Synthetic Fiber-Reinforced Concrete Testing for the Mitigation of

Plastic Shrinkage Cracking

Study Performed for

A1 Concrete Fibers, LLC

By

The University of Alabama at Birmingham

Department of Civil, Construction, and Environmental Engineering

University of Alabama at Birmingham

Personnel:

Mentor: Dr. Fouad H. Fouad

Laboratory supervision: Mr. Richard Hawkins

Student: Austin Dada

May 5, 2009

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Introduction

The use of fiber for the reinforcement of concrete and concrete-like matrices has been

employed since ancient times. Fiber-reinforced concrete became an attractive area for research

following the discoveries of the health hazards of high tensile strength asbestos fibers used in the

1960s and 70s. Materials such as ceramics, plastics, and gypsum products have featured the use

of fibers in order to enhance properties such as tensile and compressive strength, elastic modulus,

crack resistance and control, durability, and many other related properties.

There are four basic types of fiber-reinforced concrete: steel, glass, synthetic, and natural.

The first major inquiry into the use of steel fiber-reinforced concrete in the United States was

made in the 1960s and since then, a great deal of research has been carried out on the use of steel

fibers for residential and industrial applications. Preliminary studies on the use of synthetic

fibers as concrete reinforcement proved to be unsuccessful and deemed steel and glass fibers as

more sufficient means of concrete reinforcement. However, as more understanding of synthetic

fibers was gained and new synthetic fiber types and fabrication methods emerged, the use of

synthetic and natural organic fibers has been deemed to be just as effective as the use of steel and

glass for reinforcement pertaining to certain applications.

Traditional means for fiber reinforcement of concrete featured steel rebar reinforcement

or prestressed steel reinforcement, providing a uniform reinforcement and structural integrity to

concrete that exhibits general weakness in the area of tensile strength. Fiber reinforcement of

concrete is randomly distributed and even throughout the concrete matrix. It may also be used in

conjunction with conventional reinforcement. The flexibility and freedom to control the

methods of manufacturing and placing fiber-reinforced concrete generally proves an economic

advantage over conventional reinforcement methods. Fiber-reinforced concrete has a wide

variety of applications in construction due to the improved physical and mechanical properties of

the material. Synthetic fiber reinforced concrete has been utilized extensively in floor slabs and

slabs on grade. Synthetic and composite fibers for use in the reinforcement of concrete used in

construction elements have also been used. Synthetic fiber-reinforced products can exhibit high

ductility while still retaining their integrity.

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Objective and Scope

The objective of the proposed work was to determine the behavior of concrete reinforced

with two different types of synthetic polypropylene fibers. The ASTM C1579 test standard was

followed in order to evaluate the plastic shrinkage cracking of the FRC.

Test Specimens

Two different synthetic fiber textures (ASTM C 1116 Type III) were used in the

respective mix designs of the specimens to be tested. It was decided upon that the performance

of the different fibers were to be compared after sufficient testing. The two fiber textures to be

used in the fiber-reinforced concrete samples were named A1-Micromesh and A1 Microfibers

(fibrillated fiber in varying lengths, ¼” to ½”). Control specimens were also molded, containing

no fibers, and tested in order to determine the effect of the respective fiber textures on the base

mix design and properties. The major test performed was the plastic shrinkage cracking test. A

battery of standard concrete tests were also performed on FRC and plain concrete specimens to

aid in the evaluation of the test results.

Two concrete slab specimens were created for each dosage, including the control dosage.

The dosage rates tested were ¾ lb, 1 lb, and 1 ½ lb of fiber per cubic yard of concrete also

including the control dosage with no fibers. The specimens were subjected to the same

environmental test conditions as per ASTM C1579. The two specimens per dosage were made

for each fiber texture, both of which were tested on separate occasions.

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Table 1: Type of tests performed

Fiber Type Major Test Supporting Tests Fiber

Dosage

(lb/cu yd)

References

A1 Micro Mesh Plastic shrinkageASTM C1579

SlumpUnit weightCompressive strengthTime of Setting

¾, 1, 1 ½ FM 150NER 414

A1 Tough Mesh

Plastic shrinkageASTM C1579

SlumpUnit weightCompressive strengthTime of Setting

¾, 1, 1 ½ FM300ESR 1165

Materials and Apparatuses Used

ASTM C1579

Plyform mold (14 X 22 X 4 in.)

