Ultra-High Strength Concrete Mixtures Using Local Materials
Srinivas Allena1 and Craig M. Newtson2
1 New Mexico State University, Civil Engineering Department, P.O. Box 30001, MSC 3CE, Las Cruces, NM 88003-8001; PH: (575) 571-9490; FAX: (575) 646-6049; email: [email protected] 2New Mexico State University, Civil Engineering Department, P.O. Box 30001, MSC 3CE, Las Cruces, NM 88003-8001; PH: (575) 646-3034; FAX: (575) 646-6049; email: [email protected] Abstract This paper presents the development of ultra high strength concrete (UHSC) using local materials. UHSC mixture proportions were developed using local materials so that UHSC may be made more affordable to a wider variety of applications. Specifically, local sand with a top size of 0.0236 in. (600 μm), and locally available Type I/II cement and silica fume were used in this research. Each of these material selections is seen as an improvement in sustainability for UHSC. Two mixtures (one with and one without fibers) were recommended as the UHSC mixtures. The greatest compressive strengths obtained in this study were 24,010 psi (165.6 MPa) for UHSC with steel fibers and 23,480 psi (161.9 MPa) for UHSC without fibers. The compressive and flexural strengths obtained from the UHSC mixtures developed in this work are comparable to UHSC strengths presented in the literature. Producing this innovative material with local materials reduces the cost of the material, improves sustainability, and produces mechanical performance similar to prepackaged, commercially available products. Introduction
In the past several years, improvements have been occurring in concrete technology. Sustainable use of supplementary materials and revolutionary developments in superplasticizing admixtures have facilitated improvements in the mechanical properties and durability of concrete. For example, researchers are using silica fume and high range water reducing admixtures to produce high density concrete. In addition to the use of mineral and chemical admixtures, applying pre-setting pressure and using post-setting heat treatment can also be used to produce dense microstructure. Other techniques intended to increase concrete density include using numerical packing models (de Lerrard and Sedron 1994). Numerical packing models predict the packing density of a particle mixture. Packing density is the volume of solids in a unit volume (Ferraris and de Larrard 1998). Using these methods, compressive strengths greater than 29,000 psi (200 MPa) have been achieved (Richard and Cheyrezy 1995).
In addition to high strength, the concrete should exhibit greater durability characteristics. This means that the concrete should be high strength and high performance. One of the materials developed in recent years is ultra high strength concrete (UHSC) also known as reactive powder concrete (RPC). This material possesses a compressive strength greater than 21,750 psi (150 MPa). The concept of RPC was first
developed by Richard and Cheyrezy (1994) and was first produced in the early 1990s by researchers at Bouygues׳ laboratory in France (Dili and Santhanam 2004). This new material is usually produced with cement, fine quartz sand, silica fume, steel fibers and high range water reducing admixture (HRWRA). Very low water-to-cementitious materials ratios are used to produce this kind of concrete. In a more accurate sense, UHSC is not concrete because it contains no coarse aggregate (Collepardi et al. 2007). The steel fibers provide ductility to UHSC. This material differs from conventional concrete not only in terms of strength, but also in terms of durability. UHSC is more durable because the low water-to-cementitious materials ratio results in very low porosity (Roux et al. 1996). The possibility of achieving high strength, durability, and improved ductility with the use of ultra high strength concrete encourages researchers and engineers to use this modern material in many practical applications like nuclear waste containment structures, high rise structures, long span bridges, and walkways.
