The effects of defoaming agent and air entraining agent combined with polycarboxylic acid superplasticizer on gas content, mechanical properties and chloride penetration resistance of freshly mixed concrete were studied. The test results show that when the content of defoamer is appropriate, the large bubbles caused by polycarboxylic acid superplasticizer can be effectively reduced and the bubbles can be stabilized. With the increase of air entrainment agent, the gas content of concrete increases and the warp loss decreases. The compressive strength and chloride ion penetration resistance of concrete initially increase with the increase of air entrainment agent, but when the amount of air entrainment agent exceeds a certain dosage, it decreases with the increase of air entrainment agent. The optimal dosage of air entraining agent is about 0.03‰.



0 Introduction



The initial state of freshly mixed concrete has great influence on pumpability, constructability, mechanical properties and durability of concrete after hardening. At present, adding water-reducing agent is one of the measures to improve the initial state of concrete. Polycarboxylic acid superplasticizer has good water reducing effect and small slump loss, so it has been widely used. However, compared with traditional naphthalene superplasticizers, polycarboxylic acid superplasticizers are more sensitive to the change of concrete raw materials as an entrain type superplasticizer, and the air content of the mother liquor itself is generally high, and most of them are harmful bubbles. At present, in engineering practice, defoamer is usually used first to reduce the harm of harmful bubbles in the mother liquor of polycarboxylic acid superplasticizer, and then air entraining agent is used to adjust the quantity and quality of bubbles inside the concrete (that is, "first elimination and then introduction") to achieve the purpose of improving the performance of concrete. However, the reasonable dosage of defoamer and entrained air agent has not been studied deeply.




In this experiment, the amount of defoamer in the test was first determined through the defoamer test, and then the influence of the amount of air entraining agent on the performance of polycarboxylic acid superplasticizer concrete was studied, which provided a certain reference for the practical application of polycarboxylic acid superplasticizer "first elimination and then primer" technology.





1 Raw materials and test methods




1.1 Raw Materials





Cement: Conch P·042.5 grade cement






Sand: Ganjiang river sand, medium sand, fineness modulus 2.7, mud content 0.5%;






Gravel: Hubei Wuxue gravel, 5~20mm, 5~10mm, 10~20mm= 4:6, continuous grading;






Water: common domestic water;






Water reducing agent: polycarboxylic acid high performance water reducing agent PCA(standard type) produced by Jiangxi Ditte Technology Co., LTD., solid content 40%, water reducing rate 25%;






Defoamer: Japan imported Takemoto oil A;






Air entrainment agent: Degusse concrete efficient compound air entrainment agent B.






1.2 Test methods






1.2.1 Concrete mix ratio






The concrete mix ratio is designed according to JGJ55-2011 "Ordinary Concrete mix ratio design Regulations".











1.2.2 Defoamer test





The content range of bamboo oil A was 0.1‰, 0.3‰, 0.5‰, 0.7‰ and 0.9‰. Five groups of tests were conducted to determine slump, gas content and observe the defoaming situation, so as to determine the fixed content of defoaming agent in subsequent tests, in which the content of A accounted for the dosage of PCA










It can be seen from Table 4 that with the increase of defoamer content, slump and gas content of concrete decrease correspondingly, indicating that bubbles in concrete can be well eliminated under the action of A, resulting in the reduction of gas content and the increase of viscosity of concrete, resulting in the reduction of concrete slump. When the content of A is 0.1‰ and 0,3 ‰, the gas content of freshly mixed concrete is higher and there are more large bubbles in concrete, and the bubbles are unstable. When the content is 0.5‰, the gas content is high and the bubbles are unstable. Although the gas content at 0.9‰ is 0.1% lower than that at 0.7‰, the bubbles at 0.9‰ are unstable. Therefore, the fixed dosage of A in the subsequent test was selected as 0.7‰.