Environmental Chamber

Air Channeling Fan Box Tunnels [4]

Industrial Fans [4]

Penetration Testing Machine

2 sheet metal restraints (1.25 in.)

Sheet metal stress riser (2.5 in.)

Oil

Adjustable speed fans (airflow speed > 10 mph)\

Temperature sensors (for recording ambient air and concrete temperatures to the nearest

1oF)

Anemometer (for measuring air velocity to the nearest 1 mph)

Vibrating platform

Scale or Balance

6 FRC specimens molded from the dimensions of the plyform mold (4 in. depth)

o 2 for each dosage rate

2 control specimens molded from the dimensions of the plyform mold (4 in. depth)

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Procedure for Molding of Beams and Mix Design

The mix design that was used in the formation of the test specimens is listed in the appendix to

this write-up.

Six FRC slabs of dimensions 14” X 22” X 4” were produced, where 2 slabs would

contain a certain dosage of fiber. Two control slabs of the same dimensions were made for

comparison purposes. The freshly mixed molds were also consolidated for 1 minute and 30

seconds using the vibrating table to avoid material segregation. Each slab specimen was then

screeded three times with an angled iron screed. The surface was then troweled with a

predetermined number of passes in order to smooth the slabs for ease of crack observation. The

slabs were then ready for testing in the fan box apparatus where two specimens of the same

dosage were placed under the same fan box.

Three 4” x 8” cylinders from each dosage were also produced from each concrete mix for

each fiber with three other control cylinders containing no fibers. The cylinders were then tested

for compressive strength to verify the strength of the mix and to identify any discernable effect

on strength by the fibers.

Table 2: Number and dimensions of test specimens for each fiber type tested

Fiber Type Compressive Strength

Cylinders [4” X 8”]

Plastic ShrinkageFRC Slabs [14” X 22” X 4”]

Control (No Fiber) 3 2

A1 Tough Mesh 9** 6*

A1 Micro Mesh 9** 6*

* Two specimens per dosage rate

** Three specimens per dosage rate

Plastic Shrinkage Cracking Test Procedure

The ready-molded FRC and control slabs were placed under the fan boxes in the selected

exposure area, downstream of the fans. The fans were then activated and the time of day

recorded. During the test, both the control and the FRC specimens were exposed to identical

drying conditions with a temperature of 92-95oF and 20-30% humidity. Times of setting

specimens were also created as per ASTM C403 and after the measure of final setting by the

same ASTM, the evaluation of cracking began. At the beginning of the test, and at every

subsequent 30 minute interval, the air temperature, relative humidity, and airflow velocity were

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measured and recorded at outlet of air from the fan boxes. The time at which the first crack

occurred was also recorded for each specimen. Observations were continued for a minimum of 3

hours or until final set had occurred where measurement of cracking began. Cracks were

measured with the aid of a crack comparator where the largest width along each crack length was

measured and recorded. Transparencies were placed on the specimens that cracked and the

length of each crack was traced upon the transparencies.

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Figure 1. Schematic of Plastic Shrinkage Mold

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Data Collection

The following data recorded includes the time of setting graphs for each fiber type tested,

compressive strength data for each fiber type, and the width of the cracking from each test

specimen as well as the temperature and humidity records for the testing.

Time of Setting Testing for A1 Microfiber

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Time of Setting Testing for A1 MicroMesh

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Compressive Stength Data for A1 Microfiber Specimens

Dosage AgeLoad (lb)

Strength (psi)

Control 28 53590 4264Control 28 53220 4235Control 28 53140 4229Control 28 52510 4178Control 28 56790 4519Control 28 55500 44163/4 lbs 28 53380 42503/4 lbs 28 52150 41503/4 lbs 28 51390 40903/4 lbs 28 51720 41203/4 lbs 28 55160 43903/4 lbs 28 53590 4264

1 lb 28 57300 45601 lb 28 57310 45601 lb 28 58840 46821 lb 28 58120 46251 lb 28 56850 45241 lb 28 57780 4598

1.5 lb 28 58790 46781.5 lb 28 60340 48021.5 lb 28 60550 48191.5 lb 28 59580 47411.5 lb 28 59490 47341.5 lb 28 62180 4948

