Standard Methods used to Measure Drying Shrinkage of Concrete by Nathaniel Gant

Standard Methods used to Measure Drying Shrinkage of Concrete Nathaniel R. Gant ABSTRACT: Concrete is one of the most common materials used in construction today. When engineers design a structure, they must consider properties of different types of concrete batches including drying shrinkage. In order to measure drying shrinkage of concrete, technicians use length comparators according to ASTM C596. An alternate method using DEMEC gauges has been devised to measure the shrinkage of four sides of standard shrinkage bars. The newly devised method and the ASTM were compared on three concrete mixes: grout, standard concrete, and lightweight concrete. This report contains the results of an experiment performed to reveal if the alternate method of measuring shrinkage yields similar results. Keywords: Concrete, drying shrinkage, ASTM C596, DEMEC

INTRODUCTION Many man-made structures in the world are made of concrete. It is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges, roads, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences, poles, and even boats. Composite steel and concrete beams have been used in the United States for more than 100 years (Subcommittee 2002). Concrete is composed of water, cement, and aggregate. Aggregate is generally coarse gravel or crushed rocks such as limestone or granite, and finer materials such as sand. Cement is composed of a combination of calcium, silicon, aluminum, iron and small amounts of other materials; gypsum is added to regulate the setting time of the concrete (Portland Cement 2013). Lime and silica make up about 85% of the mass of cement. Portland cement is the most common type of cement used for concrete construction today. When the correct ratio of cement, water, and aggregates are blended together, a chemical reaction

creates a strong bonded material capable of yielding tremendous compressive loads. The curing process gives the concrete what it needs to gain strength properly. Concrete strength depends on the growth of crystals within the matrix of the concrete. These crystals grow from a reaction between Portland cement and water known as hydration. Figure 1 shows the different phases of hydration with time.

Figure 1. A digital model of cement hydration. Phases are color coded: Black=water (pores), Red = C3S, Blue = C2S, Yellow = C-S-H gel (Jennings & Thomas 2008).

If there is not enough water, the crystals are not able to grow, thus the concrete does not fully develop its potential strength. The units of these measurements are in the micro range. If there is enough water, the crystals grow out like tiny rock-hard fingers wrapping around the sand and gravel in the mix and intertwining with one another (Jennings & Thomas 2008). Temperature is critical during the curing process. If the temperature is too cold, the hydration process will slow down. When the temperature is below 40 degrees Celsius, hydration typically stops (Curing Concrete 2013). If the temperature is too hot, the hydration process will increase 2

rapidly (Curing Concrete 2013). Because hydration occurs too quickly, the crystals do not have enough time to generate properly. Concrete curing under normal conditions undergo some amount of shrinkage. Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. Loss of moisture in the hydrated cement paste results. This effect on the concrete can lead to cracks, internal warping, and external deflections making shrinkage very important to an engineer for the design of any structure. Shrinkage occurs on all concrete structures including slabs, beams, columns, bearing walls, prestressed members, tanks, and foundations. The composition of concrete contributes directly to its drying shrinkage. Different compositions and fineness of cements have variable effects on the shrinkage of cement paste. The more that aggregate takes up by volume, the more shrinkage is reduced, which ultimately requires less cement for equivalent strength (Groom and Suprenant 1991). Most engineers determine how much concrete will shrink based on experimental measurements of samples. Two common instruments used to measure shrinkage are demountable mechanical strain gauges and length comparators. The demountable mechanical strain gauge, or DEMEC gauge, consists of a dial gauge attached to a bar with fixed conical points mounted at each end of the bar. A pivoting movement of one conical point is measured by the dial gauge. A setting bar is used to position pre-drilled stainless-steel discs that are attached to the structure using an adhesive. Each reading is taken by inserting the conical points into the holes of the discs and reading the dial. Figure 2 shows a picture of a DEMEC gauge.

Figure 2. Picture of DEMEC gauge (Testing Machines & Tools Company 2013).

