CHAPTER 7 PROPERTIES OF SCC AT FRESH STATE

Transcription

1 84 CHAPTER 7 PROPERTIES OF SCC AT FRESH STATE 7.1 GENERAL In the construction of structures having thin sections, pour-heights of more than acceptable free fall forconventional concrete, combined with dense reinforcement and inserts, it is essential that concrete used has high deformability with moderate viscosity to ensure uniform dispersion of concrete constituents during transportation, casting and thereafter until settling. It should have the ability topass through the congestion and occupy complete space in the form. Concrete should becapable of retaining fresh concrete properties for longer duration to cater to the time requirements of other concreting operations from transportation to final finishing. In fact, the most important property is the ability of concrete to resist segregation, i.e. the stability of fresh concrete. It depends on the cohesiveness and viscosity of the mix. The mix should be capable of withstanding the expected/possible variations in the amount of mixing water, moisture content of ingredients or in the proportions of coarse and fine aggregate, to remain cohesive and free flowing. 7.2 CASTING THE TEST SPECIMENS To prepare the concrete mix, coarse aggregate was placed inside the concrete mixer followed by fine aggregate. Then 20% of the total quantity of water was added. The concrete mixer was allowed to rotate a few times after which fly ash and cement were added. Approximately 40% of the total

2 85 quantity of water was poured into the concrete mixer and the materials were mixed for 1 minute. Superplasticizer and VMA were added to the remaining quantity of water and added to the mixer. Mixing was continued for another 2 minutes. After mixing, tests were conducted to determine the properties of fresh concrete as per EFNARC Guidelines. Slump flow, L box, U box and V funnel test was used to evaluate the fresh concrete properties of SSC. 7.3 FRESH CONCRETE PROPERTIES After mixing, the mix must be tested to determine the properties of fresh concrete as per EFNARC guidelines. The details of the tests are given in Table 7.1 and the equipments are in Figure 7.3. Slump flow, L box, U box, and V funnel test were used to evaluate the fresh concrete properties of SSC. A concrete mix can only be classified as SCC if the requirements for all the following three workability properties are fulfilled (EFNARC, 2002) i) Filling ability ii) Passing ability and iii) Segregation resistance

3 86 Table 7.1 List of test methods for workability properties of SCC and acceptance Criteria for SCC S. No Property of SCC Test Method Unit Typical range of values Minimum Maximum Slump Flow Test Mm Filling ability T 50cm Slump Flow Test Sec 2 5 V-Funnel Test Sec 6 12 Orimet Test Sec 0 5 L-Box Test h 2 /h Passing Ability U-Box Test (h 2 - h 1 ) mm 0 30 Fill Box Test % J-Ring Test Mm Segregation Resistance V-Funnel Test at 5 minutes GTM Screen Stability Test Sec 0 +3 % Filling Ability It is the ability of SCC to flow into all spaces within the formwork under its own weight. Tests such as slump flow test, T 50 slump flow test, V-funnel test and orimet test are used to determine the filling ability of fresh concrete. Self-compacting concrete must be able to flow into all the spaces within the formwork under its own weight. This is related to workability as measured by slump flow or Orimet test. The filling ability or flowability is the property that characterizesthe ability of the SCC of flowing into formwork

4 87 and filling all space under its own weight, guaranteeing total covering of the reinforcement. The mechanisms that govern this property are high fluidity and cohesion of the mixture Passing Ability It is the ability of SCC to flow through tight openings, such as spaces between steel reinforcing bars, under its own weight. Passing ability can be determined by using U-box test, L-box test, Fill-box test and J-ring test methods. Self-compacting concrete must flow through tight openings such as spaces between steel reinforcing bars under its own weight. The mix must not block during placement.the passing ability is the property that characterizes the ability ofthe SCC to pass between obstacles, gaps between reinforcement, holes and narrow sections, without blocking. The factors that govern passing ability are moderate viscosity of the paste and the properties of the aggregates, principally maximum size of the coarse aggregate. Such properties provide uniformity of the mixture during transport, placement and consolidation.the mechanisms that govern this property are the viscosity and cohesion of the mixture Segregation Resistance Segregation resistance is the property that characterizes the ability of the SCC to avoid the segregation of its components such as the coarse aggregates. SCC must meet the filling ability and passing ability with uniform composition throughout the process of transport and placing. Latest developments in accordance with the objectives of the European SCC project aim to limit the admixtures used for general purpose SCC to only one by using new types and combinations ofpolymers. Experience has shown that

