Part 14 - Testing of Hardened Concrete

There are several reasons why testing of hardened concrete is important: (1) test can investigate the fundamental physical behavior of concrete such as elastic properties and strength characteristics; (2) When physical laws are not fully understood testing can simulate expected conditions to evaluate performance; (3) tests to determined physical material constants like the modulus of elasticity; and (4) quality control.

Common characteristics of concrete like strength and durability should not be considered fundamental material properties. Variables like specimen geometry and preparation, moisture content, temperature, loading rate, and the type of testing device will affect the mechanical behavior. Therefore, when defining some mechanical property it is necessary to specify the test used to determine the value. Also, there is no unique relationship between mechanical properties obtained from different test. In light of these restrictions, a series of "standard" tests have been proposed. There are several organizations such as ASTM, British Standards Institute (BSI), and the Canadian Standards Association (CSA) which publish standards. These "standard" tests are continually revised as new technologies develop. It is estimated that 1/3 of the ASTM tests are revised annually.

Small representative samples of concrete do not in any way guarantee the quality of the concrete. Studies have shown that there is not a very good correlation between strength of concrete determined by ASTM standard tests and the strength of the concrete in the structure. However, there are many reasons to continue standard testing: (1) test help ensure proper batching and proportioning; (2) provide statistical information on properties; (3) reveal problems associate with the materials; (4) helps ensure high production standards are maintained; (5) documented testing will help identify any structural problems that arise; and (6) strength test may be used as a guide for construction operations.

Test for Compressive Strength

The most common test preformed on concrete is for compressive strength. There several reasons for this: (1) it is assumed that the most important properties of concrete as directly related to compressive strength; (2) concrete has little tensile strength and is used primarily in compression; (3) structural design codes are based on compressive strength; (4) the test is relatively simple and inexpensive to perform.

  • ASTM Cylinder Test -- The normal compressive specimen in North America is a cylinder with length to diameter ratio of 2:1. Molds may be reusable, made of heavy-gauge metal or single-use, made from sheet metal or waxed cardboard. Cardboard molds have been found to yield slightly lower strength (+/-3%) than other types. Specimen should be cast on a firm level surface, free from vibration. If the slump is more than 3 inches, concrete is consolidated by rodding; if the slump is less than 1 inch, the concrete is consolidated by vibration. Poorly compacted cylinders will have lower strength. If the specimen is to be rodded, it should be filled in three equal layers, each rodded 25 times with 5/8 inch diameter steel rod with a rounded end.

If specimens are to used for quality control the cylinders must be stored at 605F to 805F for the first 24 hours in such a way that moisture loss is prevented. The cylinder are then removed and stored in a standard moist room or in saturated lime water (73 F) until tested. If cylinder are made to estimate form removal they should be stored as near to the part of the structure in question as possible.

Capping cylinder reduces the effects of concentrated stresses under loading. Testing should be done as soon as capping is completed. Sulfur caps lose strength and pourablity with used and therefore should not be reused more than five times.

Determination of compressive strength using ASTM C39 states tolerances for the testing machine. Since strength is dependent on loading rate, the specimen should be loaded at a controlled rate of 20 to 50 lb/in2/s or a deformation rate of 0.05 in/min.

  • Cube Test -- Cube test, standard in Great Britain and Germany, uses a6in cubic mold, which is filled in three layers, rodded 35 times with a 25mm square rod or compacted with a vibrator. The cube is tested at right angles to the position casted and therefore required no capping or grinding. The loading rate is 33 lb/in2/s.

  • Prism Test (ASTM C116) -- Develop to test compressive strength from broken portions of beams tested in flexure. This primary a research test and is not an alternative to the cylinder test.

Factors Affecting the Measured Compressive Strength

Compression tests assumed that a pure state of uniaxial loading. However, this is not the case, because of frictional forces between the load plates and the specimen surface. The affect is to restrain the specimen from expanding. As specimen length to diameter ratio decreases the end effects are more important resulting in higher apparent compressive strengths. The use of rubber of lubricant between the specimen and the loading plate can induce lateral tensile load at the end of specimen. This will cause vertical splitting and reduce apparent strength.

A hard or stiff plate will concentrate stress at the outer edges whereas a softer plate will have higher stress at the center. This same concepts of hard and soft at applicable to the testing machines themselves. A soft machine will release the stored energy of its deformation to the specimen as it fails whereas a hard machine will not.

As l/d decreases below a value of 2 the strength increases. At ratios above 2 the effect is more dramatic. Also, this phenomena is significant in high-strength cement.

Specimen size is important for the simple fact that as the specimens become larger it is more likely to contain an element that will fail at a low load..

Rate of loading as discussed above is quite important to the test compressive strength. In general, the higher the loading rate the higher the measured strength. The reasons for this are not completely clear, however, it is thought that under slow loading rates more subcritical cracking may occur or that slow loading allows more creep to occur which increase the amount of strain at a given load.

Most concrete specimens are tested in a saturated state. Concrete that has been dried shows an increase in strength, probably do to the lack of lubricating effect moisture has on the concrete particles. Higher temperatures at the time of testing will lower the apparent strength of the concrete.

