Designing Concrete Structures:

Acceptance of Concrete Compressive Strength Test Results According to ACI 318-14

E702.3

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ACI E702.3 Example Problem Acceptance of Concrete Test Results Luke M. Snell

Example Problem: Acceptance of Concrete Compressive Strength Test Results According to ACI 318-14, “Building Code Requirements for Structural Concrete and Commentary.”

Problem Statement Evaluate if concrete is acceptable based on compressive strength cylinder test results. The project specification states that the concrete compressive strength is required to be 3500 psi at 28 days. Given Information Table 1 provides a set of 15 test results performed on 6 x 12 in. cylinders at 28 days. Table 1

Test Cylinder 1 (psi) Cylinder 2 (psi)

1 4100 4320

2 4320 4190

3 4310 4310

4 4420 4380

5 4200 4160

6 4250 3810

7 3880 4040

8 3570 3680

9 3570 3210

10 3780 3780

11 3680 2980

12 3300 3740

13 3470 3210

14 2770 2750

15 3200 3480

Assumptions

Reference Sampling frequency is adequate; the minimum frequency is:

a. Once a day for a given class b. Once for every 150 yd3 c. Once each 5000 ft2 of surface area for slabs or walls

ACI 318, 26.12.2.1(a)

Samples are taken on a random basis – concrete was not to be sampled due to appearance, convenience, or other biased criteria

ACI 318R, 26.12.2.1(a)

Each set of cylinders was cast from a different batch of concrete ACI 318R, 26.12.2.1(a)

No water was added to the concrete after the samples were taken ACI 318R, 26.12.2.1(a)

Qualified field testing technicians performed the tests on the fresh concrete

ACI 318, 26.12.1.1(c)

Qualified laboratory technicians performed all required laboratory tests ACI 318, 26.12.1.1(d)

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ACI E702.3 Example Problem Acceptance of Concrete Test Results Calculations References Concrete strength is considered to be satisfactory as long as averages of any three consecutive compressive strength test results (Arithmetic Average) remain above f’c and as long as no individual strength test (ST) falls below f’c by more than 500 psi when f’c is 5000 psi or less, or by more than 0.10 f’c when f’c is greater than 5000 psi. Determination of Compressive Strength Strength test results calculation is the average strength of two 6 x 12 in. or three 4 x 8 in. cylinder tests from the same batch tested at 28 days or at test age designated for determination of f’c. For 6 x 12 in. cylinders:

• Cylinder 1 + Cylinder 2 = Strength test result 2

Note: For 4 x 8 in. cylinders the equation becomes:

• Cylinder 1 + Cylinder 2 + Cylinder 3 = Strength test result 3

Strength Test Results

• Strength test No. 1: 4100 + 4320 = 4210 2 The strength test results for each of the 15 tests are as shown (Table 2): Table 2

Test Cylinder 1 (psi)

Cylinder 2 (psi)

Strength Test Result (ST) (psi)

1 4100 4320 4210

2 4320 4190 4255

3 4310 4310 4310

4 4420 4380 4400

5 4200 4160 4180

6 4250 3810 4030

7 3880 4040 3960

8 3570 3680 3625

9 3570 3210 3390

10 3780 3780 3780

11 3680 2980 3330

12 3300 3740 3520

13 3470 3210 3340

14 2770 2750 2760

15 3200 3480 3340

ACI 318, 26.12.3.1(b)(1)(2) ACI 318, 26.12.1.1(a)

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Arithmetic Average of Strength Test Calculation

• ST1 + ST2 + ST3 = Arithmetic Average

3 Arithmetic Average of Strength Test Results

• Using the first three strength test results: 4210 + 4255 + 4310 = 4258 psi 3

Thus, the arithmetic averages for the tests are shown (Table 3) with each result compared to f’c: Table 3

Test Cylinder 1 (psi)

Cylinder 2 (psi)

Strength Test Results (ST) (psi)

Arithmetic Average (psi)

1 4100 4320 4210 -

2 4320 4190 4255 -

3 4310 4310 4310 4258

4 4420 4380 4400 4322

5 4200 4160 4180 4297

6 4250 3810 4030 4203

7 3880 4040 3960 4057

8 3570 3680 3625 3872

9 3570 3210 3390 3658

10 3780 3780 3780 3598

11 3680 2980 3330 3500

12 3300 3740 3520 3543

13 3470 3210 3340 3397

14 2770 2750 2760 3207

15 3200 3480 3340 3147

Determination of value to verify strength test results are acceptable: Specified strength is 3500 psi therefore, to be acceptable, no strength test result can be below f’c – 500, or 3500 – 500, which is equal to 3000 psi. Each Strength Test Result (ST) in Table 2 is to be compared to 3000 psi. Test Results Evaluation Determine if either the arithmetic average or the strength test results fail to meet satisfactory strength levels.

