Chemical prestressing of concrete structures; state of the art review

Chemical prestressing was developed during the 1960s; however, and due to technical difficulties at that time, the technology was not considered as an alternative to conventional prestressing; instead, it was only used as a shrinkage compensating method. Still, recent development in concrete technology encouraged several researchers to revisit this technology, where significant achievements were made. This paper aims to present a state‐of‐the‐art review about the recent development in the chemical prestressing technology. This includes reviewing existing field applications, existing types of expansive additives, influence of different expansive additives, influence of curing methods, mix proportioning, mechanical properties, and structural behavior. Furthermore, a thorough discussion was performed and suggestions were made for future researchers to help further contribute to the development of this prestressing technology. It was concluded that although the chemical prestressing technology seems very promising, more research is required before using it in practical applications.


| INTRODUCTION
Prestressing of concrete structures can be defined as the introduction of internal compressive stresses inside the concrete, which have an opposite nature to the tensile stresses developed by the applied loads.And, as concrete is well known to have a high compressive strength and a relatively negligible tensile strength, the introduction of compressive stresses extends the stress range of the member significantly. 1 The compressive stress can be introduced into the concrete by stressing the tendons either before the casting of concrete and releasing after the hardening of concrete (pretension), or by tensioning the tendons shortly after the casting of concrete (posttension).In both cases, the compressive force is usually introduced into the concrete by tensioning the tendons using a hydraulic jacking system and anchors. 2 The prestressing technology has been around since the 1870s, however; and due to stress losses resulted from the poor quality of the materials used at that time, the method was not fully adopted till the 1930s by a French engineer named Eugene Freyssinet. 3,4he prestressing technology offers numerous advantages to structural concrete such as the reduction in cross section for the same applied loads.This is because the compressive stresses allow for the full utilization of the cross section.Furthermore, and in general, prestressing eliminates completely the existence of flexural cracks, especially under service loads.This leads to several advantages including; better esthetics, enhanced corrosion protection of reinforcement, and less maintenance.In addition, the bending moment, resulted from the eccentricity of the applied prestressing force, produces camber in the structural member, leading to much less deflections at service loads.On the other hand, there are also several challenges associated with using conventional prestressing such as; it requires high-quality control, initial and longterm losses of prestressing force, and relatively high cost.][7] Another method of introducing compressive stresses into the concrete is known as the chemical prestressing method.In this technique, an expansive additive is added to the cement leading to a positive volume change within the concrete.If a proper bond is available between the reinforcement and the concrete, such volume change will stretch the reinforcement leading to tensile stresses.2][13] Furthermore, and unlike conventional prestressing, chemical prestressing does not impose any additional cost, as the cost of expansive additives are approximately similar to that of normal cement.However, chemical prestressing requires significant specific knowledge and high-quality control, which were not available at that time on an industrial level.Furthermore, the level of prestressing produced by chemical prestressing was limited and could not match that produced by mechanical prestressing. 14In addition, the amount of expansion required to obtain a sustainable prestressing force in reinforced concrete (restrained expansion) ranges between 200 and 1000 μm/m. 156][17] Therefore, chemical prestressing was not considered as an alternative to conventional prestressing at that time; instead, the technology was used as a shrinkage compensating method.
On the other hand, and due to the recent rapid development in concrete technology, several researchers have revisited the chemical prestressing technology, where significant developments were made.Chemical prestressing was utilized for steel reinforcement 8,9 and Fibers Reinforced Polymers (FRP) reinforcement. 18,19Furthermore, during the past decade, a relatively new form of reinforcement was introduced.The reinforcement is known as textile carbon fiber reinforced polymer (CFRP) reinforcement, and it consists of a flexible two-dimensional grid made of CFRP fibers.1][22] However, it is extremely challenging to prestress textile FRP reinforcement using the conventional mechanical prestressing.Because such reinforcement is very sensitive to the transverse force resulting from the mechanical anchorage.In addition, it is not feasible to prestress all the rovings simultaneously. 23Therefore, chemical prestressing can provide a promising alternative for such reinforcing system.Based on the above, this paper aims to present a state-of-the-art review about the recent development in the chemical prestressing technology.This includes reviewing existing field applications, existing types of expansive additives, influence of different expansive additives, influence of curing methods, mix proportioning, mechanical properties, and structural behavior.This will provide researchers with a comprehensive overview about chemical prestressing which can contribute further in the development of this promising prestressing technology.

