Use of Viscosity-Modifying Admixture to Reduce Top-Bar. Effect of Anchored Bars Cast with Fluid Concrete by K. H. Khayat. With the increasing use of highly ...
ACI MATERIALS JOURNAL
TECHNICAL PAPER
Title no. 95-M16
Use of Viscosity-Modifying Admixture to Reduce Top-Bar Effect of Anchored Bars Cast with Fluid Concrete
by K. H. Khayat With the increasing use of highly fluid concrete to facilitate the casting and consolidation of congested reinforced concrete sections, it is essential to ensure adequate stability of fresh concrete to reduce micro-structural defects at the interface between the hardened cement paste and embedded reinforcement. Accumulation of bleed water under the reinforcement and minute separation offresh paste from the reinforcement due to segregation and settlement can significantly reduce bond with anchored or overlapped reinforcement. The reduction in bond with horizontally embedded bars located in the upper sections of structural elements as opposed to those located near the bottom is known as the top-bar effect (top-bar factor). This paper aims at investigating the effectiveness of incorporating viscosity-modifying admixture to enhance the stability of high-slump concrete and reduce the top-bar factor of anchored bars. A total of 25 specimens measuring 500, 700, and 1100 mm in height were cast with various mixtures with slump values of220 and 190 mm and self-consolidating mixtures with slump flow values of 600 to 690 mm. The specimens were prepared to evaluate the effect of viscosity-modifying admixture content, specimen height, and mode of consolidation on external bleeding, surface settlement, segregation, and relative bond strength to horizontally embedded bars. The findings indicate that regardless of the slump, specimen height, and mode of consolidation, the reduction in surface settlement (that is related to bleeding and segregation) resulting from incorporating a viscosity-modifying admixture (welan gum) can significantly reduce the top-bar factor. Highly stable, self-consolidating concrete with low settlement had low top-bar factors comparable to specimens cast with 190- and 220-mm slump concrete. The use of0.07 percent welan gum in concrete made without any silica fume developed greater stability and lower top-bar factor than similar concrete containing 0.035 percent of welan gum and 8 percent silica fume replacement. Keywords: average bond strength; bleeding; segregation; self-consolidating concrete; settlement; stability; top-bar effect; viscosity-modifying admixture.
INTRODUCTION Fresh concrete consists of an unstable dispersion of cement paste and aggregate. The settlement of relatively heavy solid constituents can result in an upward displacement of part of the free water in the fresh concrete in the form of bleeding. Some of the bleeding water reaches the upper concrete surface, while the remaining water can be trapped in bleed channels and under various obstacles, such as aggregate and reinforcement. Rising bleed water causes variation in water-to-cement ratio (w/c) between the top and lower portions of a cast section that affects the in situ porosity and 158
mechanical properties. Hoshino 1 reported that the increase in external bleeding of concrete specimens cast with mixtures made with w/c of 0.5, 0.6, and 0.7 can reduce the compressive strength at the upper portions of the specimens without exhibiting much strength reduction at the lower portions. Accumulation of bleed water under horizontally embedded reinforcement bars can also increase the local w/c under the bar and weaken the quality of the bond. 2 Through an interactive action between adjacent solid particles in a dense suspension, the segregation of aggregate can lead to settlement of unhydrated cement grains along with some early hydration products in the plastic concrete. The settlement of fresh concrete around rigidly held reinforcement can reduce the effective projection of the lugs and lead to a minute separation between the lower portions of reinforcement and concrete that contributes to reducing bond. Surface settlement can also lead to the formation of surface cracks above top-cast bars because of the restraining action of the bar to the settling of plastic concrete. Such surface settlement cracking that reduces the quality of the interface between the reinforcement and concrete increases with the increase in free water content that reduces the viscosity and increases the risk of bleeding and settlement. 3 Surface settlement cracking increases with the increase in bar size and the reduction in the cover over the bar. 3 With the increase in specimen height, greater amount of bleeding, segregation, and surface settlement can be obtained because of the increase in the quantity of fresh concrete contributing to such bleeding, segregation, and settlement. 1•4 As a result, bond strength between top-cast bars and surrounding concrete that is a function of the interface between the bar and concrete can be significantly lower than that with bars embedded at the lower portion of cast
ACI Materials Journal, V. 95, No.2, March-April 1998. Received March 4, 1996, and reviewed under Institute publication policies. Copyright © 1998, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the January-February 1999 ACI Materials Journal if received by October I, 1998.
