Dr. Rakesh Kumar, Senior Principal Scientist & Pankaj Goel, Senior Technical Officer, Rigid Pavements Division,CSIR-Central Road Research Institute (CRRI), New Delhi, give information on the Performance of Curing Compounds in Strengthening the Concrete Mix to Be Used in the Construction of Rigid Pavements.


India has a road network of more than 6 million km that comprise fully access-controlled modern highways, expressways, narrow national highways, and unpaved rural roads. About 2 percent of the total road length of the country is of cement concrete. Due to numerous technical advantages such as pothole-free surface, a longer service life with little to no maintenance cost for the first 10 years or so, better safety in all weather conditions, long-term economy, and other such proven benefits, thousands of kilometers of cement concrete pavements are being constructed every year in various parts of the country and under different government schemes.

To ensure construction of roads that give better performance, the Government of India encourages use of innovative materials and technologies.

Concrete pavement engineering is concerned with the selection of design, construction materials, and construction practices to ensure satisfactory performance over the designed life of the pavement. Continuous research and development related to the improvement of the field performance of cement concrete mix in light of the increasing scarcity of its basic ingredients, including curing water, have led to the development of new and innovative materials, including different types of curing compounds.

Proper curing of a cement-based product is a must to develop the desired properties in it. Curing of a cementitious product consists of maintaining an appropriate temperature and moisture content in the concrete for a defined period to ensure successful hydration of cement for the development of the desired properties in concrete as ceasing curing stops the process of strength gaining.

A concrete pavement slab has a high surface-area-to-volume ratio, which makes it difficult to maintain a uniform moisture content throughout the pavement. Plastic shrinkage occurs when the rate of water loss from the surface exceeds the rate at which bleed water is available. The occurrence of plastic shrinkage cracks on the cement concrete pavement surface is very common, particularly during the construction in the summer season.


The 4 primary factors that affect the occurrence of plastic shrinkage cracks are concrete temperature, air temperature, wind velocity, and relative humidity (RH). Plastic shrinkage cracking will occur if the concrete is too stiff to flow but has not yet developed enough strength to withstand the tensile stresses that develop due to non-uniform shrinkage. Plastic shrinkage cracks are typically 25 - 50 mm deep and run parallel to one another 0.3 to 1 m apart (Figure 2). Precautionary measures should be taken to prevent plastic shrinkage cracking when the rate of evaporation of the bleed water exceeds 1 kg/m2 /hr [5]. Generally, a curing compound is used to help prevent or to minimize the occurrence of shrinkage cracking by reducing the evaporation of water from the freshly laid concrete. A curing compound should be applied as soon as bleed water ceases to collect on the pavement surface. If the compound is applied too soon, tensile stresses develop where the bleed water collects between the pavement surface and the membrane thereby creating pinholes in the membrane. Shrinkage cracks can also develop if the curing compound is applied too late after an excessive amount of evaporation has already occurred.

India has the second largest road network in the world. Due to several technical advantages such as saving of light for illumination of the pavement’s surface, low maintenance requirement, wide opportunities for utilization of industrial by-products, post-consumer recycled materials, opportunities to incorporate several sustainability-related aspects, and finally the longevity of rigid pavement, lesser consumption of materials like stone aggregates by eliminating or reducing the thickness of the underneath pavement layer/layers, unaffected at high ambient temperature and heavy rains, once constructed, concrete roads do not require aggregates, cement, etc. for regular maintenance, no periodical increase in road surface level for many decades, such pavement has become very popular for roads subjected to moderate to heavy traffic loads besides airport pavements and industrial flooring.

Several thousand kilometres of concrete roads have been built in the past few years in many cities of India, besides highways and expressways. Such a huge activity has opened a large market for highway construction materials including curing compounds. Several manufacturers of curing compounds claim a significant improvement in concrete strength properties without sufficient research data. Hence, the study is mainly focused to educate and disseminate the knowledge generated on the performance of curing compounds on the strengths developed in paving concrete to eliminate the requirement of conventional water curing in the construction of concrete roads.

