Introduction
Cement grouts contain cement and water with or without sand and admixtures. Admixtures are added to improve properties of grout like flowability, permeability, strength etc. Super plasticizers (SP) are the most common admixture used in grout mixes. Sand if used will require more water and super plasticizer to provide the requisite workability. Cement grouts have many areas of application in construction. It is utilized as a ground improvement technique attained by injecting into the ground. Grouting has lot of applications in repairing masonry structures. Grouting using cement slurry to fill up the voids and cracks in the poor quality porous concrete is the most simple and economical remedial measure. Grouting technique is widely used in filling post-tensioning ducts of pre stressed concrete structures. It is important that the grout mix used must be flowable to ensure proper filling of the ducts and should have low bleeding to limit the free water nearby the tendons. In short a grout should satisfy the ACI definition as “a mixture of cementitious material and water, with or without aggregate, proportioned to produce a pourable consistency without segregation of the constituents.”
Literature Review
Krishnamoorthy et al. [
1] investigated on cementitious grout containing supplementary cementitious materials (SCMs) viz., fly ash, silica fume, GGBS. Studies conducted on flow, strength and durability characteristics show that cementitious grout containing SCMs in grouting operations can be successfully used for repairing concrete structures. Wei-Hsing Huang [
2] has presented the study on cement-fly ash grouts containing polypropylene (PP) fiber and super plasticizer. Improved resistance against crack, sulfate attack and volume changes due to wet–dry cycle were observed with PP fiber addition, but it resulted in higher viscosity and permeability. The adverse effects of PP fiber were compensated by the addition of SP through enhanced flowability and reduced permeability. In the study conducted by Ma et al. [
3], cement grouts containing bauxite and gypsum as mineral admixtures were used. The resulting grout mixes had good fluidity, short setting time, and high strength. For gypsum-bauxite grout better composition was suggested as, 0.3 water/binder ratio and approximately 15% mineral admixture content.
The laboratory study carried out by Khayat et al. [
4] on grouts containing cement, water, silica fume and high-range water reducer reveals that high-performance cement grout can be achieved by 8% cement replacement with silica fume, 0.4
w/
c ratio and 1.6% High range water reducer (HRWR). Silica fume addition resulted in reduced bleeding, increased strength and improved corrosion resistance. Addition of HRWR is responsible for the enhancement of fluidity, stability, electrical resistivity and strength of the grouts. The investigation undertaken by Shannag [
5] on high-performance cement grouts focuses on structural repair application of grouts. Natural pozzolan and silica fume were added to achieve high-performance. This study showed that incorporation of silica fume and natural pozzolan results in grout with high flowability, zero bleeding, high strength and satisfactory shrinkage. Akthem et al. [
6], conducted studies on the physical properties of cement grout containing silica fume and superplasticizer. This research reports that mixes containing 10% silica fume and approximately 1% superplasticizer meets the objective of obtaining flowable, non-bleeding and high strength grouts. Bastien et al. [
7] studied the properties of low water cement ratio cement grouts with superplasticizer and low proportion of precipitated silica (3% by weight of cement). Rheological properties were also investigated. Grout mixes with zero bleeding, good flowability and high compressive strength satisfying the requirements for post-tensioning usage could be obtained.
The review indicates the desirable properties that grouts should possess as good flowability, reduced bleeding, not too short initial setting time, adequate strength and durability. The objective of this paper is to study the influence of ultra fine slag addition on the mentioned properties.
Materials used and scope of study
From the studies reported earlier, the common grout composition has been adopted for the present study. Cement grout containing cement, water, and superplasticizer, has been used in the first phase. Though use of silica fume in grouts is reported, partial substitution of cement with the Ultra-fine slag (UFS) mineral admixture is attempted in the present study. The typical properties of UFS [
8] are presented in Table 1. This material is economical compared to silica fume. UFS is added into the grout mixes in various proportions as 0, 5%, 10%, and 15% replacing the cement.
To ensure high flowability without increasing the water content, superplasticizers are added to grouts [
2,
6]. Due to its compatibility with all cement types, Sulphonated Naphthalene Formaldehyde (SNF) superplasticizers have been widely used in grout mixes. Being conventionally used in most of the grouts, SNF was chosen for the present study. In addition, grouts with poly carboxylate ether (PCE) plasticizer are also investigated, since only limited studies are reported with this generation of superplasticizer. Table 2 shows the SP dosages selected for the grout mixes with SNF and PCE. Superplasticizer dosages were chosen as percentage by weight of cementitious material, the same being based on the recommended dosage range of the manufacturer.
Based on similar studies reported in literature, water to cementitious ratio (
w/
c) were selected as 0.3, 0.35 and 0.4 in order to prepare grout mixes of flowable consistency. These
w/
c ratios selected are reported [
9] to be suitable for filling post tensioning ducts. 53 grade Ordinary Portland cement conforming to IS: 12269 [
10] was used for preparing grout mixes.
