Lane 1 PERFORMANCE OF SLAG CEMENT IN VIRGINIA CONCRETES

Transcription

1 Lane PERFORMANCE OF SLAG CEMENT IN VIRGINIA CONCRETES D. Stephen Lane Associate Principal Research Scientist Phone: -- Virginia Center for Transportation Innovation and Research 0 Edgemont Road Charlottesville, VA Word Count: + tables + figures = 0

2 Lane Abstract The Virginia Department of Transportation began allowing the use of slag cement in hydraulic cement concretes in and its use has steadily increased since that time. Several years ago a study was conducted to investigate the performance of bridge decks constructed with hydraulic cement concretes containing straight portland cement and portland cement plus slag cement with a specified w/cm of 0.. At the time of the field work, these decks were - years old. In addition to general observations of deck condition, cores were extracted from the decks for petrographic examination of internal condition and determination of concrete transport properties using electrical conductivity, and rate of water absorption. Overall, the slag cement concretes have performed well with a tendency toward lower transport properties and showing no inherent tendency for scaling. Some evidence of alkali-aggregate reactivity was observed with evident damage in one case, although it is not clear whether an insufficient amount of slag cement was used or this was a case where slag cement would simply be ineffective. Overall the performance and experience with slag cement has been favorable.

3 Lane INTRODUCTION AND BACKGROUND Slag cement is produced by finely grinding granulated, iron blast-furnace slag. In the past, this material was commonly referred to as ground, granulated blast-furnace slag, GGBFS, or ground slag. It is a byproduct of the production of iron from iron ore that, when properly processed, possesses hydraulic properties. When used with portland cement as a constituent of the cementitious material in hydraulic cement concrete, it provides reduced permeability, improved resistance to alkali-silica reaction and sulfate attack and higher later age strength. (-) Early-age strength development and setting times can be retarded in cold weather and when used as high proportions of the cementitious material. A major milestone in the use of slag cement in the United States was the installation of production facilities at Sparrows Point, Maryland in the early s. () In the Virginia Department of Transportation (VDOT) published a report () of a laboratory investigation of the use of slag cement in concrete and included it as an admixture for use in hydraulic cement concrete in its specifications. In, in an effort to avoid damage resulting from alkali-silica reaction, VDOT specifications for hydraulic cement concretes were revised to require the use of a pozzolan or slag cement in concrete when the alkali content of the portland cement exceeded 0.0% Na O equivalent. () This change was also viewed as a positive move to improve the resistance of concrete to chloride ion penetration and resulting chloride-induced corrosion of imbedded steel. In 00, VDOT initiated a study to evaluate the performance of bridge deck concretes produced with a maximum w/c of 0. to more recent concretes with a maximum w/cm of 0. produced with either straight portland cement or portland cement with either pozzolans or slag cement with respect to key factors relevant to the durability of concrete. This paper summarizes the findings that relate to concrete produced with a maximum 0. w/ccm and were produced with either straight portland cement or portland cement with slag cement. METHODOLOGY As part of studies to investigate the relative performance of concretes used in bridge deck construction, cores were collected in 00 from a number of bridge decks across Virginia. (,) Decks spanned two construction periods, - and - reflecting two different concrete specifications. In the earlier period, the cementitious material consisted only of portland cement with a maximum specified w/c of 0.. In the later period, cementitious materials included portland cement only, portland cement with fly ash, and portland cement with slag cement a maximum specified w/cm of 0.. The decks were selected so that a roughly equal number of structures representing the three groups (portland cement, 0. w/c; portland cement, 0. w/c; and portland cement with fly ash or slag cement, 0. w/cm) came from each of six climatic (physiographic) regions across Virginia. () This paper summarizes the results obtained from the slag cement concretes and portland cement concretes (maximum 0. w/c) obtained from the same VDOT districts. This subset includes four of the nine VDOT districts, Hampton Roads, Lynchburg, Northern Virginia, and Staunton. The decks in the slag cement group ranged in age from

