A.32
COMBINED EFFECT OF SHRINKAGE REDUCING AND EXPANSIVE AGENTS ON AUTOGENOUS DEFORMATIONS
OF HIGH-PERFORMANCE CONCRETE
M.S. Meddah1-Postdoctoral Rearcher; M. Szuki2- Consulting Engineer; R. Sato1- Professor
1Dept. of Social and Environmental Eng, Graduate School of Engineering, Hiroshima University, Japan
2P.S.Mitsubishi Construction Co.,Ltd.
ABSTRACT: Nowadays, high-performance concrete (HPC) has become widely ud throughout the world in different construction filed. However, this type of concrete with low water-binder ratio (w/b) and ultrafine cementitious materials is known by high nsitivity to early-age cracking which is strongly related to a large amount of autogenous shrinkage. This paper investigates the reducing effect of autogenous shrinkage provided by the incorporation of a combination of shrinkage reducing agent (SRA) and expansive additive (EXA). The early-age development of both autogenous shrinkage and the induced stress of silica fume HPC mixtures made with three different w/b and using a combination
of (SRA + EXA) were examined. The results have shown that for the control mixes, up to 90% of the ultimate autogenous shrinkage strains was exhibited during the first 24 hours. However, adding a combination of (SRA + EXA) results in a significant reduce of both autogenous shrinkage and the induced stress. Furthermore, the u of such a combination has resulted in a very low shrinkage even non shrink HPC with a gradually internal stress development compared to the reference concrete.
KEYWORDS: autogenous deformations, internal capillary stress, shrinkage reducing agent, expansive additive, anti-shrinkage agents, silica fume.
1. INTRODUCTION
High-performance concretes made with low w/b and ultrafine cementitious materials are known by high magnitude of autogenous shrinkage especially, at early-age. Autogenous shrinkage is considered as the main mechanism induced internal capillary tension that might result in early- age cracking which is still the major problem relating to the u of HPC in reinforced concrete structure.
Over the last decades, veral techniques have been suggested to reduce autogenous shrinkage and conquently, the internal capillary stress that might increa the high nsitivity to early-age cr
acking. High belite or low heat Portland cement [1,2], super-absorbent polymer particles inclusions [3] as well as expansive additives (EXA) bad on calcium sulfoaluminate or free lime [1,2] and shrinkage reducing admixtures [2,4,5] have been successfully and extensively ud in inhibiting shrinkage. Even recently the internal water curing provided by the incorporation of pre-saturated lightweight aggregate (LWA) has revealed a high efficiency for autogenous shrinkage reducing and conquently, the induced capillary stress [6,7,8,9], still the anti-shrinkage agents widely ud to mitigate both autogenous and drying shrinkage of concrete.
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The magnitude of autogenous shrinkage and conquently, the capillary tension within the solid hydrating cement paste corresponds to the surface tension induced by the formation of the menisci at the interfaces between the water-filled and empty pores. Therefore, reducing such type of shrinkage required reduces of surface tension into capillary pores.
It is well known that SRA is designated to reduce the surface tension of water into pore solution of cement paste, whereas EXA are assigned to increa the volume of concrete by the transformation of a mixture of monosulfo-aluminate (C4A3S), lime (C) and anhydrite (C S) into ettringite (C3A. 3C S
H32); or the transformation of lime into calcium hydroxide (CH) [10]. Since the internal stress in the pore solution are directly proportional to its surface tension, shrinkage reducing agent (SRA) which lowers this surface tension could also be ud to mitigate autogenous shrinkage and early-age cracking in low w/b concretes. Therefore, the main functions of anti-shrinkage admixtures is to reduce capillary tension in pore solution that develops within concrete as it dries by both internal water consumption or due to external evaporation.
The results of Hori et al. [1] indicate that the u of EXA reduces autogenous shrinkage, while calcium sulfoaluminate with high amount of free lime is more efficient than conventional calcium sulfoaluminate. Meanwhile, veral authors have reported that the u of combination of SRA and EXA is more advantageous and reliable in shrinkage reducing than the mono-incorporation of either SRA or EXA parately [11] as well as using ternary system blended with low-heat cement and (SRA + EXA) [12].
Furthermore, results have shown that neither expansive agent nor SRA when ud parately can definitely avoid the risk of cracking induced by shrinkage [10]. However, the u of combination of SRA and CaO-bad expansive agent enable a synergistic effect in terms of more efficiency regarding the reduction of shrinkage.
The objective of the prent study is to elucidate the synergistic effect of such a combination (SRA + EXA) on both autogenous strains and stress development for silica fume HPC.