| FIELD APPLICATION AND CODES OF PRACTICE
In the United States and after its development during the 1960s, expansive concrete was used to avoid drying shrinkage and cracking.A number of concrete structures were constructed using expansive concrete during that period such as the Baltimore underground parking garage, a 500-m-long experimental section of pavement and many more. 24According to Hoff, 25 in 1972, an estimated 380,000 tons of expansive concrete were used across the United States.The report also recommends using expansive concrete in precast applications such as; structural elements, concrete pipes, and tilt-up construction, as such technique is also capable of prestressing the reinforcement.2][33][34] According to McLean et al., 26 about 600 bridge decks in the United States were constructed using shrinkage compensating concrete.The survey illustrates that those bridge decks exhibited superior crack control than conventional bridge decks.This has led to an increased life cycle and less maintenance cost for those bridges.Although, the use of expansive concrete as a shrinkage compensating method continues in the United States, there is no evidence within the literature regarding the use of expansive concrete as a prestressing technique.
After its development, expansive concrete gained increasing popularity in Japan.According to Nagataki, 35 who conducted a review on expansive concrete use in Japan, expansive cement production reached about 1% of the total cement production in Japan in 1976.During that period, expansive concrete was first used as a shrinkage compensating method; however, its use was expanded later to include self-prestressing.The recommended values of expansive admixture were (25-30 kg/m 3 ) for shrinkage compensating applications and (40-60 kg/m 3 ) for self-prestressing applications.The field application of expansive concrete included: slabs on grade with 25-40 kg/m 3 of expansive admixture and tunnel lining with up to 60 kg/m 3 of expansive admixture.Furthermore, nearly 85% of the manufactured culverts up to 1976 were made of self-prestressed concrete, which makes around 170,000 tons of expansive concrete. 36After that period, the use of expansive concrete continued to increase in Japan.By the year 1995, about 30% of castin-place concrete and 70% of precast concrete manufactured were of self-prestressing concrete. 16Currently, the use of expansive concrete in Japan is increasing and it is expected to increase further as reported by the Japanese Technical Committee on expansive concrete. 37nlike Japan, the use of expansive concrete in Europe is limited to shrinkage compensating.Examples of field applications can be found in many European countries.In Italy, for example, expansive concrete was used to construct the jointless walls in the Museum of Arts in Rome, which was completed in 2006.After this successful project, expansive concrete was also used in other applications, see Ref. 38 In other countries such as Germany, Norway, Denmark, and Lithuania, expansive concrete is being currently used in jointless slabs on the ground.The expansive additive increases the volume of the slab, and thus reduces the tensile stresses resulted from the restrained contraction of the concrete. 39urrently, there is only one guide in the world for chemically prestressed concrete structures.The guide was issued by the Japanese Society of Civil Engineers (JSCE) in 1994 under the title "Recommended practice for expansive concrete," and it is valid up to date. 15From the start, the guide distinguishes between chemical prestressing and shrinkage compensating.It states that if the rate of expansion after 7 days of measurements is between 0.15 and 0.2 mm/m, the concrete can be classified as a shrinkage compensating concrete; however, if the expansion rate is between 0.2 and 1.0 mm/m, the concrete will have the ability to stretch the reinforcement causing a prestressing force, and thus it can be classified as self-prestressing concrete (also see Figure 1).Although, the guide recommends using trial mixtures to obtain the required expansion rate, it also states that an expansion rate of around 0.7 mm/m can be obtained by adding around 30 kg/m 3 of expansive additive to the concrete mixture, while additions above 65 kg/m 3 can produce an expansion rate of around 1.0 mm/m, which can provide good prestressing force.To measure the rate of restrained expansion, the guide refers to JIS A 6202, which is updated regularly, where the last update was published in 2017. 40The American Concrete Institute (ACI) also provides a guide for expansive concrete ACI 223-21. 41And although, it is only for shrinkage compensating concrete, it also can be very beneficial for expansive concrete used for self-prestressing applications, as it provides details regarding the mixture design of expansive concrete.Furthermore, it also describes various structural members such as bridge decks, airport pavements, posttensioned structures, slabs on ground and walls, as well as, various restraints and reinforcement ratios.