5 88 such an admixture may have to be added to generate and maintain compacting concrete using less liable materials. 7.4 EVALUATION OF SELF-COMPACTABILITY OF FRESH CONCRETE A number of test methods such as slump-flow, U-flow, V-flow time, L-box, and J-ring tests are in use for the evaluation of self-consolidating properties of the concrete. These test methods have two main purposes. One is to judge whether the concrete is self-compactable or not, and the other is to evaluate deformability or viscosity for estimating proper mixture proportioning if the concrete does not have sufficient self-compactability. The most commonly used methods for this purpose are discussed briefly in the following sections Slump Flow Test The slump flow test aims at investigating the free, unrestricted deformability of SCC. This test evaluates the flowability of SCC, measuring the horizontal free flow of SCC in the absence of obstructions. The test method is based on the conventional slump test. Slump-flow testing is the simplest and the commonly adopted test method for evaluating the flowability of SCC. This test measures the horizontal free flow of SCC inthe absence of obstructions. An Abram s slump cone is filled with concrete without any tamping. The cone is lifted and the diameter of the concrete after the flow has stopped is measured as shown in Figure 7.1 The mean diameter in two perpendicular directions of the concrete spread is taken as the value of slumpflow. It gives no indication of the ability of the concrete to pass between reinforcement without blocking but may give some indication of resistance to segregation. The higher the slump flow value, the greater is its ability to fill

6 89 formwork under its own weight. Acceptable range for SCC is from 650 to 800 mm (EFNARC, 2005). SCC is characterized by a slump-flow of 660 to 720 mm. Measurement of slump-flow indicates the flowability of SCC and determines the consistency and cohesiveness of the concrete by Campion, J & Jost, P (2000). Figure 7.1 Slump cone apparatus Figure 7.2 Measurement of slump flow

7 90 About 6 litre of concrete is needed to perform the test, sampled normally. Moisten the base plate and inside of slump cone. Place base plate on level stable ground and the slump conecentrally on the base plate and hold down firmly. Fill the cone with the scoop. Do not tamp, simply strike off the concrete level with the top of the cone with trowel. Remove any surplus concrete from around the base of the cone. Raise the cone vertically and allow the concrete to flow out freely. Simultaneously start the stopwatch and record the time taken forthe concrete to reach the 500 mm spread circle. This is T 50 time. Measure the final diameter of the concrete in two perpendicular directions. Calculate the average of the two measured diameters. This is slump flow in mm as shown in Figure 7.1. The slump-flow test measures the capability of concrete to deform under its own weight against the friction on the surface of the base plate with no other external resistance present by Sonebi, M et al. (2000). According to Nagataki and Fujiwara (2005), a slump-flow ranging from 500 to 700 mm is considered as a proper slump required for a concrete to qualify for selfconsolidating concrete. At more than 700 mm, the concrete might segregate and at less than 500 mm the concrete is considered to have insufficient flow to pass through congested reinforcement. According to Bartos (2000), the slump-flow test can give an indication of filling ability and susceptibility to segregation of the self-consolidating concrete. Flowing time from the initial diameter of 200 mm to 500 mm, designated as T 50, is sometimes used for a secondary indication of flow. A time of 3 to 7 seconds is acceptable for general applications and 2 to 5 seconds for housing applications. The results of the slump flow test on fresh SCC mixes M1 to M8 are listed in Table A1.1 to Table A1.8 in the appendix.

8 V-Funnel Test Figure 7.3 V-funnel test apparatus V funnel test was developed in Japan and used Ozawa et al. (1996). The equipment consists of a V-shaped tunnel, with a rectangular cross section as shown in Figure 7.2. The top dimension is 495 mm by 75 mm and the bottom opening is 75 mm by 75 mm. The total height is 572 mm with a 150 mm long straight section. Though the test is designed to measure flowability, the result is affected by concrete properties other than flow. The inverted cone shape will cause any liability of the concrete to block to be reflected in the result. High flow time can also be associated with low deformability due to high paste viscosity and with high inter-particle friction.the different test apparatus are seen in Figure 7.3. Figure 7.4 Apparatus for testing the workability of SCC