• Arithmetic averages (Table 3) for test numbers 13, 14, and 15 are below f’c.

ACI 318, 26.12.3.1(b)(1) ACI 318, 26.12.3.1(b)(2)

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• Strength test results (Table 2) for test number 14 are below 3000 psi (f’c – 500). Individual cylinder test results that fall below required values are not to be used in this evaluation.

Note: Test 9 has one cylinder test result below 3500 psi (3210 psi). The average of the two cylinders tested is 3390 psi (the strength test result), which is above 3000 psi, so the concrete is acceptable. Test 11 has a cylinder test result of 2980 psi. The strength test result is 3330 psi, so the concrete is acceptable.

Steps to be taken: 1. Increase Average of Strength Test Results

Steps to be taken will be dependent on circumstances and could include one or more of the following:

• Increase in cementitious materials content

• Reduction in, or better control of, water content

• Use of a water-reducing admixture to improve the dispersion of cementitious materials

• Other changes in mixture proportions

• Reduction in delivery time

• Closer control of air content

• Improvement in the quality of testing, including strict compliance with ASTM C172, ASTM C31, and ASTM C39.

Note: Changes in operating procedures or small changes in cementitious materials content or water content should not require a formal resubmission of mixture proportions; however, changes in sources of cement, aggregates, or admixtures need to be accompanied by evidence that the average strength level will be improved.

2. Investigate Low Strength Results

When the strength test results fall below acceptable levels, steps are to be taken to ensure that the structural adequacy of the structure is maintained.

• Building official should apply judgment as to the significance of the low test results; however, lower strength may be tolerated under many circumstances; this is a matter of judgment on the part of the licensed design professional and building official.

• If further investigation is deemed necessary, the following methods of investigation may be used:

In-place testing Strength testing of cores, in extreme cases

Note: In-place tests of concrete may be useful in determining whether a portion of the structure actually contains low-strength concrete. Unless these in-place tests have been correlated with standard strength test results for the concrete in

ACI 318, 26.12.3.1(c) ACI 318, 26.12.3.1(d)

ACI 318R, 26.12.4

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the structure, they are of value primarily for comparisons within the same structure, rather than as quantitative estimates of strength. In-place tests include probe penetration (ASTM C803), rebound hammer (ASTM C803), or pullout test (ASTM C900).

3. Determine if concrete is adequate using coring, if required

When coring is required, three cores are to be taken for each strength test that falls below f’c by more than the limit allowed for acceptance (ASTM C42).

• Concrete represented by cores is considered structurally adequate if the average of the three cores equals or exceeds 85 percent f’c with no single core being less than 75 percent of f’c.

• Additional core testing is permitted if the core compressive test results are considered erratic.

• If additional core tests fail to meet required strength test levels and the structural integrity remains in doubt, the responsible authority is permitted to order a strength evaluation in accordance with ACI 318 Chapter 27 for the questionable portion.

Note: For f’c of 3500 psi, the average core strength must exceed 85 percent of

3500, or 2975 psi. No single tested core can be below 75 percent of 3500 psi, or

2625 psi.

Additional Reading

• ACI 214R-11––Guide to Evaluation of Strength Test Results of Concrete

• ACI 228.1R-03––In-Place Methods to Estimate Concrete Strength

• ACI 228.2R-13––Report on Nondestructive Test Methods for Evaluation of Concrete in Structures

• ACI 301-16/301M-16––Specifications for Structural Concrete

• ACI 437R-03––Strength Evaluation of Existing Concrete Buildings

ACI 318, 26.12.4.1(a) ACI 318, 26.12.4.1(d) ACI 318, 26.12.4.1(e) ACI 318, 26.12.4.1(f)