Views: 2 Author: Site Editor Publish Time: 2023-02-16 Origin: Site
Abstract: By changing the content of hydroxypropyl methylcellulose ether (HPMC) in normal temperature curing ultra-high performance concrete (UHPC), the effect of cellulose ether on the fluidity, setting time, compressive strength, and flexural strength of UHPC was studied. , axial tensile strength and ultimate tensile value, and the results were analyzed. The test results show that: adding no more than 1.00% of low-viscosity HPMC does not affect the fluidity of UHPC, but reduces the loss of fluidity over time. , and prolong the setting time, greatly improving the construction performance; when the content is less than 0.50%, the impact on compressive strength, flexural strength and axial tensile strength is not significant, and once the content is more than 0.50%, its mechanical The performance is reduced by more than 1/3. Considering various performances, the recommended dosage of HPMC is 0.50%.
Key words:ultra-high performance concrete; cellulose ether; normal temperature curing; compressive strength; flexural strength; tensile strength
With the rapid development of China's construction industry, the requirements for concrete performance in actual engineering have also increased, and ultra-high performance concrete (UHPC) has been produced in response to the demand. The optimal proportion of particles with different particle sizes is theoretically designed, and mixed with steel fiber and high-efficiency water reducing agent, it has excellent properties such as ultra-high compressive strength, high toughness, high shock resistance durability and strong self-healing ability of micro-cracks. Performance. Foreign technology research on UHPC is relatively mature and has been applied to many practical projects. Compared with foreign countries, domestic research is not deep enough. Dong Jianmiao and others studied the fiber incorporation by adding different types and amounts of fibers. The influence mechanism and law of concrete; Chen Jing et al. studied the influence of steel fiber diameter on the performance of UHPC by selecting steel fibers with 4 diameters. UHPC has only a small number of engineering applications in China, and it is still in the stage of theoretical research. The performance of UHPC Superiority has become one of the research directions of concrete development, but there are still many problems to be solved. Such as high requirements for raw materials, high cost, complicated preparation process, etc., restricting the development of UHPC production technology. Among them, using high-pressure steam The curing of UHPC at high temperature can make it obtain higher mechanical properties and durability. However, due to the cumbersome steam curing process and high requirements for production equipment, the application of materials can only be limited to prefabrication yards, and cast-in-place construction cannot be carried out. Therefore, it is not suitable to adopt the method of thermal curing in actual projects, and it is necessary to conduct in-depth research on normal temperature curing UHPC.
Normal temperature curing UHPC is in the research stage in China, and its water-to-binder ratio is extremely low, and it is prone to rapid dehydration on the surface during on-site construction. In order to effectively improve the dehydration phenomenon, cement-based materials usually add some water-retaining thickeners to the material. Chemical agent to prevent segregation and bleeding of materials, enhance water retention and cohesion, improve construction performance, and also effectively improve the mechanical properties of cement-based materials. Hydroxypropyl methyl cellulose ether (HPMC) as a polymer Thickener, which can effectively distribute the polymer gelled slurry and materials in cement-based materials evenly, and the free water in the slurry will become bound water, so that it is not easy to lose from the slurry and improve the water retention performance of concrete .In order to reduce the impact of cellulose ether on the fluidity of UHPC, low-viscosity cellulose ether was selected for the test.
In summary, in order to improve the construction performance on the basis of ensuring the mechanical properties of normal-temperature curing UHPC, this paper studies the effect of low-viscosity cellulose ether content on normal-temperature curing based on the chemical properties of cellulose ether and its mechanism of action in UHPC slurry. The influence of fluidity, coagulation time, compressive strength, flexural strength, axial tensile strength and ultimate tensile value of UHPC to determine the appropriate dosage of cellulose ether.
1. Test plan
1.1 Test raw materials and mix ratio
The raw materials for this test are:
1) Cement: P·O 52.5 ordinary Portland cement produced in Liuzhou.
2) Fly ash: Fly ash produced in Liuzhou.
3) Slag powder: S95 granulated blast furnace slag powder produced in Liuzhou.
4) Silica fume: semi-encrypted silica fume, gray powder, SiO2 content ≥ 92%, specific surface area 23 m²/g.
5) Quartz sand: 20~40 mesh (0.833~0.350 mm).
6) Water reducer: polycarboxylate water reducer, white powder, water reducing rate ≥ 30%.
7) Latex powder: redispersible latex powder.
8) Fiber ether: hydroxypropyl methylcellulose METHOCEL produced in the United States, viscosity 400 MPa s.