| STRAIN MEASUREMENT METHODS
One of the most promising methods of measuring the deformation of expansive concrete is distributed fiber optic sensors (DFOS).3][44] Such sensors will not only allow measuring the strain along the whole length of the structural member, but also along the reinforcement.Thus, it provides a valuable insight into the strain and stress development inside the structural members in a timely manner.Furthermore, it can provide a real-time monitoring of the structural element during its service life.There are many types of DFOS depending on the required application.However, Zdanowicz 23 and Zdanowicz and Marx 10 used a quasi-continuous fiber optics system to measure the strains inside chemically prestressed concrete slabs.The system consists of a glass wire with additional protective cladding.The wire has multiple segments of different densities as shown in Figure 2.Each segment can reflect a certain range of the light spectrum.Once the length of the segment increases due to concrete expansion, the reflected wavelength will also change.The wavelength is then collected and interoperated by the acquisition system.This principle is known as Rayleigh scattering. 23The results have shown that it is possible to measure the strains inside the concrete and reinforcement accurately and in a timely manner.Furthermore, the system was also able to detect cracks propagations and locations (see Figure 3).To determine the prestressing force, the tensile strain measured inside the reinforcement using DFOS is used to determine the tensile force.This is done by multiplying the tensile strain developed inside the reinforcement by the modulus of elasticity of the reinforcement.Then, the resulted tensile stress is multiplied by the area of reinforcement to obtain the tensile force developed inside the reinforcement due to the expansion of concrete.This tensile force will be transferred to the concrete as a compressive force due to equilibrium.The prestressing stress can then be obtained by dividing the compressive force by the cross-sectional area of the structural member.The strain inside the reinforcement can also be measured using linear variable differential transformer sensors (LVDTs).However, and unlike DFOS, such technique can provide only localized strain measurement.
Another way to measure the restrained expansion of concrete is the ASTM C878-14 method. 45And although, the method is intended to measure the expansion of shrinkage compensated concrete, it also can be used to measure the restrained expansion of self-prestressing concrete.The method includes casting a concrete element with dimensions of 80 Â 80 Â 240 mm.In addition, F I G U R E 2 Fiber optic segment with multiple sensors. 23I G U R E 3 Development of strain inside the concrete and textile reinforcement of one of the specimens tested by Zdanowicz. 23DFOS, distributed fiber optic sensors.a threaded steel bar is embedded centrically inside the concrete.The threaded steel bar has a diameter of around 5 mm, and thus it makes a reinforcement ratio of 0.47%.Furthermore, the bars are also connected to two steel end plates using fixed screws (see Figure 4).Once the concrete is set, the concrete element and the steel plates are removed.The concrete element with its threaded steel bars is then placed in a steel frame with a dial gauge as shown in Figure 4.Then, readings of expansive concrete can be recorded, and expansion can be measured.

| EXPANSIVE CEMENTS AND ADMIXTURES
According to ACI 223-21, 41 expansive cement is a system that comprises of Portland cement and an expansive additive.When this system is mixed with water, it forms after setting, a paste that increases in volume to a degree much greater than that produced by only using Portland cement.In the United States, expansive cements are divided into four types depending on their chemical composition as follows; type K, type M, type S, and type G. 41 Except for type M cement which was developed in Russia, all the other types were developed in the United States. 14Both types K and M cements can be manufactured as an expansive cement or as an expansive additive.On the other hand, type S is only manufactured as an expansive cement, while type G is only manufactured as an expansive additive.Unlike expansive additives, expansive cement does not require any additional additives to cause the expansion of concrete; however, it does not allow for proportion adjustment, and thus restricts the flexibility in controlling the amount of expansion. 23On the other hand, expansive additives provide more flexibility in terms of controlling the required expansion, as it is possible to control their dosage.Therefore, expansive additives are widely used in self-prestressing concrete.
[48][49] However, the expansive action of ettringite is not yet fully understood.Nevertheless, there are several theories that try to explain the action of ettringite.One of the oldest theories is the theory of crystallization pressure.This theory claims that the increase in volume is due to the formation of ettringite crystals, which exert pressure on the cement matrix causing expansion. 50Another theory is the swelling theory.This theory claims that ettringite forms gel-like crystals, which swell when absorbing water, exerting swelling pressure on the cement matrix, and thus causing expansion.However, the action of these crystals depends directly on the amount of available lime, and thus they are ineffective in the absence of lime. 51ype G system, on the other hand, depends on the formulation of calcium hydroxide, or portlandite to cause the expansion.3][54] Furthermore, most of the expansion caused by calcium hydroxide take place within 1-2 days, while most of the expansion caused by ettringite takes place more slowly within 5-7 days (see Figure 5). 52The figure also shows that ettringite-based expansion correlates well with strength development.Therefore, it is advisable to use ettringitebased additives to produce self-prestressing concrete, because such additives will allow the concrete to gain the 23 F I G U R E 5 Expansive rate of ettringite-based system (S) and portlandite-based system (CaO) in compression with compressive strength development (A: normal strength gain concrete, B: rapid strength gain concrete). 52equired strength before expanding.The gain in strength is necessary to form the bond between the concrete and the reinforcement, and to resist the compressive stresses exerted by the reinforcement.However, ettringite-based systems require more curing days to reach the required expansion.
In Asia and Europe, expansive systems have different nomenclature than the United States.The most widely used type is known as calcium sulfoaluminate (CSA) additive, which is similar to type K cement in the United States, which means that it is also an ettringitebased system. 55CSA has many advantages such as its reaction produces less carbon dioxide emissions than ordinary Portland cement. 56Furthermore, CSA is an additive, which means that it is possible to control the amount of concrete expansion by controlling the amount of CSA dosage.Therefore, it is widely used in selfprestressing and shrinkage compensating concrete.

