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What are the construction characteristics of concrete mixed with various additives?
Blog What are the constru
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The prestressed concrete sleepers in railway engineering are mainly prefabricated in factories using steam curing technology, and their concrete strength grade is generally C60. Steam curing accelerates cement hydration, promotes rapid early-strength development, and speeds turnover of rail sleeper molds, thereby shortening production cycles and improving production efficiency.
Previously, many companies used naphthalene-based water reducers. However, they significantly affect the durability of concrete preforms, and their production uses raw materials such as concentrated sulfuric acid and formaldehyde, resulting in high energy consumption and adverse environmental effects. Polycarboxylate water reducers offer high water reduction rates and cleaner production. From both economic and environmental perspectives, researching the use of polycarboxylate water reducers instead of naphthalene-based water reducers is highly significant.
Cement: P.O52.5 cement produced by Wanhua Branch of Shandong Donghua Cement Co., Ltd.
Mineral powder: S95 slag powder produced by Shandong Lubi Building Materials Co., Ltd.
Fine aggregate: Yishui River sand, with a fineness modulus of 2.8, a mud content of 1.5%, and a mud block content of 0.3%.
Coarse aggregate: Linqu crushed stone, with a particle size range of 5- 20 mm, a needle-like particle content of 2%, and a crushing index of 6%.
Water-reducing agents: polycarboxylate superplasticizer, naphthalene-based water-reducing agents.
The mixing performance of dry and hard concrete is determined using the consolidation factor method in GB/T50080 Test Method for Performance of Ordinary Concrete Mixtures.
The demoulding strength and 28-day strength of the specimens were determined using the “GB/T50081 Test Method for Physical and Mechanical Properties of Concrete”.
Measure cement expansion using an NJ-60B slurry mixer according to the GB8077 Test Method for Uniformity of Concrete Admixtures.
After loading to the inspection load value using the “TB/T Prestressed Concrete Pillow Static Load Crack Resistance Test Method”, stabilize for 3 minutes, then continue loading until cracking occurs to pass the test. Test piece maintenance system: The molded test piece is placed in a steam curing pool with the rail sleeper for maintenance. It is left to stand for 3 hours, and the temperature is raised to 58 ℃ at a rate of 15 ℃/h. After 12 hours of constant temperature, the temperature begins to decrease. When the test piece temperature is close to room temperature, it is demolded for compressive strength testing.
The mixing performance of dry and hard concrete will affect the compaction effect and appearance quality of the concrete, mainly reflected in the compaction factor JC. Under the premise of a water-cement ratio of 0.26, when the solid content of polycarboxylate superplasticizer is 21%, the mixed concrete is in the same state as daily production, but the state loss is fast (workshop temperature of 28 ℃), which is not conducive to production; When the solid content is 21.5%, the compaction factor is smaller than that in daily production, indicating that the concrete is thinner. However, compared with concrete mixed with naphthalene-based water-reducing agents, the loss is still faster.
Experimental data show that concrete with polycarboxylate superplasticizer has higher demolding strength. Polycarboxylic acid water reducers have a higher water-reduction rate and can reduce the water-cement ratio, which naphthalene-based water reducers cannot achieve.
After the rail sleeper is vibrated and formed, it is lifted into the curing kiln and steam cured. The compressive strength of the concrete reaches 45MPa or above before demolding can be carried out to avoid damage to the rail sleeper end and shoulder during the demolding process, and to improve the stability of product quality. Compared with naphthalene-based water-reducing agents, under the same conditions, the addition of comb-shaped polycarboxylate water-reducing agents with ultra-long side chains increased the early strength of fresh concrete by 16% and the 28-day strength by 6%, without significant adverse effects on the durability of concrete, meeting the construction needs of rail sleeper production sites.
At the same time, fresh concrete mixed with polycarboxylate superplasticizer is relatively soft, more suitable for fabrication and leveling, and requires less labor. This is because polycarboxylate superplasticizer has air-entraining properties, introducing small bubbles into the concrete; these bubbles isolate the rolling ball, reduce the shear viscosity of internal movement, make the concrete fluffier, and improve its durability. Figure 1 shows the appearance of the rail sleeper produced using polycarboxylate superplasticizer after demolding. The product has a clean appearance, uniform color, and fewer air holes at the ends.
By conducting comparative tests on concrete mixed with naphthalene-based water reducers and polycarboxylate-based water reducers, some experience has been gained in the application of polycarboxylate-based water reducers in rail sleeper concrete. The summary is as follows:
Polycarboxylate water reducers have improved concrete mixing performance to a certain extent, achieving a higher water-reduction effect at low dosage, and the demolding strength is 16% higher than that of naphthalene water reducers; however, early concrete loss in summer is too fast, which is not conducive to assembly-line production.
Polycarboxylate superplasticizer meets the production requirements of prestressed concrete sleepers in terms of demolding time, demolding strength, durability, and other aspects.
Moreover, fresh concrete workability is good, and the sleepers have a cleaner appearance.
What are the construction characteristics of concrete mixed with various additives?
Blog What are the constru
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