Curing Compounds & Concrete Pavement Surface
Curing compounds are basically used to help prevent shrinkage cracking by reducing the evaporation of water from the freshly laid cement concrete. The major types of curing compounds are of the following categories:
  • Synthetic resin based
  • Acrylic-based
  • Wax-based
  • Chlorinated rubber based
All curing compounds are membrane-forming. This membrane helps in retaining moisture in the concrete. Cement concrete pavements have a wide exposed surface to the atmosphere. The conventional methods of water curing of concrete pavements are ponding, sprinkling, and wet covering as shown in Figure 3.


For curing with membrane-forming curing compounds, the compounds containing either a wax or resin that is emulsified in water or dissolved in a solvent are applied on the pavement surface. The water or solvent constituent evaporates leaving the wax or resin to form a membrane over the surface of the pavement (Figures 4-5). This membrane helps retain moisture in the concrete.


Characteristics of a Good Curing Compound
The most important characteristics of a curing compound to be used on concrete pavement slabs are as listed below:
  • Water retention
  • Reflectance
  • Drying period
  • Long term setting
  • Non-volatile matter
Application of a high-water retention curing compound with a uniform coverage of adequate thickness is very critical for obtaining the maximum benefits of such compounds. A curing compound with good water retention characteristics is of no use unless it is applied correctly. The application rates for a curing compound varies from compound to compound. It is unfortunate to see that in fields the importance of proper application of a curing compound is generally overlooked. Several factors affect the application rates. The application rates for a textured surface like tinning is lesser than a broomed textured pavement surface.

It has been noticed that the inspections of curing process are limited only to counting the empty barrels rather than ensuring an adequate volume of curing compound has been put through the sprayer. Many factors such as wind and dirty/worn nozzles will affect how much of the cure ends up on the pavement surface and the ability to obtain a uniform coverage (Figure 6). In automatic sprayer the nozzle type, spacing, nozzle spacing and boom height, nozzle orientation and cart speed and wind shield play important roles in ensuring uniform coverage of a curing compound.


This applies to all pavement surfaces regardless of the type of tining or texturing performed.

Limitations of Curing Compound
Some of the important limitations of the effectiveness of curing compounds are as given below:
  • If layer is punctured or damaged: Curing compound is not effective
  • If water is lost by self-desiccation: Curing compound is not effective
  • Time of application: If too early then it dilutes with the available water, if applies after pores in concrete dries out then no to less effect.
  • Minimum w/c ratio: 0.23 (hydration) +0.15 (filling of voids and gel pores) = 0.38.
Experimental Study on Performance of Curing Compounds in Strengthening of Concrete
Two types of curing compounds that are easily available in the Indian market - a resin-based and a wax-based (Table 1) were used in this study. A paving grade concrete with mix proportions of ordinary portland cement, sand, coarse aggregate, and free water cement ratios of 1:1.54:2.98:0.4 were used for the preparation of concrete specimens. A high range water-reducing admixture (HRWRA) naphthalene-based was used to achieve the desired workability in concrete mix. Standard cubes of 150 mm and beams of 100 x 100 x 500 mm were prepared for the evaluation of compressive and flexural strengths. One set of 9 cubes and 3 beams were submerged in water for curing while the rest of two sets i.e. 18 cubes and 6 beams were coated with curing compounds as per the instruction of the manufacturers.

Table 1. Technical specification of curing compound
  Wax-Based Resin-Based
Sp. Gravity 1.02 1.10
Form Wax-based liquid Water-based liquid
Color White Pink
Coverage 4-6 m²/ltr 10-15m2/kg
Drying time Approx. 120min. at 300C Approx. 40-50 min./coat
Toxicity Non- Toxic Non-Toxic, Non-VOC
Application/Coating of Curing Compound on Specimens
A low pressure hand-held sprayer was used to apply the curing compounds on the surfaces of the concrete specimens (Figure 7). The first coat of curing compound was sprayed over the top surface of the specimen just after disappearing of bleeding water from the surface. The concrete specimens were demolded after 24 hours of casting. Curing compound was applied at remaining five surfaces of the cubes and beams. The rate of application was 5 m2 per litre for wax-based and 13 m2 per litre for resin-based curing compound respectively. The second and final coat of curing compound was applied at the same application rate to all the surfaces just after 30-45 minutes after the first application. After the coating, some specimens were kept under sunlight and other were kept under sunshade outside laboratory till testing ages.