In the present research, an attempt has been made to develop UHSC mixtures with locally available materials. The materials used in this work included Type I/II portland cement, silica fume, fine sand (passing ASTM No. 30 sieve), 0.5 in. (13 mm) long steel fibers, and HRWRA. Mixtures with and without steel fibers were prepared and tested to study the effects of steel fibers on compressive strength and tensile strength. Factors such as age and curing regimen were also investigated. Background
UHSC is composed of cement, fine sand, quartz powder, micro silica, steel fibers and HRWRA. When used in optimum dosages, the HRWRA reduces the water- to-cementitious materials ratio while improving the workability of concrete. The addition of micro silica enhances the mechanical properties of the paste by filling voids, enhancing rheology, and producing secondary hydrates. The quartz powder is useful for its reactivity during heat treatment (Dili and Santhanam 2004). The constituents of the mixture and proportions (by fraction of cement mass) proposed by various investigators (Richard and Cheyrezy 1995, Shaheen and Shrive 2006, and Matte and Moranville 1999) are presented in Table 1.
Roux et al. (1996) demonstrated that the mechanical properties of RPC are obtained by lowering the water-to-cementitious materials ratio, using HRWRA’s, and including silica fume. The lower water-to-cementitious materials ratio reduces the porosity of the cement paste and improves durability. Richard and Cheyrezy (1995) recommended the following principles to develop UHSC:
• Removal of coarse aggregate to enhance homogeneity of the concrete. • Use of silica fume for pozzolanic reaction. • Optimization of the granular mixture for enhancement of compacted density. • Application of presetting pressure for better compaction. • Post-setting heat treatment to enhance the mechanical properties of the
microstructure. • Addition of steel fibers to achieve ductility.
Materials used in UHSC such as quartz dust and steel aggregate are often shipped long distances, internationally in many cases, increasing the cost of the material. Additionally, strict requirements on the chemistry of the cement and silica fume increase the cost of commercially available, prepackaged UHSC products such as Ductal. Expensive materials such as ground quartz and fibers that are not available locally are used in the production of Ductal, resulting in increased cost of the final product. Therefore, the present work focused on developing UHSC mixture proportions using local materials so that UHSC may be made more affordable to a wider variety of applications. Specifically, local sand with a top size of 0.0236 in. (600 μm), and locally available Type I/II cement and silica fume were used in this research. Each of these material selections is seen as an improvement in sustainability for UHSC. Reduced member sizes are also possible with UHSC, which reduces the volume of concrete used to produce a given structural element. Experimental Program Materials. Type I/II portland cement, silica fume, and fine local sand (0.00295-0.0236 in [75-600 μm]) from Las Cruces, New Mexico were used. Table 2 shows the grain size distribution for the fine sand. The chemical compositions of the Type I/II portland cement and silica fume are provided in Table 3. Steel fibers that were 0.5 in. (13 mm) long were used to provide ductility. To achieve the desired workability, a commercially available polycarboxylate-based HRWRA (Glenium 3030 NS from BASF Chemicals) was used. Concrete Mixtures. Table 4 shows the mixture proportions, the water-to-cement ratio and the water-to-cementitious materials ratio (including silica fume) for a group of 7 mixtures investigated during the development of the UHSC. These mixtures were divided into five categories. Several other mixture proportions were also evaluated. However, only these mixtures are presented for the sake of brevity. The mixtures were categorized as follows:
A : The mixture in this category used Type I/II portland cement and HRWRA. The aggregates for this mixture were thoroughly washed and dried prior to use. Steel fibers were used to fill 1.5% of the volume. However, no silica fume was used in this mixture.
B : The mixtures in this category also used Type I/II portland cement and HRWRA and the aggregates were thoroughly washed and dried prior to use. Silica fume and steel fibers were both used in these mixtures.
C and D: The mixtures in these categories used Type I/II portland cement and HRWRA. The aggregates were thoroughly washed and dried prior to use. Silica fume was used in these mixtures. However, steel fibers were not used in these mixtures. The mixture in category D had lower water-to-cement and water-to-cementitious materials ratios than the mixtures in category C.
