Ultra-High Performance Concrete: A State-Of-The-Art Report for The Bridge Community

CHAPTER 5. DURABILITY AND DURABILITY TESTING

INTRODUCTION

The use of UHPC in any infrastructure application requires the UHPC to have adequate resistance to deterioration caused by the environment to which it is exposed. This chapter reports on the durability of UHPC based on the parameters and tests generally used to determine the durability of conventional concrete.

PERMEABILITY

In the United States, the permeability of concrete is generally assessed using AASHTO T 277 (ASTM C1202)—Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration.^(215,216) Tests conducted by Graybeal in accordance with ASTM C1202 resulted in values of less than 40 coulombs at 28 days for steam-cured specimens and values of 360 and 76 coulombs at 28 and 56 days, respectively, for untreated specimens.⁽²²⁾ Materials with values less than 100 coulombs are considered to have negligible chloride ion penetrability.

Ahlborn et al. reported rapid chloride permeability values of less than 100 coulombs for both air-cured and heat-treated concretes.⁽¹⁰⁸⁾ Bonneau et al. reported values of 6 to 9 coulombs for two different mixes.⁽¹⁰⁵⁾ Thomas et al. reported values of zero to 19 coulombs at an age of 28 days.⁽²¹⁷⁾ Ozyildirim reported values of 19 and 35 coulombs.⁽⁴⁵⁾

Chloride penetration tests in accordance with AASHTO T 259 were also reported by Graybeal.^(22,218) This test involves ponding a 3-percent sodium chloride solution on the surface of the concrete for 90 days and then determining the penetration of the chlorides into the concrete. Although there tended to be higher levels of chloride ions near the surface, the amount of chloride that penetrated into the concrete was extremely small.

Different tests for permeability are used in other countries. One measure of chloride penetration is the value of its chloride diffusion coefficient. Reported values are as follows:

• 2 x 10^-15 m²/second in steady-state conditions and 3 x 10^-11 m²/second in non-steady-state conditions. ⁽²¹⁹⁾
• 1.3 x 10^-13 m²/second at 28 days.⁽²¹⁷⁾
• 2.3 x 10^-13 m²/second in a non-steady-state condition.⁽²²⁰⁾

Gao et al. tested the permeability of UHPC using pressure testing. No water leakage occurred when the hydraulic pressure was increased from 14.5 to 232 psi (0.1 to 1.6 MPa) at a rate of 14.5 psi (0.1 MPa) per 8 hours.⁽⁹¹⁾ After removing the pressure, moisture had penetrated 0.11 inches (2.7 mm) into the specimens.

The effects of microcracks induced by loading on chloride penetration have also been investigated. Graybeal examined the penetration of a 15-percent sodium chloride solution into the tension face of a beam.⁽²²¹⁾ The beam was subjected to 500,000 cycles of repetitive loading over 154 days to a maximum tensile stress 14 percent above the first cracking load. The solution penetrated to a depth of 0.12 inches (3 mm) on the side faces and 0.2 inches (5 mm) on the tensile face of the beam. The steel fibers crossing crack planes did not show any visible signs of section loss or tensile failure.

Aarup loaded small reinforced beams with a cover to the reinforcement of 0.4 inches (10 mm) to produce various levels of bending stresses.⁽²³⁾ Over a period of 4 years, during which the beams were repeatedly exposed to a salt solution for 2 days and dried for 5 days, no correlation between loading of the beams and chloride diffusion was observed and no corrosion occurred. Measured diffusion coefficients for unloaded and loaded beams ranged from 2x10^-14 to 1x10^-15 m²/second.

Charron et al. reported the results of permeability tests on UHPC specimens previously subjected to various levels of tensile deformation.⁽²²²⁾ Based on the test results, the maximum residual tensile strain whereby the water permeability remained low was determined to be 0.13 percent.

FREEZE-THAW RESISTANCE

The standard test for freeze-thaw resistance in the United States is AASHTO T 161 (ASTM C666)—Resistance of Concrete to Rapid Freezing and Thawing. ^{(223, 124)} AASHTO T 161 has two procedures. Procedure A involves rapid freezing and thawing in water while Procedure B involves rapid freezing in air and thawing in water. Tests of UHPC beginning 5 to 6 weeks after casting and using Procedure A were reported by Graybeal.⁽²²⁾ Specimens subjected to steam curing prior to testing and untreated specimens showed very little deterioration throughout 690 cycles of freezing and thawing. The specimens that were untreated continued to hydrate and gain strength during the testing sequence.

