APPLICATION OF SELF-CONSOLIDATING CONCRETE FOR BRIDGE REPAIR

Transcription

1 APPLICATION OF SELF-CONSOLIDATING CONCRETE FOR BRIDGE REPAIR Xian-feng Wang The Sixth Construction Group of Shanxi Province, , Shanxi, P. R. China Abstract: A highway bridge in province Shanxi was constructed during a hot summer, which leads to serious concrete defects. A Grade C60 self-consolidating concrete (SCC) was designed to repair the defects. This paper describes the design, testing and construction of the self-consolidating concrete. Experimental results indicated that the use of two types of sand with different modulus of fineness could produce SCC mixtures with excellent workability of concrete. The sand with lower modulus of fineness was needed in the case of no thickening agent was added. Quick and efficient construction is very important to self-compacting concrete in order to retain the flowability of SCC, especially at high temperature season. Moist curing is also very important for obtaining good quality and durability of concrete. 1 INTRODUCTION Self-consolidating concrete (SCC) is recognized as one of the greatest advances in the concrete industry over the past 20 years. Since the birth of SCC in the 1980 s, it has been used for a variety of applications due to its good flowability. SCC facilitates the use of creative ways to prefabricate precast, prestressed concrete elements to achieve high production, while increasing plant safety and workers health and job satisfaction. Prefabricated SCC elements of different shapes and sizes have been produced economically and successfully in plants and then brought to the job site for safe and speedy construction. Cast-in-place SCC has also been used in many cases where it would have been very difficult for people to cast and vibrate concrete. A highway bridge in Shanxi Province was built during a hot summer seanson, which resulted in serious concrete defects due to the fast slump loss under high temperature environment, as shown in Fig.1. A reinforced wall was designed to retrofit the defects, and a C60 (SCC) was selected for this purpose. This paper reports the design, testing and construction of SCC for the repair of concrete defects. 801

2 Fig. 1 Quality examination and reinforcement of the highway bridge 2 MATERIALS, MIXING PROPORTIONS AND TESTING METHODS 2.1 Raw materials Grade 42.5 ordinary Portland cement was used the cement. Fly ash was used as a mineral admixture incorporated into the self-consolidating concrete. Two types of river sand, A and B, with different fineness modulus were used as fine aggregates. The combination of the two kinds of sand can effectively reduce porosity, weeping and segregation of the concrete mixtures. Sand B has higher fineness modulus and contain 12% (by mass) sand particles larger than 5mm. The coarse aggregate was 5~16mm crushed stone with continuous gradation. The main characteristics of these materials are summarized in Table 1. Two types of superplasticizer used in the project are described in Table 2. The four mixture proportions used in the program is given in Table 3. Table 1 Details of materials used in the program Materials Characteristics Ordinary Portland cement Grade 42.5 Fly ash Grade I Fine aggregate A Modulus of fineness 2.3 Fine aggregate B Modulus of fineness 2.9 Coarse aggregate 5~16mm Crushed stone Table 2 Descriptions of two types of superplasticizer used Superplasticizer Description BNS (solid) Naphthalene sulphonate-based super plasticizer PCA (liquid) Polycarboxylic acid type super plasticizers(ester type) 802

3 Table 3 Mixture proportions of SCC used in the project (kg/m 3 ) Material number NO.1.1 NO.1.2 NO.2.1 NO.2.2 Cement A sand B sand Coarse aggregate Fly ash Water Superplasticizer 7.2(BNS) 7.2(PCA) 8.6(BNS) 8.6(PCA) 2.2 Flowability of cement paste and concrete A paste with a W/C ratio of 0.25 produced by cement, fly ash, superplasticizer and water is poured into a truncated cone mold (upper diameter 36mm, down diameter 60mm, height 60mm) until the mold is filled, then redundant paste poured into the mold is scraped off with a scraper. The mold is lifted upright to make the paste flow onto a glass plate, and at the same time a stopwatch begins to record the time up to 30 seconds. After the 30 seconds, sizes of two perpendicular diameters of the flowed paste were measured and their average value was taken as fluidity of the cement paste. The measurement of fluidity of concrete was concrete in the same way as that for paste. The measurement procedures are given Fig.2. Fig 2 Photos derived from fluidity measurement of concrete No.1.2, which was taken continuously at speed of 0.75 second per picture 2.3 Stability of mixtures Measurement of stability of mixture No.1.2 was conducted using a hard three-pair bucket( as shown in Fig 3, inner diameters 115mm, external diameters 135mm, pair height 803

4 100mm). Firstly, the concrete mixture was poured into the stability-test bucket before the bucket was filled; secondly, after keeping the bucket with the mixture still for 1minute, used a scraper to scrape redundant mixture off, but be sure not to pressure the mixture; thirdly, put the stability-test bucket onto a jumping table, rotated the rocking handle one time per second before the jumping table had jumped 25 times; finally, separated the three pairs of stability-test bucket, and put the mixture in each pair into 5 mm circular aperture sieve and washed it with water to get coarse aggregates, weighted their masses after drying water of their surface with a piece of sponge, then mass of upper, middle, and lower pair was got, named as m 1, m 2, m 3, respectively. The percentage of coarse aggregate vibration segregation can be calculated according to the following equation: m3 m1 f m = 100% m Where: f Percentage of coarse aggregate vibration segregation (%) m m average mass of three pairs (g) m 1 mass of upper pair (g) m mass of lower pair (g) Construction and Curing Concrete mixture was mixed in a 0.5m 3 forced mixer. The feeding sequence is: sand, cement, coarse aggregates and water. After mixing 60 seconds, the additive was added and 60s mixing was set to get a self-compacting concrete. The reason of adding the admixture after other materials is due to water content of aggregates fluctuated slightly. In this way, the change of water content in aggregates could be observed easily and adjusted in time, and consistent concrete mixtures could be achieved easily by adding superplasticizer when mixing. Concrete with high cementitious materials and low W/C (<0.4) needs special curing. Because of hydration of cement, concrete need more water to compensate the loss of water. Without further water supply, hydration of cement will stop at a certain degree. It will influence concrete quality and properties when relative humidity in concrete is less than 80%, especially within 7 days. Thus, plastic film covering method is not very good to keep enough water supply. In this case, water-spraying or water-immersing curing is needed to get more water used for hydration and better concrete properties [3-4]. 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Fluidity of Paste and Concrete The measured fluidity of the pastes is shown in Fig. 4. It can be seen that the fluidity of the paste increases with increased dosage of super plasticizer. It is interesting that the time 804

