Cutting-edge Spray-Lock Concrete Protection’s SCP technology has been used on many projects in South Africa and in other countries on the continent to enhance concrete strength and durability. Carl White, Managing Director of SprayLock Africa, explains how SCP mitigates chlorides in reinforced concrete. SprayLock Africa are the exclusive African agent for United States-based SCP’s products.

 

SCP treatments combat corrosion of concrete reinforcing steel in five important ways. Firstly, SCP products significantly reduce the transmission of chlorides through concrete. This potentially extends the corrosion initiation period of steel reinforcing or “rebar” by many years compared to reinforced concrete that has not been treated with SCP. Moreover, it acts as a waterproofing agent, restricting access of the corrosion reaction to water and significantly preventing oxygen availability. SCP increases the electrical resistivity to levels where corrosion is unlikely to occur. Lastly, the technology purges some of the available chlorides from the concrete.

 

SPRAY-APPLIED COLLOIDAL SILICA-BASED TECHNOLOGY

Colloidal silica with a pozzolanic potential greater than that of silica fume

SCP comprises of post-initial set applied colloidal silica of an extremely small particle size, providing a tremendous amount of pozzolanic potential. The pozzolanic potential of SCP’s colloidal silica is greater even than that of undensified silica fume¹. It fills the void space of concrete with Calcium Silicate Hydrate, or “C-S-H”, which is the same reaction product that provides concrete strength and durability.

 

This restricts the movement of water through concrete – even under hydrostatic pressure. In so doing, it reduces water-borne contaminant ingress and chloride transport considerably to protect reinforcing steel or rebar from corrosion.

 

CHLORIDE INDUCED CORROSION OF STEEL

An examination of concrete reinforcing steel or “rebar” corrosion in concrete

It is important to consider chlorides when examining concrete reinforcing steel or “rebar” corrosion in concrete. This is because they penetrate the passive layer around the concrete reinforcing or “rebar”. The passive layer is a film created by the concrete’s high alkalinity or “pH” through which oxygen cannot penetrate. Chloride ions increase the solubility of this passive layer of the reinforcement bar or “rebar” in reinforced concrete, and it yields at a certain threshold concentration². Once the passive layer of the reinforcing steel or “rebar” in concrete has been penetrated, corrosion no longer requires chlorides to progress. The corrosion action is typically self-sustaining, requiring only the reinforcing steel or “rebar”, water and oxygen to continue. After the passive layer has been breached by chlorides, the pore structure of the concrete becomes the primary consideration. This occurs together with the electrical resistivity that the pore structure provides³.

 

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1 Singh, L.P., Karade, S.R., Bhattacharyya, S.K., Yousuf, M.M., & Ahalawat, S. (2013) “Beneficial role of nanosilica in cement based materials – a review,” Construction and Building Materials 47, 1069-1077.

2 Hussain, R.R. (2014) “Passive Layer Development and Corrosion of Steel in Concrete at the Nano-scale”. Journal of Civil & Environmental Engineering 4:e116

3 Neville, Adam (1995) “Chloride Attack of Reinforced Concrete: An Overview.” Materials and Structures 28, 63-70.

 

SCP’S PERFORMANCE IN LABORATORY AND FIELD TESTS

Years before chlorides reach concrete reinforcing steel or “rebar”

SCP’s efficacy in improving concrete strength and durability by mitigating chloride-induced corrosion has been proved in many laboratory and field tests. These have been undertaken in South Africa by SprayLock Africa and in the United States by our principal, Spray-Lock Concrete Protection.

 

One of the tests undertaken in a laboratory environment to demonstrate the efficacy of SCP technologies in improving concrete strength and durability included chloride bulk diffusion testing. Chloride bulk diffusion testing entails “ponding” salt water on top of concrete to assess chloride concentrations at various depths. Undertaken according to AASHTO T-259 or ASTM C1543 standards, it informs calculations to produce an average diffusion coefficient (D). These are used to predict the time that it will take for chlorides to reach the reinforcing steel or “rebar” and penetrate the passive layer around it. Life 365 and other state-of-the-art software is used to model these projections.

 

By reducing the chloride bulk diffusion coefficient, years are added to the period that it takes for chlorides to reach the concrete reinforcing steel or “rebar”.

 

Table 1 contains the results of chloride bulk diffusion scenarios in conventional concrete during tests that were undertaken at three American laboratories.