Tensile Strength

There is as yet no standard test for directly determining tensile strength. However there are two common methods for estimating tensile strength through indirect tensile tests. The first, is the splitting test carried out on a standard cylinder specimen by applying a line load along the vertical diameter. It is not practical to apply the a true line load to the cylinder because the side are not smooth enough and because it would induced high compressive stresses at the surface. Therefore, a narrow loading strip made of soft material is used.

Another way of estimating tensile strength is the flexural test. A specimen beam 6 x 6 x 20 inches is molds in two equal layers each rodded 60 times, once for each 2 in2 of top surface area. The beam may be vibrated and should be cured in the standard way. This test tends to overestimate the true tensile strength by about 50%. This can be explained by the fact that the simple flexural formula used is based on a linear stress-strain distribution whereas concrete has a nonlinear distribution. This is an important test because it model how a concrete beam is normally loaded.

Bond Between Concrete and Reinforcement

For bond between steel reinforcement and concrete to be effective, there must be an adequate frictional bond between the two materials. As concrete ages and shrinks there may be a decrease in bond strength, or if the concrete cracks or is very permeable, some corrosion of the steel may take place. There are not standard test for reinforcement, however, a pull-out test has been developed for comparison of different concretes (ASTM C234). The test consist of a 6 inch cube with a No. 6 (19-mm-diameter) deformed steel bar embedded in it. The bar is loaded at a rate not greater than 5000 lb/in^2/s. The load and slip at recorded at intervals until (1) the steel is yielded; (2) the concrete splits; or (3) a slip of at lest 2.5mm occurs at the loaded end.

Modulus of Elasticity

To estimate the modulus of elasticity from the nonlinear behavior of concrete the chord modulus of elasticity, Ec, is measured. A standard cylinder specimen is fitted with a strain gauge and slowly loaded, 5 lb/in^2/s. in compression. Stress is recorded at a value of strain of 0.0005 in/in and at 40% of the ultimate load. Using these values the chord modulus of elasticity can be calculated. A dynamic measure of the modulus of elasticity may be found by a nondestructive test in which the concrete specimen is vibrated at its natural frequency.

Triaxial Strength

ASTM C801 is a standard test for triaxial loading of concrete, in which two of the three principal stresses are always equal. The most important results form this test are the compressive and shear strengths.

Accelerated Tests

When the standard compression test was first introduced, construction practices and cement quality were not what they are today. We also realize that the strength of a standard cylinder test is a true representation of the concrete strength. Considerable work has been done to enable the engineer to predict the potential 28-day strength within a few hours after casting. In general, 1- or 3-day strengths cannot be used to predict 28-day strength because these early strengths are sensitive to the fineness of the cement, curing time and temperature, and admixtures. There are three accepted method for accelerating curing time: (1) the warm water method; (2) the boiling water method; and (3) the autogenous method. It should be noted that the values obtained from this three test are equal to each other or to normal 28-day strengths. These test are becoming increasing more common as a quality control measure.

Assessment of Concrete Quality

It is possible that a situation may arise that the actual strength of a structure is desire. In this case a study of the concrete structure strength and the placement of reinforcing bars may be necessary. The common way of measuring the strength of concrete in a structure is to cut a core sample using a rotary diamond drill. These cores, which may contain some steel, are soaked in water and tested the standard way. There are a number of problems associated with this kind of test: (1) core cylinder strength are generally lower than standard cylinders due to construction site curing; (2) Damage may occur due to the vibration of the core drill; (3) the ratio of core strength to cylinder strength is not constant (1.0 for 3 ksi to 0.7 for 9 ksi); (4) Core strength is dependent on the location the sample was taken from; (5) The concrete cast in the field is an anisotropic material due to the affects of bleeding.

Nondestructive Quality Test

This test are useful to: (1) quality control; (2) determination of the time for form removal; and (3) help assess the soundness of existing concrete structures.

  • Surface Hardness Methods -- One of the oldest nondestructive tests, developed in Germany in the 1930's. Basically, the surface is impacted with a mass and the size of the resulting indention is measured. The accuracy of these type of tests is only 20 to 30%.

  • Rebound Hardness -- The most common nondestructive test is the rebound test. The test measures the rebound of a hardened steel hammer impacted on the concrete by a spring. This method has the same limitations as the surface hardness tests. The results are affected by: (1) surface finish; (2) moisture content; (3) temperature; (4) rigidity of the member being tested; (5) carbonation of the surface; and (6) direction of impact (upward, downward, horizontal). Most useful in checking the uniformity of concrete.

  • Penetration Resistance -- Resistance of concrete to penetration by a steel probe driven by a given amount of energy is measured. This test is not affected by surface hardness or carbonation as the above tests, however, the mix proportions and material properties are still important.

  • Pull-Out Test -- Pull-out test determine the force required to pull a steel insert out of concrete which it was embedded during casting. This test is a measure of the shear strength of the concrete which can be correlated with compressive strength. This test is better than those previously discussed, however, the test may be planned in advance and the assembly embedded in the concrete during casting.

  • Ultrasonic Pulse Velocity -- This test is based o the fact that the velocity of sound is related to the elastic modulus. The device is accurate to about + 1%. The position of the testing equipment can affect the measurement, method A given the best results. There are several factors which affect this test: (1) surface smoothness; (2) travel path of the pulse; (3) temperature effects on the pulse velocity; (4) moisture content; (5) presence of steel reinforcing bars; and (6) age of concrete.

This web site was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.