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The Efficiency Of A Specific Surface Treatment Based On Lithium And Silicon

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ABSTRACT

For a concrete pavement, the permeation specifications of the surface have a crucial influence on its durability. In this accelerated laboratory research, a surface treatment that combines lithium silicate chemistry with a reactive silicon catalyst was tested to typify the product longevity under traffic and against salt scaling. The river gravel and limestone were used in two different mixture designs. The abrasion test was conducted according to the ASTM standards and the mass loss was recorded at different time intervals. A standard modified test method was directed using a diluted deicer simulated by 4 wt% CaCl2 solution during 15 cycles of freeze/thaw test. A model was performed to relate the efficiency against abrasion cycles of the surface treatment to the longevity of a concrete pavement based on the abrasion coefficient. During the experimental procedures, the untreated concrete specimens were used as the control sample. Regarding the abrasion and freeze/thaw tests, the results showed a less cumulative loss damage, which confirm the benefits of using the product to treat the concrete surface. The type of aggregates used had a significant effect on the results where the mix design with limestone showed less damage while exposed to freeze/thaw cycles compared to the mix design with river gravel. The calculation method showed an increase by 14% of the ultimate load application to failure for treated specimens, which indicates an increase in longevity of the pavement.

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INTRODUTION

The durability of the concrete has been a main issue regarding its material deterioration with time exposed to traffic loading and environmental damaging effects. The abrasion of the surface due to repeated tire rotations and the freeze/thaw cycles exposed to deicer chemicals, both affect the surface of a concrete pavement.

One of the most important wear cases is the one that occurs on concrete road surfaces due to automobiles and heavy trucks. The impact-cutting type of wear added to the rubbing action exacerbate the condition of the pavement. In some situations, where chains on trucks or automobiles tires are used, the contact between the tires and the concrete surface is brought with an important impact that causes to cut the surface. This regular action on the pavement rapidly deteriorates the surface, causing consequently roads safety issues. Since the abrasion process occurs at the surface, the quality of the cement paste is one of the main concern. Hydrated cement paste contains microscopic particles of calcium hydroxide. This by-product of the hydration process of the cement is soft which can be eroded away quickly, leaving microscopic pits in the surface of the concrete. Many laboratory tests have been used to evaluate the abrasion resistance of the concrete that depends on the paste hardness, the aggregates abrasion resistance and the aggregate/paste bond.

In addition, the freeze/thaw resistance is related to many factors such as: Pore sizes in concrete, type of aggregates, water to cement ratio, the use of non-air-entrained concrete and even any finishing method on wet concrete. During the freezing cycle, the water entrapped in the pores solidifies and turns into ice. This phenomenon expands the water by 9% and creates important tensile stress. Scaling due to several freeze/thaw cycles refers to a local flaking of a finished surface. This damage is distinguished by a removal of mortar, which leads to a direct exposure of the concrete to moisture penetration and aggressive salts. Worldwide, the use of deicer chemicals has been employed in order to increase the roads safety. The effect of a deicer on ice is related to the physical bonds formed between the solute and the solvent. Two main properties are in perspective: the freezing point depression and the boiling point elevation. Adding salt both increases the boiling temperature of the water and decreases its freezing temperature at the same time. The combined effect of freeze/thaw cycles and deicer chemicals on the scaling resistance is lower than the resistance to frost alone. The use of deicer chemicals can decrease the scaling resistance as moisture tends to move towards zones with higher salt concentrations via osmosis. Indeed, the aqueous solution at the surface causes expansive forces inside the concrete microstructure.

The use of different surface treatments to protect the concrete surface from abrasion and freeze/thaw cycles exposed to deicer chemicals has been widely elaborated. It is an economical way to protect the surface compared with other methods, such as decreasing the water to cement ratio or adding admixtures. Many studies showed the effect of surface treatment on mechanical and physical properties of concrete. It is widely acknowledged that a surface treatment has poor effect of the strength of the concrete since it cannot improve the porosity and quality of the whole concrete element. In this accelerated laboratory research, the surface treatment applied was a concrete wear resistant surface hardener with anti-scale protection and low viscosity surface treatment. It penetrates the wear layer of the concrete and reacts with the free lime C-S-H (calcium silicate hydrate) to form an insoluble wear and moisture protective surface.

Relating the results obtained in a laboratory environment to the field application was a main concern treated by several studies. E. Horszczaruk presented a theoretical model of testing the abrasion of concrete caused by the movement of rubble carried by water in hydraulic structures and modelling the natural mechanisms present in the environment. The results showed that the concrete wear caused by interaction with the aggregate-water mix is closely related to a parameter that describes its structural composition and the work of the abrasive mix. Another study that extents the theoretical model developed by E. Horszczaruk, expresses the material loss as a function of the work done by the wheel and the total abrasion time, in addition to the composition of the concrete. This probabilistic model aims to predict the volume abraded of the pavement. Knowing the model parameters representing the material characteristics and the traffic configuration, a prediction of the concrete pavement lifetime can be obtained.

