Shen Chen Liang 1, Yang Yu Qi 2, Gao Wen Xia2

( 1. Bei Jing Yu Shu Zhuang Component Plant mixing Plant, Bei Jing, 100070; 2. Bei Jing Da Xing Concrete Mixing Plant of Municipal Construction Engineering Company, Bei Jing, 102600)

[Abstract] The exposed concrete exterior facing of traditional buildings is often decorated after the construction of the outer wall of the building, so the appearance quality of the concrete surface is not controlled as the main index. With the development of the times, the owners and architects have the requirements of returning to the original appearance of the building: the exterior facing of the building is the natural color of concrete. This increases the requirements for the construction quality control of concrete external facing. This paper mainly discusses the factors affecting the appearance of fair faced concrete.

[Key words] Fair faced concrete mix proportion formwork release agent curing

0 Preface

Fair faced concrete refers to that the structural concrete is directly used as the external finish of the building after the formwork is removed, and the surface will not be decorated after hardening, and the visual effect is the natural color of the concrete. Fair faced concrete is adopted by more and more architects in the world because of its dignified and elegant texture and unique decorative effect. This decorative art has been used in world-famous buildings such as Sydney Opera House, Japan's National Grand Theatre and Paris prehistoric Museum. Due to its high requirements for concrete materials and construction technology, in China, fair faced concrete is mostly used in municipal engineering, traffic engineering, water conservancy engineering, aviation engineering, etc., and is rarely used in civil construction engineering. As a decorative surface, fair faced concrete has high requirements for beauty, color difference, surface bubbles and so on. Therefore, corresponding measures should be taken in the preparation, production, construction, curing and protection of concrete. Combined with the fair faced concrete project in which the author once participated, this paper discusses the relevant problems.

1 Mix proportion

The performance of concrete depends on the selected raw materials and mix proportion. When used as fair faced concrete, the concrete mixture shall have good workability without segregation and bleeding; After hardening, the concrete shall have a smooth surface with uniform color.

1.1 Bleeding

Concrete is mixed with a variety of raw materials. When using high-quality raw materials, carefully designed mix proportion and strict production control, high-quality concrete materials should be obtained. This traditional concept, which has been tested by practice for many years, has been challenged at present. This is the bleeding phenomenon of fresh concrete from time to time, and there are problems in the quality of concrete.

The water added during concrete mixing has two functions: one is to ensure the progress of cement hydration process; Second, make the fresh cement slurry or concrete mixture have enough fluidity to facilitate pouring and molding. Under the current construction conditions, the water demand to meet the requirements of the latter is often greater than the minimum water demand required by the former. This excess water is easy to produce to the separation of different components in the process of concrete transportation, pouring, vibration and the static period before solidification. It is often manifested as the separation of water - the phenomenon of seepage inside or outside, which is called bleeding. Under most construction conditions, bleeding is required to be minimized. Due to the diversity of raw materials and the fluctuation of production control and construction control, the bleeding phenomenon of fresh concrete occurs from time to time. Losing control of any link may lead to bleeding of fresh concrete. In serious cases, there are phenomena such as large bleeding volume, fast bleeding speed and long bleeding duration. The concrete with low strength grade (high water/cementitious ratio) is more prone to bleeding than the concrete with high strength grade (low water/cementitious ratio), and the concrete construction quality is difficult to meet the appearance requirements of fair faced concrete.

1.1.1 Influence of cement

The implementation of the new cement standard puts forward new requirements for the strength and fineness of cement. In order to produce cement meeting the new standard, the cement plant has taken measures such as changing the mineral composition and increasing the specific surface area to improve the early strength and final strength of cement. One of the measures to improve the early strength is to increase the C₃A content in cement minerals to accelerate cement hydration, but the hydration process of C₃A depends on the quality and content of gypsum, which directly affects the adaptability between admixture and cement. Therefore, the change of gypsum quality and content is not conducive to stabilizing the adaptability of admixture and cement. Large specific surface area is conductive to accelerating the hydration process of cement and improving the strength of cement, but the water consumption of standard consistency will increase with the increase of specific surface area of cement. At the same time, the water retention of cement depends on the particle distribution of cement. The current technical level of cement plant can determine good cement particle gradation through tests. However, in production, in order to achieve good particle gradation will often affect the cement output. At the same time, the national standard has no mandatory requirements for cement particle gradation index, and many cement manufacturers are unwilling to invest in this aspect. It is inevitable that the fluctuation of cement will affect bleeding.

