Influence of Surface Scaling on the Performance of Cement Concrete Roads

Dr. Rakesh Kumar, Principal Scientist, Rigid Pavements Division, CSIR-Central Road Research Institute (CRRI), Mathura Road, New Delhi

A well designed and properly constructed cement concrete road generally outperforms the structural and functional performance parameters set for it. On the other hand, premature distresses such as surface defects i.e. scaling, pop-outs, crazing, map cracking, loss of texture etc, and structural cracks are not very uncommon on a newly constructed cement concrete road. Among all the surface defects of newly constructed cement concrete road, the scaling draws a quick attention of the road users as well as road authorities. Scaling of a concrete surface is defined as the loss of surface mortar and mortar surrounding the aggregate particles (Figures 1-3). It is deterioration of the upper concrete slab surface. Scaling may occur anywhere over the pavement slab [Miller and Bellinger, 2003]. As shown in the figures 1-3, the aggregate is usually exposed and often stand out from the concrete. Normally the depth of scaling varies from 3 mm to 13 mm [FHWA-HRT-13-092, 2014, N.J. Delatte 2014, ACI 224.1R]. Scaled concrete surface gives a very ugly look of the concrete slab (Figure 1). It is measured as the square meters of affected area. Light scaling does not expose the aggregate. In moderate scaling, the aggregate are exposed and the mortar loss depth may be 3 to 10 mm. But, in case of severe scaling, more surface areas are lost and the aggregate is clearly exposed and projected from the surface. Severe scaling of a pavement slab is considered as a type of functional distress as it affects riding quality and safety of the traffic. It is common to observe scaling problem of varying severity in several pavement slabs. Such situation reduces the riding comfort and increase a chance of accident due to sudden application of break or for lowering the speed of a moving vehicle at a high speed.


In fact, scaling does not have any significant effect on structural performance of a pavement, however, it does reduce the overall performance of a pavement. Risk of scaling is higher on the pavement concrete surfaces that have not been finished properly. In few cases of scaling, it has come to know that the road owners have ordered reconstruction or replacement of concrete slabs inspite of the fact that it does not reduce the structural performance of the slabs. According to IRC:SP 83, 2008, distress rating system and repair action for concrete pavements, scaling shall be measured in percentage of total surface area of slab. Table 1 presents the measured parameter, degree of severity, assessment rating and repair action for scaling distress of a concrete pavement.

It is very uncommon to have a scaled pavement concrete surface with scaled depth more than 25 mm for a pavement concrete mix having maximum size of aggregate of 25 mm. Therefore, reconstruction of scaled pavement slab is seldom necessitated. It is important to note that the FHWA, does not recommend any severity levels for scaling of the concrete pavement slab surface.

Measured parameter	Degree of severity	Assessment rating	Repair action
Area in square meter expressed in %. r=scaled surface area/ total slab surface area. Maximum depth of damage (h)	0	Nil, not discernible	No action
	1	r < 2%	Local repair of damaged areas
	2	r = 2 – 10 %
	3	r = 10 – 20 %	Bonded Inlay
	4	r = 20 – 30 %
	5	r > 30 % and h > 25 mm	Reconstruction of slab

Causes of Scaling of Concrete Pavement Surface

The major causes of scaling of concrete pavement slabs include;

• Poor quality concrete mix (which may be due to high water to cement ratio over 0.5, excessive slump for prevailing job, premature finishing i.e. finishing operation carried out while bleed water is present, too much fines in fine aggregate, dusty aggregate, inadequate curing etc.). On many occasions, especially during the construction of concrete pavement by semi-mechanised method of construction in summer, sprinkling of water on the surface of concrete for finishing can be seen. Such situation also results in scaling of the surface of pavement slabs
• In adequate slope to properly drain water away from the slab. Author has come across such problem on concrete pavement with two lanes and three lanes each way cast on different times, for the same chainage the scaling of concrete was observed on the surface of outer slab and some times in the mid slab in the areas near outer slabs. The reason found was as discussed earlier
• Application of excessive de-icing salts (such situation encounter in snow bound climatic areas which are subjected to freezing and thawing condition)
• Use of non-air entrained concrete for freezing and thawing condition

Evaluation of Damage

Irrespective of the cause of damage, it is imperative to establish the extent of the damage and determine if the major portion of the concrete is of suitable quality on which a sound repair could be build. Based on this information, the type and extent of repair are chosen.

Repair of Scaled Areas of Concrete Pavement Slabs

Premature reconstruction of infrastructures such as pavements, buildings etc, corresponds to an unacceptable waste of materials, work energy, and an unnecessary emission of greenhouse gases (GHGs). Therefore, proper repair and/or rehabilitation without compromising the overall performance of the pavement are encouraged. Scaled areas of concrete pavement slabs can be satisfactorily repaired by a thin concrete or mortar overlay, provided the surface is sound, durable and clean. Numerous patching materials are available for this purpose. Texture and colour of the repair will have to match the surrounding concrete. The repaired pavement slab surface shall be as strong as the base surface to which it is bonded. Therefore, the surface should be free from dirt, paint or oil and sound. To accomplish this, use of a hammer and chisel, sand blasting, high-pressure water jet (Figure 4) etc, are used to prepare for resurfacing with a repair material.