5 used in the fluidity test is different to pastes with different superplasticizers, the paste with BNS cost more time than that with PCA. This phenomenon clearly suggests that paste with BNS has larger viscosity and lower flowability at low water cement ratio was used as the W/C of paste to compare dispersion ability of the two superplasticizers in the case of low W/C. From the result, it seems that the liquid PCA has better dispersion ability than solid BNS in the case of low W/C when the dosage is more than 1.2%; however when the dosage is lower than 1.2%, the situation is contrary. The measured fluidity of concrete mixtures is summarized in Table 4. Similar to measurement of paste fluidity, liquid PCA has better dispersion ability than solid BNS in the case of lower, however when the dosage is larger, the situation is contrary. Fluidity (mm) BNS PCA Dosage of the two superplasticizers (%) Fig. 4 Fluidity of the paste with different superplasticizers dosages Table 4 Measured fluidity of concrete mixtures Number Superplasticizer Dosage Flowability T500 Type (%) (mm) (s) Time (s) No.1.1 BNS No.1.2 PCA No.2.1 BNS No.2.2 PCA Stability of SCC mixtures The measured stability results of concrete mixtures are listed in Table 5. As observed during the measurement and shown in Table 5, the mixture demonstrated an excellent flowability. No segregation was observed. Actually, it also demonstrated a very good slump flowability retention with time. 805

6 Table 5 Stability of concrete mixture Method Results at different times (T=24 o C) 5min 30 min 60 min Slump spread (mm) T 500 (s) Stability (%) Strength Development and Durability of Concrete Strength development of the self-consolidating concrete is plotted in Fig.5. As shown in the figure, the actual strength of the concrete can meet requirement of designed strength. Due to Province Shanxi is very cold and temperature can reach -20 o C in winter, property of freeze-thaw cycle is very important to concrete. Therefore, property of freeze-thaw cycle of in concrete in this program was measured, and results are listed in Table 6. The results show that the concrete has a good resistance to freeze-thaw cycle Strength (Mpa) Age(d) Fig.5 Strength development curve of the self-consolidating concrete 4. CONSTRUCTION Table 6 Property of concrete after 150 freeze-thaw cycles Mass loss Modulus of elasticity loss 1.2% 8.6% The self-compacting concrete was pumped into molds with a concrete pump with a 68mm caliber. This pumping method not only increased construction efficiency, but also saved construction time. The time from mixing concrete to construction site was less than 20minutes, which avoided slump flow loss and guaranteed properties of fresh concrete. 806

7 In this program, wet bamboo formworks were used and moistened after finishing their combination to make the formworks wet. It should be noted that before construction, redundant water onto surface of formworks should be wiped off. Wet sacks covering method was used and plastic films are covered on the wet sacks to protect water from evaporating rapidly. At the same time, water-spraying method was used within 7 days. When stopping the water-spraying curing, a curing agent was coated onto surface of concrete for prevention of concrete carbonation. 7 days of water curing can almost make concrete obtain designed strength, in this case, concrete gets high inner moisture and high permeability resistance and external water can not easily influence development of inner structure. 5. FIELD EXAMINATION Carbonation of the concrete was also measured after 1 year of construction, and the carbonation depth was 0.17mm. Compared with the conventional concrete in field, carbonation depth of the concrete was much lower. This indicated that the concrete had a good quality. This clearly suggested that wet curing of concrete at first period of concrete construction. After wet curing, the curing agent was coated onto surface of concrete was a good measure to for prevent concrete from carbonating. 6. CONCLUSIONS a) During choosing materials of self-compacting concrete, choosing more than 2 kinds of fine aggregate can obtain excellent workability of concrete. Sand with lower Modulus of fineness is needed in the case of no adding thickening component. b) Superplasticizer PCA is a better choice for self-compacting concrete at low W/C ratio (<0.3), it increases fluidity of concrete much more than traditional BNS additives do. c) Quick and efficient construction is very important to self-compacting concrete, which can get less slump loss elapsed time, especially at high temperature season. The best pouring method of self-compacting concrete is pumping, which can obtain good stability and duration of concrete. d) Wet curing is a good curing method at first period of concrete construction, this is very important to get high durability of concrete. After wet curing, the curing agent was coated onto surface of concrete was a good measure to for prevent concrete from carbonating. REFERENCES [1] The Chinese National Standards GB/T , Concrete addictive property test, [2] The Standard of Civil Engineering Society of China, Design and construction self-compacting concrete CCES , [3] Copeiand, L. E. and Bragg, R. H., Self Desiccation in Portland Cement Pastes, Research Department Bulletin RX052,Portland Cement Association, cement.org/pdf_file/rx052.ptf,1955,13 pages. [4] Henderson, E. and Turrillas, X. and Barnes, P., The formation, stability and microstructure of calcium sulpoaluminate hydrates present in hydrated cement pastes, using situ synchrotron energy-dispersive diffraction, Journal of Materials Science, 30 (5) (1995)