1.1.2 Mineral admixtures

Mineral admixture is a component of concrete. When selecting mineral admixture, consider the activity of admixture and good filling of cement space in concrete. Therefore, when making fair faced concrete, mineral powder should preferably be grade S95 or above; Grade I fly ash shall be selected. So that the powder in the concrete also has a good gradation.

 

1.1.3 Admixtures

The fluctuation of cement quality leads to the change of the performance of fresh concrete. In order to improve the working performance of concrete, the admixture should adapt to this change in time; On the other hand, the fluctuation of admixture quality can not be ignored. If the adaptability of admixture and cement is not good, the concrete will have bleeding, large slump loss, change of setting time and so on. The traditional concept of adjusting admixtures to adapt to cement will continue to guide the work of admixture companies.

Admixtures with a single variety of water reducing agent often can not solve the bleeding caused by cement fluctuation. It is necessary to combine two or more water reducing components from different water reducing types of admixtures. At the same time, different chemical components controlling concrete slump have different effects on concrete bleeding, which requires the admixture supply plant to often take cement from different manufacturers to detect whether it is necessary to change the chemical components controlling concrete slump. Adding an appropriate amount of high-quality air entraining agent can alleviate the occurrence of concrete bleeding. The judgment of high-quality air entraining agent should mainly be determined by hole spacing coefficient. In addition, due to the requirements of concrete appearance, thickened components should not be used to solve the problem of concrete bleeding. When using admixtures in the mixing plant, the dosage must be determined according to the test. Insufficient dosage will lead to insufficient water reduction rate, resulting in the water cement ratio exceeding the trial water cement ratio, reducing the concrete strength, and failing to control the slump loss over time. Excessive admixture will cause greater harm, resulting in sensitive water consumption per cubic meter, easy segregation of concrete, poor integration of various raw materials, and normal setting of concrete, resulting in delay of construction period.

1.1.4 Variation of sand fineness modulus, etc

During the construction of fair faced concrete, it is found that when the sand fineness modulus changes (or the grading is not good) and the mud content changes greatly, the sand rate and the amount of admixture shall be adjusted in time, otherwise, the concrete bleeding may occur and the decoration effect may be affected; When the quality of admixture changes greatly, so that the water demand of concrete changes, the production mix proportion shall be adjusted in time. Otherwise, in order to ensure the slump of concrete, the production personnel will often adjust the water consumption of concrete or the amount of admixture, which will increase the trend of concrete bleeding. Reducing or eliminating concrete bleeding is the first step for fair faced concrete construction.

1.2 Color consistency

Fair faced concrete not only requires no bleeding of the mixture, but also requires uniform color after formwork removal. From a large number of production practices, it is found that in order to ensure uniform color, the same raw materials and mix proportion should be used as far as possible.

1.2.1 Water/cementitious ratio

A large number of engineering practices have proved that the appearance color of concrete with different amount of cementitious materials or different water cementitious ratio is different after hardening. In addition to bleeding, the amount of powder wrapped in aggregate is not enough for concrete with less amount of cementitious materials. Therefore, for fair faced concrete engineering, the amount of cementitious materials should not be less than 350kg / m3, and the water consumption per cubic meter of concrete should not exceed 160kg / m3. When the amount of cementitious materials reaches more than 500kg / m3, Measures shall be taken to prevent the concrete from being sticky and difficult to construct.

For example, mineral powder and silica fume with certain fineness are added. For a single project, the amount of rubber material and water consumption should be consistent on the same visual surface, and the components of each cementitious material should not change greatly, so as to reduce the impact of the change of mix proportion on the appearance. Different cementitious ratio shall not be adopted due to different strength grades.

1.2.2 Aggregate

Except that different water binder ratio will lead to the change of concrete color after hardening, the influence of aggregate on the appearance can not be ignored. The appearance color of hardened concrete is different when river pebble crushed aggregate and lime stone crushed aggregate are used. Due to the great difference of gradation, the appearance color of concrete may also be greatly different for the same aggregate. Therefore, for concrete works on the same visual surface, the same type of aggregate should be used to reduce the impact of aggregate change on the appearance of concrete.

1.3 Formation of surface pores

As we all know, a large amount of air will be introduced into concrete during mixing, and a certain amount of air（air） will be introduced into admixtures, which is the main reason for the air content of ordinary concrete. After the concrete is poured and vibrated, a large amount of air can be discharged. In the process of air discharge, when the bubbles encounter the formwork surface, a certain amount of bubbles will remain on the concrete surface. After the concrete is hardened and demoulded, a certain amount of bubbles appear on the structural surface. This is because when the concrete is liquefied by vibration, it is a high flow liquid, while the formwork is a solid. In the process of air floating, it will produce a certain resistance on the surface of different phases. The size and distribution of bubbles on the concrete surface will be very different when different materials of formwork or different types of release agents are used.

