Water Cement Ratio
The water-to-cement (w/c) ratio is a crucial factor in determining the strength, durability, and overall performance of hardened concrete and cement-based materials. A high w/c ratio can result in poor strength, excessive porosity, and inadequate durability, while a low w/c ratio can cause voidage and microcracking. Determining the w/c ratio retrospectively is often desirable for quality control, condition assessment, and forensic investigation, but it can be challenging once the concrete has set.
The physicochemical method of estimating the w/c ratio from capillary and chemically bound water measurements and partial chemical analysis of soluble CaO and SiO2 has limitations, including inaccuracies in cement measurement and errors caused by absorption into voidage, cracks, and porous aggregates. Petrographic optical microscopy methods, such as those described in ASTM C856, NT 361-1999, APG SR2, and BS 1881-211, use indicators of apparent w/c ratio to estimate capillary porosity indirectly. The fluorescence intensity observed under UV light after impregnation with epoxy resin containing fluorescein dye is compared visually or quantified using image analysis to a set of reference concretes with known w/c ratios.
An alternative method using field-emission scanning electron microscopy (SEM) in the backscattered electron (BSE) mode allows for direct measurement of capillary porosity, air voids, hydration products, and residual unreacted cement, enabling back-calculation of the original water and cement contents prior to hydration. While microscopy techniques are used in many commercial and research laboratories, the precision and accuracy of these techniques for assessing the w/c ratio need further evaluation.
The Applied Petrography Group (APG) initiated discussions for an inter-laboratory study in 2015 to investigate the validity and precision of optical fluorescence microscopy, visual assessment, and backscattered electron microscopy for determining the w/c ratio in hardened concretes, comparing the results obtained by participating laboratories to the BS 1881-124 method.
A study was conducted to assess the accuracy of microscopy techniques in determining the water/cement ratio of hardened concrete. Eleven laboratories analyzed five CEM I concrete mixes with unknown water/cement ratios (0.35, 0.40, 0.45, 0.50, and 0.55) using four techniques: visual assessment of paste characteristics, visual comparison of fluorescent intensity, quantitative assessment of fluorescent intensity, and quantitative backscattered electron microscopy. The study produced 100 individual determinations of the water/cement ratio, making it the largest of its kind. The findings are as follows:
(a) The estimated water/cement ratios were within ± 0.025 of the target mix values for 37% of the results, within ± 0.05 for 58% of the results, and within ± 0.1 for 81% of the results. The errors ranged from − 0.058 to + 0.23, or from − 14% to + 43% of the mix values.
(b) Statistical analysis showed that 73 out of 80 data pairs were correctly ordered, and 14 out of 20 sets of water/cement ratio estimates from 7 labs gave the exact correct order of mix water/cement ratios from low to high.
(c) The performance of individual labs varied, with the average error ranging from 0.01–0.15. Four labs consistently performed well with low errors for all specimens: Labs 07 (using VA + FM-V), 14b (using VA), 05 (using FM-Q), 14a (using FM-Q), and 11 (using BSE), with errors no greater than 0.05.
(d) Optical microscopy-based methods tended to over-estimate the water/cement ratio. VA and FM-V tended to over-estimate by ~ 10%, while FM-Q showed increasing error with an increase in the water/cement ratio (up to ~ 20%). In contrast, the BSE method under-estimates (by up to 9%) at high water/cement ratios.
(e) The largest errors occurred in labs that carried out visual assessment (VA, FM-V) without proper reference standards. Therefore, a qualitative standard-less approach is not recommended. The use of reference standards is essential for optical microscopy-based methods.
(f) Microscopy techniques produced much lower errors compared to the BS 1881-124 physicochemical method reported in Concrete Society inter-laboratory precision trial. Within the optical petrographic methods (VA, FM-V, FM-Q), labs that used reference standards performed better than those that did not. The BSE method gave the lowest range and average error.
In conclusion, microscopy techniques can provide meaningful information about mix composition and accurate estimates of the water/cement ratio of hardened concrete. Ongoing work is being done to test these methods on a wider range of contemporary concrete mixes and water/cement ratios and to determine their accuracy and precision.