This repair material may be of:

• Cement-based mortar
• Resin-based mortar
• Polymer modified cementitious mortar

Among all the above mortar types, polymer modified cementitious mortars have an upper-hand for concrete repair due to their overall compatibility with the parent concrete. Polymer modified cementitious mortars are high-performance repair materials which work monolithically with the parent concrete due to similar physical properties such as modulus of elasticity, shrinkage, and co-efficient of thermal expansion (COTE). These mortars are also lower in cost than resin-based mortar like epoxy mortar. The most widely used polymer mortar for the repair of scale is styrene butadiene rubber (SBR). SBR is added to modify the ordinary cement mortar. Wang and Zhang 2015, used three kinds of polymer i.e. SBR (styrene butadiene rubber) dispersion (SD 623), SAE (styrene acrylic ester) dispersion (S 400 F), and SAE powder (FX 7000), for modifying the ordinary cement-based mortar and studied their relative performance in terms of durability of repair system i.e. tensile bond strength, acid-base erosion, freeze-thaw cycle, wet and dry cycle effect and microstructure observation by SEM. Through their study, they concluded the best performance for SAE (styrene acrylic ester) powder followed by SBR (styrene butadiene rubber) dispersion (SD 623). Figures 5 and 6 show repaired pavement slabs SBR-based polymer modified cementitious mortar. The cost of such repairs in this case was approximately Rs.1200 to Rs.1400 per meter square. A simple calculation for the repair of a concrete pavement slab of 4.5 m x 3.5 m with 0.30 m thickness is presented in Table2.

Dimensions of pavement slab	Surface area of slab, m2	SBR based cementitious mortar repair cost per m2 (Rs.)	Repair cost of 100% scale surface of a slab (Rs.)	Typical construction cost per slab	Repair cost/ Construction cost, (%)
4.5 m x 3.5 m x0.30 m	15.75	1200	18900	37800	50
		1400	22050		58

The data presented in the above table is collected from an actual field. It clearly shows that the properly applied repair by a polymer-based cementitious mortar cost is very high and it amounts to about 60% to the construction cost of a concrete slab. Improperly repaired scaled surface of pavement develop cracks as shown in Figure 7.

Pavement Performance

A pavement’s performance is generally described in terms of structural and functional performance. Structural performance is a pavement’s ability to carry the imposed traffic and environmental loads. On the other hand, a functional performance of a pavement is its ability to provide users a comfortable ride for a specified range of speed. Both structural and functional performance is considered in assessing overall pavement performance. Concrete pavement engineering is concerned with selection of design, materials, and construction practices to ensure satisfactory performance over the designed life of the pavement. Well designed and constructed pavements, as a general rule, develop distresses gradually over time under traffic and environmental effects. But, use of poor materials, improper mixes and/or poor construction practices causes pavement distresses even before the pavement is put into service consequently affecting the performance of the pavement.

Conclusions

• Among all the surface defects of newly constructed cement concrete road, the scaling draws a quick attention of the road users as well as road authorities.
• Severe scaling of a pavement slab is considered as a type of functional distress as it effects riding quality and safety of the traffic.
• Risk of scaling is higher on the pavement concrete surfaces that have not been finished properly.
• Scaling problem can be avoided by using proper concrete mix design, proper finishing, and by adopting correct and adequate curing method.
• Scaled pavement surface areas of concrete slab can be cost-effectively repaired with a proper polymer-modified cementitious mortar by an expert repair work team.
• Scaling does not have any significant affect on structural performance of a pavement however it does slightly reduce the overall performance of a pavement since the functional performance get slightly reduced.

Acknowledgements

The permission of the Director-CRRI “Prof. Satish Chandra” to publish this work is gratefully acknowledged. The support and help provided by site engineering, contractor and road owners for the photographs and information regarding the cost of repairing of the scaled surface of pavement slabs are thankfully acknowledged. The help provided by Mr. Adarsh Kumar and Ms. Garima during the preparation of the manuscript is highly acknowledged,

References

1. Miller, J.S., and Bellinger, W.Y. 2003. Distress Identification manual for the long-term pavement performance program (4th revised Ed.), FHWA-RD-03-031, Washington, D.C.
2. Wang, R., and Zhang, L. 2015. Mechanism and durability of repair systems in polymer-modified cement mortar, Advances in Materials Science and Engineering, Vo.2015, Article ID 594672.
3. IRC SP: 83, 2008. Guidelines for maintenance, repair and rehabilitation of cement concrete pavements.
4. Federal Highway Administration (FHWA) 2014. Distress identification manual for long term pavement performance program (5th Edn), FHWA-HRT-13-092, May 2014. https://www.fhwa.dot.gov/publications.
5. Norbert J. Delatte, 2014. Concrete Pavement Design, Construction and Performance, 2ne Edn., CRC Press, London.
6. American Concrete Institute Committee 224.1R, Causes Evaluation, and Repairs of Cracks in Concrete Structures, ACI 224.1R, Farmington Hills; MI, American Concrete Institute.