TECHNICAL FIELD

This invention relates to the field of road construction, and particularly to a copolymer soil subgrade binder, and to utilizing ampholytic co-polymers in combination with soil to form improved subgrade soil binder compositions.

BACKGROUND ART

Road subgrade soil stabilization has a major influence on pavement construction and durability. The performance of pavements depends crucially on the strength and stability of the supporting soil layer, referred to as the subgrade. A pavement subgrade failure can be catastrophic, i.e., hazardous to traffic and expensive to rectify. This is known and commercial subgrade binders are available; the most common of which are cement or polymer based materials. The most common stabilization binder for soils in pavements is Portland Cement (PC). Several studies have demonstrated, but generally for agriculture applications, that a polymer based alternative can be a viable choice.

The majority of polymers that are currently considered as soil stabilizers are based on polyacrylamide (PAM) or its derivatives. Polyacrylamide and its chemical family are generally water soluble and known to be environmentally friendly. However, there is a continued need for a greener, technically sound, and commercially viable, alternative to soil stabilization that is easily adaptable to soils of various compositions. Such methods and compositions would be extremely useful in increasing the lifetime of roads and decreasing the cost of road maintenance.

Thus, a co-polymer soil subgrade binder solving the aforementioned problems is desired.

DISCLOSURE OF INVENTION

The co-polymer soil subgrade binder relates to methods and compositions utilizing polyacrylamide co-polymers and ampholytic terpolymers in combination with soil to form improved subgrade soil binder compositions for supporting asphalt roads and pavement surfaces. In one embodiment, the invention relates to a co-polymer soil subgrade binder, designated TP AM, having the formula:

wherein the relative proportional range of m, p, and q are controlled to vary between 0 and 1.

In one embodiment, the invention relates to a method of stabilizing the soil layer below pavements, comprising treating soil with the co-polymer soil subgrade binder of the above formula so as integrate the binder with the soil to form a mixture for supporting roads and pavement surfaces.

In one embodiment, the invention relates to a mixture of soil and a polymer binder, said polymer comprising at least one monomer selected from the group, for example, comprising acrylamide, sodium acrylate, and acrylamide propyl trimethyl ammonium chloride. In one embodiment, said polymer has the structure shown above.

In one embodiment, the invention relates to a layered composition comprising: a) a first layer containing a mixture of soil and a polymer binder, said polymer comprising at least one monomer selected from the group comprising acrylamide, sodium acrylate, and acrylamide propyl trimethyl ammonium chloride; b) a second layer of granular materials (viz., aggregates) on top of the first layer, typically known as the base or sub-base; and a third ;layer of either asphalt or cement concrete layer on top of the base.

In one embodiment, the invention contemplates a method of stabilizing the soil layer below asphalt-based pavements comprising; a) providing i) a co-polymer soil subgrade binder of the formula (1), ii) soil; and b) treating said soil with said co-polymer soil subgrade binder so as integrate said binder with said soil to form a mixture. In one embodiment, the method further comprises step c) molding said mixture into a desired shape. In one embodiment, the method further comprises step d) curing said mixture. These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 shows the structural formula of polyampholyte polymer "TP AM".

Fig. 2 shows another embodiment of the TP AM polymer of the current invention.

Fig. 3 shows the toughness of the soil subgrade, the subgrade treated with 9.0% cement, and the subgrade treated with 2.0% TP AM: results after 1, 7, and 28 days of curing.

Fig. 4 shows the variation of toughness a at different curing ages for representative Qatar subgrade soil treated with and without analytical PAM and it synthesized variants.

Fig. 5 shows the unconfined compressive strength (UCS) of the soil and the soil treated with Portland Cement and the polymers variants after 1 , 7 and 28 days of curing.

Fig. 6 shows the toughness of the soil and the soil treated with Portland Cement and different polymers after 1 , 7 and 28 days of curing.

Fig. 7 shows a cross section of an exemplary pavement structure and layer thickness.

Fig. 8 shows an sample wherein an asphalt layer is cured on the stabilized soil mixture layer.

Similar reference characters denote corresponding features consistently throughout the attached drawings.

BEST MODES FOR CARRYING OUT THE INVENTION

The co-polymer soil subgrade binder discloses ionic variations of polyacrylamide copolymers: cationic poly(acrylamidopropyl trimethyl ammonium chloride), anionic hydrolyzed poly(acrylamide) and the ampholitic terpolymer poly(acrylamide-co-sodium acrylate-co-(3-acrylamidopropyl) trimethylammonium chloride) and method for the stabilization and improvements of mechanical properties of pavement subgrade soils.