E: The mixture in this category used Type I/II portland cement and HRWRA and the aggregates were thoroughly washed and dried prior to use. Both silica fume and steel fibers were used in this mixture. The mixture in this series had a water-to-cement ratio and a water-to-cementitious materials ratio that was lower than those used for the series B mixtures. Mixtures within each category were assigned a name. Mixture names were four
digit symbols where the first letter referenced the category for the mixture, the second letter referred to the source of the aggregate, and the last two numbers referred to the percentage increase in water and cementitious materials content over the base mixture. For example, mixture BL20 was a mixture from category B that used aggregates from Las Cruces, New Mexico and had 20% more water, cement, and silica fume than the base mixture (BL00). Specimen Preparation. Sand used in this study was passed through a No. 30 sieve to obtain the desired particle sizes. The aggregates were then thoroughly washed over a No. 200 sieve. After washing, the sand was oven dried at 110°F (44°C) to achieve 0% moisture content.
The constituents of each mixture were mixed for 15 to 20 minutes using a laboratory pan mixture. The dry constituents were mixed for 2 minutes and then 75% of the water was added. After thorough mixing, the HRWRA was added and the remaining 25% of the water was added to the mixture. Steel fibers were added at the end. This process of mixing seemed to improve the action of the HRWRA.
Compressive strength specimens included 2 in. (50 mm) cubes and 4 by 8 in. (100 by 200 mm) cylinders. To avoid problems with end preparation of cylinder specimens, only 2 in. (50 mm) cubes were used as compressive strength specimens for mixture categories C, D, and E. Modulus of rupture testing was conducted on 3x4x16 in. (75x100x400 mm) prisms. All specimens were consolidated using a high frequency vibrating table. Curing Regimens. Three curing regimens were investigated. For the first regimen, concrete specimens were cured at room temperature, 65°F (20°C), for the first 24 hours. After demolding, the specimens were moist cured at a temperature of 73.4°F (23°C) and a relative humidity of 100% until the day of testing and this curing regimen was designated as MC.
In the second curing regimen, concrete specimens were cured at room temperature, 65°F (20°C), for the first 24 hours. After demolding, the specimens were heat cured in a water bath at 122°F (50°C) until the time of testing. This curing regimen was designated as WB.
For the third curing regimen, concrete specimens were cured at room temperature, 65°F (20°C), for the first 24 hours. After demolding, the specimens were heat cured in a water bath at 122°F (50°C). Then, the specimens were removed from the water bath and dry cured at 392°F (200°C) for two days prior to testing. This curing regimen was designated as OV.
Compression Testing. Compression tests were conducted on 2 in. (50 mm) cubes and 4 by 8 in. (100 by 200 mm) cylinders to evaluate the compressive strength of UHSC. After the specified curing period, cylindrical specimens were tested according to ASTM C 39 and cube specimens were tested according to BS 1881-116. Modulus of Rupture Testing. Prismatic specimens that were 3x4x16 in. (75x100x400 mm) were tested to evaluate modulus of rupture. Modulus of rupture was determined using a third point loading test on prism specimens according to ASTM C 78. Specimens were rotated 90 degrees from the orientation in which they were cast to measure the flexural strength. Consequently, the 4 in. (100 mm) dimension was aligned with the direction of loading. Compressive Strength Results and Discussion
Average compressive strengths of specimens produced from each mixture category are presented in Table 5. To investigate the repeatability of the compressive strengths of these mixtures, testing was conducted on three specimens cast from each of four different batches. Compressive strengths were measured at 7, 14, and 28 days. The aggregates were thoroughly washed and dried prior to use to remove any dust. A better coating of cement paste over the sand particles was achieved by removing the dust, resulting in increased compressive strengths. Category A. In the process of developing UHSC, mixture AL00 was produced as the first trial mixture. AL00 was produced with a water-to-cement ratio of 0.28 and no silica fume was used in this mixture. Compression tests were conducted on 4 by 8 in. (100 by 200 mm) cylinders after 7 days of moist curing. Mixture AL00 produced a compressive strength of 6940 psi (47.86 MPa). This compressive strength was considered low in comparison to the usual compressive strength range for UHSC. The low compressive strength of mixture AL00 was initially attributed to the absence of silica fume. Therefore, the subsequent mixtures (categories B, C, D, and E) in the research were produced using silica fume. Category B. Mixtures in category B were produced by modifying the mixture proportions of AL00. To produce mixture BL00, silica fume was used to replace 24.5% of the cement that was used in AL00. BL00 contained 8.6% less sand and 36.84% less HRWRA than AL00. The steel fiber content was maintained at 1.5%. The water-to-cementitious materials ratio of BL00 was equal to the water-to-cement ratio of AL00. Cylindrical specimens produced from mixture BL00 were moist cured until 7 days and then tested for compressive strength. The compressive strength of specimens produced from BL00 was 6880 psi (47.45 MPa) which is marginally lower (0.86%) than the compressive strength from mixture AL00.
