Woc 2010 Low Strength Concrete

Troubleshooting Low Compressive Strength Test Results by Jeffrey L. Groom, P.E. WOC Seminar TU-139

AIA Credits Hanley Wood is a Registered Provider with The American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members available on request. This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing or dealing in any material or product. Questions related to specific materials, methods and services will be addressed at the conclusion of this presentation.

Copyright Materials This presentation is protected by US and International copyright laws. Reproduction, distribution, display and use of the presentation without permission of the speaker is prohibited.

© (San Juan Construction, Inc.) 2010

Learning Objectives Define what is a low strength test result Understand the potential causes of low strength test results Learn how to evaluate low strength test results Know what to do if low strength tests are confirmed Learn how to establish in-place strengths

Strength Test for Acceptance Strength test – average of two cylinders made from the same concrete (3 cyls if using 4x8’s) Made according to ASTM C 31 – Practice for making & curing concrete test specimens

Tested (28 days) according to ASTM C 39 – Test method for compressive strength of cylindrical concrete specimens

This assumes standard or laboratory cured cylinders, not field cured.

Should We Expect Some Low Strength Tests? Answer:

YES

Low strength tests will occur about

once in 100 tests due to normal variability

Concrete Acceptance according to ACI 318 – Building Code Requirements for Structural Concrete ACI 318 5.6.3.3 – Strength level of an individual class of concrete shall be considered satisfactory if both of the following requirements are met: (a) Every arithmetic average of any three consecutive strength tests equals or exceeds f’c; (b) No individual strength test (average of two cylinders) falls below f’c by more than 500 psi when f’c < 5000 psi; or by more than 0.10f’c when f’c > 5000 psi.

Acceptable Strength Example for f’c = 4,000 psi Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

4,110 3,840 4,420 3,670 4,600

4,260 4,080 4,450 3,820 4,570

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

4,190 3,960 4,440 3,750 4,590

__ __ 4,200 4,050 4,260

Acceptable Strength Example for f’c = 4,000 psi Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

4,110 3,840 4,420 3,670 4,600

4,260 4,080 4,450 3,820 4,570

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

4,190 3,960 4,440 3,750 4,590

__ __ 4,200 4,050 4,260

Acceptable Strength Example for f’c = 4,000 psi Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

4,110 3,840 4,420 3,670 4,600

4,260 4,080 4,450 3,820 4,570

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

4,190 3,960 4,440 3,750 4,590

__ __ 4,200 4,050 4,260

Acceptable Strength Example for f’c = 4,000 psi Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

4,110 3,840 4,420 3,670 4,600

4,260 4,080 4,450 3,820 4,570

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

4,190 3,960 4,440 3,750 4,590

__ __ 4,200 4,050 4,260

Low Strength Example f’c = 4,000 psi Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

3,620 4,240 4,080 4,100 3,390

3,550 4,560 4,120 4200 3,110

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

3,590 4,400 4,100 4,150 3,250

__ __ 4,030 4,220 3,830

Things to consider when experiencing low compressive strength: Increase the cementitious materials content (determine psi/lb of cementitious materials) Change mix proportions Reductions in, or better control of fresh physical properties Reduction in delivery time Improve the quality of testing

Determine strength per pound of cementitious materials Cement = 564 lbs/CY Average compressive strength = 3800 psi Strength per pound (psi/lb) = 3800/564 = 6.7 psi/lb of cement

Slump + Air < 10

D ate: O ctob er 29, 2002 R ep ort:H5966- C T - XXXX

R ep ort of C o ncrete C om pressive S tren gth Tests Project:

Su pplier:

L ocation:G arage level 3 and 4 slabs at Lin es 1, 2, 3, 4, and 5 o n Line A ; and also L in es 1 and 2 on Lin e Cast: 7/12/2002 Specim ens M ade: TW

Ticket:329294 T ru ck:306

Batch :13:30 T est Tim e:14:45

C o ncrete Mix: 136168

P h ysical P ro p erties o f C o ncrete (AST M C 1 43, C 231, C 138, C 1 06 4) Spe cified Strength (f'c)

Slum p

Air C ontent

D ensity

T em perature

4500 psi @ 28 days

6.5 in.

9.0 %

132.4 p cf.

85 deg . F

C o m p ressive S treng ths (AS T M C 39) Sp ecim en N um b er

Age in Days

1

7

2 3

4

28 28

56

Diam eter Area (inch es) (inches * 2) 6.00

6.00 6.00

6.00

28.27

28.27 28.27

28.27

Com m ents: H eld one 28 d ay cylinder for a 56 day break.

L oad (lb s.)

Co m p. Percen t Streng th, psi o f f'c

77,500 Average:

2740 2740

69

98,000 101,500 Average:

3470 3590 3530

88

105,750 Average:

3740 3740

94

T yp e o f Fracture

Investigate Quality Control Testing

Testing Factors Field Procedures      

Sampling Casting of Specimens Initial Storage Transportation Specimen Size Mold Type

Laboratory Procedures

Field Procedures Sampling 





Sample based on type of transportation vehicle. Combine sub-samples to make composite sample. Representative Sample (middle third of load)

Which is Better?