 

Table 1: Chloride Diffusion Reduction Percentage with SCP Treatment

Study	Water to cement ratio	Reduction in chloride diffusion coefficient (D) %
MIDDLE TENNESSEE STATE UNIVERSITY	0:57	69
University of Tennessee at Chattanooga (average of 7 tests) 4	0:45	69
Tennessee Department of Transportation	0:40	75
Florida Department of Transportation	0:40	55

 

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Collet, P., Rollins, A.B., Andres, V. (2016) “Concrete Porosity Reduction by Colloidal Silica Nano Technology” Conference Proceedings, CONSEC 16, Lecco, Italy.

 

GREATER RESISTANCE TO CHLORIDE-INDUCED CORROSION

The Effect of SCP Treatments on Concrete with Inherent Chlorides

It can be noted that there are higher numbers in the surface resistivity and half-cell resistance rows. This indicates a greater resistance to chloride-induced corrosion. In the row addressing corrosion potential, the resistance to chloride-induced corrosion increases as the result approaches zero. Further explanation of the threshold levels of each test are contained in Tables 2.1, 2.2, and 2.3.

 

Tables 2.1 and 2.2: Half Cell Potentials (left) and Half Cell Resistance (right) Thresholds

Half-Cell mV (Cu)	Chloride-Induced Corrosion Potential		Resistance to Chloride-Induced Corrosion (kOhm)	Support Corrosion
> -200	Low		≥ 50	Not Likely
-200 to -350	Uncertain		< 50	Yes
< -350	High

 

Table 2.3: Surface Resistivity Thresholds

Surface Resistivity (kOhm/cm)	Corrosion Risk
≥ 20	Low
10 – 20	Low
5- 10	High
< 5	Very high

 

Spraylock Concrete Protection has also tested the efficacy of SCP on concrete containing inherent chlorides in various field studies. Of particular interest are those that were undertaken with leading United States Departments of Transport in the United States.

 

The first field test was undertaken with the Florida Department of Transportation, or “FDOT” and entailed testing the efficiency of SCP on four concrete bridges. Half-cell corrosion potentials, surface resistivity and water-soluble chloride content testing was performed before and after treatment with the colloidal silica-based technology. The results are seen in Table 3.

 

Half-cell corrosion potential testing

 

Treating a bridge deck with SCP

 

Table 3: FDOT Bridge Case Studies Testing Results

	Wilcox Road Bridge	Fred Howard Park Bridge	Palma Sola Boulevard Bridge	Blind Pass Bridge
Surface Resistivity (kOhm/cm) Before/After	NA/425	NA/NA	102/245	176/228
Half-Cell Resistance (kOhm) Before/After	12/828	34/39	22/31	27/32
Corrosion Potential (mV) Before/After	-534/–	-59/-65	-563/-322	-223/-219
Chloride Content (% by weight of concrete) Before/After	–/–	–/–	0.0890/0.0605	0.0347/0.0203

 

Reinforced Concrete Beams with High Levels of Chloride Concentration

The second field study was performed with the Tennessee Department of Transportation (TDOT) involving a series of AASHTO Type III reinforced concrete I-beams. These precast-concrete elements were manufactured by a prestressed precast concrete company that unknowingly used bore hole water as mixing water. They were, therefore, contaminated with chlorides. The reinforced concrete beam that had the highest levels of chloride concentration was selected for treatment with SCP products. Surface resistivity of the reinforced concrete beam was performed before and after treatment with SCP. The results of the tests are contained in Table 4.

 

Table 4: TDOT Prestressed/Precast Concrete Bridge Beam Testing Results

	Before/After Treatment
Surface Resistivity (kOhm/cm)	31.44/43.31

 

To interpret the changes in tested values for both the Florida and Tennessee Department of Transports case studies, evaluation against commonly accepted industry threshold levels is useful for half-cell potentials, half-cell resistance and surface resistivity.

 

The technology has also proved its efficacy in waterproofing and protecting concrete bridges, among other reinforced concrete structures, in South Africa.

 

A case in point is the use of SCP to waterproof and protect bridges on the N14 highway. The technology was specified by Boganala Consulting Engineers and applied over 2 720 m² by DSC Zendon, the approved applicator of the technology.

 

DSC Zendon was also appointed to treat 1 040 m² of concrete on highway bridges in the Diepsloot area. SCP was specified for purging and protection purposes by WSP in Africa, the consulting engineer.

 

 

Describing the Effects of SCP on Reinforcing Steel Corrosion Cycle

 

In any corrosion mitigation project, SCP recommends testing levels of chlorides at various depths, and performing half-cell and surface resistivity measurements before and after treatment. Results should be evaluated to determine if treated results are consistent with requirements for reinforcing steel preservation for the project.