Length comparators measure length changes of hardened cement paste, mortar, and concrete prismatic specimens (Length Comparator). Specimens are made with embedded studs on each end of a 3 in. x 3 in. x 11 in. cementitious block. Indicators are mounted on a sturdy upright support that is attached to a solid triangular base. The studs fit into the indicator and the length of the specimen is measured. Length comparators are commonly used for ASTM C596; a test method used to determine the change in length on drying of mortar bars containing hydraulic cement and graded standard sand. Figure 3 shows a picture of a typical length comparators used to measure shrinkage.

Figure 3. Picture of length comparator with dial indicator (Humboldt 2013). 4

This report compares the effectiveness of DEMEC gauges and length comparators on various types of concrete batches. The standard ASTM C596 test with the length comparator was performed on three types of concrete mixes. An alternate method based on DEMEC gauges was simultaneously run on the same set of samples. The method was based on applying DEMEC points on each of the four sides of the specimens. The surface strain was then recorded on each side. Figure 4 shows a picture of the DEMEC disc glued on the surface of a specimen and the steel stud embedded at the end of a specimen.

DEMEC

ASTM Figure 4. Picture of the steel disc glued on the side of a specimen and the steel stud embedded at the end of a Batch 1 specimen (Gant 2013).

The two methods measure strains on a given specimen at two different locations. The average measurement of DEMEC discsâ&#x20AC;&#x2122; surface strains on the specimens were compared to the

internal strain measurements of the embedded studs in the specimens. The question is whether both locations will have similar results.

EXPERIMENT This experiment comprised of three different concrete batches tested for shrinkage using the two methods. Each batch had three separate molds labeled A, B, and C. The same three specimens for each batch, as shown in Figure 4, were used for each testing method. Batch 1, the grout batch, contained roughly 31.4 pounds of sand, 14.9 pounds of Portland cement, and 6.69 pounds of water. These proportions were based on a typical grout mix (UFL 2013). Using a 5-gallon bucket, trowel, and drill powered mixer, the ingredients were mixed to an adequate workable consistency for approximately twenty minutes. Figure 5 shows preparation of Batch 1 materials prior to mixing.

Figure 5. Batch 1 materials prior to mixing (Gant 2013).

Batch 2, composed of sand and small aggregate, contained roughly 46.5 pounds of #68 stone (VA DOT Specifications), 25.4 pounds of sand, 7.5 pounds of Portland cement, and 3.4 pounds of water. All materials were locally obtained from Rockbridge County, Virginia; the mix approximated a typical mix used in the concrete industry in Virginia. Lastly, Batch 3, a typical concrete batch composed of large aggregate, small aggregate, and sand, consisted of lightweight pieces with a nominal maximum size of 3/8 in. These materials were obtained from a local manufacturer in Rockbridge County, Virginia. The concrete had an initial slump of 8.5 in. and was very workable. This mix was bought from a distributor and delivered in a concrete truck. Batch 1 was mixed on 13 Feb 2013, Batch 2 was mixed on 1 Mar 2013, and Batch 3 was mixed on 4 Mar 2013. The first two batches were mixed by hand in the concrete materials lab at VMI as shown in Figure 6. Batch 3 was delivered by truck and placed in the hydraulics lab at VMI for logistic reasons.

Figure 6. Mixing of Batch 2 by hand using trowel (Gant 2013). 7

After mixing, the first two batches, they were poured into rectangular molds inside a cure room and covered with wet burlap and plastic. The cure room humidity was kept between 60100% and the temperature remained between 70-98째F over the twenty-eight day period. The third batch was delivered and poured in the hydraulics lab and kept at the normal room temperature around 70째F .

We burlap was used to approximate 100% humidity on the

specimens. The molds measured 3 in. x 3 in. x 11 in. as shown in Figure 7.

Figure 7. Molds and cylinders of Batch 1 filled in the cure room (Gant 2013).