The ability of conventional concrete to resist freeze-thaw damage can also be assessed by measuring certain parameters of its air-void system. Air-void analyses of UHPC reported by Graybeal are shown in table 13.⁽²²⁾

Table 13. Air-void system parameters

Despite having an air-void system that might not be suitable with conventional concrete, the UHPC performed adequately in freeze-thaw testing.

Bonneau et al. reported that the durability factor of three different mixes was equal to or greater than 100 when tested using ASTM C666 Procedure A. ⁽¹⁰⁵⁾

Acker and Behloul reported tests with 400 cycles of freezing and thawing that showed no degradation.⁽¹³⁰⁾ Similar results were obtained by Ahlborn et al. and Piérard et al.^(108,220) Magureanu et al. reported that UHPC samples displayed higher values for compressive strength, static modulus of elasticity, and dynamic modulus of elasticity after 1,098 freeze-thaw cycles compared with control specimens.⁽⁶⁹⁾

Based on their research, Müller et al. concluded that UHPC mixes show an extremely high freeze-thaw resistance to water with or without deicing salts. ⁽²²⁴⁾ They attributed this to the very low moisture uptake by the UHPC.

SCALING RESISTANCE

The standard test for evaluation of scaling resistance in the United States is ASTM C672—Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals.⁽²²⁵⁾ In this test, the surface is exposed to a salt solution and subjected to daily freeze-thaw cycles.Generally, 50 cycles are sufficient to evaluate a surface. Graybeal reported that after 215 cycles, no surface scaling of UHPC specimens had occurred.⁽²²⁾ Bonneau et al. reported very low amounts of scaling for three mixes after 50 cycles.⁽¹⁰⁵⁾

Schmidt et al. reported a scaling rate of 100 g/m² (3 oz/yd²) after 56 cycles of freezing and thawing compared with the normal acceptance limit for their test of 1,500 g/m² (44 oz/yd²) after 28 cycles.⁽¹¹⁵⁾ Measurements of sound velocities showed no internal damage from freeze-thaw testing. Specimens that received no heat treatment showed a higher freeze-thaw resistance compared with heat-treated specimens.

Cwirzen et al. examined the effect of heat treatment on the durability of UHPC.⁽²²⁶⁾ The test results for specimens without steel fibers showed low surface-scaling values after 56 freeze-thaw cycles in all specimens. After 150 freeze-thaw cycles, the heat-treated specimens showed an increase in surface scaling. The relative dynamic modulus of the heat-treated specimens dropped below 50 percent after 200 cycles, whereas the non-heat-treated specimens showed a very small change. The presence of steel fibers restrained the internal damage but caused higher surface scaling.

CARBONATION

Carbonation of concrete is a process by which carbon dioxide from the atmosphere penetrates the concrete and reacts with various hydration products. Depth of carbonation is typically measured by applying phenolphthalein solution to the surface of the concrete and measuring the depth of the color change.⁽²²⁷⁾

Small-scale beams of UHPC placed in a carbonation chamber and subjected to flows of 5- or 100-percent carbon dioxide showed no signs of carbonation after 2 years.⁽²¹⁹⁾ On the other hand, Müller et al. reported that mechanically induced microcracks were observed to be partly or completely filled by carbonation.⁽²²⁴⁾

Piérard et al. reported a carbonation depth of 0.006 to 0.008 inches (1.5 to 2.0 mm) after a 1-year exposure to a 1-percent CO₂ atmosphere. ⁽²²⁰⁾ The duration of the test is generally limited to 56 days.

ABRASION RESISTANCE

Graybeal reported tests for the abrasion resistance of UHPC.⁽²²⁾ The tests were conducted in accordance with ASTM C944—Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method on 6-inch (152-mm)-diameter cylinders that were cured using one of four curing methods.⁽²²⁸⁾ Three concrete surfaces were used—cast against a steel mold, sand blasted, and ground. The double test load was used. The results clearly indicated much higher abrasion resistance of steam-cured specimens compared with untreated specimens. For the steam-cured specimens, the surfaces cast against the steel mold had higher abrasion resistance than the sand-blasted or ground surfaces.

