Experiment No: 13

Making and Curing Concrete Test Specimens

 

Introduction

 

The measure of compressive strength is the most frequently performed test on hardened concrete. The compressive strength is a fundamental structural design criterion that guarantees the ability of the structure to withstand the designed load. Consistently, the compressive strength increases as the water-cement ratio decreases. With a direct correlation between the water-cement ratio and concrete quality, compressive strength is frequently used as an indicator of quality, including longevity and resistance to deformation caused by weathering. Hence, designers often require a substantial compressive strength of the concrete to guarantee superior quality, even if this strength is not necessary for providing structural support. The compressive strength f′c of normal-weight concrete ranges from 20 to 40 megapascals (3000 and 6000 pounds per square inch).

 

Scope

The scope of this test includes the methods for making and curing cylinder specimens using representative samples of fresh concrete specifically for a construction project. This lab report details the standard method for making and curing concrete cylinders according to ASTM C31. The procedure applies to concrete cylinders prepared for strength tests, such as compressive strength testing, which is typically conducted on specimens molded and cured in either field or laboratory settings.

 

Purpose

 

To determine how to make and cure concrete cylindrical and beam specimens.

 

ASTM Designation

 

ASTM C31—Making and Curing Concrete Test Specimens

 

Terminology

 

hydraulic cement —

a cement that sets and hardens by chemical reaction with water and is capable of doing so under water.

Portland cement—

a hydraulic cement produced by pulverizing clinker, consisting essentially of crystalline hydraulic calcium silicates, and usually containing one or more of the following: water, calcium sulfate, up to 5 % limestone, and processing additions.

Workability –

Workability of concrete is defined in ASTM C-125 as the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. The term manipulate includes the early-age operations of placing, compacting, and finishing. The effort required to place a concrete mixture is determined largely by the overall work needed to initiate and maintain flow, which depends on the rheological property of the lubricant (the cement paste) and the internal friction between the aggregate particles on the one hand, and the external friction between the concrete and the surface of the formwork on the other.

water-cement ratio —

the ratio of the mass of water, exclusive only of that absorbed by the aggregates, to the mass of portland cement in concrete, mortar, or grout, stated as a decimal.

Concrete —

a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate; in hydraulic-cement concrete, the binder is formed from a mixture of hydraulic cement and water.

concrete, fresh —

concrete that possesses enough of its original workability so that it can be placed and consolidated by the intended methods.

concrete, hardened —

concrete that has developed sufficient strength to serve some defined purpose or resist a stipulated loading without failure.

Consistency (fresh cementitious mixture)—

the relative mobility or ability to flow.

Curing —

action taken to maintain moisture and temperature conditions in a freshly-placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.

Pozzolan —

a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.

cementitious material (hydraulic) —

an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under water.

 

Significance and Use

The significance of making and curing concrete cylinders lies in their role as representative samples of the larger concrete structure. These cylinders allow for the testing of compressive strength, which is critical for evaluating the structural integrity and durability of the concrete. The results from the cylinders are used to verify that the concrete mix complies with the design specifications and industry standards.

This practice specifies standardized procedures for producing and curing Portland cement concrete test specimens. Specimens can be used to determine strength for mix design, quality control, and quality verification.

 

Apparatus

Cylinder Molds:

Cylindrical molds made of steel or another nonabsorbent and nonreactive material. The standard specimen size used to determine the compressive strength of concrete is 152 mm (6 in.) diameter by 304 mm (12 in.) high for a maximum aggregate size up to 50 mm (2 in.). [When the nominal maximum size of the coarse aggregate exceeds 50 mm [2 in.], the concrete sample shall be treated by wet sieving, ASTM E11] Smaller specimens, such as 102 mm (4 in.) diameter by 203 mm (8 in.) high, are sometimes used. The maximum absorption and elongation limits for a cylinder with a nominal mold height of 4 inches are 2.7 g and 0.008 inch, respectively. And for absorption and elongation limits for a cylinder with a nominal mold height of 6 inches, they are 6 g maximum and 0.012 inch maximum, respectively.

Tamping Rod:

A round, smooth, straight, steel rod having a diameter complying to the specifications in Table 1.  The rod should be at least 4 in. longer than the mold depth but not exceed 24 in. in overall length. The tamping ends should be rounded to a hemispherical tip of the same diameter.

TABLE 1 Tamping Rod Diameter Requirements

Diameter of Cylinder	Diameter or Rod mm [in.]
<150 [6]	10 ± 2 [3⁄8 ± 1⁄16 ]
≥ 150 [6]	16 ± 2 [5⁄8 ± 1⁄16 ]

 

Scoop:

A scoop is a large enough size to represent each amount of concrete obtained from the sampling receptacle and small enough to prevent spillage during mold placement.

Finishing Tools:

A handheld float or a trowel.

Moist cabinet or room:

Moist cabinet or room with not less than 95% relative humidity and 23 ± 1.7°C temperature or a large container filled with lime-saturated water for curing.

Vibrators (if used):

To ensure proper concrete vibration, use internal vibrators with a frequency of at least 150 Hz [9000 vibrations per minute] while the vibrator is functioning in the concrete. The diameter of a circular vibrator should be no more than one-fourth the diameter of the cylinder mold or one-fourth the width of the beam mold. Other shaped vibrators should have a perimeter comparable to the circumference of a good round vibrator. The total length of the vibrator shaft and vibrating element should exceed the depth of the portion being vibrated by at least 75 mm [3 in.].

Mallet:

A mallet with a rubber or rawhide head weighing 0.6 kg ± 0.2 kg shall be used.

Procedure

 

Making:

 

·        Weigh the required amount of coarse aggregate, fine aggregate, portland cement, and water according to the required specification.