ACI Materials Journal I March-April 1998
ACI member K. H. Khayat is an associate professor of civil engineering at the Universite de Sherbrooke, Quebec, Canada. His research interests include high-performance concrete, rheology, repair, and grouting.
specimens. This tendency to weaken bond strength between concrete and horizontally anchored or overlapped bars located in upper sections of structural elements is known as the top-bar effect. The top-bar effect is recognized in design codes by applying a modification factor to top-cast bars to increase the anchorage and spliced lengths. For example, the ACI Building Code Provisions (1995) and the AASHTO Bridge Specifications (1989) require the increase of the recommended development lengths by 30 and 40 percent, respectively, for normal steel whenever more than 305 mm of concrete is cast below the reinforcement. 5•6 The effect of bleeding, segregation, and settlement on weakening bond between concrete and rigidly held reinforcements is related to several factors, including the stability of the concrete, the position of the embedded bar, the extent of vibration consolidation, the leakage of the formwork, and the roughness of formwork.?- 10 The quality of the interface between concrete and embedded reinforcement, especially when large size bars are used, is significantly weakened by the increase in bleeding, segregation, and settlement. Such reduction in bond is aggravated by the increase in slump and the period during which the concrete remains in a plastic state, especially when the concrete does not exhibit adequate stability. The combined effect of increasing slump (and hence instability of conventional concrete) and depth of concrete cast below anchored bars on the development length ratio l 2/l 1 needed to maintain a constant Pl pullout force is shown in Fig. 1. JI The l 21l 1 value corresponds to the top-bar factor and is shown to increase with the depth of concrete cast under the top-anchored bar and the increase in slump. Therefore, special attention is required when using fluid concrete to ensure adequate stability andreduce the top-bar effect. The stability of fresh concrete refers to its ability to resist bleeding, sedimentation, and segregation that depends on the cohesiveness and viscosity of the mixture. The viscosity decreases with the increase in free water and reduction in the concentration of fine particles. The stability decreases with the increase in consolidation effort as the vibration reduces the viscosity of the mixture and hence its ability to maintain the various constituents in a homogeneous dispersion. 4 The fluidity, or deformability, of concrete refers to the ease of the fresh mixture to undergo changes in shape under its own weight. The fluidity decreases with the increase in inter-particle friction between the various solids and is closely related to the yield value of the mixture. The yield value decreases with the increase in w/c and incorporation of high-range water reducer (HRWR). Unlike water addition, the use of an HRWR decreases especially the yield value and to a lower degree the viscosity of the mixture. Maintaining a moderate viscosity is necessary to ensure good stability of highly fluid concrete. For a mixture with a constant slump, w!c, and sand content, the effect of HRWR on reducing stability is limited when a HRWR is incorporated to enhance fluidity. The use of HRWR enables the reduction of water ACI Materials Journal I March-April 1998
Fig. ]-Relationships between top-bar effect, casting height, and slump (after Ref 11)