EXPERIMENTAL PROCEDURES

Concrete constituents, mix designs, and curing

Two mix designs were proposed for this study. An ASTM specification C 150-07 Type I/II Portland cement was used. Two types of coarse aggregates were present with a maximum size of 25 mm. The fine aggregates consisted of natural, clean silica sand with a maximum size of 4.75mm. Class C fly ash was added as a 40% cement replacement (ASTM C 618). The aggregates specifications were determined according to the ASTM C 127 and ASTM C 128. The mix proportion of concrete is summarized in Table 1.

TABLE 1: Concrete mixture proportions

Mixture Component Quantities lbs/ft3

Mix design 1 Mix design 2

Cement 14.62 14.62

Fly Ash 6.27 6.27

River Gravel 65.37 0

Limestone 0 65.31

Sand 46.04 43.23

Net Water 8.29 8.57

w/c 0.44 0.44

At first, the coarse aggregates were washed to clean them from all dirt and then were added to the sand in the mixing container. The cement and fly ash were added to the mix. When a homogenous mixture was obtained, the water is finally added. The batch was remixed periodically until the concrete was homogenous and reached the desired consistency according to the ASTM C 192. As recommended for any freeze/thaw test, the design air content for the two mix designs was fixed at 6.0%. For each mix, the concrete specimens were cast in plastic squares 6 × 6 (in2) and in cylinders molds 4 × 8 (in2). Table 2 summarizes the specimens used for this study for each mix.

TABLE 2: Specimens

Shape Dimensions (in2) ID Number of specimens Test Standard

Control Specimens Square 6 × 6 CN/F 1 Abrasion ASTM C 944

Square 6 × 6 CF(15) 1 15 cycles Freeze/thaw ASTM C 672

Treated Specimens Square 6 × 6 TN/F 2 Abrasion ASTM C 944

Square 6 × 6 TF(15) 2 15 cycles Freeze/thaw ASTM C 672

All specimens cured in a controlled room at 20 ˚C and 99% Relative Humidity for 28 days. The specimens with river gravel reached a compressive strength of 4,900 psi. The specimens with limestone had a compressive strength of 5,900 psi at 28 days. A dike about 25 mm wide and 20 mm high was placed along the perimeter of the top surface of the specimens. This dike adhered to the edge of the specimens by using a waterproof epoxy.

Surface treatment

After 28 days, the specimens were air dried at 23 +/- 2˚C and 50% relative humidity for 24h. The surface treatment was applied at a rate of 175 ft2/gallon to the top surface of the specimens and let dry for 24h.

Abrasion test

The abrasion test is based on the ASTM C 944 since it is a better simulation for roads under the action of chains and metal wheels. This test consists of a drill press with a chuck capable of holding and rotating the abrading cutter at a speed of 200 rpm and exerting a force of a normal load of 44 ± 0.4lbf. In this study, a constant load is used during 3 cycles of 2, 4 and 6 minutes. At the end of every cycle, a high pressure air blower was used for cleaning up the loose dust and the specimen is weighted. The cumulative mass loss is consequently determined.

Salt scaling test

A modified method of the ASTM C 672 was used to perform this test. After 28 days, 6 mm of a solution of a 4 wt% deicer calcium chloride was added at the surface of the specimens. The specimens were alternately place in a freezing environment at -18˚C for 16h then allowed to thaw at 23˚C for +/- 8h. Unlike the procedure according to the ASTM C 672 that uses a visual rating system, a modified approach recommended by SHRP (Strategic Highway Research Program) and CSA (Canadian Standards Association) was performed. Water was added between each cycle to maintain the proper depth of solution. After every five cycles, not only a visual assessment was made, but also the specimens were flushed with tap water and the eroded aggregates and paste were washed into a funnel with a filter paper inserted. The filter paper was dried at 38˚C overnight and the weight is recorded. The cumulative weight loss from the exposed aggregate surface is considered to be an indication of the severity of the deicer chemical and the efficiency of the surface treatment used. The test was conducted for 15 cycles.

