CEEN-043 LABORATORY 4 AGGREGATES FOR PORTLAND CEMENT CONCRETE

Transcription

1 CEEN-43 LABORATORY 4 AGGREGATES FOR PORTLAND CEMENT CONCRETE

2 Aggregates for Portland Cement Concrete Overview Aggregates generally occupy 7 8% of the volume of concrete and can therefore be expected strongly influence the concrete properties. Aggregates are granular materials derived, for the most part, from natural rock and minerals. A mineral can be defined as a naturally occurring solid substance with an orderly internal structure. Rock, whether igneous, sedimentary or metamorphic, is generally composed of several types of minerals. Weathering of exposed rock produces particles of sand, gravel, silts and clays. Aggregates used in the production of Portland Cement Concrete (PCC) must conform certain standards for engineering use. ASTM s C-33, C-131, C-666, C-117, C-29, C-127, C-128 and C-29 address fundamental characteristics of aggregates be used in PCC and set guidelines for acceptance or rejection of an aggregate source. Gradation Gradation is the particle size distribution of an aggregate as determined by sieve analysis (ASTM C-136). The seven standard sieve sizes called for in ASTM C-33 for fine aggregates range in size from the #1 sieve (1 um) 3/8 in. The thirteen standard sieve sizes used for coarse aggregate gradation have openings ranging from.46 in. 4 in. The gradation and grading limits are usually expressed as the percentage of material passing a given sieve. Table 1 shows these limits for each size number. There are several reasons for specifying gradation limits and maximum aggregate size. The gradation and max size will affect relative aggregate proportions as well as cement and water requirements, durability, economy and workability. In general, aggregates that do not have a large deficiency or excess of any size and exhibit a smooth gradation curve will produce the most satisfacry results. The effect of using a variety of aggregate sizes is a reduction of the voids between aggregates. During the early years of concrete design, it was thought that the smallest percentage of voids (greatest density of aggregates) was the optimum condition for concrete. It is now known that this is not the best target for the designer. A low void ratio is desirable for both the quality and economy of concrete, but not the lowest possible void ratio. The significance of aggregate gradation can be best appreciated by considering concrete as a slightly compacted assembly of aggregates bound gether with cement paste, with the voids between the aggregates completely filled with cement paste. It therefore follows that the amount of paste depends on the amount of void space that must be filled and the tal surface area of the aggregates that must be coated. It is important note that though a theoretical gradation curve which gives the minimum void space can be arrived at using particle geometries, this solution would not produce a workable concrete. A compromise must be worked out between workability and economy.

3 Size Number Nominal Size 1 3. in 1. in 1 Table 1 Grading Requirements for Coarse Aggregates (ASTM C-33) Amounts Finer Than Each Laborary Sieve, Weight Percent Passing 4 in 3. in in 2 2. in 1. in in 1 in 37 2 in # 4 2. in in 1. in in ¾ in in # in ½ in in 3/8 in in # in ¾ in ¾ in 3/8 in 1 67 ¾ in # 4 7 ½ in # 4 8 3/8 in # 8 1 ½ in 3/8 in No 4 No 8 No

4 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Fine Aggregate Grading ASTM C-33 permits a wide range of gradations for fine aggregates, but specifications by other interested organizations may be more restrictive. The most desirable fine aggregate gradation depends on the type of work, the richness of the mix (cement content) and the maximum size of the coarse aggregate. In leaner mixes or when small size coarse aggregates are used, a grading that approaches the maximum recommended percentage passing each sieve is desirable for workability. In general, if the w/cm ratio is kept constant and the ratio of fine--coarse aggregate is chosen correctly, a wide range in grading can be used without a measured effect on strength. The amount of fine aggregate passing the # and #1 sieves affects workability, finishability, and bleeding, and adequate fine material is needed for good cohesiveness and plasticity. Most specifications allow 1% 3% of the fine aggregate pass the # sieve. The lower limit may be sufficient for easy placement conditions or where the concrete is mechanically finished as in pavements. However, for hand finished floors or where a very smooth surface texture is desired, fine aggregate with at least 1% passing the # sieve and at least 3% passing the #1 sieve is desirable. It is important note that C-33 limits the percent passing the #1 sieve between 2% and 1%. The use of a single parameter assess and describe a gradation curve can be useful in checking the uniformity of grading. The fineness modulus is such a parameter and is defined as the cumulative percent retained on standard sieve # s 1,, 3, 16, 8, 4, 3/8, ¾, 11/2, 3, and 6 divided by 1; or : E (Cumulative % retained on std. sieves) 1 Fineness modulus is an index of the fineness of an aggregate; the higher the FM the coarser the aggregate. The fineness modulus for fine aggregate should lie between 2.3 and 3.1. The fineness modulus can be used check the consistency of grading when relatively small changes are be expected, but it should not be used compare gradations from two different aggregate sources. Fineness modulus is a crude measure of grading; two aggregates with the same fineness modulus can have very different gradation curves. The fineness modulus of fine aggregate is require for mix proportioning since sand gradation has the largest effect on workability. The finer the sand, the greater the number of particles available improve workability. This allows an increase in the amount of coarse aggregate and a decrease in the amount of fine aggregate. C-33 requires that the FM not vary by more than.2 from the value used in mix design. Larger variations can cause unacceptable changes in workability and will require reproportioning. The FM of coarse aggregate is not used in mix design.