9) Steel fiber: straight copper-plated microwire steel fiber, diameter φ is 0.22 mm, length is 13 mm, tensile strength is 2 000 MPa.
After a lot of experimental research in the early stage, it can be determined that the basic mix ratio of normal temperature curing ultra-high performance concrete is cement: fly ash: mineral powder: silica fume: sand: water reducing agent: latex powder: water = 860: 42: 83: 110:980:11:2:210, the steel fiber volume content is 2%. Add 0, 0.25%, 0.50%, 0.75%, 1.00% HPMC of cellulose ether (HPMC) content on this basic mix ratio Set up comparative experiments respectively.
1.2 Test method
Weigh the dry powder raw materials according to the mixing ratio and place them in the HJW-60 single-horizontal shaft forced concrete mixer. Start the mixer until uniform, add water and mix for 3 minutes, turn off the mixer, add the weighed steel fiber and restart the mixer for 2 minutes. Made into UHPC slurry.
The test items include fluidity, setting time, compressive strength, flexural strength, axial tensile strength and ultimate tensile value. The fluidity test is determined according to JC/T986-2018 "Cement-based Grouting Materials". The setting time test is according to GB /T 1346—2011 "Cement Standard Consistency Water Consumption and Setting Time Test Method". The flexural strength test is determined according to GB/T50081-2002 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete". Compressive strength test, axial tensile strength and The ultimate tensile value test is determined according to DLT5150-2001 "Hydraulic Concrete Test Regulations".
2. Test results
The fluidity test results show the influence of HPMC content on the loss of UHPC fluidity over time. It is observed from the test phenomenon that after the slurry without cellulose ether is stirred evenly, the surface is prone to dehydration and crusting, and the fluidity is quickly lost. , and workability deteriorated. After adding cellulose ether, there was no skinning on the surface, the loss of fluidity over time was small, and the workability remained good. Within the test range, the minimum loss of fluidity was 5 mm in 60 minutes. Analysis of the test data shows that, The amount of low-viscosity cellulose ether has little effect on the initial fluidity of UHPC, but has a greater impact on the loss of fluidity over time. When no cellulose ether is added, the fluidity loss of UHPC is 15 mm; With the increase of HPMC, the fluidity loss of mortar decreases; when the dosage is 0.75%, the fluidity loss of UHPC is the smallest with time, which is 5mm; after that, with the increase of HPMC, the fluidity loss of UHPC with time Almost unchanged.
After HPMC is mixed with UHPC, it affects the rheological properties of UHPC from two aspects: one is that independent micro-bubbles are brought into the stirring process, which makes the aggregate and fly ash and other materials form a "ball effect", which increases the workability At the same time, a large amount of cementitious material can wrap the aggregate, so that the aggregate can be evenly "suspended" in the slurry, and can move freely, the friction between the aggregates is reduced, and the fluidity is increased; the second is to increase the UHPC The cohesive force reduces the fluidity. Since the test uses low-viscosity HPMC, the first aspect is equal to the second aspect, and the initial fluidity does not change much, but the loss of fluidity over time can be reduced. According to the analysis of the test results, it can be known that adding an appropriate amount of HPMC to UHPC can greatly improve the construction performance of UHPC.
2.2 Setting time
From the change trend of the setting time of UHPC affected by the amount of HPMC, it can be seen that HPMC plays a retarding role in UHPC. The larger the amount is, the more obvious the retarding effect is. When the amount is 0.50%, the setting time of the mortar is 55min. Compared with the control group (40 min), it increased by 37.5%, and the increase was still not obvious. When the dosage was 1.00%, the setting time of the mortar was 100 min, which was 150% higher than that of the control group (40 min).
The molecular structure characteristics of cellulose ether affect its retarding effect. The fundamental molecular structure in cellulose ether, that is, the anhydroglucose ring structure, can react with calcium ions to form sugar-calcium molecular compounds, reducing the induction period of cement clinker hydration reaction The concentration of calcium ions is low, preventing further precipitation of Ca(OH)2, reducing the speed of cement hydration reaction, thereby delaying the setting of cement.
