INFLUENCE OF CEMENT COMPOSITION AND ADMIXTURE DOSAGE ON PROPERTIES OF RAPID-SETTING SCC FOR REPAIR APPLICATIONS – LESSONS LEARNED Prashant V. Ram and Jan Olek Purdue University, School of Civil Engineering, West Lafayette, IN-47907 Abstract This paper reports on the refinement efforts of previously developed rapid-setting selfconsolidating concrete (RSSCC) for field repair applications. The physical and chemical properties of cement were observed to play a significant role in the early age strength gain and the flow properties of the RSSCC. In an attempt to understand the underlying mechanisms responsible for the observed strength gain and flow changes, the first phase of the study focused on evaluating the influence of the amount of superplasticizer (SP) addition on hydration behavior of pastes. The hydration process was monitored by measuring the temperature increase resulting from the hydration process, performing mini-slump cone test and monitoring the rate of strength gain of paste mixes along with the amount of calcium hydroxide formed. The objective of the second phase of the study was to evaluate the robustness of the RSSCC mix developed for potential bridge-deck repair applications. During this stage of the study, it was seen that the early age strength requirements could not be met, mostly due to the low level of the tolerance of the proposed mix to variations in properties of constituent materials. Current efforts are being directed towards developing a simpler yet robust mix. INTRODUCTION Self consolidating concrete (SCC) mixes are very sensitive to any deviations from target mixture proportions or mixing techniques when compared to regular concrete (1, 2). Recent research on SCC indicated that the main factors affecting the robustness of SCC mixes include: type of mixing equipment, mixing duration, sequence of admixture addition and also variations in the moisture content and gradation of the aggregates used (1, 3). Since (due to the time constraints and small amount of materials involved) it is difficult to ensure the quality control of field-produced repair mixes, it is very essential to develop mixes with a high degree of robustness with respect to the uncertainties present on-site. These uncertainties include variations in moisture content of aggregates used, poor control of the amount of mix-water added and changing ambient temperature conditions (4). Motivation During the laboratory phase of the original study (3) the RSSCC mixes displayed significant sensitivity to variations in aggregate moisture content. Specifically, the use of saturated aggregates resulted in mixes with higher 6 hr. compressive strength and lower slump flow values than the use of dry aggregates. However, when working on development of the field version of RSSCC for potential application in bridge deck repair, the previously observed sensitivity to aggregate moisture content was no longer encountered. In addition, the 6 hr. compressive strength values of the new mixes were

significantly lower than those of the originally developed mixes. This discrepancy in the behavior of the original and new mixes prompted the research into the effect of physical and chemical properties of the cement used in these mixes on their early age strength gain and flow characteristics. The effect of the superplasticizer dosage on the flow and strength gain was also investigated. EXPERIMENTAL Materials Type III Cement was used in this study. The fine aggregate used was natural sand with specific gravity 2.63 and absorption of 1.85%. The coarse aggregate used was locally available pea-gravel with specific gravity 2.64 and absorption of 2.34%. A calciumnitrate (CN) type accelerator and a poly-carboxylate (PC) based SP were used. The supplementary cementitious materials used in the concrete mixes were densified silicafume and micro-fine fly ash. The RSSCC mixture proportions are shown in Table 1. Material Quantity Type III Portland cement (kg/m3) 485 Micro-fine fly ash by weight of cement (%) 7.5 Silica fume by weight of cement (%) 10 w/cm 0.31 Poly-carboxylate based SP (% by weight of cement) 2.15 Non-chloride accelerator (% by weight of cement) 8.88 Air-entraining Agent (% by weight of cement) 0.02 Sand (kg/m3) 928 Pea Gravel ((kg/m3) 581 Water (kg/m3) 176 Table 1. RSSCC Mixture Proportions

Robustness of RSSCC The effect of variations in aggregate moisture content on the rate of strength gain and workability of the RSSCC mixes was studied. The slump flow test was performed according to ASTM C 1611 (5) and the compressive strength was evaluated in accordance with ASTM C 39 (6). The results were compared to the finding of the earlier study on RSSCC (3). Effect of Physical/Chemical Properties of Cement on Hydration Paste specimens were prepared using the cement from the earlier and current studies to evaluate the rate of compressive strength gain. The compressive strength test was performed according to ASTM C 109 (7). A water-cement ratio of 0.31 was used and the dosages of the accelerator and the SP were 8.88% and 2.15% respectively. Thermogravimetric analysis was performed to compare the degree of hydration. Cement-Superplasticizer Interaction The effect of SP dosage on the hydration of cement was evaluated by measuring the rate of strength gain, temperature development (temperature signature) of the pastes and the mini-slump flow (mini-slump cone dimensions: 60-mm high, 20-mm top diameter, 40mm bottom diameter). The paste specimens were prepared with a water-cement ratio of