Approximately twenty-four hours after the concrete specimens cured, they were demolded per ASTM C596. When the specimens were removed from the mold, each specimen was inspected to confirm that the steel stud for the length comparator tests was firmly embedded at the ends of the specimen. Using a hot glue gun and DEMEC setting bar, stainless steel discs were adhered onto all four sides of each specimen centered about the length and width as shown in Figure 4. Once the specimens were demolded, they were immediately set up for measurements as shown in Figure 8. Initial shrinkage readings were taking using the DEMEC gauge and length comparator complying with ASTM C596.

Figure 8. Batch 1 specimens demolded and prepared for initial measurements (Gant 2013).

For the ASTM C596, a standard rod was used to get a zero mark for the length comparator and then the specimen length was recorded. The difference in length of the standard rod and specimen indicated the amount of shrinkage in the specimen. Figure 9 shows a measurement taken following ASTM C596 procedures.

Studs

Figure 9. Shrinkage measurement recorded using length comparator (Gant 2013).

For the DEMEC gauge, the conical points on the gauge were fitted into the hole of the steel discs, a slight pressure was applied to the gauge to get a snug fit, and then the length measurement was recorded. Since the DEMEC gauge was set to a zero mark initially, the length recorded each time revealed the amount of shrinkage. Figure 10 shows measurements taken using the DEMEC gauge.

. Figure 10. Shrinkage measurements of the Batch 1 using the DEMEC gauge (Gant 2013).

Measurements for each batch were taken at varying intervals because concrete shrinks rapidly during the first week then it begins to taper. Measurements were taken each day during the first week of curing. After the first week of curing, measurements were taken once a week until the batched cured for 28 days. Using the shrinkage data recorded, analysis was done to discover trends in the two measuring methods to ultimately reveal whether the methods gave similar results.

RESULTS From the experiment, wide ranges of supporting data were collected for each material type. Data was collected for a minimum of twenty-three days to compare early age shrinkage. 10

Using the raw data, statistical analyses was conducted to determine the total change in length, total strain, standard deviation of total strain for each batch trial, and average total strain for each batch. Table 1 shows the difference of length in each batch trial for the ASTM method and DEMEC method. Table 1. Total length change per batch per trial.

Batch

ASTM

DEMEC A

Batch 1a Batch 1b Batch 1c Batch 2a Batch 2b Batch 2c Batch 3a Batch 3b Batch 3c

mm *

0.00322 0.00890 0.01045 0.01710 0.01470 0.06015* 0.00810 0.00930 0.04060*

0.02491 0.02200 0.00518* 0.02800 0.01260 0.02010 0.01980 0.01640 0.03340*

Total Length Change Per Trial DEMEC B DEMEC C DEMEC D mm

0.01330 0.49460* 0.01503 0.02360 0.01280 0.01640 0.03380* 0.01410 0.01220

0.03079 0.00279 0.00718 0.01900 0.02060 0.01840 0.02000 0.02310 0.01440

AVG DEMEC

0.01570 0.00022* 0.02360 0.02880 0.01880 0.02380 0.03360* 0.01510 0.01380

0.01797 0.02340 0.01573 0.02485 0.01620 0.01968 0.01990 0.01718 0.01347

Table 1 shows the total length change of each batch and trial. By removing some values as outliers, shown with an asterisk in Table 1, batch trials showed close relationships. In order to compare the shrinkage measurement methods effectively, strain was calculated to serve as a common dimensionless value. In this instance, strain equates to the change in length divided by original length. For the ASTM length comparator, the original length was the distance between to two steel studs embedded inside the concrete specimen. By measuring the length of the concrete specimen and the distance each stud stuck out, the original length was obtained. The original length used for the DEMEC gauge was the length between the

conical points on the setting bar. Using the data in Table 1, Table 2 was created to reveal the total strain per batch trials.

