EXTENDING THE LIFE OF EXISTING CONCRETE STRUCTURES PENETRATING HYDROGEL TREATMENT EFFICACY

MURRAY G GILBERTSON1; JOHN B MANDER2

1 Managing Director, G-Group Consulting, New Zealand 2Inaugural Zachry Professor, Texas A&M University, USA

SUMMARY Extending the life of existing concrete structures remains critical on both economic and environmental grounds; one must endeavor to reduce the permeability. The degree of permeability reduction particularly in the cover concrete, is such that coupled with the associated chloride reductions for coastal and deicing environ structures, the service life of field structures can be economically increased. Timing of treatment can also mitigate costly long-term repairs. Penetrating hydrogels, a nanotechnology, has been extensively used over the past 20 years but with little scientific underpinning. This paper presents some of the early test results done to investigate the efficacy of permeability using a falling head permeameter system. It is shown that for uncracked concrete the penetrating hydrogel reduces the permeability by some two orders of magnitude. However, for cracked concrete, the effectiveness is reduced. Nevertheless, a reduction of at least 50% can be expected for crack widths up to 0.5 mm. INTRODUCTION The permeation characteristics of cover concrete are important for the durability of concrete structures and to premature deterioration (Krafcik et al. 2017; Thomas 2018). This deterioration, occurs more often than asset owners care for, as highlighted in relatively young concrete structures demonstrating premature deterioration. Often this is caused by inadequate cover to the steel or deficient concrete exhibiting alkali silica reaction (ASR) or delayed ettringite formation (DEF). In particular, coastal and de-icing salt environs accelerate this deterioration by impairing the steels natural passivation layer. The premature breakdown of concrete exposed to chlorides is an international problem both economically and environmentally. Concrete structures are normally constructed with a life expectation in excess of 50 to 100 years. New Zealand has many existing concrete coastal structures that are less than 40 years old which exhibit incipient or progressive deterioration. With our limited means to fund new works, as an industry, it is not desirable but rather critical, that we achieve both affordable and increased durability from our existing concrete structures. Penetrating hydrogels, a nanotechnology, can offer a solution for extending the service life of existing concrete structures. This paper outlines a laboratory permeability investigation and quantifies the degree to which a penetrating hydrogel can modify the concrete permeability. A previously hydrogel treated field structure investigation has then been compared with these laboratory findings.

HYDROGEL TECHNOLOGY Penetrating hydrogels, when spray applied to existing concrete structures, decrease the permeability of the concrete matrix and increase the tortuosity. This effectively immobilizes the moisture movement within the cover concrete and beyond the rebar. Much research and resources have gone into improving new concrete design mixes using various additives such as fly ash, micro-silica, and polymers. Research is widespread on additives such as superabsorbent polymers (SAP) additives, and hydrogels are one similar subcategory if used in a liquid admixture form. SAPs are able to absorb and retain up to several hundred times their dry weight in fluid. During the early curing phases of concrete, SAPs release their water for the hydration process to help mitigate autogenous shrinkage. Research on SAP has mainly focused on this internal agent focus (Lee et al. 2016). Far less attention has been on how the penetrating hydrogels class may be used on existing concrete structures of any age. For existing concrete structures, penetrating hydrogels come in liquid form and are spray applied. Hydrogels are a network of polymer chains that attract water, that is they are hydrophilic. Colloidal silica based penetrating hydrogels can also reinforce the formation of calcium-silicate-hydrates (C-S-H) phases in hardened concrete and form pore blocking internal gels (Medeiros et al. 2012). Acrylic acid (AA) and acrylamide (AM) hydrogel types may be suitably “tuned” for multiple beneficial effects such as curing improvement, C-S-H builders, and gel forming pore blockers (Krafcik et al. 2017). The spray applied proprietary colloidal penetrating hydrogel product used in this investigation appears to have been targeted toward lowering curing strains (new concrete), mechanical strength improvement (relatively new concrete), and permeability reduction (any age concrete) by the gel pore blocking attributes. The C-S-H improvement phases generally tend to diminish with the age of the structure. It has a catalytic penetrating ability, and being hydrophilic, the pores are blocked by way of the gel formation deep into the concrete matrix. Penetrating hydrogel compositions are aimed at extending the service life durability of concrete by inhibiting internal moisture movement within the concrete matrix. Bowers (2012) in his patent writes: “The clear solution is 0% solids before application and becomes 100% solids after curing and gelling. It is a highly reactive catalytic agent that reacts with free alkali and/or alkaline hydrates to internally produce a silica hydrogel. The hydrogel contains distinct spherical shaped particles that have pores that cannot be penetrated by water since they have dimensions smaller than that of a water molecule. It fills the pore spaces and voids around the aggregate. The treated concrete is permanently and hydrostatically sealed from within. The colloidal silicate composition never becomes crystalline but rather is amorphous in the absence of water. The hydrogel will rehydrate upon addition of water.” LABORATORY EXPERIMENTAL INVESTIGATION Low-pressure laboratory testing of concrete slices undertaken over a six-month period was conducted to compare hydrogel treated concrete versus untreated concrete in terms of permeability. A 5 m (49 kPa pressure) falling head purpose-built apparatus was used for this investigation. Flow rates monitored over time infer the coefficient of permeability for each slice. Flow rates were investigated before and after treatment with the penetrating hydrogel. This low-pressure investigation would typify concrete structures in tidal zones, retaining structures, basements of buildings, and general wharf/bridge/building decks and walls/columns exposed to heavy rainfall. A common concrete mix design and materials were provided by Winstone Aggregates, New Zealand. A specified concrete strength of f’c = 30MPa was used with a water/cement ratio of approximately w/c = 0.45. All concrete was manufactured by small mixers to exacting