The compressive strengths of AL00 and BL00 were lower than expected. However, it was felt that increasing the volume of the paste may improve the compressive strength. Consequently, the quantities of cement, silica fume, and water were increased by 20% over the base mixture (BL00) to produce mixture BL20. Since the percent increase was the same for cement, silica fume, and water, the water-to-
cementitious materials ratio did not change. The percent volume of paste was 71.3% for BL20 which was 11.9% more than the paste volume for BL00 (59.4%). The specimens (4 by 8 in. [100 by 200 mm] cylinders) produced from mixture BL20 were moist cured and tested for compressive strength at 7, 14, and 28 days. The compressive strengths at 7, 14, and 28 days were 7080 psi (48.83 MPa), 8090 psi (55.79 MPa), and 9210 psi (63.52 MPa), respectively.
The compressive strengths obtained from the mixture BL20 were still too low for UHSC. In another attempt to increase compressive strength, it was decided to accelerate the hydration reaction by employing post setting heat treatment. To accomplish this, specimens produced from BL20 were cured according to curing regimen WB. The elevated curing temperature (122°F [50°C]) resulted in increases of 19%, 6.7%, and 4.8% in compressive strengths measured at 7, 14, and 28 days, respectively. Category C. For the next step in trying to improve the compressive strength, the water-to-cementitious materials ratio was reduced to 0.22 (from 0.28 for BL00) to produce mixture CL00.The reduced water-to-cement ratio was produced by increasing the cement content by 25.31% and decreasing the water content by 7% (compared to BL00). No steel fibers were used in mixture CL00, and the sand content in CL00 was reduced by 5.5% compared to BL00.
At this stage, it was decided to use cube specimens for compression testing to avoid problems with the end preparation of cylinder specimens. Compressive strengths for all remaining mixtures were measured using 2 in. (50 mm) cube specimens. The size of the cubes was selected such that the cube dimension was at least four times the size of the largest particle used in the concrete (0.5 in. [13 mm] steel fibers). CL00 specimens were cured according to curing regimen WB and tested at 7, 14, and 28 days. The compressive strengths measured at 7, 14, and 28 days were 14,080 psi (97.10 MPa), 14,250 psi (98.28 MPa), and 16,250 psi (112.06 MPa), respectively.
To produce mixture CL20, CL00 was further modified by increasing the paste volume. The percent volume of paste was 62.85% for CL00 and 75.44% for CL20. Cube specimens produced from mixture CL20 were also cured according to curing regimen WB and tested for compressive strength at 7, 14, and 28 days. The compressive strengths at 7, 14, and 28 days were 13,560 psi (93.52 MPa), 14,360 psi (99.03 MPa), and 15,200 psi (63.52 MPa), respectively.