4 Lane years and the portland cement group, - years. At the time of construction, the decision to use slag cement was at the discretion of the concrete producer. Records were not available to indicate the percentage of slag cement used as a portion of the cementitious material, but 0-0% was a common value at that time. Information on the concretes is included in Table. TABLE. Description of Samples Type (SC/PC) District Structure no. Year No. of cores SC Lynchburg 00 SC Lynchburg SC Hampton Roads SC Staunton 0 SC No. Virginia 0 SC No. Virginia 0 PC Lynchburg 0 PC Hampton Roads PC Staunton PC Staunton PC No. Virginia PC No. Virginia When the cores (nominal 0-mm diameter) were received in the laboratory, they were examined for general condition and characteristics. A 0 mm thick specimen was cut from the top of two of the cores for measuring the transport properties of the concrete. Petrographic specimens were cut from the third core or from the remaining pieces of the two cores when only two cores were available. When the third core was used or the core was of sufficient length, the slab was cut parallel to the core axis (Figure ). If the core was too short to provide both a transport property specimen and a slab of sufficient length parallel to the core axis, the petrographic slab was cut perpendicular to the axis. One face of the slab was then finely lapped for examination using a stereoscopic microscope. Point count and linear traverse methods were used at a magnification of 0x to determine the volumetric proportions and the parameters of the air void systems of the concretes. Detailed examinations of the specimens were conducted at magnifications ranging from x-0x to assess the general condition and characteristics of the concretes. Two methods were used to measure transport properties: ASTM C, a sorptivity (rate of absorption) test, and a determination of the electrical conductivity of the concrete using ASTM C 0 equipment. (,) Transport property specimens were first conditioned following standard procedures defined in ASTM C to bring the capillary pore system of the concrete into a standard unsaturated state. The conditioning process followed was days in an environmental chamber at 0 o C and 0% relative humidity followed by weeks in an individual sealed container at standard laboratory room temperature. Because of the variable nature of the top (riding) surfaces of the cores due to tining or saw cutting, the saw cut surface of the specimen was used as the absorptive face during testing.

5 Lane Following the sorptivity testing, specimens were vacuum-saturated and the electrical conductivity determined. Following this testing, the specimens were dried to a constant mass in a forced-air oven at 0 o C to determine the concrete s vacuum-saturated absorption and the relative moisture state that existed in the specimen following the moisture conditioning immediately prior to the sorptivity testing. 0 Figure. Transport property disk (wrapped with tape) and petrographic specimen slab cut parallel to the core axis from the same core. RESULTS AND DISCUSSION For this study, the most relevant results of the point-count and linear traverse determinations are the paste content, air content and spacing factor. The paste content is important since the paste is the primary phase responsible for the transport properties, and it is the phase that is afforded protection from freeze-thaw damage by the air void system. These values are shown in Table and comments on the general condition and characteristics of the concretes are given in Table. More detailed results can be found in reference (). The paste content for a standard VDOT concrete ( lbs/yd cementitious materials) at the maximum w/cm (0.) is %. Specimens for two of the slag cement concretes, L-00 and NV-0 exhibited elevated paste contents. The general condition and characteristics of L-00 suggest that the elevated paste content in this case is a sampling issue. In the case of NV-0, evidence of bleeding and retempering and the general poor condition suggest the elevated paste content is reflective of excess water.

6 Lane TABLE. Paste content, air content and spacing factor of concretes Structure Paste content % AC %, (P-C) AC % (LT) Spacing factor, mm L-00 (SC) L- (SC) HR- (SC) S-0 (SC) NV-0 (SC) NV-0 (SC) L-0 (PC) HR- (PC) S- (PC) S- (PC) NV- (PC) NV-0 (PC) TABLE. General condition and petrographic characteristics Structure Comments L-00 (SC) Overall good condition, some scaling L- (SC) Overall good condition, surface scaled, short paste cracks HR- (SC) Overall good condition, some scaling S-0 (SC) Overall poor condition, evidence of retempering, fair-poor paste aggregate bond, cracks throughout associated with dolomitic coarse aggregate, frequent paste and aggregate cracks NV-0 (SC) Overall poor condition, evidence of bleeding and retempering, poor consolidation, plastic cracks, and surface cracks NV-0 (SC) Overall poor condition, evidence of retempering, poor paste-aggregate bond, fine map cracking on surface, spotty evidence of ASR with chert particles L-0 (PC) Overall fair condition, evidence of retempering, ASR product, voids filled or lined, occasional aggregate cracks HR- (PC) Overall good condition, plastic settlement cracks at 0 and mm S- (PC) Overall fair condition, voids filled/lined with ettringite S- (PC) Overall poor condition, surface scaled w/ cracks to 0mm, ASR product associated with dolomite and chert, frequent paste and aggregate cracks NV- (PC) Overall fair condition, evidence of retempering, poor paste-aggregate bond, surface cracking and some scaling, coalescing air voids NV-0 (PC) Overall fair condition, surface scaled, occasional paste and aggregate cracks, evidence of ASR product around chert` All of the slag cement concretes had air void systems that would be expected to provide protection from freeze-thaw damage. Scaling was noted in most cases (Figures and ), but was no more prevalent in the slag cement concrete than in the portland cement concretes and had not created a performance concern. One of the portland cement concretes (S-) had an excessively high spacing factor (0. mm) owing to considerable infilling of voids with ettringite, which is suggestive of high transport properties. Two of the slag cement concretes, S-0 and NV-0, exhibited evidence of alkali-aggregate reaction. The evidence was more subtle in NV-0, principally