5 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Maximum Aggregate Size When referring aggregates for Portland cement concrete, it is important distinguish between the terms maximum aggregate size and nominal maximum size. ASTM C-126 defines the maximum size of an aggregate as the smallest sieve size through which all of a particular aggregate must pass. The nominal maximum size is defined as the smallest sieve size through which the major portion of the aggregate must pass. The nominal max size sieve may retain % 1% of the aggregate depending on the size number. For example, a number 67 aggregate has a maximum size of 1 in. and a nominal max size of ¾ in. 9 1 percent of this aggregate must pass the ¾ in. sieve and all of the aggregate must pass the 1 in. sieve. The choice of nominal max aggregate size is determined by the specific requirements of the job. If the aggregate size is o large, the concrete at any given cross section of a member may not be representative of the entire material because of the location of an overly large aggregate particle. To guard against this type of possible problem, the following general guidelines should be followed. The maximum aggregate size should not exceed: 1. One fifth the narrowest dimension of a concrete member 2. Three fourths the clear spacing between reinforcing bars 3. One third the depth of a slab The larger the maximum aggregate size, the lower the paste (cement) requirements for the mix. For a given workability and cement content, the strength of concrete increases with increasing aggregate size because the w/cm ratio can be lowered. However, with larger aggregate sizes, increased internal stresses tend lower strength. This effect is only noticeable in rich mixes; concrete with low cement contents, which are typical of mass concrete, show continuing increases in strength. If the w/cm ratio is not lowered, strength will drop as aggregate maximum size increases. As a result, high concrete strengths (>1 ksi) may require the use of a maximum aggregate size as small as 3/8 in. In general, an increase in maximum aggregate size will improve concrete durability because there is less paste subject either chemical or physical attack. It is also true that if an aggregate is susceptible freeze-thaw damage, a reduction in aggregate size will improve durability.

6 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Moisture Contents Since aggregates have some porosity, water can be absorbed in the body of the aggregates. Water can also be retained on the surface of the aggregate particle as a film of moisture. Thus, sckpiled aggregates can have variable moisture contents. It is necessary have information about the actual moisture content at the time of batching since if there is a tendency for the aggregate absorb water, it will be removed from the paste resulting in a decrease in the w/cm ratio and a decrease in workability. Conversely, if excess water is present at the surface of the aggregate, extra water will be added the paste and the w/cm ratio will be higher than desired. Moisture States It is both necessary and convenient define four moisture states of an aggregate as follows: 1. Oven-dry (OD). All moisture is removed from the aggregate by heating in an oven at 1 degrees C a constant weight. 2. Air-dry (AD). All moisture is removed from the surface of the aggregate, but internal pores are partially full 3. Saturated-surface-dry (SSD). All pores are filled with water, but there is no film of water on the surface. 4. Wet. All pores are completely filled with water and there is a film of water on the surface. Of these four states, only two, the OD and SSD states, correspond specific moisture contents, and either of these states can be used as reference states for calculating moisture contents. The AD and wet states represent the variable moisture contents that will exist in sckpiled aggregates. The SSD state is the better choice as a reference state for the following reasons: 1. It represents the equilibrium moisture state of the aggregate in concrete; that is, the aggregate neither absorbs nor gives up water the paste. 2. The moisture content of the aggregates in the field is usually much closer the SSD state than the OD state. 3. The bulk specific gravity (BSG) of aggregates is more accurately determined by the displacement method in the SSD state. 4. The moisture content can be calculated directly from measurements of apparent BSG using the displacement method. One disadvantage of using the SSD state is that it is not easy obtain a true SSD condition and it requires skill and practice do so. Many people prefer use the OD state as a reference point because of this.