content necessary to enhance fluidity and results in a dispersion of cement grains that leads to a decrease in sedimentation velocity, hence reducing settlement and bleeding. 12 When an excessive dosage of HRWR is added to the concrete, the cohesiveness can decrease, resulting in greater segregation and heterogeneity with depth compared to a stiffer concrete made with a similar w!c. 13 High-slump concrete made with HRWR can therefore exhibit a greater top-bar effect than a lower slump concrete of an equivalent w!c in conventional concrete, as indicated in Fig. I. In addition to the greater free water content resulting from the use of HRWR to enhance fluidity (constant w/c) that can result in more bleeding, the HRWR can delay the setting time, hence necessitating a stable concrete for longer duration. Brettmann et al.? found that fluid mixtures containing HRWR can develop lower bond strength to reinforcing steel than lowslump concrete because of the longer period during which the concrete remains in a plastic state where bleeding and settlement can continue to take place. Highly stable mixtures that can flow readily into place with limited consolidation effort can be obtained by combining proper concentrations of viscosity-modifying admixture (VMA) and HRWR. Another way to enhance stability of fluid concrete is to use fine materials, such as silica fume, to provide greater cohesiveness. The incorporation of a VMA affects the aqueous phase of the cement paste where chains of the water-soluble polymer can imbibe some of the free water in the system, thus reducing the free water content and enhancing the viscosity of the paste. 14 The increase in VMA dosage increases significantly the viscosity of the aqueous phase that improves the capacity of the paste to suspend solid particles. It also increases the yield value of the paste that limits the deformability of the concrete. Therefore, HRWR is needed to maintain a relatively low yield value; hence, any loss in fluidity can be regained without significantly reducing the stability of the concrete. 4 •12.1 4 Highly flowable concrete can then be proportioned to obtain high deformability and adequate stability to facilitate the casting and consolidation of congested reinforced sections. In a previous publication,4 the effect of incorporating welan gum as a VMA was found to significantly reduce external bleeding and surface settlement of concrete mixtures made with 0.50 water-to-cementitious material ratio (w!cm) cast with slump levels up to 220 mm. Similar results were obtained for mixtures made with w!cm of 0.50, 0.60, and 159
Table 1-Summary of test program Slump,mm
Specimen height, Anchored length, mm(no. of db specimens)
Rodded concrete, percent
Vibrated concrete, percent VMA = 0, 0.035, 0.07 (segregation, bond)
220
500 (6)
5
VMA = 0, 0.035, 0.07 (bleeding, settlement, segregation, bond)
220
700 (6)
5
VMA = 0, 0.035, O.o? (bleeding, settlement, segregation, bond)
VMA = 0, 0.035, 0.07 (segregation, bond)
220
1100(6)
5
VMA = 0, 0.035, 0.07 (bleeding, settlement, segregation, bond)
VMA = 0, 0.035, O.o? (segregation, bond)
190
700 (3)
2.5
VMA = 0, 0.035, 0.07 (settlement, bond)
-
S flow 650 ±50
700 (4)
2.5
Self consolidation* VMA = 0.05 to O.G75 (settlement, bond)
-
* Self-consolidating concrete cast without rodding or external vibration
0.70 and slump values of 180 mm. Regardless of slump, w!cm, column height, or degree of consolidation, the incorporation of proper combinations of VMA and HRWR was shown to significantly reduce bleeding, settlement, and segregation. Relationships between external bleeding, surface settlement, and segregation coefficient were derived for specimens of various heights subjected to moderate hand consolidation. These relationships are as follows: 4
the consolidation and ensure proper filling of structural sections, it is important to consider the effect of high fluidity on stability and top-bar effect. The data presented in this paper shed some light on the benefit of incorporating proper concentrations of VMA and HRWR to secure highly stable, yet flowable, concrete to reduce the top-bar effect. This paper can then be useful to engineers considering the use of flowable concrete in deep structural sections whenever the risk of bleeding, segregation, and settlement can be significant.