The next step in attempting to increase compressive strength was to try to accelerate the pozzolanic reaction of the silica fume. To accelerate the pozzalonic reaction of the silica fume, concrete specimens produced from CL00 were cured according to curing regimen OV. A similar curing regimen was adopted by Reda et al. (1999). Curing of concrete specimens at autoclave temperatures between 392ºF and 572ºF (200ºC and 300ºC) has been shown to accelerate the pozzalonic reaction and help in the formation of dense calcium silicate hydrate compounds (Shaheen and Shrive 2006). In this study, curing at 392ºF (200ºC) was achieved in an oven. There were concerns that dry curing may artificially inflate the compressive strengths. However, no adverse effects were observed in flexural strength testing. This seems to indicate that strength gains caused by oven dry curing at 392ºF (200ºC) were primarily due to the accelerated pozzolanic reaction, not drying. Compressive strengths of the specimens produced from CL00 (OV) were greater than the compressive strengths of specimens
produced from CL00 (WB) by 2.13%, 15.8%, and 5.35% at 7, 14, and 28 days, respectively. Category D. The water-to-cementitious materials ratio of CL00 was reduced to 0.20 to produce mixture DL00. To improve the workability of DL00, the HRWRA dosage was increased from 5 gal/yd3 (24.70 l/m3) to 6 gal/yd3 (29.64 l/m3). Specimens produced from mixture DL00 were cured according to curing regimens WB and OV. Compressive strengths achieved with mixture DL00 (OV) were 20,010 psi (138.55 MPa), 22,210 psi (153.17 MPa), and 23,480 psi (161.90 MPa) at 7, 14, and 28 days, respectively. These are increases of 39.2%, 34.6%, and 37.2% from CL00 at 7, 14, and 28 days, respectively. The greater compressive strengths are attributed to the reduced water-to-cementitious materials ratio. Category E. Mixture EL00 was produced by introducing steel fibers to the mixture proportions from DL00. The volume of sand was reduced by the volume occupied by steel fibers in mixture EL00. Specimens produced from EL00 were cured according to curing regimen OV. The greatest compressive strengths among all the mixture categories used in this investigation were achieved using mixture EL00. The compressive strengths achieved at 7, 14, and 28 days were 21,180 psi (146.06 MPa), 23,420 psi (161. 52 MPa), and 24,010 psi (165.60 MPa), respectively. The greater compressive strength of the fiber reinforced UHSC mixture is consistent with results reported by Reda et al. (1999). Mixtures DL00 (OV) and EL00 (OV) were identified as UHSC and will be considered for further research. Strength Gain Versus Time. Compressive strength gain with time was investigated by considering the plain (CL00 and DL00) and fiber reinforced (EL00) concrete mixtures. All of the specimens were cured according to curing regimen OV. The ratios of compressive strength at 7 and 14 days with respect to 28 day strength (assuming compressive strength at 28 days is 100%) of mixtures CL00, DL00, and EL00 are presented graphically in Figure 1. For mixtures CL00 and DL00, compressive strengths at 7 days were 84.0 and 85.6 percent of 28-day compressive strength, respectively. This percentage was 88.2 for mixture EL00. Compressive strengths for CL00 and DL00 at 14 days were 96.4 and 94.6 percent of 28-day compressive strength, respectively, whereas this percentage was 97.5 for mixture EL00. This strength development is more rapid than the strength development of normal strength concrete and is attributed to the high cementitious content of the UHSC mixtures and post-setting heat treatment that increased the rate of the hydration reaction. Curing Regimens. Curing regimen significantly influenced the compressive strength of UHSC. Specimens cured according to curing regimen OV exhibited the greatest strengths. Compressive strengths of specimens from mixtures DL00 cured according to curing regimen OV and tested at 7 days were 20.0% greater than compressive strengths of specimens cured in a water bath. However, this increase in strength was only 7.22% at 28 days. The greater compressive strength exhibited by oven dried specimens is attributed to acceleration of the hydration reaction when the specimens were kept in water bath 122°F (50°C) and the formation of secondary calcium silicate hydrate (CSH) from the pozzolanic reaction of silica fume when the specimens were kept in an oven at
392°F (200°C) for two days prior to testing. These observations are similar to those reported by Shaheen and Shrive (2006). Effects of Steel Fibers. The effects of steel fibers on compressive strength of UHSC were investigated by considering mixtures DL00 (without fibers) and EL00 (with fibers). The specimens from both of these mixtures were cured according to the OV curing regimen. Figure 2 illustrates the influence of steel fibers on compressive strength of UHSC mixtures. The average percentage increase in compressive strength due to the presence of steel fibers was 5.43, 5.45, and 2.25 percent at 7, 14, and 28 days, respectively. Modulus of Rupture Results and Discussion One concern with UHSC specimens cured at elevated temperatures was that drying of the sample might artificially inflate the compressive strength of the sample and decrease the flexural strength. However, it was also believed that steel fibers would improve the ductility of the concrete and could also increase the flexural strength of the concrete. To investigate these potential effects, modulus of rupture was measured on 3x4x16 in. (75x100x400 mm) prismatic specimens.