| Cement
For self prestressing concrete, usually expansive additives are added to the cement to obtain the required expansion.When ettringite-based expansive additives are used, the expansion usually starts after the hardening of the cement paste, and continues for around 5-7 days, where most of the expansion take place. 52Therefore, rapid hardening cements are not recommended, as they lead to a rapid strength gain, which lead to high early stiffness, and thus restrict the expansion.However, high strength normal hardening cements of classes 52.5 and 42.5 are desirable, because they provide high strength concrete with normal strength gain.High strength is necessary to withstand the compressive stresses applied by the reinforcement as a result of the expansion.Furthermore, an experimental investigation conducted by Carballosa et al. 57 has shown that as the alumina (Al 2 O 3 ) content in the cement increase, the rate of expansion also increases (see Figure 6).This was attributed to the fact that the rate of ettringite formulation is controlled by the rate of dissolution of (AI) compounds.It is important to note that the legends la, lb, and ll in Figure 6, refer to two different cements with three different alumina (Al 2 O 3 ) contents namely; CEM I 52.5N-a,CEM I 52.5N-b and CEM II/A-P 42.5R, respectively.Also, the symbol K refers to the type of expansive additive used, while the % denotes the percentage replacement of the expansive additive by weight of cement.

| Dosage of expansive additive
As mentioned earlier, for self-prestressing concrete, expansive additives are more favorable than expansive cement, as they provide more flexibility in term of controlling the rate of expansion by using different dosages.Furthermore, type K expansive additives such as CSA are also usually favored, given that CSA is an ettringite-based additive, which means that most of the expansion takes place within the first 5-7 days as discussed earlier, giving the concrete the required time to gain the appropriate strength.However, the relationship between the amount of expansive additive and the rate of expansion is rather nonlinear. 57Generally, replacement ratios of expansive additive (CSA) below 10% of weight of cement lead only to shrinkage compensating concrete.While replacement ratios between 10% and 20% can provide the necessary expansion to cause self-prestressing.However, replacement ratios above 20% may lead to a very high expansion, and thus cause the deterioration of concrete. 23,58Figure 7 shows the results of an experimental investigation conducted by Wyrzykowski et al. 58 Different replacement ratios of the expansive additive CSA were used namely; 5%, 10%, 15%, and 20%.All specimens were cured underwater for around 1000 days.The figure clearly shows the nonlinear relationship between the amount of expansive additive and the rate of expansion.The figure also illustrates high expansion values especially for 20% replacement ratio.This is because the specimens were unrestrained and were cured underwater during the whole test period.On the contrast, restrained specimens with dry curing conditions will exhibit much less expansion rates. 23,58[59] F I G U R E 6 The relationship between the amount of alumina and free expansion at 14 days of age using type K expansive additive. 57

| Limestone powder
Generally, limestone powder has many benefits when used as a filler with Portland cement.1][72][73] Therefore, limestone powder is usually used with ettringite-based expansive additives such as CSA.However, in order to achieve the full potential of limestone powder, certain conditions must be fulfilled.The size of the limestone powder must be kept around 5 μm.Vance et al. 74 reported that using fine limestone powder of particle size around 5 μm improves the rheological properties of concrete, while larger particle sizes can reduce such properties.In addition, limestone powder with ultra-fine sizes increases the compressive strength of concrete. 61According to Ghafoori et al., 62 the compressive strength of concrete containing limestone powder of particle size around 5 μm was considerably higher than that with particle sizes of 10 and 20 μm.This was attributed to the improvement in packing density.Furthermore, the amount of limestone powder has also a significant influence on the properties of the produced concrete.6][77] On the other hand, replacing Portland cement by more than 15% of limestone powder decreased the compressive strength of concrete.The decrease can reach up to 86% at 45% replacement. 64Furthermore, Tongaroonsri and Tangtermsirikul 65 have shown that it is possible to reduce the total shrinkage of concrete up to 19% by using only 10% replacement of limestone powder.Based on the above, it can be concluded that when used for self-stressing concrete, the particle size of limestone powder must be around 5 μm and the replacement ratio must not exceed 10% of the weight of cement.