Laboratory tests & results
The laboratory studies were conducted in two phases. First phase consisted of assessment of fresh state properties of all the grout mixes, viz. flowability, bleeding and rheological properties. Satisfactory mixes meeting the requirements were selected and tested in the second phase for hardened state properties, viz. compressive strength and shrinkage.
Fresh state properties
Flowability
Flowability is an important property indicating workability of grout to ensure efficient pumping and injection. In the study, grout flowability is evaluated using Mini slump test and Marsh cone test. Mini slump test (Fig.1) was carried out using a mini slump cone of top diameter 19 mm, bottom dia 38 mm and height 57 mm as proposed by Kantro [
11]. The grout mix was poured into the cone until it gets completely filled. The mix is allowed to spread by lifting the slump cone. The spread diameters in orthogonal directions were measured and average spread diameter calculated.
Figure 2 and 3 represents the results of mini slump test using SNF and PCE based superplasticizers for various proportions of UFS respectively.
From Fig. 2. it can be inferred that average spread diameter is increasing with 5% UFS proportion at higher w/c ratios. The values are in the range of 14-17 cm Mixes of 0.3 w/c were showing tendency to become stiff with addition of UFS and hence 10% cement replacement was not attempted. This stiffening is possibly due to the large surface area of UFS which requires more moisture content to wet the surface area. Similarly, the mixes of 0.4 w/c at high SP dosage were highly flowable causing uncontrollable segregation of materials. Hence it was also eliminated for 10% UFS addition.
Figure 3 shows the effect of UFS incorporation at various PCE dosages. The addition of UFS shows marginal effect in the average spread diameter. At 10% replacement of OPC with UFS, the mixes were showing good spread without exhibiting tendency for separation of constituents. The mixes at higher w/c ratios were highly flowable with segregation, possibly due to the combined effect of higher water content and PCE super plasticizer. Hence trials with 0.35 and 0.4 w/c ratios were eliminated for 10% and 15% UFS additions.
A viscometer study was also conducted on the mixes to examine the rheological properties of grout mixes. A coaxial cylinder viscometer was used for the purpose (Fig. 4). The apparent viscosities were worked out at eight rotational speeds varying between 30 and 65 rpm with shear rates of 27.9 to 60.45 s-1.
Yield stress was obtained by fitting shear stress and shear rate values into Herschel Bulkley model. Relationships for different levels of ultra-fine slag replacement with average spread diameter and also yield stress were examined through various grout mixes. Figure 5 represents a typical graph.
From the graph it can be observed that higher the yield stress, lower the spread in mini slump. As the mix becomes stiff with 10% slag addition, yield stress increases and thus the ability of the mix to spread reduces. The above graph (for 0.35w/c and 0.6% SP dosage) indicates that, with increase in slag content the mix exhibits increased initial resistance to flow thereby resulting in lower spread diameters.
Marsh cone test (Fig. 6) was conducted on a steel cone attached to a stand by filling 1000 mL of grout mix into the cone with the nozzle closed. The time of efflux was noted for 500ml of the mix to flow through the nozzle as suggested by Jayasree and Gettu [
12].
Figures 7 and 8 represents the influence of replacement of OPC with UFS on the flow time for SNF and PCE mixed grouts. Flow times for workable mixes were less than 35 s.
Figure 7 shows that flow time decreases as the water content, SP content and UFS content increases. From Fig. 8 it can be noted that at 0.3 w/c ratio, 5% replacement of OPC with UFS showed increasing trend in flow time at low PCE dosage whereas and at high dosages a decreasing tendency was seen. Contrary to this, at 10% replacement a general reduction in flow time was seen. As mixes at 0.35 and 0.4 w/cm were too flowable with segregation tendencies, 10% and 15% cement replacement with UFS were not attempted.
Based on the viscometer study, plastic viscosities were obtained by fitting the shear rate and shear stress in Bingham model. Figure 9 illustrates a typical graph showing the relationships for different levels of ultra-fine slag replacement with flow time and also plastic viscosity. The trend in the curves shows that with increase in viscosity, flow time also increases i.e., flowability decreases. The decrease in flowability can be explained by the fact that, as the amount of fines increases, water demand also increases due to large surface area and hence affecting the workability [
13].
Test results indicate that there is a moderate enhancement of flowability characteristics in the mixes with UFS incorporation. Based on the flowability study, mixes were identified for bleeding test. Mixes exhibiting either too low workability or uncontrollable segregation were not taken up for next level of testing. Bleeding studies were confined to mixes of w/c ratios of 0.35 and 0.4 with respect to SNF and w/c ratios of 0.3 and 0.35 with respect to PCE.
Bleeding
Bleeding is a type of segregation wherein a layer of water accumulates at the surface of the grout during the early stage of cement hydration. Excessive bleeding can weaken the grout by increasing porosity, thus affecting the durability. Bleeding test is carried out by following the method described in ASTM C 940 [
14]. 800-mL of fresh grout mix is to be poured into a 1000-mL graduated cylinder and it is covered. The height of free water is recorded after complete sedimentation. This height is expressed as a percent of the original height of the grout, referred as ‘percent final bleed’.
