PASSING ABILITY OF SCC IMPROVED METHOD BASED ON THE P-RING

Transcription

1 PASSING ABILITY OF SCC IMPROVED METHOD BASED ON THE P-RING K D Chan*, Leppo Concrete Sdn Bhd, Malaysia K C G Ong, National University of Singapore, Singapore C T Tam, National University of Singapore, Singapore 35 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2010, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 35 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2010, Singapore PASSING ABILITY OF SCC IMPROVED METHOD BASED ON THE P-RING K D Chan *, Leppo Concrete Sdn Bhd, Malaysia K C G Ong, National University of Singapore, Singapore C T Tam, National University of Singapore, Singapore Abstract An improved method of assessing passing ability has been developed to mitigate several aspects to the current method of assessing the passing ability of concrete by means of J-Ring test. The identified approach is intended to improve the testing method by using a modified J-Ring, named as a P-Ring. It has a larger diameter to accommodate the conventional methodology of conducting a slump flow test. In addition it is able to assess more than one degree of obstruction to the free flow provided in the standard J-ring test equipment. A series of tests was conducted to gather information on the change in slump flow, J-Ring flow and P-Ring flow to compare the capabilities in assessing passing ability of concrete. Three operators performed the three different tests simultaneously over time since the end of mixing to cover a range of slump flow from its initial high flow diameter to the low end of the range for SCC. Only the results of the P-Ring configuration with similar clear spacing between bars and similar extent of circumference obstruction are reported here to illustrate the better sensitivity of the P-Ring at the low end of the slump flow range. A new approach to quantify passing ability in term of a passing ability index (PAI) is introduced. PAI is defined as the ratio of the reduced flow diameter by either the J-Ring or P-Ring to the slump flow diameter measured at the same time since the end of mixing. Keywords: self-compacting concrete, passing ability, slump flow, J-Ring flow, P-Ring flow, passing ability index (PAI) 1. INTRODUCTION The most significant property of self-compacting concrete (SCC, also known as self-consolidating concrete), is its high level of consistence (workability) in its fresh state. The three major factors are flowability, filling ability and stability (resistance against segregation). The first is indicated by the slump flow test, the second by passing ability tests and the third by segregation test. Although there are other test methods for assessing passing ability, e.g. L-box, the most common test method is the

3 J-Ring test. All the three properties have been developed in the American practice under ASTM standard test methods. They are: ASTM C a Standard Test Method for slump flow of self-consolidating concrete ASTM C a Standard Test Method for passing ability of self-consolidating concrete by J-ring ASTM C Standard Test Method for static segregation of self-consolidating concrete using column technique In this paper, the passing ability of SCC using J-ring only is first examined for the various issues related to the use of this test method. The short comings are discussed and ways to overcome them are developed, leading to a modified ring, the P-ring. In addition, the assessment of passing ability is defined in terms of a new passing ability index (PAI). This is defined as the ratio of the reduced flow diameter in the P-ring test to the slump flow diameter obtained at the same test period after the end of mixing. 2. J-RING TEST The principle of the J-ring test method is likely to be from Japan, but no specific reference is known (ERNARC, 2002), but the test method is not found in the later publication of the SCC European Project Group (ERNARC, 2005). In addition to the particular ring adopted by ASTM C1621 (2009), there are other combinations of bar size and bar height. In general, the ring diameter is around 300 mm. Details of the ring adopted by ASTM C1621 (2009) is shown in Figure ASTM C1621 (2009) Figure 1 J-Ring Apparatus (ASTM C1621, 2009) The test method is for a sample of fresh concrete to be placed into the commonly used slump cone, either in the normal upright or inverted position located centrally within the J-ring. The concrete is placed in a single lift without any tamping or vibration as for the slump flow test. Similarly, the two diameters of the concrete mass are measured in approximately orthogonal directions after the

