A NEW TEST METHOD FOR WORKABILITY OF SELF-COMPACTING CONCRETE

Transcription

1 A NEW TEST METHOD FOR WORKABILITY OF SELF-COMPACTING CONCRETE LONG Guangcheng (1), XIE Youjun (1) and LIU Yunhua (1,2) (1) School of Civil Engineering and Architecture, Central South University, China (2) Station of inspection of Construction Engineering quality, Chenzhou city, China Abstract In order to evaluate simply and exactly properties of fresh self-compacting concrete (SCC), a new test apparatus is developed and thus a new test method for workability of fresh SCC is presented in this paper. This new method reflects the workability of fresh SCC by testing three parameters of outflow time, dynamic flow-spread value and dynamic height difference of fresh concrete between inside and outside of J-ring. Experiments are carried out to examine the workability such as flowability, passing capability and segregation resistance of fresh SCC mixtures by use of this new method. Comparatively, value of slump and slump-flow of corresponding fresh SCC are also tested by use of slump cone. Preliminary results indicates this new test method is valid in evaluating workability of fresh SCC mixture and can reflect generally the workability including flowability, passing capability and segregation resistance of SCC mixture. And this new method is also convenient compared with other test methods. This work is of importance for establishing a simple and valid test method for workability of fresh concrete, especially for self-compacting concrete. 1. INTRODUCTION It is of great importance for workability of fresh concrete to its application in practice. Many efforts are focused on developing a reasonable test method to evaluate of workability of concrete due to the need of application in industry and engineering and etc[1]. Self-compacting concrete (SCC) is a special concrete with excellent deformability and segregation resistance, and the distinctive self-compacting ability, which can flow through and fill the gaps of reinforcement and corners of molds without any need for vibration and compaction during the placing process. Therefore, compared with ordinary concrete, it is very difficult to detect difference in mixes, which will permit an identification of a good (and bad) SCC for a given purpose or application, and determine whether the concrete was self-compacting. In present, there are many methods developed to test workability of fresh SCC in the world, such as slump and slump-flow test, Orimet test, V-funnel test, J-ring test, L-apparatus test, U-apparatus test, wet-sieving segregation test and penetration test and etc[1~6]. However, most of these methods come from the result in laboratory and no extensive verifications for corresponding test methods are carried out by widely application in engineering practice. On other hand, single existed method only can reflected part of the 197

2 properties of fresh SCC. In order to evaluate fully the workability of fresh SCC, two or three test methods must be used. This is also the reason why so many test methods have been developed. Therefore, it is complication and poor reproducibility in evaluating workability of fresh SCC. And most of all, existed methods can only reflect the workability of fresh SCC under static state or vertical movement in located space condition and can not reflect workability of fresh SCC under dynamic state in practice. On basis of this, an integrative apparatus was developed and thus a test method was put forward to assess workability of SCC in this paper. 2. MATERIALS Portland cement denoted as Grade 42.5 according to Chinese Standard is used. The specific gravity of cement is 3.14 and its chemical composition is shown in Table 1. The specific gravity of fly ash used is 2.32 and its blaine specific area is 520m 2 /kg. The chemical compositions of fly ash are given in table1. The maximum sizes of coarse aggregates (crushed limestone) used are 20 mm and 16mm, respectively. Its size distributions of coarse aggregate are shown in table 2. River sand is used as fine aggregate. The fineness modulus of sand is The superplasticizer used is a naphthalene-based sulfonates produced by Shanghai Huawang chemical corporations with a commercial brand of Mighty100.Tap water is used for mixing SCC. Table 1 Chemical composition of cement and fly ash Item Chemical compositions (%) Type SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 I.L cement Fly ash Table 2 Particles distribution of crushed stone and river gravel / w % Item Size distribution (D max =20mm) / % Size distribution (D max =16mm) / % Type Crushed stone Gravel In investigated experiments, the effects of such factors as volume amount of paste, dosage of superplasticizers, water to binder ratio and etc. on workability of fresh self-compacting concrete are considered. The mix proportions for all samples and the corresponding results of workability are shown in Table APPARATUS AND TEST PROCEDURE The apparatus developed is shown in Figure 1, which consists of truncated cone with valve in bottom end, steel J-ring, steel slab and pushcart. The dimension of steel J-ring can refer the reference [1].The corresponding test procedure is as followings: Firstly, the apparatus is installed according to figure 1a and it is sure that the valve located in bottom end of truncated cone is closed. Then, the fresh self-compacting concrete sample is filled in truncated cone until the sample reached the up-brim of cone. And the up-surface of sample is mopped off by spatula. After ten seconds, turning on the valve and letting fresh concrete flow down to the steel slab, and meanwhile the elapsed time T 1 which sample flow 198