9 92 To determine the flow time, about 12 litre of concrete is needed to perform the test, sampled normally.set the V-funnel on firm ground. Moisten the inside surfaces of the funnel. Keep the trap door open to allow any surplus water to drain. Close the trap door and place a bucket underneath. Fill the apparatus completely with concrete without compacting ortamping; simply strike off the concrete level with the top with the trowel. Open within 10 sec after filling the trap door and allow concrete to flow out under gravity. Start the stopwatch when the trap door is opened and record the time for the discharge to complete (the flow time). This is taken tobe when light is seen from above through the funnel. This is called the V-flow time by Ferraris, Cet al. (2000). A flow time of less than 6 seconds is recommended for a concrete to qualify as a self-consolidating concrete. After this the funnel can be refilled concrete and left for 5 minutes to settle. If concrete shows segregation, then the flow time will increase significantly. The whole test has to be performed within 5 minutes. The results of the V-funnel test on fresh SCC mixes M1 to M8 are listed in Table A1.1 to Table A1.8 in the appendix. To determine the flow time at T 5 minutes, do not clean or moisten the inside surfaces of the funnel again. Close the trap door and refill the v- funnel immediately after measuring the flow time. Place a bucket underneath. Fill the apparatus completely with concrete without compacting or tapping, simply strike off the concrete level with the top with the trowel. Open the trap door 5 minutes after the second fill of the funnel andallow the concrete to flow out under gravity. Simultaneously start the stopwatch when the trap door is opened and record the time for the discharge to complete (the flow time T 5 minutes). This is taken to be when light is seen from above through the funnel. The results of the V-funnel test at T 5 minutes on fresh SCC mixes M1 to M8 are listed in Table A1.1 to Table A1.8 in the appendix..

10 L-Box Test Figure 7.5 L-box test apparatus L box test assesses the flow of concrete and also the extent to which it is subjected to blocking by reinforcement. The L-box test method uses a test apparatus consisting of a vertical section and a horizontal section as shown in Figure 7.4. The apparatus consists of a rectangular section box in the shape ofan L, with a vertical and horizontal section, separated by a moveable gate. Reinforcing bars are placed at the intersection of the two areas of the apparatus. The gap between the reinforcing bars is generally kept at 35 and 55 mm for 10 and 20 mm maximum-size coarse aggregates, respectively. The details of the L box are in Figure 7.4. About 14 litre of concrete is needed to perform the test, sampled normally. Set the apparatus level on firm ground, ensure that the sliding gate can open freely and close it. Moisten the inside surfaces of the apparatus, remove any surpluswater. Fill the vertical section of the apparatus with the concrete sample. Leave it to stand for 1 minute.lift the sliding gate and allow the concrete to flow out into thehorizontal section.the time taken by the concrete to flow distances of 200 mm (T-20) and 400 mm (T-40) in the horizontal section of the apparatus, after the opening of the gate from the

11 94 vertical section, is measured. When concrete stops flowing, the distance h 1 and h 2 are measured. Calculate h 2 /h 1, the blocking ratio.the whole test has to be performed within 5 minutes.this test gives an indication of the filling, passing and segregation-resisting ability of the concrete by Sonebi, M et al. (2000). The results of the L-Box test on fresh SCC mixes M1 to M8 are listed in Table A1.1 to Table A1.8 in the appendix U-Box Test Figure 7.6 U-box test apparatus The test is used to measure the filling ability of self-compacting concrete.the U-flow test examines the behaviour of the concrete in a simulated field condition. It is one of the most widely adopted test method for characterization of SCC. This test simulates the flow of concrete through a volume containing reinforcing steel. This test is considered more appropriate for characterizing self-compactability of concrete. The apparatus consists of a vessel divided by a middle wall into two compartments, shown by R1 and R2 in Figure 7.5. An operating with a sliding gate is fitted between the two sections.reinforcing bars with nominal

12 95 diameters of 13mm are installed at the gatewith centre to centre spacing of 50mm.This creates a clear spacing of 35mm between the bars. About 20 litre of concrete is needed to perform the test. Set the apparatus level on firm ground, ensure that the sliding gate can open freely and then close it. Moisten the inside surfaces of the apparatus, remove any surpluswater. Fill the one compartment of the apparatus with the concrete sample.this test is performed by first completely filling the left chamber of the U-flow device, while the sliding doors between chambers are closed. The door is then opened and the concrete flows past the rebars into the right chamber. Leave it to stand for 1 minute, Lift the sliding gate and allow the concrete to flow out into theother compartment. After the concrete has come to rest, measure the height of the concrete in the compartment that has been filled, in two places and calculate the mean (h 1 ). Measure also the height in the other compartment (h 2 ). Calculate h 1 -h 2, the filling height.the whole test has to be performed within 5 minutes. Self-consolidating concrete for use in highly congested areas should flow to about the same height in the two chambers. If the filling height is at least 70% of the maximum height possible, then the concrete is considered as SCC. Therefore, a concrete with a final height of more than 200 mm is considered self-consolidating concrete by Ferraris, Cet al. (2000). This test measures filling, passing and segregation properties of self-consolidating concrete. The results of the U-box test on fresh SCC mixes M1 to M8 are listed in Table A1.1 to Table A1.8 in the appendix.

13 CONCLUSIONS To obtain the expected performance of the SCC mixes at fresh state, a perfect quality control is essential. This ensures fresh concrete properties in Slump flow test, L box test, U box test, V funnel test. Even small variation in the water powder ratio or the moisture content of the aggregates may lead to inaccurate results.