SC = -0.244 + 0.436 SETTL (R 2 = 0.95) BLD = -0.0767 + 0.021 SC (R 2 = 0.83) BLD = -0.105 + 1.082 x 10-2 SETTL (R 2 = 0.95) where SC = segregation coefficient (percent), SETTL (cm/m 3) =maximum surface settlement, in cm/H (m) x A (m 2), 3 3 BLD (cm /m ) =maximum external bleeding, in cm3/H (m) x A (m 2 ),
H (m) = height of cast column, A (m 2) = cross-sectional area of cast column. Given the above results, the objective of the investigation reported in this paper was to evaluate the effect ofVMA content, silica fume addition, concrete fluidity, height of concrete cast under top anchored bars, and mode of consolidation on stability and top-bar effect. RESEARCH SIGNIFICANCE High levels of bleeding, segregation, and settlement can adversely affect the interface between the hydrated cement paste and aggregate particles as well as the distribution of capillary porosity along a vertical concrete section. Such instability can lead to heterogeneous distribution of concrete properties, including impermeability, mechanical properties, and bond strength to embedded reinforcement. The majority of research carried out on the effect of concrete composition and casting height on reducing the bond to horizontally embedded reinforcement has involved the use of conventional concrete of limited slump. As the technology of concrete placement changes towards using fluid concrete to facilitate
160
EXPERIMENTAL PROGRAM The effectiveness of incorporating a VMA to enhance stability of fluid and highly flowable concrete and reduce the top-bar effect is investigated. As summarized in Table 1, 18 concrete mixtures were prepared with 220 mm slump and contained various concentrations of VMA corresponding to 0, 0.035, and 0.07 percent by mass of cementitious materials. The mixtures were used to cast specimens measuring 500, 700, and 1100 mm in height to evaluate the effect of concrete type and height on stability and bond to horizontally embedded anchored bars. The mixtures containing 0.035 percent of VMA were prepared with a blended silica fume cement, whereas those made without any VMA and those with 0.07 percent of VMA did not contain any silica fume. Nine of the 18 specimens were hand-consolidated, and the remaining specimens were subjected to excessive external vibration. This was carried out to evaluate the effectiveness of incorporating a VMA to enhance stability and reduce top-bar factor in fluid concrete when subjected to excessive consolidation. External bleeding, surface settlement, segregation, and bond stress vs. net slip of reinforcing bars positioned at the top and bottom of the cast specimens were determined for the hand-rodded specimens. Only the segregation coefficient and bond to reinforcement were determined for the externally vibrated specimens. The anchored length of the reinforcing bars was fixed to five times the bar diameter (5 db)Three other specimens measuring 700 mm in height were cast with 190-mm slump concrete that was consolidated with hand rodding. Four similar specimens were cast with various highly flowable, self-consolidating concrete (SCC). Surface settlement and bond strength developments between concrete with top and bottom reinforcing bars ·were determined ACI Materials Journal I March-April 1998
Table 2-Mixture proportions and properties of 220-mm slump concrete Rodded
Rodded
Rodded
Vibrated
Vibrated
Vibrated
VMA, percent of CM
Type of consolidation
0
0.035
0.07
0
0.035
0.07
VMA, percent of water
0
0.07
0.14
0
0.07
0.14
360
330
360
360
330
360
Cement, kg/m 3 Silica fume, kg,tm 3
0
30
0
0
30
0
kg/m 3
180
180
180
180
180
180
900
900
900
900
900
900
960
960
960
960
960
960
Water,
Coarse aggregate, kg/m 3 Sand,
kg/m3
HRWR,L/m 3
2.1
2.1
3.3
2.1
2.1
3.3
Slump,mm
220
220
220
220
220
220
f c'. MPa
44
46
43
44
46
43
Age, days
32
34
28
32
34
28
Specimen height, mm
500
500
500
500
500
500
External bleeding, mL
19.5
4.5
0
-
-
-
Surface settlement, mm
6.75
4.0
3.25
-
-
Segregation coefficient, percent
-
-
-
-
8.06
5.19
2.98
Specimen height, mm
700
700
700
700
700
700
External bleeding, mL
26.0
6.0
0
-
-
-
Surface settlement, mm
8.5
4.5
4.0
-
-
Segregation coefficient, percent
9.00
4.89
4.29
9.41
7.77
5.13
1100
1100
1100
-
Specimen height, mm
1100
1100
1100
External bleeding, mL
46.0
9.5
0
-
-
-
Surface settlement, mm
12.5
9.0
6.75
-
-
-
Segregation coefficient, percent
8.22
6.23
4.53