7 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Absorption and Surface Moisture To calculate the amount of water that aggregate will add or subtract from the paste, it is convenient define three quantities. These are absorption capacity, effective absorption, and surface moisture. Absorption capacity (A) represents the maximum amount of water the aggregate can absorb. It is calculated from the difference in weight between the SSD and OD states, expressed as a percentage of the OD weight: A = W SSD -W OD x1 W OD where W SSD and W OD represent the weight of the aggregate in the SSD and OD states respectively. The absorption capacity is used in mix proportioning calculations and can be used convert from the SSD the OD system or vice versa [W SSD = W OD (1+A/1)]. Most normal weight aggregates (fine and coarse) have absorption capacities in the 1%-2% range. Abnormally high absorption capacities indicate high porosity aggregates, which may have potential durability problems. The effective absorption (EA) represents the amount of water required bring an aggregate from the AD state the SSD state, expressed as a fraction of the SSD weight: EA = W SSD -W AD W SSD x1 Thus, the weight of air-dry aggregate batched will be equal the desired weight of SSD aggregate (WSSD) minus W abs, while the mix water will be increased by W abs. If an aggregate is close the OD condition when batched, it will take some time for the aggregate absorb all of the water necessary reach the SSD condition. In such cases, the effective absorption may be taken indicate the amount of water the aggregate absorbs in 3 minutes. Further absorption beyond this time will be slow, and will be accompanied by a gradual decrease in the workability of the concrete.

8 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 The surface moisture (SM) represents water in excess of the SSD state, also expressed as a fraction of the SSD weight: SM = W wet -W SSD x1 W SSD It is used calculate the additional water (W add ) added the concrete with the aggregate. W add = (SM) W SSD 1% In this case, the weight of wet aggregate will equal WSSD plus W add, while the mix water will be decreased by W add account for surface moisture. The above equations can be expressed in general terms, since both the AD and wet states represent the possible conditions of sckpiled aggregates. The moisture content (MC) of the aggregate is given by: MC = W sck -W SSD x1 W SSD where W sck is the weight of the aggregate in the sckpiled condition. If the moisture content is positive, it is surface moisture; if negative it is effective absorption. Thus: W MC = (MC)W SSD 1 where W MC is the tal moisture associated with the aggregates and is positive when the moisture content is positive and negative when the moisture content is negative.

9 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 OBJECTIVE: EQUIPMENT: To determine the grain size distribution and weight/volume properties of aggregates used in the making of Portland cement concrete (PCC). A set of sieves (1", 3/4", ½, 3/8", Nos. 4, 8, 16, 3, 1 & 2, and pan), brushes for cleaning sieves, balance, large pans, sieve shaker, mold, glass plate, volumetric flask, tampers and drying fans and ovens. ASTM REF: C 29, C 127, C 128, C , C 122 PROCEDURES: Part A - Specific Gravity & Absorption of Coarse Aggregate 1. Obtain an 8 gram sample of soaked coarse aggregate and roll it in a large absorbent cloth remove all visible films of water. Take care avoid removal of water from aggregate pores during this operation. Determine and record the mass of a clean moisture tin. Place approx. 1 grams of SSD coarse aggregate in the tin and record the mass of tin + sample the nearest.1g. Place the moisture tin in a drying oven at 11 +/- C. Remove the moisture tin + sample from the oven periodically and check its weight the nearest.1 g. Continue drying and weighing until the aggregate has reached a constant weight, signified by two consecutive weight readings, separated by at least 1 minutes, giving identical weights. 2. Weigh the remaining SSD aggregate sample (in air) the nearest.1 gram and record this weight on the data sheet in the space provided. Immediately after weighing in air, place the SSD sample in the specific gravity sample basket and determine its weight submerged in water. Record this information on the data sheet. Part B - Specific Gravity & Absorption of Fine Aggregate 1. Partially fill the specific gravity vessel with D. I. water and record the mass the nearest.1 g on the data sheet in the space provided. Take care remove any water that may have splashed on the outside of the vessel before weighing. 2. Remove the fine aggregate test sample from the water and spread it out on a flat, nonabsorbent material. Dry the sample with a gently moving current of warm air until the sample approaches a free-flowing condition. Continue drying and test at frequent intervals with the Cone Test until the sample has reached a saturated surface dry condition.

