This part of BS 1881 describes sampling procedures, treatment of samples and analytical methods to determine the cement content, aggregate content, aggregate grading, original water content, type of cement, type of aggregate, chloride content, sulfate content and alkali content of a sample of concrete.
Petrographic optical microscopy methods such as those described in ASTM C856 [8], NT 361-1999 [9], APG SR2 [10] and BS 1881-211 [11] are based on indicators of apparent w/c ratio such as variation in capillary porosity, residual unreacted cement and portlandite content [2,3,4, 8,9,10,11,12,13]. Assessment of capillary porosity is a key feature and this is estimated indirectly by impregnating the sample with epoxy resin containing fluorescein dye and subsequent examination under UV light [14, 15]. In the absence of other influencing factors, the observed fluorescence intensity is an indirect measure of intruded resin and capillary porosity. The w/c ratio of the unknown concrete is then estimated by visually comparing its fluorescence intensity to a set of reference concretes of similar type made with known w/c ratios [4, 9, 10, 16]. Fluorescence intensity can also be quantified using image analysis [17,18,19,20], which is potentially more sensitive and precise than visual comparison.
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Microscopy techniques are used in many commercial and research laboratories concerned with petrographic examination of cement-based materials. However, very little independent research has been carried out to evaluate their precision and accuracy for assessing w/c ratio. The validity of some of these techniques has been questioned [24,25,26,27]. Therefore, an industry-wide study to assess these techniques is very much needed. In late 2015, the Applied Petrography Group (APG) an affiliate to the Engineering Group of the Geological Society of London, began discussions concerning the organisation of an inter-laboratory study to address this need. The main objectives of the study are to: (a) investigate the validity of optical fluorescence microscopy, visual assessment and backscattered electron microscopy for determining w/c ratio in hardened concretes; (b) examine the precision of the results obtained for a series of concretes within a normal range of w/c ratios; and (c) compare the results obtained by participating laboratories and to the BS 1881-124 method.
A steering committee was set up by the APG to organise and oversee the round-robin tests, and report on its findings. This was chaired by Mr Richard Fox and Dr Alan Poole was the Secretary. They were empowered to seek expert advice as necessary, and have received numerous comments from APG members on the round-robin exercise. In October 2016, the committee agreed after wide consultation that five test concrete mixes should be prepared for the trial. These were simple concrete mixes containing no chemical admixtures or mineral additions. The same mix design was used throughout with the only variable being the w/c ratio. Test specimens were prepared by the Tarmac Technical Centre at Lutterworth, UK in May 2017 and distributed to participants for examination with their in-house microscopy methods.
Initially contact was made with 29 laboratories across the UK and Europe to enquire whether they would be interested in participating in this round-robin study. Eleven laboratories agreed to participate and specimens were dispatched to the following:
Five concrete mixes with w/c ratios ranging from 0.35 to 0.55 were prepared (Table 1). Mixes were proportioned to have sufficient workability to be cast without excessive voidage or honeycombing at w/c 0.35, but sufficient cohesion and segregation resistance at w/c 0.55. The mix design was based on absolute volume and standardised with the main variable being w/c ratio. Preliminary details and requirements of the mix design were discussed by APG members, but the final design was developed by the Chair and Secretary, in close consultation with Mr Michael Thomas (Technical Optimisation Manager, Tarmac Ready Mix Technical Centre, Lutterworth) who carried out trial mixes to ensure feasibility. It is worth emphasising that the w/c ratios were not disclosed to participants.
Participants were required to prepare their sub-specimens for microscopy and determine the w/c ratio using their routine in-house methodology. All participants prepared polished thin-sections or blocks impregnated with fluorescein-dyed resin. These were examined either with optical petrographic microscopy or with scanning electron microscopy. However, there were variations in the resin, fluorescein dye, specimen size, equipment, number of images and magnification adopted, as would be expected. Details of the methods used by each participant are summarised in Table 2.