Average flexural strengths from mixture categories A, B, D, and E, measured at 7 days, are presented in Table 6. It can be seen from Table 6 that the greatest flexural strength was obtained with mixture EL00 (2655 psi [18.31 MPa]). This strength is consistent with flexural strengths obtained by other investigators for UHSC produced without dry curing. Again, this indicates that the dry curing at 392°F (200°C) served to accelerate the pozzolanic reaction of the silica fume.
A significant effect of steel fibers on the flexural strength of UHSC was also observed (Table 6). The flexural strength of the UHSC with fibers (EL00) was greater than that of the plain mixture (DL00) by 68.3%. The flexural strengths of plain and fiber reinforced UHSC mixtures and the percentage increase in flexural strength due to addition of steel fibers were consistent with the literature (Collepardi et al. 2007 and Dili and Santhanam 2004).
Conclusion
This paper summarizes several steps in the development of UHSC using local materials. The conclusions drawn during the course of this work are:
1. UHSC was developed with materials locally available in Las Cruces, New Mexico that produced a compressive strength of 24,010 psi (165.6 MPa) and a flexural strength of 2655 psi (18.3 MPa).
2. The strength properties of UHSC produced with local materials were similar to those provided by prepackaged, commercially available products such as Ductal.
3. Prolonging the mixing period increased the workability of the mixtures. 4. Specimens cured with oven drying attained greater strength than specimens that
were moist cured or cured in a 122°F (50°C) water bath. The greater compressive strength exhibited by oven dried specimens was attributed to the acceleration of the hydration reaction when the specimens were kept in a water bath 122°F
(50°C) and the formation of secondary calcium silicate hydrate from the pozzolanic reaction of silica fume when the specimens were kept in an oven at 392°F (200°C) for two days prior to the testing.
5. Compressive strength of fiber reinforced UHSC (EL00) was greater than the compressive strength of plain UHSC (DL00) at all ages. The percentage increases in compressive strength due to steel fibers at 7, 14, and 28 days were 5.43, 5.45, and 2.25 percent, respectively.
6. The flexural strength of UHSC containing steel fibers was 68% greater than the flexural strength of UHSC that did not contain fibers.
Recommendations for Future Work
1. To improve sustainability, fly ash should be considered for use as a partial replacement for silica fume and cement.
2. Efforts should be made to reduce temperature of 392°F (200°C) that was used in the present work for the OV curing regimen. This would reduce energy costs.
3. Polypropylene fibers should be used in place of steel fibers to reduce the shipping costs.
4. Shrinkage should be characterized throughout the curing process. 5. Mechanical properties such as modulus of elasticity and split tensile strength of
UHSC should be investigated. 6. Durability issues such as delayed ettringite formation, alkali-silica reaction,
freeze-thaw durability, and corrosion resistance of UHSC should be investigated. References
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Table 1. Mixture proportions (by fraction of cement) of UHSC from literature
Constituent
Richard and Cheyrezy (1995) Shaheen and Shrive ( 2006)