0.31 and the accelerator dosage was maintained at 9%. The SP dosage varied from 1.25% to 2.15%. RESULTS AND DISCUSSION Robustness Study The effect of aggregate moisture content on the slump-flow and compressive strengths observed in the earlier (3) and current studies has been compared in Table 1. Slump Flow (mm) Compressive Strength (MPa) Aggregate Earlier Current Earlier Study Current Study Condition Study Study 6 Hr 24 Hr 6 Hr 24 Hr 2 SSD 673 720 19 60 6.3 61 Dry 787 740 7.6 60 6.5 60 % Change +14.5% +2.7% -60% 0% +3.1% +1.6% Table 2. Comparison between Earlier (3) and Current Study

It can be observed from Table 1 that the aggregate moisture condition had a significant effect on the slump flow and the 6 hr. compressive strength in the earlier study. In that earlier study, when dry aggregates were used as compared to 2xSSD aggregates, a 14.5% increase in the slump flow and a 60% decrease in the 6 hr. compressive strength were observed. In the current study, the aggregate moisture content had no significant effects on any of the properties discussed in Table 2. Also, the 6 hr. compressive strength results from the current study were significantly lower than those obtained in the earlier study. The 24 hr. compressive strengths were very comparable. This could possibly indicate a delayed hydration in the specimens prepared for the current study. These results prompted research into identifying the underlying cause behind the discrepancies observed between the earlier and current study. The cement used in the earlier study (3) was from a different manufacturer from the one used in the current study and differences in chemical and physical characteristics of these cements were suspected to be contributing to the observed flow and strength discrepancies. Effect of Cement Used A comparison of the physical and chemical properties of the cement used in the earlier and current studies is shown in Table 3. Cement A* Cement B** Fineness (cm2/g) 6210 6050 Tricalcium Silicate (C3S)% 59 57 Dicalcium Silicate (C2S)% 14 13 Tricalcuim Aluminate (C3A)% 10 8 Tricalcium Aluminoferrite (C4AF)% 7 7 Table 3. Comparison of Cement Properties *Used in earlier study **Used in current study

Cement A was finer than Cement B and the composition of C3S, C2S and C3A were also higher in Cement A when compared to Cement B.

A comparison between the compressive strengths of the paste specimens prepared using the cement used in the earlier and current studies are shown in Table 4. Compressive Strength (MPa) 6 Hr 24 Hr Cement A 4.75 67.4 Cement B 2 66.3 % Change -57.9% -2.1% Table 4. Effect of Cement used on rate of strength gain

It is evident from Table 4 that the cement used definitely played a significant role in the early age strength gain. At 6 hrs, the specimens prepared using Cement A developed strengths almost 60% higher than the specimens prepared using Cement B although their 24 hr. compressive strengths were very comparable. Figure 1 shows the 6 hr. TGA plot of the specimens prepared using Cement A and B.

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Cement A Cement B

Carbonate Accelerator Effects CH

Figure 1. 6 Hr. TGA Plots on Paste Specimens

The peaks observed around 250°C and 325°C are attributed to the accelerator used (based on input from the manufacturer of the accelerator). The peak around 420°C is due to the loss of water from calcium hydroxide and the peak at 675°C is due to the carbonate in the cement. After 6 hours, the specimens prepared using Cement A had a higher loss of water from calcium hydroxide (0.33%) when compared to the specimens prepared using Cement B (0.26%). This confirms the fact that the Cement B has a slower early age hydration when compared to Cement A. The faster rate of strength gain of Cement A may be attributed to the fact that it has a higher fineness and also higher contents of C3S and C3A. Both of these will accelerate hydration, especially at early ages. The accelerator used in this study is calcium nitrate (CN) type and previous research indicated that the effectiveness of CN type accelerator depends primarily on the chemical composition of

the cement (8, 9). It has also been reported in the literature that the efficiency of CN type accelerator increases with increasing belite composition (9). Effect of Superplasticizer Dosage The effect of the amount of superplasticizer added on the mini-slump flow is shown in Figure 2. It can be seen from that the highest change in slump flow is associated with the increase in SP dosage from 1.25 to 1.5%. Further increases in SP dosage produce more moderate slump flow changes.