·        Mix the materials in the mixer or manually for 3 to 5 min. If an admixture is used, it should be mixed with water before being added to the other materials.

·        Perform the following tests for each sample of concrete from which specimens are made: slump, air content, and temperature of concrete.

·        Casting Cylinders: Place the mold on a level, rigid surface, free of vibration and other disturbances. Place concrete into the mold using a scoop or trowel. Fill the cylinder in three equal layers, and rod each layer 25 times. Tap the outside of the cylinder 10 to 15 times after each layer is rodded for 6-inch diameter cylinder. Strike off the top and smooth the surface. For 4-inch diameter cylinder fill the cylinder in two equal layers, and rod each layer 25 times.

·        Vibrators can also be used to consolidate the concrete instead of rodding. Fill the molds and vibrate in the required number of approximately equal layers [2 layers for all diameter cylinder]. In compacting the specimen, insert the vibrator slowly and do not allow it to rest on the bottom or sides of the mold. Slowly withdraw the vibrator so that no large air pockets are left in the specimen. The number of insertions of the vibrator per layer is 1 for 4-inch diameter cylinder and 2 for 6-inch diameter cylinder. Generally, no more than 5 s of vibration should be required for each insertion to adequately consolidate concrete with a slump greater than 75 mm [3 in.]. Longer times may be required for lower slump concrete, but the vibration time should rarely have to exceed 10 s per insertion.

·        After consolidation, finish the top surfaces by striking them off with the tamping rod where the consistency of the concrete permits or with a handheld float or trowel. If desired, cap the top surface of freshly made cylinders with a thin layer of stiff Portland cement paste which is permitted to harden and cure with the specimen [ASTM C617].

 

Curing:

Initial Curing:

Store standard-cured specimens for a period up to 48 h after molding to maintain the specified temperature and moisture conditions. For concrete mixtures with a specified strength below 40 MPa, maintain an initial curing temperature between 16 and 27°C, while for those with a specified strength of 40 MPa or greater, maintain an initial curing temperature between 20 and 26°C.

Final Curing:

After initial curing, specimens should be cured with free water maintained at a temperature of 23.0 ± 2.0°C using water storage tanks or moist rooms. When capping with sulfur mortar capping compound, the ends of the cylinder should be dry enough to prevent steam or foam pockets. Standard curing temperature is not required for up to 3 hours before testing, provided free moisture is maintained and ambient temperature is between 20-30°C.

Precautions

·        Segregation must be avoided. Over vibration may cause segregation.

·        In placing the final layer, the operator should attempt to add an amount of concrete that will exactly fill the mold after compaction. Do not add non representative concrete to an under-filled mold.

·        Avoid overfilling by more than 6 mm (1/4 in.).

 

Report

·        Record weights, slump, temperature of the mix, and air content

·        Specimen type, number of specimens, dimensions, and any deviations from the standard preparation procedure

·        Date, time and name of individual molding specimens.

·        Curing method. For standard curing method, report the initial curing method with maximum and minimum temperatures and final curing method. For field curing method, report the location where stored, manner of protection from the elements, temperature and moisture environment, and time of removal from molds.

 

LAB Assignment Questions:

Fundamental Questions:

1.      What is the purpose of making and curing concrete test specimens?

2.      Why is it important to test concrete specimens?

3.      What are the different types of concrete test specimens that can be made (e.g., cylinders, cubes, beams)?

4.      What is the significance of curing concrete specimens?

5.      How does curing affect the strength of concrete?

Materials and Tools:

6.      What materials are needed for making concrete test specimens?

7.      What is the significance of using standard molds for making test specimens?

8.      Why is proper compaction important when preparing concrete test specimens?

9.      What methods can be used to compact concrete in molds?

10.  How do you ensure that the molds are correctly prepared before casting concrete?

11.  What is the purpose of a tamping rod or vibrator in the preparation of concrete specimens?

Procedure-Related Questions:

12.  Can you describe the procedure for casting a concrete cylinder according to ASTM C31?

13.  How are concrete specimens cured after casting?

14.  What are the different types of curing methods for concrete specimens?

15.  What are the differences between moist curing, water curing, and air curing?

16.  What precautions should be taken to avoid segregation of concrete when making specimens?

17.  How do you demold concrete specimens after curing?

18.  How long are concrete specimens typically cured before being tested for compressive strength?

Handling and Transportation:

19.  What is the correct procedure for handling and transporting concrete test specimens?

20.  What are the factors that can affect the quality of concrete specimens during transportation?

21.  How do you ensure that the specimens are not damaged during the curing process or during transport to the testing facility?

Testing and Interpretation:

22.  What tests are commonly performed on cured concrete specimens?

23.  What properties of concrete are typically measured using these specimens?

24.  What is the standard size of concrete test cylinders used for compressive strength testing?

25.  How do you calculate the compressive strength of concrete from test specimens?

26.  How do factors like air content, water-cement ratio, and temperature affect the strength of concrete specimens?

27.  What are the reasons for obtaining a low or high compressive strength from a test specimen?

Safety and Standards:

28.  What safety precautions must be observed during the preparation and handling of concrete test specimens?

29.  Which ASTM or IS code is followed for making and curing concrete test specimens?

30.  What are the key limitations of ASTM C31 when making and curing test specimens?

Quality Control:

31.  Why is quality control important during the making and curing of concrete specimens?

32.  What could cause discrepancies between the results from lab-cured and field-cured specimens?

33.  How do you ensure uniformity and consistency when preparing multiple specimens from the same batch of concrete?

References

ASTM C31: Standard Practice for Making and Curing Concrete Test Specimens