3 out completely truncated cone is measured by stopwatch. The flow spread values of sample in two directions vertical each other of steel slab is measured by scale after 30 seconds when sample flow out cone entirely. The measured values are denoted as FS 1 and FS 2 and one can calculate the average FS D from the two values (a) (b) 1. truncated cone 2. bracket 3. valve 4. steel slab 5. pushcart 6. steel J-ring Figure 1 Schematic diagram of integrative apparatus for testing workability of fresh self-compacting concrete Secondly, the apparatus is installed immediately according to figure 1b when the test procedure in the first step finished. Then the sample in the same mixture is filled into the truncated cone again. And then the repetition of test procedures in first step is carried out. One can also obtained the elapsed time T 2 which sample flowed out completely the truncated 199

4 cone from beginning to the end. After 30 seconds when sample flowed out truncated cone entirely, the height difference of fresh concrete can be obtained by calculating the average value of four height differences of fresh concrete between inside and outside of J-ring from four bars located in two directions vertical each other. The height difference measured is defined as the dynamic height difference of fresh mixture denoted as Δh d. Finally, one can obtain the average elapsed time T from T 1 and T 2. Therefore, the workability of fresh SCC is evaluated by three parameters of T, dynamic flow-spread FS D and dynamic height difference Δh d. Table 3 mixing proportion of self-compacting concrete and its corresponding testing results of workability Mix V p / V a / Slump SF FS No. m 3 m 3 W/B FA/% SP/% d Δh d /mm /mm /mm /mm T /s SA No No No No No No Yes Yes Yes Yes Note:V p and V a express paste volume and coarse aggregate volume in unit concrete mixture, respectively. And the volume of fine aggregate keeps constant for all samples and is 0.32 m 3. W/B is water to binder (cement and mineral admixture) ratio. FA is the replacement of cement by fly ash by weight. SP is addition amount of high range water-reducer expressed as percent of total amount of binder by mass. SF is the value of flow spread during slump test. SA is the segregation assess of fresh concrete, No expresses no segregation and yes shows that fresh concrete occurs segregation phenomena. 4. RESULTS AND DISCUSSION 4.1 Effect of paste volume on properties of fresh concrete Figure 2 shows the results of workability of corresponding fresh concrete measured by slump cone test and the integrative method presented in this paper. From the results given in Figure 2, one can see that in investigated area the three values of FS d, SF and slump all increase gradually with increasing paste volume, respectively. The value of FS d is more than 600mm after the volume of paste is larger than This shows that increasing paste volume can improve flowability or filling capability of fresh concrete. However, compared with value of slump, the value of dynamic flow spread varies remarkably with variation of paste volume in mixture. This indicates parameter of dynamic flow spread is sensitive to evaluate the flowability or filling capability of fresh concrete. Most of all, one can find that the value of FS d differs from that of SF for the same sample though the varying tendency of FS d with increasing paste volume is the same as that of SF. And the value of FS d is less than that of SF. It seems that this result is not the same as we expected. However, one can see that SF is the flow spread of fresh concrete under static state by the force of own weight while FS d is the one when fresh mixture fallen from truncated cone at 30 cm high down to steel slab and then

5 flow spread toward to around. i.e. FS d is the flow spread of fresh concrete under dynamic state condition, which the fresh concrete held an energy besides its own weight before it flows spread toward to around. There exists an impulsing action between fresh concrete under dynamic condition. The experimental results indicate that the impulsion action between fresh concrete results in a reduction of flow spread value of fresh concrete. This may be the reason that mixture like fresh concrete is a Bingham body. The cohesion of this viscosity mixture is improved by impulsion action and thus the flow spread of corresponding fresh concrete shows a reduction. In addition, the results of Δh d and T of fresh concrete with different paste volumes measured by integrative apparatus are shown in figure 3 and figure 4. It can be seen that from results given in figure3, value of Δh d decrease gradually with increasing paste volume. This shows the passing capability of fresh concrete is improved due to the increment of paste volume. Noteworthy is that Δh d reduces remarkably after volume of paste is larger than Meanwhile, the results shown in figure 4 indicate that the time T is influenced greatly by paste volume of mixture. The reduction of viscosity of concrete mixture results from the increment of paste volume and a reduction of aggregate since viscosity of paste keeps constant, which is responsible for this reduction of T. Obviously, in terms of relationships between properties of fresh concrete and its compositions, it is followed that workability of fresh concrete is determined by such main factors as volume of paste and aggregate when properties of paste and aggregate is given. Thus workability of fresh concrete is improved due to increment of paste volume and reduction of coarse aggregate in this experiment. Results from figure2, figure3 and figure4 show that parameters measured by integrative apparatus can reflect reasonably workability of fresh concrete mixture. 750 Flow spread /mm SF FS d Slump / mm V P / m 3 V p / m 3 Figure 2 Effects of paste volume on flow spread and slump of fresh concrete 201