To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims. Reference throughout this specification to "one embodiment," "an embodiment," or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The term "asphalt," as that term is used in the specification and/or claims, refers to several types of compositions including the sticky, black and highly viscous liquid or semisolid present in most crude petroleums and in some natural deposits, a substance classed as a pitch, the manufactured asphalt product and the asphalt concrete composite material commonly used in construction projects such as road surfaces, airports and parking lots. It is not intended that this invention is limited to any particular type of asphalt.

The term "soil," as that term is used in the specification and/or claims, refers to the naturally occurring materials that are used for the construction of all except the surface layers of pavements (i.e., concrete and asphalt) and that are subject to classification tests, to provide a general concept of their engineering characteristics. It is not intended that this invention is limited to any particular type of soil.

The term "stabilization," as that term is used in the specification and/or claims, refers to the process of blending and mixing materials with a soil to improve certain properties of the soil. The process may include the blending of soils to achieve a desired gradation or the mixing of commercially available additives that may alter the gradation, texture or plasticity, or act as a binder for cementation of the soil. The current invention envisions the addition of polymers described herein as additives to aid in the stabilization of soils acting as soil binders.

The term "cement," as that term is used in the specification and/or claims, refers to a binder, a substance that sets and hardens independently, and can bind other materials together. Cement used in construction is characterized as hydraulic or non-hydraulic.

Hydraulic cements (e.g., Portland Cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g. gypsum plaster) must be kept dry in order to retain their strength. The term "rebar," (short for reinforcing bar), also known as reinforcing steel, reinforcement steel, rerod, a deformed bar, reo, or reo bar, as that term is used in the specification and/or claims, refers to a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression. It is usually formed from carbon steel, and is given ridges for better mechanical anchoring into the concrete.

As used herein, the term "Portland Cement" (abbreviated PC) refers to the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It usually originates from limestone. It is a fine powder produced by grinding Portland Cement clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards.

The term "acrylamide," as that term is used in the specification and/or claims, refers to a chemical compound with the chemical formula C ₃ H ₅ NO and the structure:

Its IUPAC name is prop-2-enamide. A polymeric version of acrylamide, termed

polyacrylamide, has the structure:

The term "sodium acrylate," as that term is used in the specification and/or claims, refers a chemical compound with the structure:

A polymeric version of sodium acrylate has the structure:

The term "acrylamide propyl trimethyl ammonium chloride," as that term is used in the specification and/or claims, refers to a chemical compound with the structure:

A polymeric version of acrylamide propyl trimethyl ammonium chloride, termed polyacrylamide propyl trimethyl ammonium chloride, has the structure:

The term "3-acrylamido-N,N,N-trimethylpropan-l-ammonium chloride," as that term in the specification and/or claims, refers to a chemical compound with the structure:

A polymeric version of 3-acrylamido-N,N,N-trimethylpropan-l-ammonium chloride has the following structure:

The ampholitic polyacrylamide variant, termed TP AM, has the structure:

It is not intended that TP AM be limited to this particular order, the order of the monomers within may vary (i.e., the terpolymer is a random polymer) while the proportion of the component monomers is within a particular comparative range. In one embodiment, the TP AM polymer can be described with the following structure, wherein the three monomers are repeated as a unit in the sequence shown (i.e., the terpolymer is an alternating polymer):

The cationic polyacrylamide variant, termed PAMTAC, has the structure:

It is not intended that PAMTAC be limited to this particular order, the order of the monomers within may vary while the proportion of the component monomers is within a particular comparative range. The anionic olyacrylamide variant, termed HP AM, has the structure:

It is not intended that HP AM be limited to this particular order, the order of the monomers within may vary while the proportion of the component monomers is within a particular comparative range.

Cationic groups include, but are not limited to the polymeric version of 3-acrylamido- N,N,N-trimethylpropan-l-ammonium chloride and sodium acrylate.

Anionic groups include, but are not limited to the polymeric version of acrylamide propyl trimethyl ammonium chloride.

This present invention builds on the work of the prior art studies showing that compaction of the polymer impregnated soils led to better strength and durability

performance as well as studies showing that longer curing periods and higher curing temperatures of up to 140 °C lead to improved unconfined compressive strengths for all samples and, in particular, that high content silica soils could produce a semi rigid type of pavement when stabilized at high temperatures.

