WOC 2010 Low Strength Concrete

Troubleshooting Low Compressive

Strength Test Results by

Jeffrey L. Groom, P.E.

WOC Seminar

TU-139

AIA Credits

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.

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.

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 testsdue 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.

Individual Cyl. Strength

Test(Avg of Cyl 1 & 2)

Avg. of 3 Strength

TestsNo. 1 No.2

1 4,110 4,260 4,190 __

2 3,840 4,080 3,960 __

3 4,420 4,450 4,440 4,200

4 3,670 3,820 3,750 4,050

5 4,600 4,570 4,590 4,260

Low Strength Example f’c = 4,000 psi

Test

No.



Avg. of 3 Strength

TestsNo. 1 No.2

1 3,620 3,550 3,590 __

2 4,240 4,560 4,400 __

3 4,080 4,120 4,100 4,030

4 4,100 4200 4,150 4,220

5 3,390 3,110 3,250 3,830

Things to consider when experiencing low compressive strength:

Increase the cementitious materials content (determine psi/lb of cementitious materials)Change mix proportionsReductions in, or better control of fresh physical propertiesReduction in delivery timeImprove 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

Date: October 29, 2002 Report: H5966 - CT - XXXX

Report of Concrete Compressive Strength Tests Project: Supplier: Location: Garage level 3 and 4 slabs at Lines 1, 2, 3, 4, and 5 on Line A; and also Lines 1 and 2 on Line

Cast: 7/12/2002 Ticket: 329294 Batch: 13:30 Concrete Mix: Specimens Made: TW Truck: 306 Test Time: 14:45 136168

Physical Properties of Concrete (ASTM C143, C231, C138, C1064) Specified Strength (f'c) Slump Air Content Density Temperature 4500 psi @ 28 days 6.5 in. 9.0 % 132.4 pcf. 85 deg. F

Compressive Strengths (ASTM C 39)

Specimen Age in Diameter Area Load Comp. Percent Type of Number Days (inches) (inches * 2) (lbs.) Strength, psi of f'c Fracture

1 7 6.00 28.27 77,500 2740 Average: 2740 69

2 28 6.00 28.27 98,000 3470 3 28 6.00 28.27 101,500 3590 Average: 3530 88

4 56 6.00 28.27 105,750 3740 Average: 3740 94

Comments: Held one 28 day cylinder for a 56 day break.

Slump + Air < 10

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)


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.



Avg. of 3 Strength

TestsNo. 1 No.2

1 4,100 3,900 4,000 __

2 3,840 4,080 3,960 __

3 3,980 3,850 3,920 3,960

4 3,670 3,820 3,750 3,880

5 4,000 4,200 4,100 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


Inspect tested cylindersPerform ACI Analysis Calculate running average of 3 tests Each test must be within 500 psi of f’c

Structural engineer review and acceptance


Inspect tested cylindersPerform ACI Analysis Calculate running average of 3 tests Each test must be within 500 psi of f’c

Structural engineer review and acceptanceACI 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.



Avg. of 3 Strength

TestsNo. 1 No.2

1 3,620 3,550 3,590 __

2 4,240 4,560 4,400 __

3 4,080 4,120 4,100 4,030

4 4,100 4200 4,150 4,220

5 3,390 3,110 3,250 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

2. No single core is less than 75% of specified strength

X average ≥ 0.85 f’c

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 values1. 2,950 psi2. 3,100 psi3. 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]

WOC 2010 Low Strength Concrete

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