Table 2. Total strain of batch per trial. Batch Batch 1a Batch 1b Batch 1c Batch 2a Batch 2b Batch 2c Batch 3a Batch 3b Batch 3c

ASTM 0.000196* 0.000746 0.000858 0.001562 0.000379 0.005840* 0.000802 0.001050 0.019512*

DEMEC A 0.003165* 0.001015 0.000455 0.003024 0.001398 0.002554 0.002516 0.002084 0.004244*

Total Strain DEMEC B DEMEC C 0.001690 0.003150* 0.000809 0.000355 0.001910 0.000912 0.002745 0.002414 0.001601 0.002618 0.002084 0.002338 * 0.004295 0.002541 0.001792 0.002935 0.001550 0.001830

DEMEC D 0.001995 0.000028* 0.002999 0.003659 0.002389 0.003024 0.004269* 0.001919 0.001550

AVG DEMEC 0.001843 0.000726 0.001910 0.002961 0.002002 0.002500 0.002529 0.002183 0.001643

In regards to Table 2, average DEMEC measurements deviated more than the ASTM values. The values with asterisks in Table 2 were considered outliers and they were not included in average and standard deviation calculations. In respect to Table 2 data, Table 3 was created to show the standard deviation of total strain for each batch.

Table 3. Standard deviation of total strain of each batch. Batch

ASTM

DEMEC A

DEMEC B

DEMEC C

DEMEC D

AVG DEMEC

Batch 1 Batch 2

0.000079 0.000837

0.00396 0.000837

0.000583 0.000574

0.00394 0.000145

0.000710 0.000635

0.000521 0.000548

Batch 3

0.000175

0.000305

0.000171

0.000560

0.000261

0.000324

In regards to Table 3, the average DEMEC measurements had higher average standard deviations than the ASTM measurements. Thus, the ASTM method produced values that were

slightly more precise than the DEMEC method. In Table 4, the average total strain for each batch was determined at the end of the test period.

Table 4. Average total strain of each batch. Batch

ASTM

DEMEC A

DEMEC B

DEMEC C

DEMEC D

AVG DEMEC

Batch 1 Batch 2 Batch 3

0.000802 0.000971 0.000926

0.000735 0.002325 0.002300

0.001470 0.002143 0.001671

0.000634 0.002457 0.002435

0.002497 0.003024 0.001735

0.001334 0.002487 0.002035

Using this data, plots were created to show the strain measurements change over time for each batch. Typical plots are shown in Figures 11, 12, and 13 for each batch. In each graph, the curvilinear relationship for the ASTM and new Average DEMEC reading data can be noticed. Most shrinkage was at the beginning and its rate of shrinkage decreased overtime creating an exponential trend for both methods throughout. This confirmed what was expected based on the behavior of typical concrete materials.

Strain vs Time (Batch 1c)

Strain

ASTM DEMEC A DEMEC C ASTM

AVG DEMEC DEMEC B DEMEC D AVG DEMEC

0.0010 0.0005 0.0000 -0.0005 -0.0010 -0.0015 -0.0020 -0.0025 -0.0030 -0.0035 0

15 20 Time (day)

Figure 11. Strain versus time of Batch 1c (Gant 2013). 13

Strain vs Time (Batch 2b) ASTM DEMEC A DEMEC C ASTM

AVG DEMEC DEMEC B DEMEC D AVG DEMEC

0.0005 0.0000 -0.0005 Strain

-0.0010 -0.0015 -0.0020 -0.0025 -0.0030 -0.0035 0

15 Time (day)

Figure 12. Strain versus time of Batch 2b (Gant 2013).

Strain vs Time (Batch 3b) ASTM DEMEC A DEMEC C ASTM

AVG DEMEC DEMEC B DEMEC D AVG DEMEC

0.0005 0.0000

Strain

-0.0005 -0.0010 -0.0015 -0.0020 -0.0025 -0.0030 0

Time (day)

Figure 13. Strain versus time of Batch 3b (Gant 2013).