for each specimen. The !5 was limited to 2.5 db for the seven specimens. Materials and mixture proportions A Type 10 Canadian portland cement (CSA3-A5-M83, similar to ASTM C 150 Type I cement) and a blended silica fume cement containing approximately 8 percent silica fume by mass were used. The SCC mixtures incorporated a Class F fly ash and blast furnace slag with Blaine fineness values of 360 and 445, respectively. A clean and continuously graded crushed limestone aggregate with nominal particle size of 10 mm was used. A well-graded siliceous sand with a fineness modulus of 2.5 was employed. The bulk specific gravities of the gravel and sand were 2.68 and 2.64, respectively, and their absorption levels were 1.3 and 1.5 percent, respectively. Welan gum was used in a powder form to enhance the stability of the concrete. Welan gum is a high molecular-weight polysaccharide produced by a fermentation process. It is a non-toxic product that is also available in a liquid form to facilitate its introduction to concrete. A liquid-based sulfonated naphthalene HRWR with 42 percent solid content was used to enhance fluidity. The sand and coarse aggregate were first homogenized in an open-pan mixer. The cementitious material's, mixing water, and HRWR were then introduced and followed by the powder VMA that was slightly diluted with part of the ceACI Materials Journal I March-April 1998
11.78
7.33
5.70
ment (1: 10). Following the introduction of all mix ingredients, the concrete was mixed for 3 minutes. Following 3 minutes of rest, the mixing was resumed for 2 additio nal minutes. Deformed reinforcing steel bars with a 25-mm effecti ve diameter were used to evaluate bond. The bars are made with Grade 60 steel (414 MPa) complying with ASTM A6 15 specifications. The deformation patterns consisted of ribs Ill· clined at 60 deg with respect to the axis of the bar. The mixture proportions of concretes made with 0. 50 w!cm, 360 kg/m3 of cementitious materials, and various concentrations of VMA are presented in Table 2. The first set of mixtures contained no VMA, while the second and third sets incorporated 0.035 and 0.07 percent welan gum, respecti vely, by mass of cementitious materials. This corresponds to 0.07 and 0.14 percent gum, by mass of water, that are considered to be relatively low and medium contents for land-cast concrete. Blended silica fume cement was only used in mixtures containing 0.035 percent VMA. The dosage rates of HRWR were adjusted to secure initial slump val ues of220mm. The proportions of the seven other mixtures used to cast 700-mm high specimens are summarized in Table 3. The first three mixtures were similar to those presented in Tabl e2 except that the HRWR dosages were reduced to yield 190 mm slump. The other four SCC mixtures were optimized to ensure high deformability to flow well among closely spac ed 161
Table 3-mixture proportions and properties of 190-mm slump concrete and
sec
Rodded
Rodded
Rodded
SCC1*
SCC2*
SCC3*
SCC4*
VMA, percent of CM
0
0.035
0.07
0.064
O.D75
0.075
0.05
VMA, percent of water
0
O.D7
0.14
0.16
0.18
0.18
0.12
360
330
360
257
307
417
342
Silica fume, kg,/m3
0
30
0
0
18
18
18
Fly ash, kglm3
0
0
0
58
0
118
175
Type of consolidation
Cement, kglm3
0
0
0
234
235
0
0
Water, kglm3
180
180
180
225
229
227
219
Blast furnace slag, kg!m3
Coarse aggregate, kg!m3
900
900
900
808
813
815
808
Sand, kglm3
960
960
960
686
690
691
686
HRWR,Um3
1.8
1.9
3.0
5.5
5.9
5.6
5.0
0
0
0
0.5
0.5
0.5
0.5
Set retarder, Um 3 Slump,mm
190
190
190
-
-
-
-
Slump flow, mm
-
-
690
650
640
600
fc',MPa
-
-
-
-
52
52
-
-
-
28
28
-
Age, days Specimen height, mm
700
Surface settlement, mm
2.73
* sec = self-consolidating concrete
reinforcement. The stability of the SCC mixtures was enhanced by combining proper concentrations of VMA and continuously graded cementitious materials. The slump flow of the sec mixtures ranged between 600 and 690 mm and refers to the mean spread diameter of concrete following the removal of the slump cone. Such measurement is more sensitive than the slump value to evaluate the consistency of
sec. Experimental procedures The 18 specimens prepared using 220-mm slump concrete were cast in plywood forms measuring 500, 700, and 1100 mm in height and 200 x 300 mm in cross section. The remaining seven mixtures were cast in 700-mm high forms with cross sections of 200 x 200 mm. The plywood forms had smooth interior surfaces and were rigid and heavily reinforced with external steel bands to prevent deformation. All joints were sealed with silicone, and the bases of the specimens were reinforced with neoprene gaskets to prevent leakage. Each specimen had two horizontally anchored reinforcing bars measuring 25 mm in diameter positioned at 75 mm from the upper and lower ends of the specimen. A rigid plastic sheathing was tightly attached to the loaded end of each bar to limit the bond between the bar and concrete to the remaining portion of the bar. This anchor length was 125 mm (5 db) for the first 18 specimens and 63 mm (2.5 db) for the remaining seven specimens, as shown in Fig. 2. The bonded length of each bar was properly cleaned to ensure adequate bond with concrete. It is important to note that in reinforced concrete members, both the concrete and the steel are simultaneously placed in tension in positive moment regions. In the test setup adopted in this study, the pull-out steel bar is subjected to tension, but