Polyacrylamides (PAM) act as soil stabilizers due to their intrinsic chemical structure. Organic polymer functional groups are known to attach to the surfaces of soil particles and subsequently bond into a polymer matrix, thus giving the subgrade a structural integrity it would not otherwise have had. Conventionally, it is believed that polymers tend to react primarily with the clay fraction of the soil but polymer interactions with sands and aggregates have also been reported. The adsorption of cationic polymers by clays occurs through electrostatic interactions between cationic groups on the polymer and the negatively charged sites on the clay surface leading to ionic interactions in the form of charge neutralization. Anionic polymers tend to be repelled by the negatively charged clay surface, but adsorption can occur through the presence of polyvalent cations acting as bridges. Non-ionic polymers adsorb primarily through Van der Waals forces and/or hydrogen bonding. In a field situation, the molecular weight and conformation of the polymer can also influence the effective adsorption, particularly if the soil surfaces are neutral or weakly charged. Polymer binders were synthesized from polyacrylamide supplied in a 50% weight/weight aqueous solution by Sigma. Sigma also sourced the synthesis components: acrylamide (AM), N,N- Dimethylformamide (DMF), tris (2-aminoethyl) amine, and the reactant CuCl compound. Aldrich supplied: 2,2'-Azobis(2-methylpropionitrile), (3- acrylamidopropyl)-trimethylammonium chloride 75% in water, and methyl-2- chloropropionate. Ordinary Portland Cement Type I was used in the present invention.

The polymer stabilizers have been compared with PAM (polyacrylamide). Three variants of polyacrylamide (PAM) were synthesized, designated PAMTAC (cationic), HP AM (anionic), and TP AM (ampholitic), respectively, and tested as pavement stabilizer with respect to natural Qatar subgrade soil.

Example 1

Synthesis of Cationic Polymer (PAMTAC)

The PAMTAC variant was synthesized via the standard atom transfer radical polymerization (ATRP) procedure. Specifically, an aqueous solution of (3- acrylamidopropyl)-trimethylammonium chloride was polymerized with methyl-2- chloropropionate as the initiator and a tris(2-aminoethyl)amine-CuCl complex as the ATRP catalyst. The product is the PAMTAC polymer with a given concentration of cationic moduli. An exemplary structure of PAMTAC is shown below:

Example 2

Synthesis of Anionic polymer (HP AM) This anionic polyacrylamide was prepared following a standard procedure (e.g. Lulu et al. 2010) in which acrylamide (AM) is hydrolyzed with NaOH solution (Fig. lc). An exemplary structure of HP AM is shown below:

Example 3

Synthesis of Polyampholyte terpolymer polymer (acrylamide-co- sodium acrylate-co-(3- acrylamidopropyl)trimethylammonium chloride), TP AM

The synthesis is based on the standard atom transfer radical polymerization (ATRP) procedure. Specifically, an aqueous solution of acrylamide (AM) was co-polymerized with (3-acrylamidopropyl)-trimethylammonium chloride (AMTAC) at a given AM/AMTAC ratio with methyl-2-chloropropionate as the initiator and a tris-(2-aminoethyl) amine-CuCl complex as the ATRP catalyst. The product is an AM- AMTAC co-polymer with a given concentration of cationic moduli. This co-polymer is then subjected to hydrolysis with NaOH solution under controlled conditions to yield a given concentration of the anionic moduli. An exemplary structure of TP AM is shown below:

Overall, the polyampholyte (TP AM) is characterized by a particular ratio of the neutral, cationic and anionic modules m, p and q (Fig. Id). A value of the ionic ratio m:q for

PAMTAC was obtained from the 13 C NMR spectra recorded at 293 K using a Bruker Avance 400 spectrometer operating at 400 MHz. Chemical shifts in deuterated water (D ₂ 0) were reported in ppm with respect to a trace of 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) used as an internal standard. It was determined that the sample had a 2: 1 ratio of neutral to cationic units. The hydrolysis of cationic copolymer of AM-AMTAC was prepared with a 1: 1 ratio of neutral to anionic units. Thus, it is assumed that the structure of the TP AM is characterized by a m:p:q ratio of approximately 1/3: 1/3: 1/3.

Soil samples were collected from subgrade layers of pavement construction projects.

The soil was characterized as a gravel-sand-silt-clay mixture following the ASTM D2487 method (ASTM 2011). The soil had 2% by volume fraction of clay particles in accordance to ASTM D0422-63 (ASTM 2007). Dolomite, Calcite and Gypsum were the major components but with a significant contribution of Palygorskite.

Six batch samples formed the basis of the invention: untreated soil; soil treated with

PC, soil treated with PAM (50% weight/weight aqueous solution supplied by Sigma); and three soils treated with the synthetic PAMTAC, HP AM and TP AM variants, respectively. The batches were cast at their optimum moisture content. Three replicate cylindrical samples were extracted from each batch and cured for up to 28 days at 35°C. This temperature corresponds to the average temperature experienced in Qatar throughout the year. Three curing periods were selected, namely: one, seven and twenty eight days. It was determined that the optimum dosage of cement to soil was 9% per dry weight of soil; the optimum dosage of the PAM and variant binder solutions was 2% per dry weight of soil.