2. EXPERIMENTAL INVESTIGATION
2.1. Materials and mixes composition
A binary binder premixed low heat high belite cement (LHSF) containing 10% of SF as partial replacement of high belite cement was ud for both reference and anti-shrinkage concrete mixtures. Crushed diaba with a maximum nominal size of 15 mm and crushed sandstone with a maximum size of 5 mm were ud as coar and fine aggregate, respectively, in saturated surface-dry conditions (SSD). Two types of anti-shrinkage agents were ud to minimize the amount of autogenous shrinkage. Expansive additive (EXA) was ud in combination with shrinkage reducing agent (SRA) bad on lower-alcohol alkyleneoxide adduct having a specific gravity of 1.0. The content of EXA added was considered as part of the total content of cement, whereas, the amount of SRA incorporated was considered as part of the mixing water content. Therefore, for the treated concretes, the w/b is equivalent to (W+SRA)/(LHSF+EXA). To obtain the targeted slump flow, a Polycarboxylate superplasticizer (SP) having a specific gravity of 1.05 was ud in all concrete mixtu
res. The mix proportions of the SF-HPC mixtures investigated in this study is prented in Table 1. Three different low w/b of 0.15, 0.23 and 0.30 have been adopted in order to perform the prent study under somewhat vere conditions. The concrete was prepared in a laboratory pan mixer having as maximum capacity of 0.50 m3. The cement, SF and EXA, and fine aggregate were mixed first, followed by the addition of water and superplasticizer, and SRA in ca. After the cement paste was reached the adequate consistent, coar aggregate was lastly added and mixed together. Immediately after the end of the mixing
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傅雷家书笔记摘抄quence, slump follow, air content and temperature of concrete were determined. For the same type of concrete, all specimens were prepared from the same concrete batch and covered with plastic sheet. All the concrete specimens (prisms and cylinders) were aled with aluminum waterproofing tape immediately after demoulding at 1 day in order to prevent any moisture loss during the whole testing period.
Concrete mixtures were properly labeled using the notations indicated in this ction. For concrete m
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ixtures without anti-shrinkage agents, the alphabetical part designated the type of cement followed by number indicated the w/b of concrete. As all the concrete mixtures containing anti-shrinkage agents were made with the same binder type (LHSF) and the same content of SRA, and for simplification, they were designated using only the last part which indicated the content of EXA for expansive agent and R for the shrinkage reducing agent followed by the number indicated the w/b of the mix.
2.2. Testing methodology
HPC mixtures investigated in the current study were subjected to two types of tests: mechanical tests, and strains and stress development.
Throughout the whole measurement period, the specimens were kept aled under the same conditions (20 ± 1 °C and 60 ± 2 % of RH). In this study, measurement of both autogenous deformations and stress were performed on two specimens and the mean value was reported. In the other hand, and for each concrete type, three cylinder specimens measuring 100 × 200 mm and 150 × 200 mm were ud for compressive and splitting tensile strengths testing respectively, at the age of 1, 3 and 7 days of aled curing according to JSI standard test.
2.2.1. Strains measurement
The measurements of autogenous strains was performed on 100 × 100 × 400 mm specimens using a strain transducer of 100 mm gauge length embedded horizontally in the centre of the prism. Its low elastic modulus 400 kgf/cm 2 allows measurement of strains that may occur during the very earliest stages of hydration reaction process. In addition to the strains measurements, this type of gauge is able to measure also temperature variation during cement hydration reaction. Prisms were recorded to a data logger system which is connected to a computer-controlled to collect data. Measurement starts immediately after concrete placement in the moulds and readings were taken every 10 min for 7 days.
2.2.2. Stress measurement
The propod testing procedure ud for stress measurements induced by shrinkage development was designed to offer partial restraint to the concrete specimen in order to simulate restrained Mix designation Constituents kg/m 3 LHSF-15 LHSF-23 LHSF-30 EX15-15 EX20-15 EX25-15 EX20-23 EX20-30 LHSF 1033 674 517 1018 1013 1008 654 497 EXA - - - 15 20 25 20 20 Water 155 155 155 149 149 149 149 149 Gravel 944 944 944 944 944 944 944 944 Sand 445 741 875 445 445 445 741 875 SP 25.8 8.76 4.65 25.8 23.3 25.8 8.76 4.97
SRA - - - 6 6 6 6 6 Slump flow, mm
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600 575 190 530 685 595 580 175 Air, % 1.8 1.5 1.8 1.6 1.9 1.3 1.0 1.8 Table 1- Mix proportions and fresh properties of the HPC mixtures investigated
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conditions similar to tho might found in real reinforced concrete structures. The loading system us a pret degree of restraint by transferring the axial force to the concrete specimen through continuous embedded reinforcing bars. A specimen size of 100 × 100 ×1400 mm with reinforcing bar (D16) embedded in the centre of the concrete specimen was ud for stress measurement. The axial force was measured with TML strain gauge placed longitudinally at the centre of the reinforcing bar. On each specimen, the concrete temperature variation was measured at the centre of the prism using embedded thermocouples (TC).