7 Lane 0 sweating of ASR gel from sand-sized chert particles into the surrounding paste. The damage was more extensive in S-0, involving dolomite coarse aggregate particles. Considerable ASR-related damage was also noted in S-, which contained a similar dolomitic coarse aggregate and sand-sized chert. Slag cement is known to be effective in preventing ASR-related damage (-) and was used in to prevent damaging ASR in a section of pavement on I- in Virginia with an aggregate that had previously reacted with a low-alkali cement. (,) The slag cement composed 0% of the cementitious material with low-alkali cement from the same source used in the damage section and no recurrence of ASR damage was noted through years service. Prevention of ASR-related damage is one of the reasons VDOT specifies the use of slag cement pozzolan in concrete. Minimum percentages of slag cement as a portion of the cementitious material is governed by the alkali content of the portland cement with which it is used. The general guidelines have evolved through the years based on research (,,) and are given in Table. The most recent guidance is focused on limiting the transport properties of concrete, which requires higher proportions of slag cement than would be needed to prevent ASR damage with lower alkali content portland cements. An issue that needs further attention is the use of slag cement with alkalicarbonate rocks. It has been reported that slag cement is ineffective in preventing damage with ACR rocks. () However, more recent work is suggesting that ACR is simply a variant of ASR which begs the question: why then wouldn t slag cement work? Figure. Structure L-, slag cement concrete on US 0 NB in Campbell Co.

8 Lane Figure. Slag cement bridge deck (L-) in 0; deck was placed in. TABLE. VDOT Guidelines for Use of Slag Cement to Prevent Damaging ASR Period Slag Cement Guidelines - (Ref ) -0% of cementitious material if pc alkali content exceeds 0.0% Na O equivalent -0 (Ref ) Minimum % with pc up 0.0% Na O equivalent Minimum % with pc up to 0.0% Na O equivalent Minimum 0% with pc up to.00% Na O equivalent 0-Pres. (Ref ) Minimum 0% with pc up to 0.% Na O equivalent Minimum 0% with pc up to.00% Na O equivalent The results of the transport property tests are given in Table. Two values are reported for the sorptivity (C ) tests, an initial rate of absorption (C i ) which reflects the rate over roughly the first six hours, and the secondary rate (C s ) which typically follows change in the slope to a slower rate. (,) Martys and Ferraris have suggested that this break indicates a shift in dominance from the capillary pores to the smaller gel pores. () Ci values ranged from. to. mm/s / x - for the slag cement concretes compared to. to. for the portland cement concretes. Similarly, Ci values were generally lower for the slag cement group, with values ranging from. to. while the portland cement group ranged from. to.. Vacuum-saturated absorptions for the two groups ranged from. to.% for the slag cement and. to.% for the portland cement concretes. With the vacuum-saturated absorption, it was possible to determine

9 Lane 0 the degree of saturation of the specimen prior to the sorptivity testing as a percentage of the vacuum-saturated absorption. This value is important because the degree of saturation of the specimen exerts a controlling influence on the sorptivity. () At the time of sorptivity testing the slag cement specimen moisture condition as a percentage of vacuum saturated absorption ranged from 0 to, while the portland cement specimens ranged from to. Recent laboratory studies have brought to light several confounding influences that should be considered in comparing sorptivity results. (,) Exposure to deicing chemicals can slow the rate of absorption. () Sorptivity is also influenced by paste content, with higher paste contents leading to higher sorptivity for a given paste quality, as well as the conditioning prior to testing, which may not overcome the concrete s preexisting moisture state. () The concretes in this study had all been exposed to deicing chemicals (NaCl) in the field, but the extent of exposure may be variable with respect to structure location. The PC concretes as a group exhibited higher sorptivity and their moisture state prior to testing was slightly lower than the SC concretes. NV-0, the SC with the highest sorptivity was at the high end of moisture state prior to sorptivity testing, although it also had a high paste content. Conversely, L-00 the other SC concrete with high paste content had the lowest sorptivity. Unfortunately, data are not available to assess its moisture state after conditioning. Nonetheless, these issues warrant careful consideration when evaluating the data. The electrical conductivity values for the slag cement concretes ranged from. to. ms/m compared to the portland cement concrete values which ranged from. to. ms/m. Table provides conductivity values and their expected C 0 electrical conductance values based on regression of a large amount of laboratory data. () TABLE. Results of Transport Property Tests Structure Sorptivity Test (C ) % of V-S Abs Vacuum- Conductivity mm/s / x - after C Saturated Abs, ms/m Ci Cs conditioning % L-00 (SC) L- (SC) HR- (SC).... S-0 (SC).... NV-0 (SC).... NV-0 (SC) L-0 (PC).... HR- (PC)..0.. S- (PC).... S- (PC).... NV- (PC).0... NV-0 (PC) TABLE. Relationship Between Electrical Conductivity and Electrical Conductance () Electrical Conductivity ms/m Electrical Conductance, Coulombs (C 0)