| Aggregate
There is not much research regarding the influence of aggregate on expansive concrete.However, and based on logic, it can be stated that the amount of aggregate must be reduced to as much as possible.This is because the expansion takes place within the paste, and thus high amount of aggregate reduces the total expansion, while high amount of cement paste increases the expansion.Although, high amount of cement paste may lead to higher autogenous shrinkage; however, such shrinkage can be reduced using shrinkage reducing admixtures.Furthermore, the size of course aggregate particles must be also reduced to as small as possible.This is because the expansion takes place within the cement paste, as it contains the CSA.And thus, using large aggregate particles can lead to unbalanced expansion, as the volume occupied by the aggregate is not expanding, while the surrounding cement paste is expanding.Consequently, this leads to the development of internal stresses, which lead to micros cracks, and hence reducing the strength of concrete (see Figure 8).However, it is important to mention that limiting the size of aggregate may cause adverse effects on the structural performance of concrete members such as reduced shear and bond strengths due to the reduction in the interlocking capability of the concrete. 78,79

| Shrinkage reducing admixtures and superplasticizers
Shrinkage is a reduction in the total volume of concrete after it hardened without the influence of external stresses.It is divided into two types: drying shrinkage, which results from water loss to the surroundings, and

F I G U R E 7
The relationship between the dosage of the expansive additive and the rate of expansion for unrestrained specimens stored under water. 58CSA, calcium sulfoaluminate; SAP, superabsorbent polymers; SRA, shrinkage reducing admixture.
autogenous shrinkage, which takes place when the volume of the hydrated paste is less than that before hydration.The total shrinkage is the summation of both types. 80In addition to the many challenges associated with shrinkage, the reduction in concrete volume reduces the total expansion of self-prestressing concrete, as such concrete must first compensate for the volume loss resulting from shrinkage before expansion.Therefore, it is recommended to use shrinkage reducing admixture to reduce shrinkage, and thus increase the total expansion of concrete. 10,58,81here are conflicting reports regarding the influence of CSA on the workability of fresh concrete.Still, whether the influence is positive or negative, the literature shows that it is slight and insignificant. 57,58,81,82egardless, using superplasticizer is a requirement, especially with high strength concrete when the water/ cement ratio is low.There are a number of superplasticizer available; however, Zdanowicz 23 has shown that polycarboxylate ether (PCE) superplasticizers are more effective in promoting expansion than other types.

| CURING OF SELF-STRESSING CONCRETE
As mentioned earlier, ettringite-based CSA is usually used as an expansive additive in self-prestressing concrete.The action of ettringite is not yet fully understood; however, currently there are two main theories that try to explain the action of ettringite namely; the crystal growth theory, which assigns the expansion of concrete to the formation of ettringite crystals, and the swelling theory, which assigns the expansion of concrete to the formulation of gel-like crystals. 50,51Both theories state that the existence of water is essential for the process of expansion.Therefore, the curing method has a significant influence on the amount of expansion.Figure 9 shows the results of an experimental investigation conducted by Wyrzykowski et al. 58 to study the influence of curing on the expansion of concrete made by replacing 20% of cement by CSA.The figure clearly shows that specimens cured under water had a significantly higher expansion than those cured using dry conditions.The figure also illustrates that once the specimen is removed from water even after 28 days, the expansion stops immediately.In fact, the concrete shrinks slightly due to the shrinkage phenomenon.This was also confirmed by other experimental investigations. 23,81,83It can be concluded that the curing process is indispensable for the expansion of concrete at least for the first 5-7 days where most of the expansion takes place.
Curing can be made with different methods.The most effective, surely, is water immersion.Still, there are other methods that can have similar effect.Klein et al. 11 have reported that curing in fog (loose-fitting membrane) can yield approximately a similar expansion rate to that when water immersion is used.On the other hand, curing with plastic sheet or sealing with foil have a very limited effect. 25,58In some cases, especially when thick sections are used, water from the surrounding may not reach deeper regions of the section, due to low permeability.Therefore, it is necessary to use a technique called internal curing.5][86] This can be done using two methods; the first is by using saturated lightweight aggregate, which release the water after the hardening of concrete, the second is by using superabsorbent polymers, which can absorb several hundred times their own weight of water.][86][87] Curing of expansive concrete during the first days after casting is vital.If not properly cured, re-expansion can take place later when members are exposed to high humidity or rain.Figure 9 shows that expansion can take place even after remaining for 4 months in dry conditions.In fact, it can reach a level like that of specimens completely immersed under water.The expansion stops immediately after removal from water and can resume after immersion till all the ettringite has reacted, see the blue dotted line in Figure 9.Nevertheless, Wyrzykowski et al. 58 reported that, although cracks were apparent on the surfaces of re-expanded specimens, no significant influence on the mechanical properties such as compressive strength and modulus of elasticity was observed.Similar results were also observed by Bertero. 88It can be concluded that it is very important to properly cure expansive concrete, especially during the first days after casting to enable full reaction of the expansive additive.
F I G U R E 9 Influence of different curing method on the expansion of concrete. 58