The sample preparations and subsequent investigations followed standard procedure. The optimum moisture content and maximum dry density of the Qatar subgrade soil (with and without binder additions) was first determined. The moisture content was using a drying oven controlled at 110 ±5°C. When treating the soil with PC, a pre-weighed quantity of cement, defined as grams per dry weight of soil, was dry mixed with the soil prior to the sequential addition of water. During soil treatment with polymers, a predetermined polymer content estimated as percentage per dry weight of soil was first dissolved in a known volume of water and then mixed with the soil.

Further, small adjustments were applied to compensate for the contribution of the liquid polymer to the determined optimum moisture values. The treated soils were molded at their optimum moisture content and then three replicate cylindrical samples were extracted from each soil and immediately transferred to an 20 oven and cured at 35°C - a curing temperature that corresponds to an average temperature experienced in Qatar throughout the year. Three curing periods were selected, namely: one, seven and twenty eight days, respectively. Once cured, the samples were weighed and the dimensions measured using a digital Vernier caliper. Hence, the bulk densities of the cured samples followed.

The samples were tested for their unconfined compressive strengths (UCS) using an electromechanical compression testing machine having a maximum load capacity of 250kN with a moving head operated at approximately 1 mm/min and equipped with a linear variable differential transformer (LVDT) set up to measure the corresponding deformation. Tests were conducted in triplicate samples representing each soil condition after sample curing of 1 day, 7 days or 28 days at 35°C. The elastic modulus and toughness were determined from the test results for each of the samples.

Figure 4 depicts the variations in toughness of untreated and treated subgrade soil samples at different curing stages. A strong argument can be made that the parameter

"Toughness" is a realistic measure to judge the efficiency of a pavement stabilizer. Based on the toughness of the subgrade samples stabilized employing 2.0% PAM vs. 2.0% TP AM, it can be clearly inferred that ionic bonding offered by TP AM enhances the efficiency of subgrade stabilization in contrast to the nonionic PAM binder. Thus, the novel polymers give superior binding performance in comparison with the standard binder, cement.

The results of the Unconfined Compressive Strengths (UCS) tests are shown in Fig. 5. A very similar trend was obtained for the elastic modulus. The level of imprecision is indicated by the error bars and was generally similar for all the different mixes with an average standard deviation of 4.98%. Fig. 5 indicates that the UCS value for the untreated soil effectively plateaued after about 7 days of curing, i.e., when the sample has dried. For the polymer-stabilized samples, there was a slower but progressive removal of moisture from the samples and hence the influence of polymers as stabilizers was not obvious until 28 days. The change of UCS as a function of time for the cement-treated soil can be attributed to an initial setting of the cement within an approximate 24 hour period followed by progressive hydration.

The performance of the polymer- stabilized soils is compared with that of the untreated soil under equivalent conditions. At 28 days of curing, the UCS of soils treated with HP AM and TP AM is about 3.6 MPa, which is more than twice that of the untreated soil. Moreover, the UCS of polymer-treated soils surpasses that for the cement-stabilized soil (2.5MPa). Further, the guidelines indicate that the UCS of the stabilized soil should be at least 0.345 MPa higher than the unstabilized soil in order for the stabilizing agent to be considered effective. This criterion is easily satisfied by the anionic and the polyampholyte binders.

Even though the UCS is the most used parameter to evaluate the mechanical properties of soils used in road construction, the UCS value does not differentiate between brittle and ductile failure. Hence, the energy dissipated up to the point of failure of the samples tested here was determined by recording the area under the corresponding stress- strain curve. This area is referred to as specimen toughness and its values for the various soils are shown in Fig.6. Shown is the "Toughness" measured at these intervals compared with corresponding results from untreated soil and soil treated with a cement binder (cement is the most popular commercial binder currently available). "Toughness" is a measure of the ability of subsoil to withstand a load, and the ability of the subsoil to flex without rupture. It is clear that TP AM is a competitive, even superior, binder to cement. Further, it has been shown that TP AM gives results superior to those from typical polymer alternatives currently on the market.