2.3 Compressive strength
From the relationship between the compressive strength of UHPC samples at 7 days and 28 days and the content of HMPC, it can be clearly seen that the addition of HPMC gradually increases the decline in the compressive strength of UHPC. 0.25% HPMC, the compressive strength of UHPC decreases slightly, and the compressive strength ratio is 96%. Adding 0.50% HPMC has no obvious effect on the compressive strength ratio of UHPC. Continue to add HPMC within the scope of use, UHPC’s The compressive strength decreased significantly. When the content of HPMC increased to 1.00%, the compressive strength ratio dropped to 66%, and the strength loss was serious. According to the data analysis, it is more appropriate to add 0.50% HPMC, and the loss of compressive strength is small
HPMC has a certain air-entraining effect. The addition of HPMC will cause a certain amount of microbubbles in UHPC, which will reduce the bulk density of freshly mixed UHPC. After the slurry is hardened, the porosity will gradually increase and the compactness will also decrease, especially the HPMC content. Higher. In addition, with the increase of the amount of HPMC introduced, there are still many flexible polymers in the pores of UHPC, which cannot play an important role in good rigidity and compressive support when the matrix of the cementitious composite is compressed. .Therefore, the addition of HPMC greatly reduces the compressive strength of UHPC.
2.4 Flexural strength
From the relationship between the flexural strength of UHPC samples at 7 days and 28 days and the content of HMPC, it can be seen that the change curves of flexural strength and compressive strength are similar, and the change of flexural strength between 0 and 0.50% of HMPC is not the same. As the addition of HPMC continued, the flexural strength of UHPC samples decreased significantly.
The effect of HPMC on the flexural strength of UHPC is mainly in three aspects: cellulose ether has retarding and air-entraining effects, which reduce the flexural strength of UHPC; and the third aspect is the flexible polymer produced by cellulose ether, Reducing the rigidity of the specimen slows down the decrease of the flexural strength of the specimen slightly. The simultaneous existence of these three aspects reduces the compressive strength of the UHPC specimen and also reduces the flexural strength.
2.5 Axial tensile strength and ultimate tensile value
The relationship between the tensile strength of UHPC specimens at 7 d and 28 d and the content of HMPC. With the increase of the content of HPMC, the tensile strength of UHPC specimens first changed little and then decreased rapidly. The tensile strength curve shows that when the content of HPMC in the specimen reaches 0.50%, the axial tensile strength value of the UHPC specimen is 12.2MPa, and the tensile strength ratio is 103%. With the further increase of the HPMC content of the specimen, the axial The central tensile strength value began to drop sharply. When the HPMC content of the specimen was 0.75% and 1.00%, the tensile strength ratios were 94% and 78%, respectively, which were lower than the axial tensile strength of UHPC without HPMC.
From the relationship between the ultimate tensile values of UHPC samples at 7 days and 28 days and the content of HMPC, it can be seen that the ultimate tensile values are almost unchanged with the increase of cellulose ether at the beginning, and when the content of cellulose ether reaches 0.50 % and then began to drop rapidly.
The effect of the addition amount of HPMC on the axial tensile strength and ultimate tensile value of UHPC specimens shows a trend of keeping almost unchanged and then decreasing. The main reason is that HPMC can be directly formed between hydrated cement particles A layer of waterproof polymer sealing film plays the role of sealing, so that a certain amount of water is stored in UHPC, which provides necessary water for the continuous development of further hydration of cement, thereby improving the strength of cement. The addition of HPMC improves the The cohesiveness of UHPC endows the slurry with flexibility, which makes UHPC fully adapt to the shrinkage and deformation of the base material, and slightly improves the tensile strength of UHPC. However, when the content of HPMC exceeds the critical value, the entrained air affects the strength of the specimen. The adverse effects gradually played a leading role, and the axial tensile strength and ultimate tensile value of the specimen began to decrease.
1) HPMC can significantly improve the working performance of normal temperature curing UHPC, prolong its coagulation time and reduce the fluidity loss of freshly mixed UHPC over time.
2) The addition of HPMC introduces a certain amount of tiny bubbles during the stirring process of the slurry. If the amount is too large, the bubbles will gather too much and form larger bubbles. The slurry is highly cohesive, and the bubbles cannot overflow and rupture. The pores of the hardened UHPC decrease; in addition, the flexible polymer produced by HPMC cannot provide rigid support when it is under pressure, and the compressive and flexural strengths are greatly reduced.
3) The addition of HPMC makes UHPC plastic and flexible. The axial tensile strength and ultimate tensile value of UHPC specimens hardly change with the increase of HPMC content, but when the HPMC content exceeds a certain value, The axial tensile strength and ultimate tensile values are greatly reduced.
4) When preparing normal temperature curing UHPC, the dosage of HPMC should be strictly controlled. When the dosage is 0.50%, the relationship between the working performance and mechanical properties of normal temperature curing UHPC can be well coordinated.