SULFATE RESISTANCE

Piérard reported no deterioration of UHPC when immersed in sodium sulfate solution for 500 days.⁽²²⁰⁾

RESISTANCE TO ALKALI-SILICA REACTIVITY

Various tests to determine the resistance of concrete to alkali-silica reactivity (ASR) are available.⁽²²⁹⁾ ASTM C1260 contains a test procedure that accelerates any ASR reaction and can be accomplished in 16 days.⁽²³⁰⁾ Using a version of this test modified to allow for steam curing, Graybeal reported levels of expansion that were an order of magnitude below the threshold value for innocuous behavior.⁽²²⁾ He concluded that there should be no concern about ASR with the UHPC that was tested. He noted that free water must be present for ASR to occur. With the low permeability of UHPC, it is unlikely that free water would be present.

MARINE EXPOSURE

Three series of UHPC mixtures were placed in a marine exposure site at Treat Island, ME.⁽²¹⁷⁾ The exposure conditions included 20-ft (6-m) tides and more than 100 freeze-thaw cycles per year. After 5 to 15 years of exposure and more than 1,500 cycles of freezing and thawing in some cases, there was no evidence of deterioration or degradation of mechanical properties. The depth of chloride penetration was much lower than observed for typical HPC in the same environment.

FIRE RESISTANCE

Behloul et al. have reported information related to the fire resistance of UHPC made with Ductal-AF^®.⁽¹²²⁾ Ductal-AF^® is specially formulated to have fire resistance. Published information includes the change in compressive strength, tensile strength, modulus of elasticity, thermal conductivity, specific heat, and coefficient of thermal expansion for specimens subjected to temperatures between 68 and 1,112 °F (20 and 600 °C).

Fire tests according to ISO 834 were also conducted on columns and beams using both loaded and unloaded specimens.^(231,122) Some specimens were steam cured while others were not. The authors reported that the results were very positive compared with conventional concrete when using the French rules for fire safety. One feature was the lack of spalling that occurs with conventional concrete. This facilitated the use of thermal modeling to predict the behavior.

Heinz et al. reported the fire resistance of UHPC 3.9-inch (100-mm)-diameter cylinders and 4.7- by 9.4-inch (120- by 240-mm) columns under load. ⁽²³²⁾ The concretes included either steel fibers or a combination of steel and polypropylene fibers. At an age of 24 hours, the specimens were heat treated in water at 194 °F (90 °C) for 24 hours. Testing followed the time-temperature curve of German standard DIN 4102-2.⁽²³³⁾ The cylinders without polypropylene fibers exhibited spalling after a few minutes. After 90 minutes, the sample was destroyed beyond recognition. In contrast, cylinders containing 0.66 percent by volume of polypropylene fibers showed no signs of spalling. However, cracks with widths of 0.012 to 0.02 inches (0.3 to 0.5 mm) were present over the whole surface of the cylinders. In testing the columns, spalling occurred after about 11 minutes. The initial period of spalling was followed by a dormant period with no further destruction until fracture of the specimens. The authors concluded that a UHPC with 3.05 percent by volume of steel fibers and 0.60 percent by volume of polypropylene provided thebest results. The effects of elevated temperatures on the residual compressive strength and modulus of elasticity were also reported by Way and Wille. ⁽²³⁴⁾

Hosser et al. also conducted tests to evaluate which combinations of protective lining and polypropylene fiber content were able to minimize spalling under fire exposure.⁽²³⁵⁾ They also measured thermal conductivity and specific heat.

Aarup reported that the behavior of UHPC 1 week after fire tests was better than for conventional concrete.⁽²³⁾ One reason stated for the improved performance was that the UHPC had a very high silica fume content and negligible calcium hydroxide content. A literature review of the behavior of UHPC at elevated temperatures has been prepared by Pimienta et al.⁽²³⁶⁾

SUMMARY OF DURABILITY PROPERTIES

The dense matrix of UHPC prevents deleterious solutions from penetrating into the matrix, and so the mechanisms that can cause conventional concrete to deteriorate are not present. Consequently, durability properties, as measured by permeability tests, freeze-thaw tests, scaling tests, abrasion tests, resistance to ASR, and carbonation, are significantly better than those of conventional concrete. For fire resistance, it appears that a special formulation may be necessary.