability of concrete through admixtures. For human factors, we can only strengthen the skill training of operators, construction personnel, and supervisors in the mixing station (plant). Through off-work time, professional training can be conducted to enhance their theoretical knowledge and skill levels, so that they can effectively handle the workability of fresh concrete during construction. Only with a team of capable construction and technical personnel can we make the project optimal.

 

Author: Wang Aili, Hubei Jingtian Road & Bridge Co., Ltd.

 

 

References:

 

[1] Zhang Meizhen: Highway Engineering Testing Technology: 2nd Edition [M]. Beijing: China Communications Press, 2005: 7.

 

[2] Ministry of Transport of the People's Republic of China Industry Standard "Code for Design of Ground and Foundation for Highway Bridges and Culverts" (JTJ024-85) [S]. Beijing: China Communications Press, 1985.

 

[3] Yuan Jie. Road Building Materials [M]. Chengdu: Southwest Jiaotong University Press, 2005.

 

[4] People's Republic of China Industry Standard "Technical Specifications for Construction of Highway Bridges and Culverts" (JTJ041-2000) [S]. Beijing: China Communications Press, 2000.