The sand used for the replication tests was selected and adjusted to conform to the ASTM C144- 11 standard in terms of gradation. A correction to the available sand found in the laboratory was made by removing the aggregate not passing the 4.75 mm sieve, which represented a very low percentage. No other adjustments were required to achieve an acceptable gradation. Figure 3.2 shows that the gradation of the corrected sand used in the CEM mixes fell between the high and low limits stipulated in the standard.
Summary Combin Range
Input w/c a/c Start Total
0.4 2.75 78 43 Batch Volume (ml) 250 Combin # 33 Admixtures % G 7101 NC534 P 20+ CNI SRA20 50 160 214.4 96.7 84.4 Output Ingredient (g)
Cement Sand Water 144.0 391.7 35.6 Admixtures (ml)
G 7101 NC534 P 20+ CNI SRA20
Figure 3.2 Gradation of the sand used in CEM mortar mixes
The mix selected for replication was taken from Korhonen (2006). It is listed in Table 4 of the reference as mix number 8. The mix corresponds to a typical concrete composition containing 474 kg/m3 cement, with w/c = 0.40, sand/cement = 1.52, and coarse aggregate/cement = 2.27. The CEM counterpart of this composition is a mortar containing 783 kg/m3 cement, with w/c = 0.36, and sand/cement = 1.59. The effective content of the batch was calculated to fill a total volume of 6 L and included 4.80 kg of cement, 1.77 L of water and 7.66 kg of sand. The purpose of Korhonen’s (2006) study was to evaluate the effect of high dose antifreeze admixtures on the freezing point, the compressive strength, the freeze-thaw durability, and the change in length. Korhonen, in an earlier work (Korhonen and Orchino 2001), justified the use of the CEM method by the need of reducing the material waste and by more convenient sample sizes.
A mixing procedure similar to that described in ASTM C305-12 (ASTM 2012a) was followed. It consisted of putting all the water then the cement into the bowl, and running a 10 L Hobart mixer at low speed for 30 sec. The mixer was stopped and the bowl sides were scraped within 15 sec, then all the sand was added at once. A protection grid placed on top of the bowl did not allow a gradual addition of the sand. The mixer was run at low speed for about 45 sec, until the mix looked homogeneous. The mixing procedure was then stopped for 90 sec and then restarted at medium speed for a final 60 sec.
The confectioned mortar was cast in 50 mm diameter x 100 mm long plastic cylinder molds in two layers, and tamped 25 times each with a brass rod. Samples were vibrated for an additional 15 seconds on the shaking table to achieve a better compaction of the relatively stiff mortar in the cylinders. All specimens were capped with plastic sheets held with rubber bands and stored in their respective curing condition. The total mixing, casting, and transferring process took approximately 50 minutes.
Two curing temperatures were selected for this preliminary test: (a) room temperature, and (b) 5°C, also defined as the lowest allowable temperature without protection. The actual temperature and relative humidity (RH) conditions were measured using a humidity thermometer (Omega, model HH311), and found to be 23.7°C and 87% RH for the curing room, and 6.15°C (varying between 7.8°C and 4.5°C) and 22% RH for the cold chamber.
A total of 30 cylinders were prepared for 10 testing conditions, as summarized in Table 3.1. Twelve cylinders were stored in the curing room, and 18 cylinders were stored in the cold chamber. After 14 days, two sets of three cylinders stored in the cold chamber were transferred to the curing room for the remaining period. Two to twenty-four hours ahead of the compression testing time, designated samples for the specific testing age were demolded and capped with sulfur to reduce the effect of uneven surfaces of the hardened mortar cylinders.
Table 3.1 Summary of compressive strength test results in (%)
Curing scheme 7d 14d 28d 56d
Curing chamber 22°C full time
Coef. Var. (%) 63.3 13.5 92.8 12.6 100.0 5.5 107.7 10.1
5°C for 14 days, then move to 22°C
Coef. Var. (%) 53.8 3.4 54.8 9.4 90.0 13.5 103.8 12.1
5°C for 28 days, then move to 22°C
Coef. Var. (%)
- - 79.1
9.5
108.0
4.6
The workability of the CEM mix was evaluated using the flow table test according to ASTM C1437-07 (ASTM 2007) and ASTM C230-08 (ASTM 2008). The flow rate was found to be 93, indicating that the mortar was relatively stiff, as compared to a flow rate of 120 reported by Korhonen (2006). It was observed after demolding the cylinders that a relatively high air pocket ratio existed in most of the samples. The relatively high stiffness of the mortar is likely one reason,
and insufficient compaction energy may have been used to achieve good consolidation. This observation was taken into consideration for the next testing phases. Samples cured in the curing room also seemed much dryer than the ones cured in the cold chamber, even though the relative humidity of the former was higher. In addition, a powdery layer appeared on the surface of the samples as if some of the cement had not reacted.
Table 3.1 shows the mean relative compressive strength obtained for each set of three samples for each curing scheme and testing age. The same results are represented in graph form in Figure 3.3. The relative strength is defined as the ratio of the strength of the specific sample set to the mean strength of the set cured at room temperature for 28 days, which was measured to be 41.4 MPa. For the sake of comparison, Figure 3.3 shows also the results of relative compressive strength of samples cured at 5°C and 20°C reported by Korhonen (1999).
Figure 3.3 Relative compressive strength results for tests replicating Korhonen (1999)
It can be seen that there was a very close match between the results obtained in the laboratory and those reported by Korhonen (1999), except for the data point of 7 days at 5°C, which had a higher strength than expected. Samples cured in cold temperature also demonstrated a higher rate of strength gain at later ages compared to those cured in room temperature.