Similar to the UCS results, Fig. 6 shows that the HPAM and TP AM clearly display higher toughness compared to the untreated soil and cement-stabilized soils especially as the curing time progresses. Higher toughness has important favorable implications on the performance and durability of the stabilized layer in the pavement structure. The anionic HPAM also performed well; this performance was not unexpected since an anionic binder would be receptive to the limestone in the Qatari soil. Previous studies have pointed out that the Qatari soil has a heavy proportion of limestone which is known to become electropositive when wetted. Nevertheless, the contribution of the polymer cationic component is significant.

In one embodiment, the invention relates to said co-polymer TP AM soil subgrade binder, wherein m, p, q proportions of the terpolymer as shown in Fig. 1 are varied within the range to suit the composition of a candidate soil. In one embodiment, the m, p, and q proportions of the co-polymer soil subgrade binder are varied within the range to suit the anionic/cationic/neutral composition of said soil. In one embodiment, m is 1/3 for all soils. In one embodiment, m is 1/3, p is between 0 and 2/3 and q is between 0 and 2/3. In one embodiment, such as for a very bentonite-rich soil, p is 2/3 and q is 0. In one embodiment, such as for a very limestone, or similar, soil, p is 0 and q is 2/3. In one embodiment, said treating soil with said co-polymer subgrade binder comprises mixing said soil with an aqueous solution containing said co-polymer. In one embodiment, said solution contains a predetermined co-polymer content estimated as percentage per dry weight of soil. In one embodiment, said polymer content estimated as percentage per dry weight of soil is between 0.1 and 10 percent.

Preparation for road paving generally includes compaction of the base or sub-base, which may comprise clay, gravel, crushed stone, and the like, either taken from the native materials or transported to the site. Frequently, the material includes crushed or otherwise particulated concrete and/or asphalt from the old roadway. Whether the material is primarily reclaimed from an old roadway surface material, taken from a new or old base on site, or is made from materials transported to the site, it is commonly tested for stability. The tested stability of a given mix of materials will be used as an important criterion in determining the thickness of the new pavement to be laid for a road having an expected type of traffic or load. Generally, a road or highway expected to have a great deal of heavy usage will require more concrete or asphalt than one built for relatively light or less frequent use, but an unstable base, in either case, can result in rapid damage to the pavement.

In preparing a roadbed from the materials at hand, or from imported materials, or from a mixture of them, the highway engineer may consider the bed material's permeability, elasticity, plasticity, cohesion, shearing strength, compressibility, shrinkage and swell, and frost susceptibility, among other properties. Each of these properties is well known in highway engineering and may be considered an important factor in the choice of the bed mix or any additives for it.

In one embodiment, the pavement structure used in this evaluation consists of five main layers: asphalt wearing course, asphalt base course, unbound granular base, stabilized subgrade, and natural subgrade as seen in Fig. 7, is it contemplated that the mixture of soil and polymers 5 of the current invention would comprise the stabilized subgrade layer. Fig. 8 shows an example of the current invention wherein an asphalt layer is cured and stabilized mixture of subgrade soil and polymers.

The various material properties and thicknesses used in the analysis are shown in Table 1. Table 1 : Factors Used in the Analysis of the Asphalt Pavement Performance

Unbound

Natural Stabilized Stabilized

Total Asphalt Granular

Subgrade Subgrade Subgrade Asphalt Binder Base

Modulus Thickness Modulus Thickness Grade Thickness

(MPa) (mm) (MPa)

(mm)

290 PG 76-10 0, 100, 200 100 200 689, 2086

The input parameters for the asphalt mixture were determined based on the gradation of asphalt mixtures shown in Table 2, and an asphalt binder classified as PG 76-10 grade. The asphalt wearing course was selected to be 40 mm thick, while the base course was selected to be 250 mm thick. The asphalt binder grade was PG 76-10. The analysis was performed with these parameters as input data and with granular base layer thicknesses of 0 (i.e., no granular layer), 100 mm and 200 mm, respectively.

Table 2: Gradations of Asphalt Mixtures

Sieve

Sieve Size Asphalt Mix Asphalt Mix Size

(mm) (Wearing) (Base)

(inch)

Cumulative %

3/4 19 0 j

retained

Cumulative %

3/8 9.5 14 32

retained

Cumulative %

#4 4.75 39 55

retained

% Passing #200 0.075 4 4

The results indicate that an ampholitic polymer, TP AM, should be considered as a viable stabilization binder. This finding suggests that even the relatively small contribution from cationic clay/polymer interactions plays a significant role in improving the soil mechanical properties. Hence, the present invention suggests that tailoring a polymer to be specifically compatible with any subgrade, provided the subgrade dominant material characteristics are known, would be very productive. Surprisingly, the present inventors found that the use of this ampholitic polymer provides better soil mechanical properties than the traditional standard, Portland cement, under equivalent conditions.

It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.