结婚大门对联大全The restrained stress development in concrete at the centre of the concrete specimen due to the restraint of reinforcement was determined using the equation (1). This equation is derived from the equilibrium of the force among concrete and reinforcement as well as Navier’s assumption, in which the stress is positive in tension and negative in compression.
c
s c A P −=σ and s s s s E A P ε= (1) where, P s = the axial force in reinforcement, E s = Young’s modulus of reinforcement,
= stress on the extreme bottom fiber, = cross-ction area of concrete.
3. RESULTS AND ANALYSIS
3.1. Mechanical properties黄庭坚作品
In RC structure element, tensile and compressive strengths are the two major mechanical properties required for structural design purpo. The inclusion of anti-shrinkage agents in concrete mixtures may have an affect on both the compressive and tensile strengths development over time. Obviously, both the highest compressive and tensile strengths were achieved by the mix made with the lower w/b (LHSF-15) and the lowest strengths were obtained by the mix LHSF-30. Figs.1 and 2 provide the average value of the compressive and splitting tensile strength results up to 7 days for both the references and the treated concrete mixtures investigated . It can be obrved that for the three w/b mixtures investigated herein, both early-age and 7 day-strength have shown a slight decre
a of the compressive and tensile strengths with the incorporation of a combination of SRA and EXA at different proportions compared to the control mixes.
阿卡莉In fact, as it can be en in Fig. 1, the inclusion of EXA and SRA has rather a controversial effect. In one hand, for the three w/b mixtures, the inclusion of (EXA + SRA) results in a slight decrea of the compressive strength compared with the reference mix, and in another hand, for the same mixture with a w/b of 0.15, higher the content of EXA, lower the decrea of the compressive strength induced. It can be expected that the high content of EXA result in an important increa of the volume of concrete via the condary ettringite formed, this could fulfill the empty space and coarr capillary pores which leads to a denr and strengthens cement paste. However, the small drop of the concrete strength may due to the microcraking that appear in the contact surface between the hardening cement paste and the condary ettringite formed by the transformation of monosulfo-aluminate into ettringite as described above.
Meanwhile, it should be noted as pointed out, that the SRA acts as a surface tension reducing within the pore solution which, basically, cannot significantly affect the strength of concrete. Therefore, such a decrea/increa of concrete’s strength is mainly related to the effect of EXA and the transformation process of monosulfo-aluminate into ettringite.
Fig.1- Compressive strength development Fig.2- Splitting tensile strength development 3.2. Autogenous deformations development
Autogenous strains of the HPC mixtures with and without anti-shrinkage agents were continuously measured on aled specimens and the results obtained are plotted in Figs. 3to6. It can be en that the w/b of concrete and the anti-shrinkage agents substantially affect both the ultimate magnitude and the development of autogenous strains. Results have shown that for the control mixtures without anti-shrinkage agents, decreasing the w/b of concrete leads to an increa of the m
agnitude of autogenous shrinkage. Lower the w/b is, greater the magnitude of autogenous shrinkage will be.In fact, lowering the w/b of concrete by 0.15 results in a significant increa in the autogenous shrinkage magnitude as shown in Fig. 3. The ultimate amount of autogenous shrinkage for the 0.15 concrete is estimated as the double of that of 0.30 concrete. As expected, the autogenous deformation behavior of concrete mixtures containing anti-shrinkage agents was clearly different than that of the control mixtures. For the same w/b of 0.15 and with the same content of SRA, increasing the content of EXA leads to a slight decrea of the magnitude of autogenous shrinkage as illustrated in Fig. 4.三的英语单词
Obviously, the mix design and especially, the w/b of concrete is the major factor affecting autogenous strains development. In fact, while the 0.30 concrete has exhibited relatively an important expansion of around 80 µε, the 0.23 concrete has shown an initial shrinkage up to 12 hours which turned into an important expansion until 1 day. Beyond 1 day, the volumetric change behavior of both 0.30 and 0.23 concretes tends to stabilize and no significant deformations were recorded (Fig. 5). In addition, concrete mixtures with the same content of EXA (20 kg/m3) and made using different w/b have exhibited different volumetric contractions behavior as shown in Fig. 6. A quite similar expansion followed by stabilization after a short and pronounced shrinkage period was
also recorded with the 0.15 concrete containing the same dosage of EXA+SRA as it can be en in Fig.4 and 6. In fact, during the first hours, the hydration reaction results in a formation of hydrate products and pore network. SRA acts as a surface tension reducing within the pore solution however, EXA starts to react once the initial monosulfo-aluminate formed. The transformation of monosulfo-aluminate into ettringite leads to an increa of the volume of concrete which explain the expansion obrved for the mixtures containing EXA. Meanwhile, the results have shown that while the combination of (SRA + EXA) could significantly reduce the magnitude of autogenous shrinkage, the completely elimination of such
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