162
I Rigid plastic sheating
I 2.5 db
Fig. 2-Pullout test setup of 200 x 200 mm specimens the surrounding concrete is in compression. The confining compressive stresses around the steel bar are however reduced by positioning the bonded region of the bar away from the loaded end of the specimen (Fig. 2). The distance was greater than 175 mm in the case of bars with anchorage lengths of 5 db and 137 mm in the case of bars with anchorage lengths of 2.5 db. The average bond stresses considered over short anchorage lengths are much higher than the actual bond strengths determined over long anchorage or spliced lengths. The average bond stresses determined in this study are, however, used to determined the top-bar factors where the bond values are considered in a comparative manner. The 700- and 1100-mm high specimens were cast in three lifts, whereas the 500-mm specimens were cast in one lift. For the hand-rodded specimens, each lift was rodded 24 times using a 20-mm diameter tamping rod. Specimens cast with 200-mm slump concrete were also subjected to external vibration that involved mounting the forms onto ·a vibrating ACI Materials Journal/ March-April 1998
50
--
• • • • Segregation vs. settlement Bleeding vs settlement •
E
c:
30
i
[j· "
G)
:a
10
•• •• •• •• • •.• 700mmd. •• ~. •• ••
40
...1
D)
Hand consolidation
20
>< ca :IE
8
~ e....
c: G)
·c:; 6 E G) 0
tl ••
4
.. u c: 0
ca
500mm
•
10
0
D)
VMA 0%
0
0.035%
D
0.07%
2
!D) G)
(/)
0 2
4
10 8 Max. settlement (mm)
6
12
14
Fig. 3-Bleeding and segregation vs. settlement of specimens cast with 220-mm slump concrete containing various VMA contents (after Ref. 4) table that was turned on for 60 seconds after the casting of each lift. The vibrating table operates at a frequency of 60 vibrations per second with an amplitude of 0.6 mm. Compared to the consolidation secured by hand-rodding, the excessive external vibration was considered to constitute severe consolidation condition that can lead to greater risk of segregation, settlement, and bleeding. Surface 8ettlement was determined for the 25 tested specimens using a special set up involving the measurement of changes in the vertical position of two orthogonal laser beams reflected off a thin mirror positioned on the upper concrete surface. 4 External bleeding was determined by collecting water accumulated at the upper surface of concrete specimens at set intervals until reaching a steady state condition that corresponded to approximately the beginning of hardening. Except for periods corresponding to bleeding and settlement measurements, the columns were covered to prevent evaporation. Standard cylinders measuring 100 x 200 mm along with the column specimens were demolded one day after casting and covered with wet burlap and plastic sheeting until the age of 7 days. They were then air cured at approximately 20 C until the age of compressive strength and pullout testing that was carried out at 31 ± 3 days. Average bond stresses with horizontally embedded bars were evaluated by carrying pullout testing using a 135 kN hydraulic jack. The specimens were set horizontally for bond testing with the jack attached to the reinforcing bar with a reaction cylinder positioned against the concrete (Fig. 2). The pullout load was applied gradually and recorded using a load cell. The deformation of the bar was measured using a linear variable deferential transformer (LVDT) located near the loaded end. A data acquisition system with a scanning frequency of one Hertz was used. The net slip was calculated as the total measured deformation minus the elastic deformation in the steel and concrete. The test was terminated when a pullout failure
ACI Materials Journal I March-April 1998
occurred, the reinforcing steel began to yield or the surrounding concrete cover failed in tension. The average bond strength was calculated as follows:
where P, db, and is correspond to the applied load, bar diameter, and anchored length, respectively. Following pullout testing, the segregation of concrete was determined by sawing the specimens along their heights and observing the distributions of coarse aggregate with particles greater or equal to 5 mm along the height. A segregation coefficient is then determined for the tested section using a sum of square approach. 4 An increase in segregation coefficient reflects an increase in the deviation of coarse aggregate distribution from the weighted average value along the observed vertical section. TEST RESULTS AND DISCUSSION
Effect of VMA dosage on settlement, bleeding, and segregation · The total bleeding, maximum surface settlement, and segregation coefficient values of the 18 specimens cast with 220-mm slump concrete are given in Table 2. The maximum surface settlements of three specimens cast with 190-mm slump concrete and four SCC mixtures are given in Table 3. The variations in bleeding, settlement, and segregation with VMA content of hand-rodded concrete are shown in Fig. 3. Regardless of specimen height, the increase of VMA dosage from 0 to 0.035 and 0.07 percent in the 220-mm slump concrete resulted in sharp reductions in bleeding, settlement, and segregation. Concrete mixtures made with Type 10 cement and 0.07 percent VMA exhibited greater stability than those made with blended silica fume and 0.035 percent VMA. The addition of 0.07 percent VMA eliminated externai bleeding and reduced settlement by approximately 50 percent com163
pared to similar concrete made without any VMA. Bleeding and settlement of rodded specimens cast with 220-mm slump concrete decreased considerably with the reduction in specimen height. However, only a slight decrease in segregation was observed when the specimen height decreased from 1100 to 500 mm. The incorporation of VMA was also effective in reducing segregation of concrete subjected to excessive vibration. Regardless of specimen height, vibrated specimens cast with mixtures containing 0.035 and 0.07 percent VMA had approximately 30 and 50 percent, respectively, lower segregation coefficients than the control concrete made without any VMA. 4 The surface settlement decreased also with the increase in VMA content for the 190-mm slump mixtures. Such mixtures were similar to the 220-mm slump concrete except for the lower HRWR dosage. Regardless of the content of VMA, the reduction in slump from 220 to 190 mm resulting from lower addition of HRWR led to significant decrease in settlement (greater stability). Despite the highly fluid nature of the sec mixtures, surface settlements of the four 700-mm high specimens were low and comparable to those of the hand consolidated, 190-mm slump concrete. Effect of VMA dosage on top-bar factor For the 18 specimens cast with 220-mm slump concrete with 5 db development lengths, the failure mode of the pullout test was caused by the splitting of the cover. Given the absence of stirrups that provide some confinement and delay crack propagation, the splitting plane propagated through the 63cmm cover. Therefore, in the absence of ultimate average bond strength values corresponding to a bond failure mode, average bond stresses at small net slips were considered in the analysis of the 18 tested specimens. On the other hand, in the case of the seven remaining specimens that had relatively short anchorage lengths of 2.5 db, a pullout bond failure took place. Typical changes in average bond stress vs. net slip of 700-mm high specimens cast with 220-mm slump concrete consolidated by external vibration are shown in Fig. 4. Both the stiffness of the stress-slip curve and the average bond stress determined at the splitting failure were greater for bottom bars than top-cast bars. The spread between the average bond stress at a given net slip for bottom and top bars was considerably different for concrete incorporating a VMA compared to that made without any VMA. Unlike the latter type of mixtures, insignificant differences in stiffness and maximum stress were obtained between bottom and top bars of concrete containing a VMA. Similar observations were obtained for the remaining rodded and vibrated specimens of different heights cast with 220-mm slump concrete containing various VMA contents. Average bond stress ratios of bottom-to-top bars (Ubo!Utop) taken at small net displacements in the elastic zone are used to compare of the effect of VMA dosage, placement height, and type of consolidation on the top-bar factor. The spread in average bond stress between bottom and top bars, or the top-bar factor, is plotted in Fig. 5 and 6 against various small net displacements for vibrated specimens measuring 500 and 1100 mm in height, respectively. Regardless of the net slip,
164
25 Top bars Bottom bars
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