6 h d /mm T / s V p / m 3 0 V p /m 3 Figure 3 Effects of paste volume on difference in height of J-ring of fresh concrete Figure 4 Effects of paste volume on outflow time from truncated cone of fresh concrete 4.2 Effects of W/B and superplasticizers on properties of fresh SCC mixture It is followed that the properties of fresh concrete are influenced greatly by W/B and superplasticizers. Table 3 gives the results of workability of fresh concrete with slight variation of W/B and amount of superplasticizers measured by integrative apparatus and slump cone test (see No.3, No.7 and No.8). From the results given in table 3, value of slump is almost the same although W/B increases by 0.01 from to or amount of superplasticizers increases by 0.1% from 1% to 1.1%. However, Δh d increases and T reduces with slight variation of W/B and superplasticizers and fresh concrete occur segregation. This indicates that workability of fresh concrete becomes bad. Obviously, viscosity of fresh mixture reduces owing to increment of W/B or dosage of superplasticizers. So the tendency of segregation between paste and aggregate increases, which results in a reduction of passing capability and resistance to segregation of fresh concrete. The result indicates that the parameters of measured by integrative apparatus in this experiment is sensitive in embodying or reflecting variation of workability of fresh concrete with slight changes of W/B or dosage of superplasticizers. 4.3 Effects of amount of fly ash on properties of fresh concrete Mineral admixture is an indispensable composition of high performance concrete. The properties of concrete in fresh state and hardened state are remarkably influenced by mineral admixture. The experimental result of effects of amount of fly ash on workability of fresh concrete is given in table 3 (see No. 3, No.9 and No.10). One can see that from the result in table3, the slump, slump flow and dynamic flow spread of fresh concrete all vary with variation of amount of fly ash. Compared with mixture with 40%FA, the workability of fresh concrete with 10%FA or zero fly ash is worse. And thus Δh d increases greatly although T reduces. This shows that addition of mineral admixture such as fly ash into mixture can improve resistance to segregation of fresh concrete as the results measured by test method proposed in this paper. 202

7 5. CONCLUSION The integrative apparatus designed in this paper is feasible to evaluate reasonably and objectively the workability of fresh SCC on basis of experimental result and related to theoretical analysis and thus a new test method is developed for assessing workability of fresh SCC. Compared with other existed methods, this method presented in this paper is simple and objective. And most of all, it can reflect dynamic properties of fresh concrete under placing construction condition in practice. The impulsion action between fresh concrete results in a reduction of the dynamic flow spread of fresh concrete, which shows the enhancement of cohesion of fresh concrete and weakening of flowability or filling capability of fresh mixture. REFERENCES [1] P. J. M. Bartos. Testing-SCC: Towards new European Standards for fresh SCC[C], In: Proceedings of 1 st international symposium on design, performance and use of self-consolidating concrete, Changsha, China, 5: [2] Hajime Okamura and Masahiro Ouchi. Self-compacting concrete: Development, present use and future[c], In: Proceedings of 1 st international RILEM Symposium on self-compacting concrete, Stockholm, Sweden, September, 1999: 3~14 [3] V.K. Bui, D. Montgomery, I. Hinczak, K. Turner. Rapid testing method for segregation resistance of self-compacting concrete[j], Cement and Concrete Research, 2,32(9): [4] M. Sonebi, M. Rooney and P.J.M. Bartos. Evaluation of the segregation resistance of fresh self-compacting concrete using different test methods[c], In: Proceedings of 1 st international symposium on design, performance and use of self-consolidating concrete, Changsha, China, 5: [5] Youjun Xie, Gaungcheng Long, Yanguang Li et al. a new method for evaluating stability of fresh self-compacting concrete[c], In: Proceedings of 1 st international symposium on design, performance and use of self-consolidating concrete, Changsha, China, 5: [6] China Civil Engineering Society. Guide to Design and Construction of Self-Compacting Concrete (CCES 02-4), Beijing: China Construction Engineering Press, 5(in Chinese) 203