| DEVELOPMENT OF PRESTRESSING FORCE
Usually, expansion starts after the concrete final setting, however, the transfer of strain from the concrete to the reinforcement depends on the bond development between the concrete and the reinforcement.Nevertheless, in most experimental investigations, measuring the expansion starts after demoulding of concrete, which is usually 1 day after casting.This means that it is highly likely that the concrete exerts some strains before the start of recording, which can result in unrecorded stresses.It is possible to solve this issue by using fiber optics sensors, which can start recording at any time after casting.Nevertheless, experimental investigations have shown that the tensile strain inside the reinforcement can be detected from Day 1 after measuring starts. 23,58,81yrzykowski et al. 58 conducted an experimental investigation to measure the prestressing force resulted from the expansion of concrete (chemical prestressing).They utilized concrete prisms with dimensions 40 Â 40 Â 160 mm 2 .Each prism was reinforced with a centrically placed threaded steel bar along the longitudinal direction.Two types of bars were used to resemble two different reinforcement ratios namely 0.9% and 2.3% (see Figure 10).The steel bars had a tensile strength of 1430 MPa and modulus of elasticity of 200 GPa, while the concrete had a compression strength of around 60 MPa.Also, CSA was used as an expansive additive with a replacement ratio of 20% by weight of cement.Additionally, LVDTs were glued on the steel bars to measure the expansion of concrete.
The experimental investigation has shown that it is possible to produce a maximum prestressing stress inside the concrete of around 6 MPa and a final prestressing force of around 2.5-3 MPa (see Figure 11).The maximum 6 MPa stress could not be maintained due to shrinkage of concrete after removing the specimens from water into drying conditions.Similar level of prestressing was also achieved by the same authors in another study where they used CFRP tendons as reinforcement instead of steel bars. 81Furthermore, Okamura et al. 36 and He et al. 89 have also produced similar level of prestressing stress (2-5 MPa) using CSA expansive additive.And although such a level of prestressing is slightly less than that achieved using conventional prestressing, 90 it seems to be a significant and promising development in the chemical prestressing technology.
Furthermore, it is important to recognize that restrained expansion of concrete is considerably lower than unrestrained expansion.Depending on the amount of reinforcement, restrained expansion could be around 30%-50% of the unrestrained expansion for the same concrete 23,58,81 (see Figure 12).
F I G U R E 1 0 Experimental setup used to measure the prestressing force resulted from the expansion of concrete. 58I G U R E 1 1 Developed compressive stress inside the concrete with different reinforcement ratios. 58I G U R E 1 2 A comparison between unrestrained and restrained expansion. 58t is well documented that conventionally prestressed concrete suffers from creep due to the high compressive stresses exerted by the reinforcement, which leads eventually to losses in the prestressing force.However, several experimental investigations have shown that CSA-based expansive concrete is less prone to creep. 12,36,91This was attributed to the fact that even when expansion seems to stop, expansive concrete still has some expansive energy, which pushs against the creep of concrete. 36t is extremely complicated to measure the compressive strain inside the concrete, which is caused by the reinforcement while the concrete is expanding.Therefore, the prestressing stress inside the concrete is usually derived from the tensile force developed inside the reinforcement.This is generally done by dividing the tensile force developed within the reinforcement by the area of concrete.Also, as the modulus of elasticity of the reinforcement increases, the prestressing force inside the concrete also increases.This is because the expansion of concrete is relatively small and limited.Therefore, such expansion will not cause high tensile force, if the modulus of elasticity of the reinforcement is low.The development length of the reinforcement is also an important factor for the transfer of the strain from the concrete to the reinforcement.Zdanowicz 23 has shown that it requires about 250-300 mm to achieve a full bond between a 12 mm steel bar and the expansive concrete, which is less than the 500 mm previously thought 92 (also see Figure 13).