1.4 Site adjustment of mix proportion

During the transportation of commercial concrete, slump loss will occur. When it arrives at the construction site, it is sometimes difficult to meet the pouring construction requirements, which requires the adjustment of concrete mixture.

 

Many mixing plants often use the sand and stone just transported from the sand and stone production plant. In some periods, the sand and stone manufacturers produce and leave the factory, resulting in great changes in the water content of the sand and stone. When transported to the mixing plant and mixed with the sand and stone stored in the mixing plant, the water content of the sand and stone is unstable, and it is difficult to control the stable slump of the concrete when leaving the station. When the slump of concrete is small when it leaves the station, resulting in that when it arrives at the construction site, the slump of concrete can not meet the construction requirements. After confirmation by the technicians of the mixing plant, it is advisable to give priority to water adjustment and comprehensively consider the situation of water addition, but it shall not exceed the theoretical water consumption of mix proportion.

When the mixing tank truck waits for a long time at the construction site, water and admixture can be adjusted at the same time on the premise of not exceeding the water consumption of the mix proportion, but the admixture shall not exceed the upper limit of the admixture recommended by the admixture manufacturer, so as to ensure that there is no great difference in the setting time of concrete.

2 Concrete formwork works

In order to obtain satisfactory fair faced concrete effect, the mixing plant should strictly control the concrete production. At the same time, the influence of structural formwork on the appearance of concrete after formwork removal can not be ignored. In order to achieve a smooth surface without pores, the construction unit shall consider the formwork material. The size and distribution of pores, on the concrete surface will be very different when different materials of formwork or different types of release agents are used.

 

2.1 Formwork material selection

In order to find out the influence of formwork of different materials on the surface appearance of concrete after hardening and formwork removal, we selected different formwork for comparative test under the same conditions. The same release agent emulsified paraffin was used in the test.

Wood formwork, steel formwork, phenolic coated formwork and PVC lined formwork are selected for the test. Under the same construction conditions, the appearance effect of concrete after hardening is as follows: steel formwork > phenolic coated formwork > PVC lined formwork > wood formwork. The above templates are newly made and have not been used. The steel formwork is easy to be processed and formed, which meets the requirements of the construction drawings for the structural size. The joints of other material formwork are easy to be connected poorly.

 

When using steel formwork for high water binder ratio, that is, low strength concrete, it is easy to produce sand lines in appearance; When wooden formwork is used for low water binder ratio, that is, high-strength concrete, the pores on the outer surface of the concrete are not easy to discharge, and the phenolic coated formwork should be a compromise choice.

2.2 Influence of release agent

The release agent is a layer of coating that is easy to separate the formwork and concrete structure after the concrete is hardened. The release agent is painted on the inner side of the formwork. In order to play a good role in demoulding and have the least impact on the appearance of concrete after concrete hardening, the release agent must be firmly bonded to the formwork. The appearance of hardened concrete is very different due to different materials of release agents. We selected the following release agents for test. The formworks are all steel formworks of the same material. The sequence of formwork paint, emulsified paraffin, diesel engine oil, water quality release agent and edible salad oil affecting the appearance of concrete is as follows: formwork paint > emulsified paraffin > edible salad oil > diesel engine oil > water quality release agent. It should be noted that after painting, the formwork paint can be used only after drying and curing for 2 days at a temperature of no less than 20 ℃. The formwork cannot be put into use again soon, otherwise the construction progress will be affected.

 

3 Influence of pouring and vibration

Before pouring concrete, first check the tightness of the formwork installation, whether the surface of the formwork is polluted during the support process, whether there is dust or other floating objects on the inner side of the formwork, and whether there is a gap between the lower side of the formwork and the lower structure. In order to prevent the concrete from running slurry on the lower side of the formwork during concrete pouring, the construction unit often paves mortar on the lower side of the formwork. If the water cement ratio of this layer of mortar and concrete is very different, or the mortar bedding layer is very thick, Bleeding is easy to occur after the lower concrete is poured. Commercial concrete is a semi-finished product. After the mixing and transportation of concrete in the mixing plant, the construction unit is required to put the concrete into the formwork and vibrate it tightly. The concrete shall be vibrated and compacted, and the hardened surface shall be smooth without defects. After the commercial concrete arrives at the site, the construction unit and the technicians of the mixing plant shall jointly inspect the workability of the concrete and confirm that the concrete meets the pouring requirements before pouring into the formwork. During pouring, the distance between the concrete blanking opening and the pouring surface shall not be too large in strict accordance with the specification requirements, otherwise the concrete is prone to segregation; The pouring thickness of each layer shall not be too thick, otherwise, the air in the concrete is not easy to be discharged. The vibration shall strictly comply with the specifications, and the rod spacing shall not exceed the liquefaction radius of the vibrating rod, otherwise it is easy to miss vibration.

