Aggregate Matters:

About this Project

Project Name: Evaluation of Concrete Piles

Location: Docks 40-41, Port of Texas City, Texas

Executive Summary: In keeping with periodic evaluation of concrete structures at the Port of Texas City, the Port, in conjunction with its Engineering Consultant, LJA Engineering, contracted C3S, Inc. (C3S) to perform concrete evaluation of the piers on Docks 40-41.

Four durability indicator tests were performed. All revealed piles to be in good condition.

• The compressive strength for samples tested ranged from 7,200 psi to 13,000 psi.
• The petrographic analysis showed dense compact concrete with no threat of deleterious reaction product or impurities.
• The scanning electron microscopy (SEM) evaluation complemented the petrographic analysis findings.
• Carbonation: the concrete cover on the embedded rebars show indication of high alkali medium capable of protecting the embedded rebars.

Based on the results of this evaluation, as summarized above, repairs that need to be performed are the visible cracks and other surface defects on affected piles.

Project Information: Rebars and cables in the concrete piles restricted the size of samples cored to perform the various tests. The coring was preceded by locating and marking out rebars locations in the piles using a GPR scanner. The most practical diameter of sample cored was about 1-5/8” instead of a minimum of 3.7” as per ASTM standard. The length of cores ranged from 4” to 10”. Some of the cores from the same location were broken into 2 pieces. A total of 19 cores were extracted for the tests. 

Tests Performed:

The following tests were performed to evaluate the current durability condition of the concrete piles in Docks 40-41. The tests are as listed below.

1. Compressive Strength of representative samples of the piles in the referenced Docks
2. Petrographic Analysis to determine the bulk macro level condition of the two principal ingredients of concrete, namely, the aggregate and cementitious matrix that hold the concrete matrix together.
3. Scanning Electron Microscopy was used to complement the petrographic analysis – looking at the concrete matrix at the micro level.
4. Carbonation test to determine the alkali condition of concrete cover on the rebars.

A. Compressive Strength, ASTM C42

The test method provides standardized procedures for obtaining and testing specimens of concrete to determine the compressive strength of the in-place concrete. Due to configuration of embedded rebars, as revealed by ground penetrating radar technique, the expected 3.7” diameter cores could not be retrieved from the piles due to potential of cutting through rebars. A 1-5/8” diameter was considered safe to avoid any potential damage to embedded rebars.

Petrographic Examination of Hardened Concrete, ASTM C 856

Petrographic analysis involve the optical examination of concrete specimen under low and highpower magnification. Detailed instructions on conducting a petrographic examination of hardened concrete/mortar can be found in ASTM C856, “Standard Practice for Petrographic Examination of Hardened Concrete”. For examination, a section of the mortar was impregnated with blue dye under vacuum. The impregnation under vacuum causes the dye to permeate every crack, micro crack, and all pores, including micro pores in the concrete sample. The impregnated concrete is cut and placed on a glass plate, ground and polished to a thickness of about 30 microns. The samples were then with by polarizing microscope using a 400X magnification.

Purpose of Petrographic Examination

The usefulness of petrographic analysis ranges from quality control to predicting future performance of concrete; others include:

• Determination of the causes of inferior quality, distress, or deterioration of concrete/mortar in a construction.
• Determination of cause(s) of a well performing concrete in service.
• Part of a survey of the safety of a structure for a present or proposed use.

Findings from Petrographic Analysis (Samples 9, 16 and 19)

1. The cement pastes in all three Samples examined were dense and uniform, indicative of concrete formulated with optimal water to cement ratio. The dense paste effectively surrounded both coarse and fine aggregates giving rise to a dense compact matrix.
2. A few of the coarse aggregates had inherent cracks in them but cracks have not extended into the dense, hardened cement paste.
3. Entrained air voids, with hydration products surrounding the inner lining, were noted in some of the samples examined as shown in Figure 3d and 4d.

C. Scanning Electron Microscopy, (SEM) ASTM C1723-10

Scanning electron microscopy (SEM), is used in the evaluation of several materials, including concrete. It uses electron beams as part of the analytical microscope system to examine concrete samples in magnifications ranging from 15X to over 5,000X. In concrete samples, SEM allows details such as fly ash particles and hydration products to be clearly viewed. SEM is also often used to compliment petrographic analysis of hardened concrete as per ASTM C 856 method.

D. Carbonation Test

Carbonation is a process in which moisture-laden carbon dioxide from the atmosphere diffuses through the pores of concrete and through a chemical reaction, converts the calcium hydroxide into calcium carbonate. This reaction lowers the pH of concrete in the areas concerned, typically on the surface of concrete. The process can continue to progressively penetrate the concrete depending on the porosity of the concrete.

The reagent commonly used is phenolphthalein. When in contact with high pH (>10) concrete, the phenolphthalein solution shows as bright pink color; with low pH (<10), the solution shows no color change. A gray concrete color is noted on the top surface of the concrete representing the carbonated area, typically measured in fractions of an inch.
Samples tested for carbonation are 2, 4, 5, 7, 9, 11, 12, 16, 19

Discussion of Results

Periodic maintenance of structures at large facilities such as POTC, are scheduled and performed to determine the structural integrity of sections of the facility depending on facility size.

In this maintenance period, the Port of Texas City selected Docks 40-41 to be evaluated for the durability of concrete piles supporting concrete platforms at the two docks.

Indicators of durability of concrete structures in service can be determined from standard tests. In this evaluation, tests chosen included compressive strength, petrographic analysis, scanning electron microscopy and carbonation test.

Compressive strength gives an indication of the ability of the constituent structure to support an expected load.

Petrographic analysis determines the condition of the concrete as seen from the constituents of the concrete using microscopy – to determine the condition of the cement paste, the aggregates,  and other ingredients of the concrete. in addition to possible formation of deleterious chemical reactions such as alkali-silica reaction products.

Scanning electron microscopy (SEM) is used as a complement to petrographic analysis, specifically determining the microstructural features of the concrete, and validation of deleterious reaction products, if any.

Carbonation test gives an indication of the level of protection of the rebars embedded in the concrete. Alkalinity of concrete above a certain level protect the rebars from corrosion, below that level the rebars become susceptible to corrosion.

The size of cores required to perform the compressive strength was compromised due to the probable damage to embedded rebars. However, ASTM specification allowed for the use of smaller sizes due to instances such as was faced in this situation. The specification allows for a notification to be made and how the report of the strength test to be presented. ASTM C42, Section 7.1.2 Note 10 states “The compressive strength of nominal 2-in diameter cores is known to be somewhat lower and more variable than those of nominal 4-in diameter cores.”

The compressive strength test was conducted on 12 samples and values ranged from 7,600 to 13,000 psi.

The petrographic analysis revealed a dense cement paste effectively covering the aggregates. The coarse aggregate appears strong and durable, but some showed a nonthreatening defect of contained inherent cracks as shown in Figure 3b.

The SEM results consistently showed dense cement paste as seen in magnifications in the range of 65X to 2,000X. No deleterious reaction products were found in any of the samples examined.

The carbonation test showed the absence of carbonated cover of concrete in the samples tested – an indication of a good protection of the embedded rebars. The intensity of the bright purple coloration indicate a higher pH of the concrete.

Based on the four durability indicator tests used in evaluating concrete samples from Docks 40-41 it is our considered opinion that repairs that need to be performed are cracks and
surface defects on some of the piles.