| MECHANICAL PROPERTIES OF EXPANSIVE CONCRETE
Many experimental investigations have shown that the compressive strength of expansive concrete either remains the same or slightly increases up to a certain level, after which unavoidable decrease starts to occur.This was explained by the fact that as concrete expands it becomes denser due to the filling and pressuring effect of the expansive additive; however, after reaching a certain level, the concrete structure starts to deteriorate, and a sudden drop in the mechanical properties of concrete takes place. 12,93he definition of this threshold differs between researchers.Klein et al. 11 and Mailvaganam 94 reported that a large and sudden decrease in compressive strength appears when the concrete reaches unrestrained expansion between 0.3% and 0.4%.On the other hand, other researchers defined this threshold by the amount of added expansive additive.Recent experimental investigations conducted by Zdanowicz 23 Wyrzykowski et al. 58 have shown that the compressive strength slightly increases when using a 5% CSA expansive additive or less; however, once the amount of replacement exceeds 10%, a decrease in compressive strength can be detected.The decrease can reach up to 15%-20% at 25% CSA replacement ratio.
Furthermore, specimens cured underwater have a lower threshold than those cured outside of water. 23,88his is because much higher expansion rates can be achieved when the specimens are cured underwater.This shows that using the first method (expansion based) of describing the threshold is more accurate than using the second method (amount of additive).Another reason which shows the advantage of using the expansion rate to define the threshold is that all the previous studies did not consider the influence of the reinforcement (restraint) on the compressive strength.For example, if the expansion rate is 0.7% for unrestrained concrete, the compressive strength is expected to drop when tested using the standard compression test; however, if reinforcement is provided for the structural element, the expansion rate can be significantly <0.7%, which mean that the compressive strength of the concrete within the structural member is still intact, as the threshold was not exceeded.Further studies are needed to define this threshold, and to study the compressive strength of restrained concrete.Similarly, the modulus of elasticity follows the same rules as the compressive strength, as the compressive strength is a function of the modulus of elasticity.

| BEHAVIOR OF CHEMICALLY PRESTRESSED MEMBERS UNDER FLEXURAL LOADS
A significant enhancement can be observed in the flexural behavior and crack resistance of concrete members when using the chemical prestressing method.Zdanowicz and Marx 10 conducted an experimental investigation to study the behavior of concrete flat plates of F I G U R E 1 3 Development length of reinforcement in chemically prestressed concrete. 23imensions 2000 Â 1000 Â 30 mm 2 , and a concrete compressive strength of 90 MPa.The plates were reinforced with CFRP textile reinforcement with an ultimate tensile strength of 2500 MPa and a modulus of elasticity of 180 GPa.CSA was used as an expansive additive with a replacement ratio of 15% of weight of cement.They reported a 53% increase in the crack resistance of CFRP reinforced expansive concrete when providing 1.27 MPa of compressive stress.This also coincides with the experimental results obtained by Okamura et al., 36 who have shown that it is possible to increase the diagonal crack resistance of steel reinforced expansive concrete up to 40% when introducing a compressive stress of around 5 MPa.
On the other hand, and in a recent experiment, Wyrzykowski et al. 81 also studied the flexural behavior of chemically prestressed concrete reinforced with CFRP bars.For this purpose, rectangular concrete plates of three different cross sections were casted.Two CFRP bars were placed in each of the concrete plates.The experimental investigation has shown that it is possible to increase the cracking load of CFRP reinforced expansive concrete up to three times that of CFRP reinforced members with normal concrete.This was achieved by a compressive stress of around 4 MPa.It is important to note that these CFRP bars had a young's modulus of around 464 GPa, which is twice that of conventional steel reinforcement, which can explain the relatively high compressive stress developed inside the concrete.Figure 14 compares the flexural behavior of chemically prestressed concrete reinforced with CFRP bars and normal concrete reinforced also with CFRP bars.The figure shows that chemical prestressing delayed the appearance of the first crack and the failure load significantly. 81This figure illustrates the promising potential of the chemical restressing technology, especially when used with high strength high modulus of elasticity reinforcement.