Figures 10 and 11 represent the effect of replacement of OPC with UFS on bleeding of grouts prepared using SNF and PCE respectively.
Analyzing Fig. 10, it can be observed that there is a considerable reduction in bleeding with UFS addition. At 0.35w/c ratio with 5% replacement of cement with slag bleeding was negligible at low SNF dosages and with 10% cement replacement with slag, bleed percent was reduced to zero at all SNF dosages. Similarly, for 0.4w/c the bleed percent have come down to zero with 10% cement replacement with UFS. The fine size of slag has aided in reducing bleed level appreciably.
Figure 11 also shows a similar trend of reduction in bleeding with increase in UFS content. However, in PCE mixes it was difficult to attain zero bleed even at 10% UFS replacement. Hence 15% replacement was investigated at 0.3 w/c ratio for the low SP dosages (0.6 and 0.85) and zero or minimal bleed were observed. Since desirable mixes with low bleed were obtained at 0.3 w/c ratio, mixes higher w/c ratios were not attempted to avoid material wastage.
In short, it could be inferred that with increase in UFS content, bleeding of grout decreases significantly. This is possibly due to the finer size of UFS. ASTM C 937 [
15] recommends a maximum bleed of 2% after 3 h of mixing as the acceptable limit. Mixes conforming to this criterion were identified.
Hardened state properties
In this phase, the study was narrowed down to mixes satisfying flowability and bleeding criteria. Consideration was given to select the satisfactory mixes with lowest w/c ratios and SP dosages with a view to get enhanced strength at lowest cost. Table 3 presents the mixes that were finalized for studying compressive strength and shrinkage.
Setting time of the grout mixes is also a factor of concern. For large scale field applications it is desirable to have longer initial setting times. Vicat apparatus was used to find out the setting time of the mixes. Generally PCE mixes exhibited slow initial setting. SNF mixes had initial setting time in the range 7 to 10 h, while PCE mixes had initial set in the range of 9 to 11 h. The final setting times of SNF and PCE mixes were in the range of 18 to 20 h and 23 to 24 h respectively. The setting times observed were within the acceptable limits as recommended in ASTM C 937 [
15].
Compressive strength
Compressive strength is an indication of grout quality with respect to its bond and shear strength. It is important to ensure the quality of grout used. The grout specimens for the compressive test (50 × 50 × 50 mm) were prepared according to ASTM C 942 [
16] and tested for their 28 day strength. The compressive strength values of the different grouts are presented in Table 3. Analyzing the compressive strength results, it was observed that, PCE mixes showed slightly higher strengths than SNF mixes. Lower
w/
c ratio of PCE mixes can be a reason for this difference. Replacement of OPC with UFS show some strength enhancement for SNF mixes, but higher enhancement in strength (24%-28%) were achieved for PCE mixes at 10 and 15% UFS incorporation. The maximum compressive strength attained was 53 MPa for PCE based grout at 15% UFS addition. Strength values obtained are conforming to IS 1343 [
17], which suggest minimum grout strength of 27 MPa for application in pre-stressed concrete. Besides, the strength values also satisfies the PTI draft requirement [
18,
19] of 35 MPa minimum strength for grout filling post tensioning ducts.
Shrinkage
Shrinkage is caused due to the loss of moisture and it can lead to loss of bond between the grout and the surface. The method prescribed in ASTM C 157 [
20] was referred for shrinkage testing. Specimens of 160 × 40 × 40 mm were cast and water cured. Readings for shrinkage were taken at 3, 7, 14, 28 and 56 days.
Figures 12 and 13 show shrinkage strains of SNF and PCE mixes respectively at different time periods.
From the graphs it is obvious that control mixes (mixes without slag content) have higher shrinkage rate than mixes with containing slag. By the incorporation of UFS, shrinkage of specimens has reduced considerably.
For SNF based grouts, shrinkage rate has reduced by 59% at 10% cement replacement with UFS, whereas for PCE based mixes a reduction of 42% at 15% replacement was attained. On the whole, PCE mixes exhibit lower shrinkage values. Increased water demand in SNF mixes, to maintain adequate flowability, can be the reason for its relatively higher shrinkage.
Conclusion
Conclusions based on the results obtained from the study on fresh and hardened state properties of cement grout can be summarized as:
1) Addition of ultra fine slag has reasonably enhanced the flowability characteristics of the grout mixes. It enhances flowability characteristics of SNF based grouts at 0.35 w/c and PCE based grouts at 0.3 w/c. Rheological study results were consistent with the flowability results
2) Due to its ultra fine size, slag incorporation in cement grouts have resulted in substantial reduction in bleeding. Zero bleed could be achieved for SNF with 10% and for PCE at 15% cement replacement with ultra-fine slag.
3) Compared to the control mixes, reasonable increase in compressive strengths and reduction in shrinkage strains could be attained by slag incorporation.
4) Within the range of test parameters, best results were obtained with 0.35w/c ratio at 5% for SNF mixes and with 0.3w/c ratio at 10 and 15% cement replacement with slag for PCE mixes.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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