Probe Penetration Test vs Core Testing

The determination of the strength of concrete in a structure may become necessary when standard cylinder strength test results fail to comply with specified values, or the quality of the concrete is being questioned because of inadequate placing or curing procedures. It may also be required in the case of older structures where changes in the quality of the concrete are being investigated. In these instances, the most direct and common method of determining the strength of concrete is through drilled core testing; however, some nondestructive techniques such as the probe penetration test have been gaining acceptance as a means to estimate the in situ strength of concrete.18,31

TABLE 2.3 Summary of Compressive Strength and Windsor Probe Test Results at Ages 1, 2, and 3 Days (Mixture No. 1)

Average Standard Deviation Coefficient of Variation (%) Test na _{1 Day 2 Days 3 Days 1 Day 2 Days 3 Days 1 Day 2 Days 3 Days}

Compressive strength of cylinders 150 = 300-mm cylinders, MPa 3 10.4 17.2 19.2 0.29 0.25 0.14 3.80 1.48 0.73 100 = 200-mm cylinders, MPa 5 9.5 16.0 18.4 0.53 0.83 0.40 5.58 5.19 2.17 Compressive strength of cores 100 = 200-mm cores, MPa 3 11.1 16.9 18.3 0.36 0.57 0.17 3.21 3.38 0.92 Penetration resistance (Windsor probe) Exposed length of probe, mm 6 17.9 29.0 33.1 2.12 3.69 2.20 11.84 12.72 6.65 Embedded length of probe, mm 61.5 50.4 46.3 2.12 3.69 2.20 3.45 7.32 4.76

a _{Number of test determinations.}

Source: Reference 29.

TABLE 2.4 Summary of Compressive Strength and Windsor Probe Test Results at Ages 1, 2, and 3 Days (Mixture No. 2)

Average Standard Deviation Coefficient of Variation (%) Test na _{1 Day 2 Days 3 Days 1 Day 2 Days 3 Days 1 Day 2 Days 3 Days}

Compressive strength of cylinders 150 = 300-mm cylinders, MPa 3 15.3 21.6 24.6 0.57 0.21 0.25 3.71 0.98 1.03 100 = 200-mm cylinders, MPa 5 14.2 20.8 24.1 0.46 0.71 0.61 3.24 3.41 2.54 Compressive strength of cores 100 = 200-mm cores, MPa 3 15.9 18.1 18.7 0 0 0.42 0 0 2.24 Penetration resistance (Windsor probe)

Exposed length of probe, mm

6 29.1 33.2 38.0 2.21 2.05 2.78 7.59 6.17 7.32

Embedded length of probe, mm

50.3 46.2 41.4 2.21 2.85 2.78 4.39 4.44 6.71

a _{Number of test determinations.}

It has been claimed that the probe penetration test is superior to core testing and should be considered as an alternative to the latter for estimating the compressive strength of concrete.25 _{It is true that the}

probe test can be carried out in a matter of minutes, whereas cores, if from exposed areas and if they have to be tested in accordance with ASTM C 42-87, must be soaked for 40 h;32_{also, the cores may have}

to be transported to a testing laboratory, causing further delay in getting the results. However, the advantages of the probe penetration test should be judged against the precision of its test results, and the following statement by Gaynor7_{should be of interest in this regard:}

Based on these tests, the probe system does not supply the accuracy required if it is to replace conventional core tests. However, it will be useful in much the same manner that the rebound hammer

TABLE 2.5 Summary of Compressive Strength and Windsor Probe Test Results at Ages 1, 2, and 3 Days (Mixture No. 3)

Average Standard Deviation Coefficient of Variation (%) Test na _{1 Day 2 Days 3 Days 1 Day 2 Days 3 Days 1 Day 2 Days 3 Days}

Compressive strength of cylinders 150 = 300-mm cylinders, MPa 3 20.1 24.8 26.1 0.81 0.31 0.19 4.04 1.24 0.71 100 = 200-mm cylinders, MPa 5 18.8 25.8 28.1 0.50 1.03 0.60 2.65 3.98 2.14

Compressive strength of cores

100 = 200-mm cores, MPa 3 19.1 24.7 24.5 0.13 0.27 0.32 0.66 1.11 1.29 Penetration resistance

(Windsor probe) Exposed length of probe,

mm

6 23.2 34.1 36.3 2.64 1.78 2.48 11.38 5.22 6.83

Embedded length of probe, mm

56.2 45.3 43.1 2.64 1.78 2.48 4.70 3.93 5.76

a _{Number of test determinations.}

Source: Reference 29.

TABLE 2.6 Summary of Compressive Strength and Windsor Probe Test Results at Ages 1, 2, and 3 Days (Mixture No. 4)

Average Standard Deviation Coefficient of Variation (%) Test na _{1 Day 2 Days 3 Days 1 Day 2 Days 3 Days 1 Day 2 Days 3 Days}

Compressive strength of cylinders 150 = 300-mm cylinders, MPa 3 20.1 25.0 26.3 0.53 0.58 0.31 2.64 2.31 1.17 100 = 200-mm cylinders, MPa 5 19.8 25.8 27.5 0.94 1.24 1.52 4.74 4.80 2.94 Compressive strength of cores 100 = 200-mm cores, MPa 3 19.7 20.6 21.6 0.16 0.82 0.63 0.81 4.00 2.94 Penetration resistance (Windsor probe) Exposed length of probe,

mm

6 31.8 36.5 38.3 3.43 3.28 1.64 10.79 8.99 4.28

Embedded length of probe, mm

47.6 42.9 41.1 3.43 3.28 1.64 7.21 7.65 3.99

a _{Number of test determinations.}

is useful. In these tests, neither the probe system nor the rebound hammer provides precise quantitative estimates of compressive strength of marginal concretes. Both should be used to locate areas of relatively low- or relatively high-strength concretes in structures.

On the other hand, it has been shown by Malhotra and Painter12_{that the standard error of estimate of}

28 day compressive strength of concrete cylinders is of the same order for both the probe and the core tests. More recently, Swamy and Al-Hamed20 _{have compared the results of the probe penetration test and}

core strength tests, and examined how these related to the results of the standard wet-cube strength test. Their work was carried out on slabs of 1800 = 890 = 125 mm, and on 50-mm cores and 100-mm cubes, and covered both normal-weight and lightweight concretes. One of their conclusions was that the probe system, as a general method of nondestructive testing, estimated the wet-cube strength better than the small diameter cores at ages up to 28 days while the cores estimated the strength of older concrete better, particularly in the lower range of strength.

Carette and Malhotra29 _{have also observed that, at early ages, probe penetration test results showed}

better correlation with standard cylinder strengths than with core strengths. This was attributed by the investigators to the variations in the temperature history of the large test slabs used.

It must be stressed that in cases where standard cylinder or cube strength is strictly the parameter of interest because of the specifications being expressed primarily in these terms, the core test which provides a direct measure of compressive strength clearly remains the most reliable means of estimating

in situ strength. In many situations, however, it has been found possible to establish, within certain

limits of material composition and testing conditions, relationships between probe penetration and strength that are accurate enough so that the probe test can be used as a satisfactory substitute for the core test.31