1.2.3 "elimination before introduction" test






Six different components of concrete were designed in the experiment. The first group only added polycarboxylic acid water-reducing agent without any defoamer or air entraining agent. From group 2 to Group 6, antifoam agent A was added to polycarboxylic acid superplasticizer first, and then air entraining agent B was added with different dosage half an hour later, and the solution was stirred until uniform. The concrete admixtures are shown in Table 5, where the admixtures of PCA and B account for the amount of cement, and A accounts for the amount of PCA.






1.2.4 Test Method






Concrete mixing was carried out according to the six groups of tests in Table 5. The gas content and the 1h gas content were tested in accordance with GB/ T50080-2002 "Standard for Performance Test Methods of Ordinary Concrete Mixtures" and GB8076-2008 "Concrete Admixtures". 62; The determination of compressive strength of concrete 3, 7, 28d is carried out according to GB/T50081-2002 "Standard of Test methods for Mechanical Properties of ordinary concrete". The electric flux is tested according to the relevant provisions of GB/T50082-2009 "Standard of Test methods for long-term performance and durability of ordinary concrete".
















2. Test results and analysis




2.1 Combined gas volume and menstrual loss






It can be seen from Table 6 and Figure 1 that the gas content of concrete without A and B is greatly lost over time, amounting to 2.0%, and the pore structure can basically be regarded as inferior bubbles. After the "first elimination and then drawing" technology is adopted, the inferior bubbles with larger bubble diameter and bad bubble shape in the concrete are eliminated by A, and the bubbles with smaller bubble diameter, more quantity and uniform distribution are introduced under the action of B. The initial and 1h air content of concrete increases with the increase of B content, and the loss of concrete air content is small. When B is 0.01‰, the initial gas content of concrete is 0.5% lower than that of the first group due to the addition of A, but when the content is not less than 0.02‰, the initial gas content of concrete can catch up with and exceed the first group. When the dosage is 0.03‰, the gas content of concrete has no time loss.






















2.2 Compressive strength of concrete






As can be seen from Table 6 and Figure 2, after adopting the "first elimination and then diversion" technology, the influence on the strength of concrete at 3, 7 and 28 days is parabolic with the difference of B content. When the content of B is 0.01‰ ~ 0.03‰, the strength of concrete at 3, 7 and 28d is increased, which indicates that A can eliminate the bubbles in concrete well, reduce the gas content of concrete and improve the strength of concrete. However, when the B content exceeds 0.03‰, the concrete strength decreases to the same level as the first group.









2.3 Resistance of concrete to chloride ion penetration





It can be seen from Table 6 and Figure 3 that the 56d electric flux value of concrete after "first elimination and then drawing" technology is significantly lower than that of the first group of concrete, which improves the chloride ion penetration resistance of concrete. This is because under the joint action of anti-bleed gas components, the pore structure system in the concrete is changed, and many capillary channels are closed, thus breaking the continuity of the capillary tubes and improving the impermeability of the concrete. However, when the B content is 0.04‰ and 0.05‰, the electric flux value of concrete increases significantly, and the chloride ion permeability resistance decreases. This is mainly due to the increase in the number of large holes in the concrete when the gas content is too large, so the chloride ion permeability resistance decreases.





3 Conclusions




(1) When the content of bamboo oil A of antifoaming agent is 0.73‰ of PCA of polycarboxylic acid system, it can effectively reduce the large bubbles produced by water reducing agent and make the bubbles stable.






(2) The initial and 1h air content of polycarboxylic acid superplasticizer concrete increased with the increase of air entraining agent content, and the loss of air content was small






(3) The strength of concrete at 3, 7 and 28 days increases when the air entraining agent content is between 0.01‰ and 0.03‰, and the strength of concrete decreases when it exceeds 0.03‰. When the amount of air entraining agent is 0.01‰ to 0.04‰, the concrete's resistance to chloride ion penetration is improved. When the amount of air entraining agent is more than 0.04‰, the concrete's electric flux value increases obviously, and the concrete's resistance to chloride ion penetration is decreased. The optimal dosage of air entraining agent is about 0.03‰.