4 concrete has passed the ring and ended its flow. The difference between the slump flow and J-Ring flow provides an indication of the passing ability of the concrete, with both tests with the cone in the same orientation. Both flow diameters with and without the J-Ring are reported to the nearest 10 mm as well as for the difference between them. Blocking assessment is indicated according to Table 1. Table 1 Blocking Assessment (ASTM C1621, 2009) Difference between Slump Flow and J-Ring Flow Blocking Assessment 0 to 25 mm [0 to 1 in.] No visible blocking > 25 to 50 mm [> 1 to 2 in.] Minimal to noticeable blocking > 50 mm [> 2in.] Noticeable to extreme blocking 2.2 Issues with J-Ring Test The current test method has several disadvantages which are discussed in the following sections. Four main issues have been identified for the modifications that are incorporated into the new P-Ring Ring diameter The diameter of the J-Ring (300 mm) is relatively small when used with the standard slump cone, better known as Abram s Cone, with a base diameter of 200 mm. In addition, it is usually provided with foot holds for the operator to stand on whilst filling the cone. Even using an alternate cone without these foot holds, it is still rather difficult to remove the cone by lifting it vertically due to the tight clearance between the base of the slump cone and the inner diameter of the J-Ring (262 mm). Together with the minimum metal thickness of 1.5 mm for the cone, the clearance between the cone and the inner diameter of the ring is less than 30 mm all round. Furthermore, the need to avoid contact between the cone and ring during lifting may unduly slow down the lifting operation (within 3 ± 1 s). This has led to the use of the inverted cone procedure. For the new P-Ring of 500 mm diameter, the clearance is increased by 100 mm enabling proper vertical lifting without hindrance. The increased diameter has the added advantage that the time to reach a free flow of 500 mm diameter (T 500 or T 50 ), an indication of the viscosity of the SCC can also be determined. In practice, the two flow tests, with and without the ring are likely to be conducted one after the other. The time between them may be 5 to 10 minutes. The change in flow characteristics, if any, may be indicated by the value of T 500, also obtained in each test Inverted cone procedure The use of the inverted cone procedure results in the slumping or flowing of a volume of concrete having the larger cross section (200 mm diameter) on top with the smaller cross section (100 mm diameter) below. Although in general, both orientations of the cone are permitted and likely to produce similar flow diameters for most mixtures, its influence can be significant for a certain range of viscosity of SCC and slump flow values. It is preferred that the normal cone can be used for both slump flow as well as J-Ring flow tests. This is made possible with the increased 500 mm diameter of the P-Ring Clear spacing Although there are some differences noted for the height of the steel bars in the survey of various rings in published literature (Chan, 2007), this factor is not significant in terms of obstruction to the flow of concrete passing through the clear spacing between the bars. The size of the bars in combination with the clear spacing between bars is a key factor in providing obstacles against the flow of the slumping fresh concrete. For a given diameter of the ring, the number of bars and the diameter of the bars (usually of the same size throughout), determine the clear spacing between bars and the percentage of the circumferential obstruction. In reinforced concrete where the maximum aggregate size does not generally exceed 20 mm and the typical reinforcement bar diameter ranges from 10 to 40 mm, the choice of 16 numbers of 16 mm diameter round bars for the J-Ring ASTM C 1621 (2009) results in a clear spacing of 42.9 mm between bars and a circumferencial obstruction of 27% if the diameter of the is 300 mm in the case of the J-Ring. This clear spacing is just over twice that of the 20 mm maximum aggregate size. In the new P-Ring a similar clear spacing 45.5 mm is provided by selecting 24 numbers of 20 mm diameter bars but with a circumferencial obstruction of 31%. However, other combinations of number of bars and bar diameters, e.g. 10 mm, 20 mm and 40

5 mm diameters, offer a range of alternative configurations of clear spacing between bars and percentage circumferencial obstruction (Chan, 2007). This enables the selected configuration to be closer to actual conditions encountered in the field to provide a more realistic assessment of the SCC used. The longer radial flow distance of around 150 mm before the slumping concrete mixture reaches the bars is also expected to be less influenced by the slight pressure head of the volume of fresh slumping concrete as it encounters the bars after flowing radially for only about 50 mm in the case of the inverted slump cone procedure. 3. P-Ring The details of the new P-ring are shown in Figure 2. The number of bars and their diameters may be varied to provide clear spacing and percentage circumferential obstruction desired to better represent the actual field conditions. Figure 2 shows the case of 14 numbers of 10 mm, 20 mm or 40 mm diameter bars. Figure 2 P-Ring Apparatus (Chan, 2007) 3. Experimental Study A limited study to assess the difference in the performance of the J-Ring and the new P-Ring in determining the passing ability of a SCC is reported in this paper. It forms part of a more extended study which will be reported separately. Details of the study have been reported fully by Chan (2007). The objective of the study reported herein is to compare the performance of the J-Ring with a typical configuration of bars (number and size) with the new P-Ring having similar clear spacing and percentage circumferencial obstruction as described in above. The P-Ring has a slightly higher clear spacing (45.5 mm compared to 32.9 mm) but also a higher percentage circumferencial obstruction (31% compared to 27%) relative to the J-Ring. 3.1 SCC The SCC used for the study has the following composition: Portland cement 400 kg Crushed sandstone fines 100 kg Fine aggregate (natural sand) 550 kg Coarse aggregates mm 880 kg 5-10 mm 250 kg Superplasticiser 5.2 litres