| RECOMMENDATIONS AND CONCLUSIONS
In this paper, a state-of-the-art review about the recent development in the chemical prestressing technology is presented.Based on this, the following conclusions and recommendations were made: • When producing expansive concrete for selfprestressing applications, using expansive additives is more advantageous than expansive cement, since unlike expansive cement, expansive additives allow for more control over the added dosage, and thus better control over the rate of expansion.• It is recommended to use ettringite-based expansive additives such as CSA to produce self-prestressing concrete, since most of the expansion takes place within 5-7 days when using such additives, and thus they allow the concrete to gain the proper strength before expanding.The gain in strength is necessary to form the bond between the concrete and the reinforcement, and to resist the compressive stresses exerted by the reinforcement.On the other hand, the expansion of calcium hydroxidebased additives takes place within 1-2 days, hence it does not allow the concrete to gain the required strength.• It is recommended to use normal hardening cement when producing self-prestressing concrete, since unlike rapid hardening cement, normal hardening cement allows for a moderate stiffness and strength gain with time.Rapid strength gain leads to high early stiffness, and thus restricts the expansion.Furthermore, experimental evidence has shown that as the alumina (Al 2 O 3 ) content in the cement increase, the rate of expansion also increases.This was attributed to the fact that the rate of ettringite formulation is controlled by the rate of dissolution of (AI) compounds.• The relationship between the amount of expansive additive and the rate of expansion is rather nonlinear.Generally, the addition of expansive additive such as CSA below 10% of weight of cement leads only to shrinkage compensating concrete.While additions between 10% and 20% is capable of providing the necessary expansion to cause self-prestressing.However, additions above 20% may lead to a very high expansion, and thus it may cause the deterioration of concrete.• Limestone powder promotes and stabilizes the formation of ettringite, and thus increases the total volume of concrete.Therefore, limestone powder is usually used with ettringite-based expansive additives such as CSA.
F I G U R E 1 4 Load deflection response of chemically prestressed concrete reinforced with high strength carbon fiber reinforced polymer bars. 81owever, in order to achieve the full potential of limestone powder, certain conditions must be fulfilled such as the size of particles must be around 5 μm and the replacement ratio must not exceed 10% of the weight of cement.Otherwise, negative effects may appear.• The amount of aggregate must be reduced to as much as possible.This is because the expansion takes place within the paste, and thus high amount of aggregate reduces the total expansion.Furthermore, the size of course aggregate particles also must be reduced to as small as possible, as large size particles may lead to unbalanced expansion, which can cause internal stresses.However, limiting the size of aggregate may cause adverse effects on the structural performance of concrete members such as reduced shear and bond strengths due to the reduction in the interlocking capability of the concrete • Shrinkage reducing admixture must be used when producing self-prestressing concrete, to reduce the shrinkage to as much as possible, as shrinkage reduces the total expansion.Furthermore, and if necessary, PCE-based superplasticizer must be used, as experimental evidence has shown that it promotes better ettringite formulation.• The curing process is indispensable for the expansion of concrete at least for the first 5-7 days where most of the expansion takes place.If not properly cured, reexpansion can take place later when the members are exposed to high humidity or rain, which may jeopardize the integrity of the concrete.• The restrained expansion of concrete is considerably lower than unrestrained expansion.Depending on the amount of reinforcement, restrained expansion could be around 30%-50% of the unrestrained expansion for the same concrete.• Experimental investigations have shown that it is possible to introduce a final prestressing stress inside the concrete up to 5 MPa using the chemical prestressing technology.And although such level of prestressing is slightly lower than that achieved using conventional prestressing, it resembles a significant and promising development in the chemical prestressing technology.• Up to a certain expansion threshold, the mechanical properties of concrete such as the compressive strength, modulus of elasticity and modulus of rapture slightly increase.However, once this threshold is exceeded, a decrease in these mechanical properties starts to appear.The definition of this threshold differs between researchers.Some researchers stated that such a threshold is reached when the unrestrained expansion reaches 0.3%-0.4%.While others related it to a certain amount of expansive additive addition.• A significant improvement can be obtained in the flexural behavior of chemically prestressed structural elements including; better crack behavior and much higher flexural strength.• Although the chemical prestressing has been studied by several researchers, there are still many aspects that need further investigation such as their longterm behavior including; the re-expansion phenomenon, influence of creep and prestressing loses.Also, further studies to optimize the concrete mixture to have higher and more reliable prestressing stress are required.Furthermore, it is also important to define the threshold at which concrete starts to deteriorate more accurately.Although the chemical prestressing technology seems very promising, more research is required before using this technology in practical applications.

F I G U R E 8
Unbalanced expansion due to large aggregate sizes.