specifications. To ensure consistency the fresh concrete was compacted on a vibrating table. The maximum aggregate size for this investigation was restricted to 12 mm. Test specimens were cut from standard 100 mm diameter concrete test cylinders. Prior to cutting, the compression cylinders were sealed on their around circumference using a double epoxy coating to ensure one-dimensional flow through the length of each slice. The test slices ranged from 30 to 50 mm in length. The top and bottom 10 mm of the compression cylinders were cut to waste such that all test findings are based on a cut surface. Any differences between aged concrete cylinders (1 to 2 years) and younger concrete cylinders (1 and 3 months) was also considered. All samples were preconditioned in a controlled temperature (230 C) and relative humidity (50%) room. The hydrogel application to the treated slices was to the manufacturer’s technical specifications. A hand held airless pressure spray unit that reflects the typical atomisation of the liquid hydrogel when applied in the field was used in the laboratory. The hydrogel applied was A2000 (Aquron Corp. Texas, USA). FALLING HEAD PERMEAMETER APPARATUS

Figure 1 presents the purpose-built falling head permeameter apparatus that was used to investigate flow rates and thereby infer the coefficients of permeability for untreated and penetrating hydrogel treated concrete slices. The purpose-built falling head test apparatus comprised six 5 m long individual vertical 7.75 mm internal diameter transparent plastic tubes providing a pressure head on each of the concrete slices as depicted in figure 1(a). The concrete slice containment apparatus used a steel bolted plate with the top rubber ring within a steel confining ring to seal the water entry area. The opening at the top rubber ring was 80mm diameter with a similar opening at the base plate. The rubber rings were used with a petroleum sealant to ensure water tightness at the rubber ring to concrete interfaces. The test procedure involved the vertical tubes being charged with water from the bottom to minimise air entrapment. The pressure sealed apparatus was charged with water, the vertical tube was attached by a quick-snap lock fastener, and any air entrapment within the pressure containment rig was released through a screw valve. The water head was then topped up and readings taken to the nearest 1 mm at regular intervals up to at least 5 days as the head dropped. The flow rate performance of some 80 test slices was investigated. From these results the flow rates were analysed and transient coefficient of permeability rates inferred as the moisture penetrated through each concrete slice. Figure 1(b) shows the condition of two slices at the conclusion of testing. The flow through the untreated concrete slice compared to absence of moisture flow on the treated slice is visually apparent. EXPERIMENTAL RESULTS Figure 2 presents some typical experimental results for untreated and hydrogel treated concrete slices. The comparative test results may be modelled to give a falling pressure head against elapsed time enabling the coefficient of permeability to be inferred for the treated (kt) and the untreated (ku) concrete. Figure 2(a) shows results from the falling head apparatus where the open symbols are the geometric mean of the data points and the solid lines are the modelled results. Note that the efficacy of the treatment, is the ratio of the time to a specified head loss of the treated to the untreated. Using figure 2(a) if we take the pressure head at 4800mm the elapsed time for water in flow for the untreated is some 30 minutes, whereas the treated requires some 4000 minutes, a factor of improvement of approximately 133.