By June 2018, 11 participating laboratories had submitted detailed reports and their sets of 5 results. Some had presented more than one set, either by using more than one method (Lab 14), or by having several petrographers applying the same method to estimate w/c ratio (Labs 02, 04 and 09). These are denoted as result set (a), (b) et cetera (see Table 2). In total, 20 complete sets of results, or 100 individual w/c ratio determinations were obtained.
Figure 3 shows the estimated w/c ratios plotted against actual mix values from all participants. Results from laboratories that did not use reference standards (Labs 01, 02, 08, 13) are plotted separately to those from laboratories that did, the latter divided into FM-V (Labs 07, 09, 12, 14b) and FM-Q (Labs 04, 05, 14a). Data from the BSE method are treated as a separate category. Figure 4 presents the errors in the estimated w/c ratios for each participant, grouped according to the method used.
Figure 5 shows the maximum, minimum and average absolute error in the estimated w/c ratio, grouped to test method. Data from the UK Concrete Society inter-laboratory precision trial [3] using the BS 1881-124 physicochemical method are also included for comparison (discussed later). Overall, the microscopy-based methods gave much lower errors than the BS 1881-124 method. Within the optical microscopy 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, the magnitude of these are similar to those reported in an earlier study [22].
Figure 6 presents the frequency distribution and cumulative histogram of absolute error from all w/c ratio determinations (100) in this study. The data show that 37% of the estimated w/c ratios are within 0.025 of the target mix values, 58% are within 0.05 and 81% are within 0.1. In contrast, only 68% of the estimated w/c ratios using BS 1881-124 are within 0.1 of the target mix values.
The application of microscopy techniques for determining w/c ratio is based on the principle that capillary porosity of cement paste increases as w/c ratio increases. Using microscopy, it is possible to directly establish the microporosity of the cement paste rather than the concrete as a whole. This is a significant advantage over other porosity based test methods such as that given in BS 1881-124 [1], which cannot distinguish capillary porosity from porosity due to aggregate particles, air voids and cracks. Furthermore, the BS 1881-124 method requires a separate determination of cement content by chemical analysis of soluble silica and calcium oxide content and this is also prone to various errors [3]. In contrast, microscopy methods do not require a priori knowledge of the aggregates and cement content, or presence of voids and cracks [2].
It is critically important for optical microscopy-based methods (VA, FM-V, FM-Q) to use suitable reference standards to which the fluorescence intensity and/or other microstructural features of the test concrete can be compared against. The references should ideally be made of the same composition (cement type, mineral additions, admixtures, aggregates), cured in the same manner (duration, humidity, temperature) and to the same degree of hydration to the concrete in question. Each mixture type needs its set of reference standards that spans the widest range of w/c ratio possible to ensure that determinations are reliable. However, this is difficult to duplicate for unknown concretes, and inevitably, there will be variations between the reference concretes used by some labs and the tested specimens. Indeed, this was noted by several participants of this study as a source of error.
There are potential sources of error unrelated to the concrete that can affect measurement of w/c ratio by fluorescence microscopy, many of which can be mitigated by technicians and petrographers skilled in the preparation and examination of thin sections. Possible errors include variation in the quality of impregnation with fluorescent resin; variation in size (thickness) of the thin section; accidental or partial removal of the fluorescent resin-impregnated zone during preparation; and variation in thin section of the test specimen compared to that of reference standards. Other potential errors are summarised in Table 3.
Another important factor is that the relationship between fluorescence intensity and apparent microporosity in thin sections becomes much less linear at the extremes. The reasons for this are that at very low w/c ratios the fluorescence levels become much harder to detect. It is also difficult to be confident that complete epoxy impregnation has been achieved and preserved in the polished sections [19, 32, 33]. At very high w/c ratios, the fluorescence becomes saturated and this makes it increasingly difficult to detect differences in microporosity. Furthermore, self-quenching of the fluorophores can occur, which reduces fluorescence intensity. To avoid this, the concentration of fluorescein dye could be reduced for very porous samples (which would also be required in the reference standards) [33]. All participants of this study used a constant dye concentration in their specimens. 2ff7e9595c
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