10 Lane CONCLUSIONS This study reports the findings on an evaluation of the performance of slag cement concretes in bridge decks after - years service. The evaluations included petrographic examinations for general condition and the determination of sorptivity and electrical conductivity. As a group, the SC concretes had lower electrical conductivity and lower sorptivity than a companion group of PC concretes, although the sorptivity values may be somewhat skewed by differences in moisture state prior to testing. The moisture condition of the test specimen has an important impact in transport property test results and should be carefully evaluated when performing comparative field studies. Three of the six SC concretes were judged to be in good overall condition based on petrographic examination. The other three were rated poor. In each of these cases, retempering and excess water appeared to play a role. In one case there was extensive damage associated with a dolomitic coarse aggregate which may reflect an inability of SC to control ACR or an insufficient amount of SC to control ASR. Scaling appeared to be no greater an issue with SC than with the PC concretes. Overall VDOT s experience with SC has been quite favorable. Exceptions to this emphasize the need to exercise good quality control practices in concrete production and construction. REFERENCES. American Concrete Institute. Slag Cement in Concrete and Mortar. ACI-R Hogan, F.J, and Meusal, J. W. Evaluation for Durability and Strength Development of a Ground Granulated Blast Furnace Slag. Cement, Concrete, and Aggregates. Vol., No.,, pp Lane, D.S., and Ozyildirim, H.C. Combinations of Pozzolans and Ground, Granulated Blast-Furnace Slag for Durable Hydraulic Cement Concrete. VCTIR 00-R. Virginia Center for Transportation Innovation and Research,.. Ozyildirim, H.C., and Walker, H.N. Evaluation of Hydraulic Cement Concretes Containing Slag added at the Mixer. VCTIR -R. Virginia Center for Transportation Innovation and Research,.. Lane, D.S. Alkali-Silica Reactivity in Virginia. VCTIR -R. Virginia Center for Transportation Innovation and Research,.. Williamson, G., Weyers, R.E., Brown, M.C., and Sprinkel, M.M. Bridge Deck Service Prediction and Cost. VCTIR 0-CR. Virginia Center for Transportation Innovation and Research, 00.. Lane, D.S. An Evaluation of the Performance of Concretes Containing Fly Ash and Ground Slag in Bridge Decks. VCTIR 0-R. Virginia Center for Transportation Innovation and Research, 00.

11 Lane 0 0. Lane, D.S. Supplanting the Rapid Chloride Permeability Test with a Quick Measurement of Concrete Conductivity. VCTIR 0-R. Virginia Center for Transportation Innovation and Research, 00.. Lane, D.S. Laboratory Comparison of Several Tests for Evaluating the Transport Properties of Concrete. VCTIR 0-R. Virginia Center for Transportation Innovation and Research, 00.. Lane, S. Research Pays Off: Alkali-Silica Reaction, Preventing Damage in Hydraulic Cement Concrete, TR News, Nov-Dec, No., 00, pp Lane, D.S., and Ozyildirim, HJ.C. Use of Fly Ash, Slag or Silica Fume to inhibit Alkali-Silica Reactivity. VCTIR 0-R. Virginia Center for Transportation Innovation and Research,.. Rogers, C.A. and Hooton, R.D. Comparison Between Laboratory and Field Experience of Alkali-Carbonate Reactive Concrete. Proceedings of the Ninth International Symposium on Alkali-Aggregate Reactions. The concrete Society, Slough, UK,, pp. -.. Martys, N.S., and Ferraris, C.F. Capillary Transport in Mortars and Concrete. Cement and Concrete Research. Vol., No.,, pp Spragg, R.P., Castro, J., Li, W., Pour-Ghaz, M., Huang, P-T., and Weiss, J. Wetting and Drying of Concrete Using Aqueous Solutions Containing Deicing Salts. Cement and Concrete Composites. In Press.. Castro, J., Bentz, D., and Weiss, J. Effect of Sample Conditioning on the Water Absorption of Concrete. Cement and Concrete Composites. Submitted for publication.