(a) Concrete slices in the falling head permeameter apparatus. The plastic tubing extended 5m above the concrete slice

(b) Typical view on the underside of the concrete slices.

The left sample is untreated, and the right treated.

Figure 1 Permeameter apparatus

(a) Falling head test results

(b) Geometric mean inferred coefficient of permeability over group type

(c) Probabilistic dispersion of results for each

Figure 2. Experimental results from the falling head permeability tests

Figure 2(b) also depicts a similar improvement whereby the inferred permeability (vertical axis) over time (horizontal axis) is reduced by some two orders of magnitude. The lower permeability coefficient curve against time is for the treated slices, with the higher permeability coefficient for the untreated concrete slices. It is clearly evident the water penetration has been significantly slowed down by the penetrating hydrogel treatment. The hydrogel has penetrated the slices and formed a permanent pore blocking gel that markedly reduces concrete permeability by increasing the concrete tortuosity as well as blocking the pores. The two lower curves in figure 2(b) are both similar penetrating hydrogels. The one annotated as A2000 is a standard hydrogel, whereas, the A7000 curve is an adaption of the A2000 to include a proprietary steel corrosion inhibitor. Clearly, both provide similar permeability reduction. As there is with any geomaterial, there is considerable scatter with the results. This scatter is readily characterized in a cumulative distribution plot as depicted in figure 2(c). The vertical axis is the cumulative distribution against the permeability coefficient horizontal axis. The left-hand side cluster of data points are the treated samples and due to their similarity both the A2000 and A7000 treated samples are included in this distribution. The right-hand side (red circles) are a plot of the untreated companion test results. Through both sets of data points a lognormal distribution has been fitted. Using the median permeability coefficient for the untreated samples in figure 2(c) it is found

that ku = 180 x 10-12 m/s with a lognormal dispersion factor of = 0.28. Similarly, the median permeability coefficient for the hydrogel treated samples is kt = 1.15 x 10-12 m/s with a

lognormal dispersion factor of = 0.33. Herein we define a penetrating hydrogel “efficacy ratio”, Reff, for the treated concrete. This may be considered by comparing the induced pressure flow rates through the concrete slices before and after treating with the penetrating hydrogel by determining the coefficient of permeability (k) for both the treated and untreated concrete slices:

Reff = ku / kt (1)

where ku = coefficient of permeability for the untreated concrete slice, and kt = coefficient of permeability the hydrogel treated concrete slice of similar thickness. For the test specimens shown in Figure 2, the efficacy ratio based on the above-mentioned permeability coefficients for the untreated vs treated specimens is Reff = 155. The Reff ratio provides a determination for the efficacy of the hydrogel treatment on that point of the concrete at the falling head versus time. As concrete is not homogeneous, significant variations may be expected in the results. For example, the pore structure and interstitial spaces will vary between slices giving a probability range Reff rather than a single crisp value. This variability has been included in the analysis. By combining the uncertainties for untreated vs. treated specimens (which is simply a root-

sum-of-the-squares calculation), the overall lognormal dispersion factor becomes = 0.43. Thus, it can now be conservatively stated: “One can be 90 percent confident that for the uncracked concrete used in this investigation, the efficacy ratio of the untreated vs treated specimens will not be less than Reff = 75”.

EFFECT OF CRACKING ON PERMEABILITY Figure 3 presents the results for another data set of six slices. Initially, three cylinders were tested untreated and those test results are given by the lowermost curve in Figure 3(a). The other three slices in the same-concrete mix group were treated with the penetrating hydrogel (A2000). Test results showed that one of the three slices had developed a small crack (surface measured at approximately 0.5 mm). As might be expected, the crack opened a pathway for an increased overall flow rate. The result for the cracked specimen is plotted as the central (blue) curve. The remaining two treated slices results are plotted as the upper (green) curve in Figure 3(a). The cumulative distributions are shown in figure 3(b). Using results analysis that conform to the methods described above, the median efficacy ratio