10 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No Determine the mass of a clean moisture tin the nearest.1 g. and record on the data sheet. Place approx. 2g. of SSD fine aggregate in the moisture tin and record the mass of the tin + sample the nearest.1g. Place the moisture tin + sample in a drying oven at 11 +/- o C. Remove the moisture tin from the oven periodically and check its weight the nearest.1 g. Continue drying and weighing until the aggregate has reached a constant weight, signified by two consecutive weight readings, separated by at least 1 minutes, giving identical weights. 4. Place approximately 1g of the SSD fine aggregate in the partially filled specific gravity vessel and record the mass of the vessel + aggregate the nearest.1g.. Fill the specific gravity vessel + aggregate approx. ½ from the p and place in the vacuum tank. Apply a vacuum the sample for approx. minutes de-air the sample. Attach the conical p the specific gravity vessel and fill until a meniscus forms at the tip of the conical p. Dry the outside of the vessel completely taking care not disturb the meniscus. Weigh the nearest.1g and record the mass of the filled vessel + aggregate. Determine and record the water temperature the nearest.1 degree C. Part C - Sieve Analysis of Coarse Aggregate 1. Weigh the nearest.1 g each sieve be used and record on data sheet. Make sure each sieve is cleaned of loose particles prior weighing. (Note: Never force particles forward through openings in sieves. If particles are imbedded, try remove them with gentle pressure in a backwards direction.) 2. Weigh the nearest.1 g a representative 2 kg specimen of coarse aggregate. 3. Separate the aggregate through a nest of sieves using the mechanical shaker. At least minutes of mechanical shaking is desirable. 4. Weigh the nearest.1 g each sieve with the aggregate retained by the sieve. Part D - Sieve Analysis of Fine Aggregate 1. Weigh the nearest.1 g each sieve be used and record on data sheet. Make sure each sieve is cleaned of loose particles prior weighing. (Note: Never force particles forward through openings in sieves. If particles are imbedded, try remove them with gentle pressure in a backwards direction.) 2. Weigh the nearest.1 g a representative g specimen of fine aggregate. 3. Separate the aggregate through a nest of sieves using the mechanical shaker. At least minutes of mechanical shaking is desirable. 4. Weigh the nearest.1 g each sieve with the aggregate retained by the sieve.

11 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Part E - Dry Rodded Bulk Unit Weight of Coarse Aggregate 1. Weight the empty mold the nearest.1 lb. Fill the mold with coarse aggregate onethird full and level the surface with the fingers. Rod the layer with 2 strokes of the tamping rod evenly distributed over the surface, but do not allow the rod strike the botm of the mold forcibly. 2. Fill the mold two-thirds full and again level and rod as above, but use no more force than is necessary penetrate the previous layer of aggregate. Finally, fill the mold overflowing and rod again in the manner previously mentioned. Level the surface of the mold with the fingers or a steel straightedge in such a way that any slight projections of the larger pieces of the coarse aggregate approximately balance the larger voids in the surface below the p of the mold. 3. Determine the weight of the mold filled with rodded aggregate the nearest.1 lb.

12 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 Report Part A - Specific Gravity & Absorption of PCC Aggregates 1. Determine the absorption capacity (A) for each aggregate type. 2. Determine the Bulk, Bulk SSD, and Apparent specific gravity for each aggregate type. 3. Comment on the values obtained above as compared typical values presented in class. Part B - Sieve Analysis 1. Plot the grain size distribution of each aggregate. 2. Determine the fineness modulus (FM) for the fine aggregate. 3. Determine the effective size (D 1 ), the coefficient of uniformity (C u ), and the coefficient of gradation (C c ) for each aggregate. 4. Determine the closest size number of the coarse aggregate(s) using Table 1. Comment on Variances from ASTM lerances (if any) Part C - Bulk Unit Weight of Coarse Aggregate 1. Determine the dry rodded bulk unit weight for the coarse aggregate tested. 2. Using the results from Part A, determine the bulk porosity of the coarse aggregate in the dry-rodded state of compaction.

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14 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 SIEVE ANALYSIS DATA SHEET Sieve Size 1 " Sieve Wt, g Coarse Aggregate Sieve + Agg Wt., g Sieve Wt, g Sieve + Agg Wt., g Sieve Wt, g Fine Aggregate Sieve + Agg Wt., g Sieve Wt., g Sieve + Agg Wt., g 3/4" 1/2" 3/8" No. 4 No. 8 No. 16 No. 3 No. No. 1 No. 2 Pan

15 CEEN 43 - Behavior & Properties of Engineering Materials Laborary Experiment No. 1 SPECIFIC GRAVITY AND ABSORPTION DATA SHEET Aggregate Type Coarse Fine Weight of Empty Pan, g Weight of Pan + SSD Coarse Agg in air, g Weight of SSD Coarse Agg Submerged, g Weight of Moisture Tin, g Weight of Moisture Tin + SSD Agg, g Weight of Partially Filled Vessel, g Weight of Partially Vessel + SSD Agg, g Weight of Filled Vessel + Agg, g Water Temp, o C Weight of Moisture Tin +OD Agg, g DRY UNIT WEIGHT OF COARSE AGGREGATE DATA SHEET Weight of Mold, lb Volume of Mold, ft Weight of Mold + Dry- Rodded Aggregate, lb 1