Influence of Slump & Air Content 1” increase in slump = 150 psi loss 1% increase in air content = 4% loss of compressive strength

Consolidation

Insufficient consolidation can lead to 60% reduction in strength.

End Planeness (75% strength reduction)

Initial Cylinder Storage Initial Storage Requirements ACI 301 states storage is contractors responsibility

Winter Curing

Summer Curing

Hot cylinders cause high early strength but low ultimate strength.

The ultimate!

Transporting Cylinders Transport 8 hours after final set Protect from jarring Prevent from freezing Transport time must be less than 4 hours.

The wrong way!

(7% strength reduction)

The right way!

Cylinder Size

Mold Material (Reused plastic molds 22% reduction?)

Laboratory Practices Cylinder Storage

Laboratory Practices Cylinder Storage Testing Techniques

Troubleshooting Low Strength Concrete Inspect tested cylinders (if possible)

Troubleshooting Low Strength Concrete Inspect tested cylinders (if possible) Perform ACI Analysis 

Calculate running average of 3 tests

Calculate Running Average of 3 Consecutive Tests (f’c=4000) Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

4,100 3,840 3,980 3,670 4,000

3,900 4,080 3,850 3,820 4,200

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

4,000 3,960 3,920 3,750 4,100

__ __ 3,960 3,880 3,920

If test results are within 500 psi of f’c and the running average is below f’c: Adjust plant procedures (QC)  

Following in-house batch procedures Moisture contents of aggregate

Adjust mix 

Calculate psi / lb of cement and adjust if necessary

Troubleshooting Low Strength Concrete Inspect tested cylinders Perform ACI Analysis  

Calculate running average of 3 tests Each test must be within 500 psi of f’c

Structural engineer review and acceptance

Troubleshooting Low Strength Concrete Inspect tested cylinders Perform ACI Analysis  

Calculate running average of 3 tests Each test must be within 500 psi of f’c

Structural engineer review and acceptance ACI 318 5.6.5.2 – If the likelihood of low-strength concrete is confirmed and calculations indicate that load-carrying capacity is significantly reduced, tests of cores drilled from the area in question…shall be permitted…….

If some results are more than 500 psi below f’c: Engineer Review  

Accept Reject  In-place testing

Low Strength Example for 4,000 psi specified strength Test No.

1 2 3 4 5

Individual Cyl. No. 1 No.2

3,620 4,240 4,080 4,100 3,390

3,550 4,560 4,120 4200 3,110

Strength Test (Avg of Cyl 1 & 2)

Avg. of 3 Strength Tests

3,590 4,400 4,100 4,150 3,250

__ __ 4,030 4,220 3,830

Methods To Determine In-place Concrete Strength Nondestructive 

Probe penetration



Ultrasonic pulse velocity



Impact hammer



Used only to locate questionable concrete, not for acceptance

Destructive 

Cores

Cores – Destructive Testing

Three Cores Required for Strength Acceptance 1. Average of three cores is equal to at least 85% of specified strength X average ≥ 0.85 f’c

2. No single core is less than 75% of specified strength X i ≥ 0.75 f’c where i = 1, 3

Core Requirements Diameter > 3.7 inches for load bearing structures Length:Diameter ratio 1:1 or greater Cap in accordance with ASTM C 617 (No unbonded caps)

0.85 Factor Accounts for … Difference in size of cylinder vs. core Drilling operation Different placement & consolidation Different curing conditions (temperature & moisture differences)

Example: Specified Strength = 3,000 psi Individual core values 1. 2,950 psi 2. 3,100 psi 3. 2,500 psi Avg. = 2,850 psi ACI 318 requirements for strength acceptance 1. 2,850 psi greater than 2,550 psi (0.85 x 3,000) 2. 2,500 psi greater than 2,250 psi (0.75 x 3,000) YES and YES

** ACCEPTED **

Summary Keep and maintain compressive strength records  

Calculate running average of three Note fresh physical properties of failing tests

Adjust mix proportions if necessary Adjust fresh physical properties if necessary Ask structural engineer to accept

Summary (continued) Check Testing Laboratory (Field)     

Obtaining sample (truck or point of placement) Make cylinders correctly Initial curing most important factor Contractor responsible for curing facility Transport Cylinders Correctly

Check Testing Laboratory (Lab)  

Centering specimen etc Laboratory Accreditation

Summary (continued) Determine in-place strength Always discuss acceptance in preconstruction meeting.

Thank You! This concludes the American Institute of Architects Continuing Education Systems Program

Any Questions? Jeffrey L. Groom, P.E. San Juan Construction, Inc. 970-497-8224 [email protected]