The vibrating concrete should be grouted on the surface. It is not allowed to vibrate at the same vibration point for a long time to avoid over vibration. The above mentioned bleeding problem of concrete itself. After improper vibration of concrete, it is easier to produce over vibration - relative bleeding. Under normal conditions, the fresh concrete is a viscous mixture of different materials. Under the external force of construction vibration, the fresh concrete is in a flow state, and the concrete is no longer viscous and difficult to flow. Vibration at the same vibration point for a long time will lead to over vibration of concrete; Concrete itself is a mixture of materials with different densities, and water is the least dense material here. The over vibration is manifested in the super liquefaction of concrete, and various components in the concrete are separated to varying degrees, especially the water floats up. In the process of water floats up, a water seepage channel is formed, which is manifested in the bleeding when the material components of the concrete mix proportion are not matched properly. After the concrete is hardened and the formwork is removed, the surface is rough and the sand is exposed, and the floating trace of water can be clearly seen. The use of high-frequency vibrator is easier to cause bleeding of concrete than that of low-frequency vibrator, and the use of vibrator with high amplitude is easier to cause bleeding of concrete than that of vibrator with low amplitude.

 

4 Influence of curing and formwork removal period

Commercial concrete is a semi-finished product. After pouring and vibrating, full and reasonable curing of concrete is the key to ensure the quality of hardened concrete. In the actual construction process, many construction units often fail to fully cure the formed concrete due to insufficient understanding or site conditions, resulting in water loss shrinkage cracking of concrete surface, especially for low water binder ratio (high strength) concrete, Curing is very important and necessary. Wet or water saturated curing should be preferred for the concrete surface. When the upper surface of the concrete is not at the same level, non-woven fabric can be paved and watered for curing; Any curing that may cause color pollution to the concrete surface is not advisable to ensure that the concrete surface is not polluted due to curing. As the facade is not easy to be cured by water saturation or humidity, the formwork shall be used for curing first. The removal time of the side formwork shall be determined according to the weather temperature and the mix proportion of the mixing plant. The strength of the concrete during formwork removal shall not be less than 30% of the design strength. In cold seasons, early formwork removal is easy to lead to water loss on the concrete surface, resulting in low concrete strength of the surface course. After water loss on the concrete surface course, it is easy to cause no surface finish. If conditions permit, on-site steam curing shall be adopted as far as possible to accelerate the hydration of active components in concrete and ensure the growth of concrete surface strength.

 

5 Repair of construction defects

During the construction process, many people participate in the concrete pouring, vibration, curing, formwork removal and other work. After the formwork removal, the surface of the concrete is prone to defects. The opinions of the designer, the owner and the supervisor shall be respected as to whether the defective concrete is repaired and the repair scheme. The safety of the structure shall be given priority, and then the beauty shall be considered; When the defect affects the structural safety, it must be repaired; When the concrete defect does not affect the structural safety, but seriously affects the appearance, it needs to be repaired for decoration, and the special repair agent adjusted by comparing with the appearance color of the original concrete can be used for repair. When the concrete defect slightly affects the appearance, it is recommended to retain the natural color and do not take any repair measures. With the consent of the architect and the owner, the outer surface of the defective concrete can be sand blasted and matted, and the impact on the overall appearance of the building should be considered.

In many developed countries, after the fair faced concrete formwork is removed, the concrete surface is sprayed with protective coating, so that the concrete surface has a certain ability to resist environmental erosion. The protective layer can be colorless or very light color, which can cover up some appearance defects. Some domestic projects have also started work in this regard. After the surface of fair faced concrete is sprayed with protective coating, the surface antifouling ability is greatly improved, which can maintain the appearance unchanged for more than 20 years, and plays a positive role in improving the durability of the structure.

 

6 Conclusion

There are many factors affecting the appearance effect of fair faced concrete, including concrete material selection, mix proportion, mixing, transportation, formwork, pouring method, vibration curing, ambient temperature and so on. In the process of concrete construction, all participating units shall fully understand the factors affecting the quality of fair faced concrete and take corresponding technical and quality measures to make a satisfactory fair faced concrete project.