Figure 2: Mini-slump flow as a function of SP Dosage

Figure 3. Comparison between 6 Hr. and 24 Hr. Strengths

The effect of the amount of SP added on the 6 hr. and 24 hr. compressive strengths is shown in Figure 3. Linear regression models provide a reasonable approximation to the mini-slump flow and the compressive strength as a function of the SP dosage. The effect of SP dosage on the 6 Hr. and 24 Hr. compressive strengths are shown in Table 4. As the SP dosage is increased from 1.25% to 2.15%, it was seen that the retarding effect was more pronounced in the 6 Hr. compressive strength with a decrease in strength of around 90% while the effect on the 24 Hr. compressive strength was not as conspicuous (decrease of around 14%). SP Dosage Compressive Strength (MPa) 1.25% 2.15% % Difference 23.35 3.07 86.9% 6 Hour 91.05 78.61 13.7% 24 Hour Table 4. Effect of SP Dosage of Compressive Strength

Table 5 provides information on the composition of the paste mixes prepared to obtain the temperature signature curves. For Mix-6 and Mix-8, the w/c was increased to 0.42 as a consistent mix could not be prepared at a low w/c of .31.

Mix SP Dosage (%) Accelerator Dosage (%) w/c Mix-1 1.25 9 0.31 Mix-2 1.50 9 0.31 Mix-3 1.75 9 0.31 Mix-4 2.00 9 0.31 Mix-5 2.15 9 0.31 Mix-6 0 0 0.42 Mix-7 1.25 0 0.31 Mix-8 0 9 0.42 Table 5. Mix composition for the temperature-signature curves

Figure 5 shows the temperature signature curves for the paste mixes. 55 50

Mix-6 Mix-7

Temperature (°C)

45

Mix-8

40

Mix-1 Mix-2 Mix-3 Mix-4 Mix-5

35 30 25 20 0

2

4

6

8

10

12

14

16

18

Time (Hours)

Figure 4. Temperature Signature Curves

Comparing the temperature signature curves of Mix-1 through Mix-5, it is evident that the increase in SP dosage has resulted in a decrease in the peak temperature and has also resulted in an increase in the dormant period, thereby shifting of the peaks to the right. An interesting observation in the temperature signature curves of Mixes 1 through 5 is that the curves have two peaks. In an attempt to understand the underlying cause for this observation, another set of paste mixes (Mix-6 through 8) with and without the SP and accelerator were prepared. It was seen that the characteristic double-peak which was observed in Mixes 1 though 5 were absent in Mixes 6 through 8. Also, the peak temperatures of these mixes were higher than those observed for Mixes 1 though 5. These observations are attributed to the interaction between SP and the accelerator. The exact nature of these interactions is not known at this point.

CONCLUDING REMARKS • •

• •

The physical and chemical properties of cement play a significant role in the workability and early age strength properties of RSSCC mixtures. The increase in SP dosage from 1.25% to 2.15% resulted in a considerable decrease in the 6 Hr. compressive strength; this effect was not as pronounced in the 24 Hr. compressive strength The PC-based superplasticizer used significantly increases the dormant period thereby significantly retarding the early age hydration. For optimal performance, the given cement-superplasticizer-accelerator systems should be pre-screened for potential adverse interactions in advance of the field placement.

REFERENCES (1) Emborg, M., (2000), Final Report of Task 8.1 (No. Proposal No. 0BE96-3801). (2) Nishizaki, T., Kamada, F., Chikamatsu, R., & Kawashima, H., (1999), Application of High-Strength Self-Compacting Concrete to Prestressed Concrete Outer Tank for LNG Storage, Paper presented at the Proceedings of the First International RILEM Symposium on Self-Compacting Concrete, Stockholm, Sweden, 629-638 (3) Deshpande, Y. S., & Olek, J., (2005, Oct 30-Nov 2), Effect Of Mixing Equipment And Mixing Sequence On Rapid -Setting Self-Consolidating Concrete, Paper presented at the Proceedings of The Second North American Conference on the Design and Use of Self-Consolidating Concrete (SCC) and the Fourth International RILEM Symposium on Self-Compacting Concrete, Chicago, USA, 897-904. (4) Ram, P., & Olek, J., (2008), Performance Evaluation of Rapid-Setting Materials for Pavement/Bridge Deck Repair Applications – A Laboratory and a Field Perspective, Proceedings of the Second International Conference on Concrete Repair, Rehabilitation and Retrofitting (Accepted for Publication) (5) ASTM C 1611, 'Standard Test Method for Slump flow of Self-Consolidating Concrete', Vol. 4.02, ASTM International. (6) ASTM C 39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, , ASTM International. (7) ASTM C 109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, ASTM International. (8) Chikh, N., Cheikh-Zouaouni, M., & Aggoun, S., (2008), Effects of Caclium Nitrate and Triisopropanolamine on the Setting and Strength Evolution of Portland Cement Pastes, Journal of Materials and Structures, Vol 41, 31-36. (9) Justnes, H., & Nygaard, E.C., (1995), Technical Calcium Nitrate as Set Accelerator for Cement and Low Temperatures, Cement and Concrete Research Vol. 25(8), 17661774.