RESULTS AND DISCUSSION

Abrasion

The abrasion resistance of surface-treated concrete is an indicator of the longevity of the surface treatment under repetitive traffic loading. However, when the surface is completely worn away, the abrasion resistance of the concrete depends on many other factors such as the abrasion resistance of the aggregates, their shape and sizes. During the whole test, the surface treatment used showed a significant impact on improving the abrasion resistance of the concrete surface. For the mix with river gravel, the surface treatment performed well during the whole test and especially during the first 4 minutes where only the surface was abraded. Concerning the mix with limestone, the control specimen showed more damage the first 2 minutes. Following that, the results were mixed. The limestone demonstrated an anisotropic behavior during the test which illustrates a lack of homogeneity underneath the abraded layer. It was predicted that the samples with limestone will exhibit less loss damage since they had a higher compressive strength than the samples with river gravel. As the compressive strength increases, the resistance to abrasion increases and consequently leads to a lower mass loss. The chemistry of this specific surface treatment based on reactive lithium and silicon enhanced the bonding with calcium hydroxide to lead to a better quality and tougher concrete surface.

Scaling resistance of concrete exposed to freeze/thaw and deicer chemical

Every 5 cycles, the surface of the specimens was washed and the cumulative weight loss was measured. This mass loss is an indicator of the longevity of the surface treatment. During the 15 cycles and for the two mix designs, the untreated specimens showed a significant mass loss compared to the treated concrete specimens. The mass loss was more significant during the 15 cycles for the specimens with river gravel than for the ones with limestone. This additional result asserts that the aggregate characteristics control the frost damage. The gravel were bigger than the limestone. Under freezing conditions, the unfrozen water in smaller aggregate particles is expelled quickly without developing damaging pressure within the structure of the concrete. Pore structure (i.e., pore size, pore shape and pore distribution) are also an indicator of the performance of an aggregate under freeze/thaw cycles.

Theoretical model

The following method can be used in prediction concrete pavement’s life under dry erosion damage regardless of the type of abrasion test used to evaluate the efficiency of a surface treatment. During the abrasion tests, the abrading cutter generated a circular wear path on the concrete surface. An effective shear stress term, τ, is proposed to quantify the average stress level on the abrading zone of concrete specimen. This shear stress will evoke a mass loss gradually resulting to a wear depth.

The averaged abrasion depth, D, is computed:

D=∆V/A

∆V= loss of material in volume during the test (m^3), ∆V=( ∆m)/ρ

∆m = Mass loss

A= Cutter contact area (m^2), which equals to the area of wear path.

ρ=2,400 Kg/m^3

A term, abrasion coefficient (c), is further defined to account for the velocity of the abrading surface. The use of c also can convert D into a dimensionless term.

c=D/vt

Where,

v=200rpm=0.9299m/s = The velocity of the abrading cutter (m/s)

t=6 minutes=360 s = test time (s)

TABLE 3: Abrasion coefficient and averaged depth

Mix design Specimen Type ID ΔV (m3) D (m) Abrasion coefficient × 10-7

River Gravel Control CN/F 2.77917E-06 0.000488 14.56468137

Treated Specimens T1N/F 2.3375E-06 0.00041 12.25005435

T2N/F 2.44583E-06 0.000429 12.81779305

Limestone Control CN/F 1.72083E-06 0.000302 9.018310953

Treated Specimens T1N/F 1.89167E-06 0.000332 9.913591217

T2N/F 1.89167E-06 0.000332 9.913591217

The abrasion coefficient is function of the strength of the material and the shear stresses induced by the tire. A large abrasion coefficient induces a more abraded surface since the average abrasion depth D is important.

The erosion model at the surface can be expressed as:

%E= (∑▒D_i )/D_allowable ×100

D_allowable = The allowable abrasion depth.

D_i = The accumulated erosion damage ratio over the design period, calculated as follow:

D_i= ∑▒n_i/N_f

n_i = The predicted number of load repetitions.

N_f = The allowable number of load repetitions to failure at a stress/strength given.

The allowable abrasion coefficient is given by:

c=D_allowable/vt

v = The traffic speed

t = The traffic load application time.

The allowable number of load repetitions N_f can be defined as:

N_f= vt/a

Where,

a = The radius contact area of the tire, given by:

a= √(P/πq)

P = The total vertical load

q = The contact pressure.

Therefore,

N_f= D_allowable/(c*a)

CONCLUSIONS

In this paper, the efficiency of a specific surface treatment based on lithium and silicon was evaluated. It was demonstrated that the application of this surface treatment enhanced the properties of a concrete surface exposed to abrasion and several freeze/thaw cycles. The characteristics of the aggregates had a significant impact on the results. The proposed model to predict the allowable number of load repetitions is related to a parameter quantified by the abrasion resistance which is a function of the material mass loss due to the abrasion test. On the field, this coefficient can be determined based on the traffic characteristics (as the speed and time) and the mass loss due to the impact between the tires and the concrete surface. Following this, an engineer can establish the allowable number of load repetitions before the allowable wear depth is reached. Applying this model to the surface treatment used, it was deduced that the allowable number of load repetitions increases by 14%. Further studies and applications methods should be performed in order to relate this proposed model to the freeze/thaw cycles and be able consequently to predict the concrete pavement lifetime.

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