for the uncracked cylinders is Reff = 215/2.5 = 86 with a composite dispersion of = 0.78. Thus the 90 percent confidence efficacy is not less than Reff > 32. In the case of the single cracked cylinder the average efficacy ratio of untreated vs treated is Reff = 2.3. Note no further statistical analysis has been performed due to lack of data at this time. Nevertheless, the hydrogel treated cracked concrete flow rate is still significantly less than the untreated uncracked concrete, demonstrating some hydrogel would have penetrated the crack zone to inhibit the flow. Although the performance remains somewhat better than the untreated curves it is clear that the crack has provided an additional pathway for the moisture ingress. DISCUSSION OF THE PERMEABILITY RESULTS Internal flow rates of the hydrogel treated concrete were significantly reduced when compared to their untreated counterparts, the former being only 1 percent of the latter. Not surprisingly, in the cracked specimen, the flow abatement was only reduced some 50 percent. Nevertheless, this may be considered as doubling the cover thickness for protecting embedded reinforcing bars against corrosion. While the cracked sample was an unintended bonus of the investigation, it remains insufficient evidence to substantiate any general conclusions. This raises the question of the effect of crack width and crack density on permeability performance. Clearly, priority needs to be given to this matter as a subject of future research. RELATING THE LABORATORY RESULTS TO FIELD OBSERVATIONS These overall laboratory permeability investigation findings can be compared to a longitudinal field study where parts of several at-risk wharf structures in Australia and New Zealand were treated with the same penetrating hydrogel (Gilbertson 2015). When the chloride concentration exceeds a minimal chloride threshold of 0.06% (by concrete weight), the structure is deemed at risk and remains a candidate for treatment. When treated with a penetrating hydrogel and later retested, the treatment may successfully reduce the chlorides below the risk threshold. Three of the six wharfs investigated were treated more than 17-years ago, and retest results have shown that the hydrogel treatment has held the chloride reductions at or near the threshold at the steel level. Thus the chloride enhanced deterioration remains essentially abated. Figure 4 shows a scatter-graph of before and after penetrating hydrogel treatment changes in the chloride readings at different concrete depths for testing on the Auckland Onehunga Wharf that was constructed in 1918. The vertical scale is the percentage chloride levels by weight of concrete.

(a) Head above specimen versus time.

uncracked treated vrs cracked treated vrs untreated

(b) Statistical distribution of concrete performance:

uncracked treated vrs cracked treated vrs untreated

Figure 3 Experimental results for the cracked falling head test

Figure 4 clearly indicates the lowering of chloride levels at the cover depth of 50 to 60 mm, to just below the 0.06% corrosion propagation threshold level (shown by the red horizontal line) within 1-year of treatment. This is one of the three structures that was retested some 13 years after the initial treatment and the chloride readings have been held over that time at the lower levels by the reduced permeability of the concrete. This multiple wharf field investigation showed internal moisture movement (permeability) and associated chloride ion movement were mitigated by the penetrating hydrogel, up to the 150mm testing depth. The chloride level reductions are largely caused by the penetrating hydrogel using the internal pore moisture and by its hydrophilic action forming it into an internal gel. This is the same process, without chloride ions being present that reduced the permeability in the laboratory pressure testing. In the case of the wharf structures the penetrating hydrogel has locked up free chloride pore ions as part of the gel formation making them unavailable to accelerate the corrosion process. Timely treatments by penetrating hydrogels, such as used in this paper, that reduce existing concrete permeability, lower chlorides at the steel, and mitigate cracking, extend the life of existing concrete structures.

Figure 4 Onehunga Wharf constructed in 1918

Chloride versus depth scatter-graph

CONCLUSIONS Based on the investigation presented herein, the following conclusions are drawn: 1. Penetrating hydrogels show considerable promise in reducing the permeability of both

uncracked and cracked concrete. The concrete used in this investigation when treated with a penetrating hydrogel had an efficacy scale that can account for variation within the concrete matrix, and micro and miso cracking.

2. Immobilizing the concrete matrix moisture flow is the best defence in reducing corrosion,

and thereby improving the longevity of concrete structures in general.

3. The penetrating hydrogels used in this study demonstrated the permeability reduced to some 1 percent of the untreated counterparts.

4. The penetrating hydrogel, when applied to a sample which had a 0.5 mm wide surface

crack, was not nearly as effective as the uncracked cases, but the flow rates were still reduced to less than 50 percent of the untreated cases. In the tension zone of reinforced concrete when tensile cracking is inevitable, treating the concrete with the penetrating hydrogel is equivalent of doubling the cover.

5. The above conclusions are supported by an historical Australasia longitudinal field study

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