Results & Observations
Compressive Strength: The compressive strength development with ages for water cured specimens and the specimens covered with resin-based curing compound but kept under sunlight and under sunshade was determined as per IS- 516 [7] and the results are shown in Figure 8. Similarly, Figure 9 shows the development of compressive strength for water cured and cured with coatings of a wax-based curing compound under sunlight and under the sunshade.


It is obvious (figure 8) that a similar compressive strength for water cured and resin-based curing compound cured under different conditions is developed up to 7 days. However, at 28th day the compressive strength developed by the water cured sample was highest followed by the sample cured with curing compound but kept in the sunshade. The least compressive strength was shown for the sample cured with curing compound but kept under the sunlight. About 12% lesser compressive strength at 28-day was noted for the sample kept under sunlight. The reduction in compressive strength was within the range for this type of curing compound (Figure 10).


Figure 9 shows the compressive strength trends similar to that of resin-based curing compound with some exceptions such as a significant reduction at all ages of testing. Here a reduction of about 20 -30% was noted for the sample cured with wax-based curing compound. The solid content, water retention capability, non-uniform coverage, and other similar factors may be responsible for such a reduction in the compressive strength of concrete.

Flexural Strength
The flexural strength of the concrete beam specimens was determined as per IS- 516 [7]. Third point loading system was used to determine the flexural strength of concrete specimens. The average of three specimens tested at 28 days is reported. Table 2 presents the flexural strength of concrete specimens test results cured under different curing conditions with resin based and wax based curing compound besides water cured specimens.

Table 2. 28-Day flexural strength of concrete samples of water cured, resin-based and wax-based curing compounds cured under sunlight and sunshade.

Flexural strength (MPa) under curing condition
Water curing Resin-based curing compound Wax-based curing compound
Under Sunlight Unser Sunshade Under Sunlight Unser Sunshade
5.3 4.8 4.6 NA NA
5.2 NA NA 4.2 4.8
Similar to the trend of compressive strength, the water cured concrete sample developed the maximum flexural strength. The sample covered with curing compound but kept under sunlight developed the least flexural strength irrespective to curing compound types. Depending on the exposure condition after application of curing compound, the flexural strength developed for the curing compound cured samples was 5% to 13% lesser than water cured concrete specimens. In general, under sunlight a reduction of just over 10% for flexural strength was noted.

Conclusions
  • A comprehensive information about the placement environment of concrete including season besides curing compound characteristics such as uniform coverage rates, solid content, water retention level etc., may help maintaining effectiveness of curing compound.
  • Depending upon the types, application coverage rate, application time, coverage uniformity etc., a reduction in strengths in comparison of water curing is encountered.
  • The resin-based curing compound showed better performance in maintaining the strengths than the wax-based curing compound.
  • Neither resin based nor wax based curing compound can be used to eliminate water curing of pavement concrete slab on the basis of their strength development criteria.
Acknowledgement
The permission of the Director-CRRI to publish this work is gratefully acknowledged. The help provided by Adarsh Kumar and Garima during the preparation of the manuscript is highly acknowledged.

Dr. Rakesh Kumar and Pankaj Goyal

References:
  1. Ministry of Road Transport and Highway, Government of India, New Delhi, Annual Report (2019-20).
  2. Kumar R., Repair of Scaled surface areas of newly constructed cement concrete pavement slabs, NBM&CW (2019) 1-6.
  3. Neville A M., “Properties of concrete”, 4th Edition, Pitman Publishing Limited, London, 1997.
  4. Kosmatka, S. H. and W. C. Panarese, 1992, Design and control of Concrete Mixtures, 13th Edition, Portland Cement Association, Skokie, IL.
  5. Final Report 2001-06, A review of the curing compounds and application techniques used by the Minnesota department of transportation for concrete pavements.
  6. Curing of concrete, Cement Concrete & Aggregate Australia. Apr.2006.
  7. IS: 516-2000, Methods of tests for Strength of Concrete, Bureau of Indian Standards, New Delhi.
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