Compressive Strength Data for A1 MicroMesh Specimens

Dosage AgeLoad (lbs)

Strength (psi)

3/4 lbs 28 63970 50903/4 lbs 28 63150 50253/4 lbs 28 61230 4873

1 lb 28 66030 52551 lb 28 65370 52021 lb 28 64540 5136

1.5 lbs 28 65700 52281.5 lbs 28 62450 49701.5 lbs 28 61330 4880

Crack Widths of A1 Microfiber Specimens

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Crack #Control

[1]3/4 lb

[1]3/4 lb

[2]1 lb [2]

1 0.15 0.08 0.115 0.082 0.125 0.175 0.275 0.1753 0.2 0.175 0.30667 0.1254 0.09 0.275 0.2 0.25 0.08 0.3 n/a 0.56 n/a 0.365 n/a 0.1757 n/a 0.33 n/a 0.158 n/a 0.3 n/a n/a9 n/a 0.175 n/a n/a

10 n/a 0.125 n/a n/a

Crack Widths of A1 MicroMesh Specimens

Crack #Control

[1] 1 lb [1]1 0.08 0.12 0.09 0.21253 0.1 0.254 0.45 0.1255 0.5 0.1256 0.45 0.087 0.45 0.098 0.365 0.089 0.4 0.125

10 0.275 0.1511 0.225 n/a

Temperature and Humidity Readings for Testing of A1 Microfiber

Control 3/4 lb 1 lb 1.5 lb

TimeWind Speed Humidity

Wind Speed Humidity

Wind Speed Humidity

Wind Speed Humidity

11:04 AM 6.8 m/s 95.5 F 7.5 m/s 96 F 7.4 m/s 97.3 F 7.5 m/s 97.1 F

11:32 AM 6.6 m/s 95.5 F 7.4 m/s 95.1 F 7.25 m/s 95 F 7.58 m/s 98 F

12:04 PM 6.7 m/s 94.5 F 7.6 m/s 94.6 F 7.1 m/s 94.6 F 7.65 m/s 94.4 F

12:38 PM 6.7 m/s 93.9 F 7.6 m/s 94.2 F 7.15 m/s 94.5 F 7.5 m/s 94.4 F

1:04 PM 6.5 m/s 94.1 F 7.1 m/s 94.6 F 6.77 m/s 95 F 7.5 m/s 95 F

Temperature and Humidity Readings for Testing of A1 MicroMesh

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Control 3/4 lb 1 lb 1.5 lb

TimeWind Speed Humidity

Wind Speed Humidity

Wind Speed Humidity

Wind Speed Humidity

12:29 AM 8.2 m/s 93.3 F 7.2 m/s 93.5 F 8.0 m/s 93.7 F 7.5 m/s 93.9 F

1:00 PM 8.2 m/s 95.5 F 7.0 m/s 95.5 F 8.3 m/s 95.1 F 7.1 m/s 94.8 F

1:35 PM 7.8 m/s 96.8 F 6.9 m/s 96.9 F 7.9 m/s 96.9 F 6.7 m/s 96.6 F

2:05 PM 7.4 m/s 93.2 F 7.0 m/s 93.3 F 8.1 m/s 93.7 F 6.3 m/s 93.9 F

References

ASTM (C1579). “Standard Test Method for Evaluating Plastic Shrinkage Cracking of Restrained

Fiber Reinforced Concrete (Using a Steel Form Insert) West Conshohocken, Pa.

ASTM. (C1116). “Standard Specification for Fiber-Reinforced Concrete.” West Conshohocken,

Pa. (Approved January 1, 2008).

ACI 544.3R-08, “Guide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel

Fiber Reinforced Concrete,” American Concrete Institute (ACI). P.O. Box 9094,

Farmington Hills, MI 48333.

ACI 544.2R-89, “Measurement of Properties of Fiber-Reinforced Concrete,” American Concrete

Institute (ACI). P.O. Box 9094, Farmington Hills, MI 48333.

ACI 544.1R-96, “State-of-the-Art Report on Fiber Reinforced Concrete,” American Concrete

Institute (ACI). P.O. Box 9094, Farmington Hills, MI 48333.

ICC-AC32, “Acceptance Criteria for Concrete with Synthetic Fibers,” ICC Evaluation Services,

Inc. Approved October 2003.

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