6 The volume batched is around 50 liters for conducting the flow tests for the period of testing. 3.2 Test programme The test programme consists of three tests as follows: (a) (b) (c) slump flow J-Ring flow P-Ring flow All the three tests were performed at the same time by three separate operators, each performing the same test starting initially at the end of mixing and at selected time intervals thereafter until the slump flow falls below 550 mm the lower limit for SCC. 3.3 Test results The three types of flow tests were carried out at intervals of about 5 to 6 minutes until around 95 minutes when the slump flow reached below 550 mm. The results are as shown in Table 2. Table 2 Results of flow tests (P-Ring with 45 mm clear spacing) Slump flow J-Ring flow P-Ring flow Passing Ability Time T 500 d d (s) (s) sf d jf d sf d jf d sf d pf pf Index (mm) (mm) (mm) (mm) (mm) PAI-J PAI-P < < > < Two types of interpretation of the test results are provided. The first is based on the difference between the flow diameters of slump flow and that of the J-Ring flow or P-Ring flow. The criteria of blocking assessment according to ASTM C 1621 (2009) (see Table 1), is that the difference between slump flow, d sf and J-Ring flow, d jf remained below 25 mm until about 95 minutes by which time the slump flow was down to 550 mm. On the other hand, the difference between slump flow, d sf and P- Ring flow, d pf values was observed to exceed 25 mm at around 60 minutes. This is to be expected as the rate of flow when the concrete reaches 500 mm (P-Ring diameter) is slower than at 300 mm (J- Ring diameter) and hence the effect of the obstruction is enhanced, i.e. the P-Ring is more sensitive to changes in passing ability than the J-Ring. The blocking assessment criteria in ASTM C 1621 (2009), applies to the full range of slump flow in SCC. In order to include the difference arising from slower slump flow rate on passing ability, another way to quantify this reduction in passing ability is proposed in terms of a Passing Ability Index, (PAI). The values for T 500 changed from an initial value

7 of slightly below 2s to over 5s after about 95 minutes when the slump flow was down from an initial value of 740 mm to 550 mm. 3.4 Passing ability index The passing ability index (PAI) is intended to take into consideration not only the reduction in flow diameter due to obstruction by the bars but also in relation to the slump flow diameter of the SCC. Hence, the derivation of PAI is as shown below: PAI = 1 (d sf d jf )/d sf = d jf /d sf (PAI-J for J-Ring test), or = 1 (d sf d pf )/d sf = d pf / ds (PAI-P for P-Ring test) The values for PAI based on the J-Ring and P-Ring tests are as shown in the last two columns of Table 2. It can be noted that the value of PAI-J = 0.95 corresponds to the cases of a reduction of flow diameter exceeding 25 mm in the J-Ring test (ASTM C1621, 2009). It is expected that at a higher slump flow, a larger reduction in flow can be tolerated. The range of slump flow for SCC is typically between 550 mm to 750 mm. For example at PAI = 0.95, the reduction in flow is about 40 mm at a slump flow of 750 mm, but only 30 mm at a slump flow of 550 mm. The PAI value of less than 0.90 corresponds to about an 80 mm reduction in the flow at a slump flow of 750 mm and only a 50 mm reduction at a slump flow of 550 mm. The trends of changing PAI with time is shown in Figure 3 which shows that the values of PAI for the P-Ring indicates a more distinct drop in PAI than for the J- Ring as the slump flow is observed to reduce with time (Figure 4). Figure 3 Passing Ability Index for J-Ring and P-Ring

8 1.02 Passing 0.98 PAI-J PAI-J PAI-P PAI-P y = 0.117Ln(x) R 2 = y = 0.231Ln(x) R 2 = Ability Index PAI Slump Flow (at the time of PAI test) - mm Figure 4 Changes in PAI with slump flow 4. Concluding Remarks The comparison of the test results between the ASTM C 1621 (2009) J-Ring and the P-Ring with 24 bars of 20 mm diameter (a configuration with similar percentage circumferencial obstruction and clear spacing of J-Ring) indicates that the reduction in flow diameter is better assessed by the P-Ring test. Several of the less desirable features of the J-Ring test are mitigated by selecting the 500 mm diameter P-Ring. In addition, the P-Ring test enables the value of T 500 to be determined as part of the test procedure. The recommended assessment method based on the Passing Ability Index (PAI) takes into consideration the slump flow diameter of SCC at the time of passing ability measurement. As no blocking is the requirement during casting, the use of the P-Ring flow diameter alone when the PAI is above 0.95 is a reasonable conservative indication of slump flow as its value is generally about 30 mm lower. 4.1 Recommendation The slump flow test is often performed when SCC is delivered to the site. The P-Ring test can be added on to the suite of tests performed to ensure that a PAI of 0.95 and above is achieved. The value of T 500 is also to be measured when carrying out both the slump flow test and the P-Ring test for comparison, as with a single operator, the two tests are expected to be conducted one after the another. When the actual arrangement of bars is significantly different from the configuration of the P-Ring with 24 numbers of 20 mm diameter bars, other more suitable configurations may be selected to better representation of actual site conditions. REFERENCES ASTM C a, (2009), Standard Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring, ASTM Annual Book of Standards, ASTM International, West Conshohocken, V , pp

9 Chan, K.D., (2007), Passing Ability of Self-Compacting Concrete, B.Eng. Dissertation, Department of Civil Engineering, National University of Singapore, EFNARC, (2002), Specification and Guidelines for Self-Compacting Concrete, The European Federation of Specialist Construction Chemicals and Concrete Systems, February 2002, 32pp. EFNARC, (2005), The European Guidelines for Self-Compacting Concrete, SCC European Project Group, The European Federation of Specialist Construction Chemicals and Concrete Systems, May 2005, 63pp.