CONCLUSION This experiential report discussed an experiment conducted to compare measuring concrete drying shrinkage with a length comparator complying with ASTM C596 and a DEMEC gauge on a micro scale. This experiment was conducted for approximately two months and the specimens were handled with great care in controlled environments. The main characteristics compared were standard deviation of total strain and average total strain for each batch. From Table 1, values that were indicated as outliers produced total strains in Table 2 that were outliers as well. Outliers were not included in the calculations for average standard deviation and average total strain because they would produce false trends in data. Outliers in the data were removed from the graphs as well, which fashioned better trendlines. Batch 1 had an average total strain of 0.000802 for the ASTM C596 length comparator method and 0.01334 for the DEMEC gauge method; the standard deviations for Batch 1 were 0.000079 for the ASTM C596 length comparator method and 0.000521 for the DEMEC gauge method. Batch 2 had an average total strain of 0.000971 for the ASTM C596 length comparator method and 0.002478 for the DEMEC gauge method; the standard deviations for Batch 2 were 0.000837 for the ASTM C596 length comparator method and 0.000548 for the DEMEC gauge method. Batch 3 had an average total strain of 0.000926 for the ASTM C596 length comparator method and 0.002035 for the DEMEC gauge method; the standard deviations for Batch 3 were 0.000175 for the ASTM C596 length comparator method and 0.000324 for the DEMEC gauge method. These final values reveal that the DEMEC gauge method produced greater average total strain measurements. For Batch 1, the average total strain for the DEMEC gauge was greater

than the value for the ASTM C596 length comparator by 0.000532. For Batch 2, the average total strain for the DEMEC gauge was greater than the value for the ASTM C596 length comparator by 0.001516. For Batch 3, the average total strain for the DEMEC gauge was greater than the value for the ASTM C596 length comparator by 0.001109. From this, it is plausible to assume that DEMEC gauges discover shrinkage measurements slightly higher that length comparators. This is an important sighting because concrete shrinkage is a critical design consideration for structures; therefore, assuming concrete shrinks more than it actually does would provide a higher factor of safety but also could results in the overdesign of some structures.. The standard deviations reveal that both methods produce precise measurements. These values represent the extent of deviation for each batch trial as a whole. For example, the standard deviation for Batch 1 was 0.000079 for the ASTM C596 length comparator and 0.000521 for the DEMEC gauge. Since these values were very small, there was little difference between measurements for the three molds for each batch meaning both methods were consistent. The results reveal that the DEMEC gauge method does gives similar results that are conservative compared to the ASTM C596 length comparator and may be an adequate alternative to use for measuring shrinkage. This is mainly because the DEMEC gauge gives shrinkage measurements larger than the actual values. This may be seen as an increase in the factor of safety for structural design. Moreover, the DEMEC gauge gives measurements with similar degrees of precision just like the ASTM C596 length comparator. Not to mention, the DEMEC gauge would be more mobile and easy to use in most environments. The DEMEC method could be used as a more economical alternative to the current ASTM and still produce reasonable values. Safety would not be compromised.

When using the DEMEC method it was found that it is important to take the average of the four sides of the specimen for the DEMEC gauge because the readings deviated a lot on each side. This can be explained from a variety of possible reasons. This is because the mold produced three smooth sides and one rough side; therefore, the steel discs were glued unevenly, which caused uneven measurements. In addition, a different amount of shrinkage occurred in the hot glue used to seal the steel discs to the specimens. Finally, concrete will shrink at different rates on different faces if the curing conditions vary throughout the depth of the specimen. Thus by taking the average, all the sides produced more accurate values. Many errors could have affected the data. Systematic error was present because the length comparator needed to be adjusted constantly so that certain specimens fit onto the apparatus. This recalibration could have caused measurements to be misread. Also, the cure room was not kept at the same humidity and temperature for the entirety of the experiment. Random errors occurred because some steel discs slid while the hot glue was cooling; consequently, not all the discs had the same spacing as the setting bar. Human error was possible because the pressure applied on the DEMEC gauge was not exactly the same for each measurement so the conical points may not have fit into the discs the same each time. Additionally, the method of reading the dials varied; therefore, measurements could have been misread. Lastly, random error was present because the eye level when reading each dial varied which could have cause incorrect measurements to have been recorded. Some cracks in structures are hard to notice; especially since many structural members are stabilizing internal forces. This experiment is useful because by knowing that these two methods of concrete shrinkage measuring produce similar results, engineers can save time, money, and lives by using the method more economical and suitable for the present condition.

REFERENCES

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